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High resolution MMR of organotin compounds and ESR study of X-ray irradiated organic single crystals Cyr, Natsuko 1967

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The U n i v e r s i t y of B r i t i s h Columbia FACULTY OF GRADUATE STUDIES PROGRAMME OF THE FINAL ORAL EXAMINATION FOR THE DEGREE OF DOCTOR OF PHILOSOPHY B. Eng., U n i v e r s i t y of Osaka P r e f e c t u r e , Japan M i S c , U n i v e r s i t y of B r i t i s h Columbia, Canada TUESDAY, DECEMBER 5, 1967, AT 3:30 P.M. IN ROOM 225, CHEMISTRY . COMMITTEE IN CHARGE of NATSUKO CYR Chairman: B. N. Moyls C A . McDowell B.A.. Dunell E. Peters R. Stewart F. Aubke W.C. L i n E x t e r n a l Examiner: G. Vincow Department of Chemistry U n i v e r s i t y of Washington S e a t t l e , Washington Research Supervisor: W.C. L i n SOME MAGNETIC RESONANCE STUDIES ABSTRACT In part I of t h i s t h e s i s , organotin compounds were i n v e s t i g a t e d using high r e s o l u t i o n proton and t i n 119 nuclear magnetic resonance technique. The t i n 119 chemical s h i f t of about f o r t y organotin compounds were measured by absorption mode f o r the f i r s t time. The t i n chemical s h i f t and t i n 119-proton c o u p l i n g constant, i n some m e t h y l t i n h a l i d e s were found to be solvent and co n c e n t r a t i o n dependent i n e l e c t r o n donor s o l v e n t s . This dependence was a t t r i b u t e d to the formation of higher than four coordinated complexes w i t h solvent molecules. E q u i l i b r i u m constants of the complex f o r -mation, the t i n chemical s h i f t s , and the t i n - p r o t o n c o u p l i n g constants of the complexes were obtained i n a few s o l v e n t s . The second-order paramagnetic chemical s h i f t s of m e t h y l t i n h a l i d e s , m e t h y l t i n c a t i o n s , and f i v e coordinated compounds were c a l c u l a t e d and compared with the observed t i n chemical s h i f t s . Good q u a l i t a -t i v e agreements between c a l c u l a t e d values and observed values confirmed that the second-order paramagnetic term i n t i n chemical s h i f t s i s dominant i n the chemical s h i f t changes i n those compounds. In part I I , X-ray i r r a d i a t e d single, c r y s t a l s of malonamide and cyanoacetamide were stud i e d by e l e c t r o n s p i n resonance technique. In both cases at l e a s t two types of r a d i c a l s were found. One was the usual e l e c t r o n type r a d i c a l the proton c o u p l i n g tensor of which had been stud i e d quite e x t e n s i v e l y i n the past. In t h i s study, besides the proton c o u p l i n g and i n the case of cyanoaccetamide, the c o u p l i n g tensor f o r the cyano-nitrogen was a l s o measured and discussed. The second r a d i c a l found both i n X-ray i r r a d i a t e d malonamid and cyanoacetamide was a CT-electron type r a d i c a l which was produced by the loss of one of the amide protons (-C0NH)„ The proton hyperfine c o u p l i n g constant was found to be almost i s o t r o p i c and very l a r g e , more than 80 gauss i n both -compounds. The n i t r o g e n coupling tensor f o r the amidenitrogen was found to be a x i a l l y symmetric w i t h the unique p r i n c i p a l value equal to 36.6 gauss i n the one (malonami.de) and 25.4 gauss i n the other (cyanoacetamide). The p r i n c i p a l value i n perpendicular d i r e c t i o n was found to be very small but could not be determined c o n c l u s i v e l y . A semi-e m p i r i c a l molecular o r b i t a l c a l c u l a t i o n was performed on the fragment of c j - e l e c t r o n r a d i c a l together w i t h p e r t u r b a t i o n through c o n f i g u r a t i o n i n t e r a c t i o n ; the large i s o t r o p i c proton coupling constants were explained t h e o r e t i c a l l y . GRADUATE STUDIES F i e l d of Study: P h y s i c a l Chemistry Topics i n P h y s i c a l Chemistry Seminar i n S p e c i a l Topics (ESR) Seminar i n Chemistry Topics i n Inorganic Chemistry Spectroscopy and Molecular S t r u c t u r e Topics i n Organic Chemistry Related Studies: L i n e a r Algebra Computer Programming D i f f e r e n t i a l Equations Modern Physics J.A.R. Coope A. Bree W.C. L i n W.A. Bryce W.R. C u l l e n H.C. C l a r k N. B a r t l e t t A.T. Kwon B.A. Dunell C. Reid A. Bree J.P. Kutney A.I. Scott F. McCapra H.A. Simmons A.G. Fowler S.A. Jennings M. Bloom PUBLICATIONS N.Cyr, L.W. Reeves, A Study of Tautomerism i n C y c l i c Diketones by Proton Magnetic Resonance, Can. J . Chem. 43, 3057 (1965) . H.C. C l a r k , N. Cyr, J.H. T s a i , Nuclear Magnetic Reso-nance Spectra of Some F l u o r i n a t e d Organotin Compounds, Can. J . Chem. 45, 1075 (1967). N. Cyr, W.C. L i n , E l e c t r o n Spin Resonance of a Sigma-type R a d i c a l Formed by X - I r r a d i a t i o n of Malonamide, Chem. Comm. 192 (1967) . 13 N. Cyr and T. Cyr, C—H Coupling Constant and Average E x c i t a t i o n Energies, J . Chem. Phys. 47, No. 8 (1967) . •y. HIGH RESOLUTION NMR OF ORGANOTIN COMPOUNDS AroD E3R STUDr OP X-RAY IRRADIATED ORGANIC SINGLE CRYSTALS NATSUKO CYR M.Sc., U n i v e r s i t y o f B r i t i s h Columbia, 196'r A THESIS SUBMITTED IN PARTIAL FUmiMEKT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of CHEMISTRY Vie accept t h i s t h e s i s as conforming t o the r e q u i r ed stand ar d THE UKCVSRSrn -OF BRITISH C LUMBIA November- 196? In presenting this thesis in p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall make i t fr e e l y available f or reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his represen-t a t i v e s . It is understood that copying or publication of t h i s thesis for f i n a n c i a l gain shall not be allowed without my written permission. Department of The University of B r i t i s h Columbia Vancouver 8, Canada i i ABSTRACT In part I of t h i s t h e s i s , orgimotin compounds were investigated, using high r e s o l u t i o n proton and t i n 119 nuclear magnetic resonance technique. The t i n 119 chemical s h i f t of about f o r t y organotin compounds were measured by absorption mode f o r the f i r s t time. The t i n chemical s h i f t and t i n 119-•proton coupling constant i n some methyltinhalides wore found, to be solvent and concentration dependent i n electron donor solvents. This dependence was a t t r i b u t e d to the formation of higher than four coordinated complexes with solvent molecules. Equilibrium constants of the complex formation, the t i n chemical s h i f t s , and the tin-proton coupling constants of the complexes, wore obtained i n a few solvents,, The second-order paramagnetic chemical s h i f t s of methyltinhaiides, methyltin cations, ond f i v e coordinated compounds viere c a l c u l a t e d and compared vdth the observed t i n chemical s h i f t s . Good q u a l i t a t i v e agreements between calculated values and. observed values confirmed that the second-order paramagnetic term i n t i n chemical s h i f t s i s dominant i n the chemical s h i f t changes i n those compounds. In part I I } X-ray i r r a d i a t e d s i n g l e c r y s t a l s of r.-al on amide and cy&uoacetamide were s t u d i e d by electron spin resonance technique. In both cases at l e a s t two types of r a d i c a l s were found. One was the u.sual 7C -electron type r a d i c a l the proton coupling tensor of which had been studied, quite extensively i n the past. In t h i s study, besides the proton coupling and i n the case of cyanoaecetanrl.de, the coupling tensor f o r the cyano-nitrogen was also measured, and. discussed. The second r a d i c a l found both i n X-ray i r r a d i a t e d malonamide and. cyanoacetamide was a 0 "-electron type r a d i c a l which was produced by•the l o s s of one of the amide protons (-COKH). The proton hyperfine coupling constant was found to be almost i s o t r o p i c and very large, more than 80 gauss i n both compounds. The nitrogen coupling tensor f o r the amide-n i t r o g e n "was found t o be a x i a l l y symmetric w i t h the unique p r i n c i p a l va lue equal to 36.6 gauss i n the one (malonamide) and 25»b gauss i n the o t h e r (cyanoacetamide) . The p r i n c i p a l v a l u e i n p e r p e n d i c u l a r d i r e c t i o n was found t o be v e r y s m a l l b u t c o u l d n o t be determined c o n c l u s i v e l y . A s e m i - e m p i r i c a l m o l e c u l a r o r b i t a l c a l c u l a t i o n was performed on the fragment o f g"-electron r a d i c a l t o g e t h e r w i t h p e r t u r b a t i o n through c o n f i g u r a t i o n i n t e r a c t i o n ; the l a r g e i s o t r o p i c p r o t o n c o u p l i n g c o n s t a n t s vrsre explained, t h e o r e t i c a l l y . i v TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS v ' i v TABLE OF OFTEN USED SYMBOLS v i ACKNOWLEDGEMENTS v i i PART I HIGH RESOLUTION NMR OF ORGANOTIN COMPOUNDS LIST OF TABLES i x . LIST OF FIGURES x CHAPTER I INTRODUCTION 2 1.1 Theory of NMR Chemical S h i f t •2 1 .2 Theory of Nuclear Spin - Nuclear Spin Coupling Constants 8 1 .3 Nuclear Double Resonance 12 l.k T i n 119 High R e s o l u t i o n NMR 17 CHAPTER I I EXPERIMENTAL 20 2.1 Instrumentation 20 2 .2 M a t e r i a l 23 CHAPTER I I I RESULTS 2h 119 3.1 Sn Nuclear Magnetic Resonance 2h CHAPTER IV DISCUSSION 39 k . l Di3.ution Study of M e t h y l t i n h a l i d e s 39 h.2 C a l c u l a t i o n of Chemical S h i f t s of Some Organotin Corapovuids l& ± M e t h y l t i n h a l i d e s l<£ i i M e t h y l t i n c a t i o n s ^2 i i i F i v e - c o o r d i n a t e d Compounds \h. 4.3 Role of 5d o r b i t a l s o f t i n PART I I ESR STUDY OF X-RAY IRRADIATED ORGANIC SINGLE CRYSTALS LIST OF TABLES x i LIST OF FIGURES x i i CHAPTER I INTRODUCTION 60 1.1 Theory of Electro n - N u c l e a r Hyperfine I n t e r a c t i o n 60 1.2 Theory of I s o t r o p i c Hyperfine I n t e r a c t i o n i n 6k ft-Electron R a d i c a l s 6k 1.3 The serai-empirical Molecular O r b i t a l Theory 67 l,k C a l c u l a t i o n o f the Hyperfine Coupling Tensor from the Observed S p l i t t i n g s 70 1.5 ESP. of R a d i a t i o n Damaged Organic S i n g l e C r y s t a l 72 CHAPTER I I EXPERIMENTAL 75 2.1 ESR Instrumentation 75 2.2 S i n g l e C r y s t a l s 78 CHAPTER I I I RESULTS 79 3.1 ESR of X-ray i r r a d i a t e d malonaraide s i n g l e c r y s t a l 79 3.2 ESR of X-ray i r r a d i a t e d cyanoacetamide s i n g l e c r y s t a l 92 CHAPTER IV DISCUSSION 99 k.l -CONH r a d i c a l 99 k.2 CNCHCONH r a d i c a l 110 BIBLIOGRAPHY 119 VI TABLE OF OFTEN USED SYMBOLS LO \c<> <o> y 4.80286*10" e.s.u. 9 . 1 0 8 3 X 1 0 " 2 8 g .10 Electron charge Electron mass Velocity of li g h t 2 .99793X10 x u cm/sec Plank's constant 6.624 x lO'^erg sec h divided by 27C 1.05443X IO"2? erg sec i,j$k Orthogonal unit vectors along x,y, and. z. directions Hamiltonian Ket representation of spin •§ with E component •*--§-Ket representation of spin -g with z component ~-|-Expectation value of an operator 0 Magnetogyric ratio H/) Externally applied steady magnetic f i e l d JA Magnetic dipole moment t I J M T ^ Electron ^ -factor Nuclear ^-.factor Bohr magneton Nuclear magneton Nuclear spin operator Electron spin operator Nuclear spin-spin coupling constant Component of J5, along an axis of quantization Component of j[ along an axis of quantization Z component of Atomic orbital Molecular orbital Spin-lattice relaxation time Spin-spin relaxation time Atomic number or nuclear charge -0.92731 X 10~ 2 0 erg/gauss 0,50504 X 10" ? ' 3 erg/gauss v i i ACKNOWLEDGMENTS The w r i t i n g of t h i s t h e s i s has been made an i n t e r e s t i n g and i n f o r m a t i v e p e r s u i t through the c o n s u l t a t i o n and advice of P r o f e s s o r W. C, L i n , who deserves s p e c i a l commendation f o r encouragement, d i r e c t i o n and general personal a t t e n t i o n , p a r t i c u l a r l y on the ESR p r o j e c t . P r o f e s s o r C. A. McDowell deserves a s p e c i a l acknowledgement f o r g i v i n g the author the opportunity t o do researches at U.B.C., and f o r g i v i n g f r e e l y of h i s time i n the reading of the manuscript i n s p i t e of the heavy a d m i n i s t r a t i v e l o a d which h i s p o s i t i o n as head of the chemistry department demands. Indebtedness i s t o be expressed to Dr. G. H e r r i n g who provided some of the more important ESR computer programmes and who a l s o had the patience t o "debug" these programmes. I should a l s o very much l i k e t o v o i c e a p p r e c i a t i o n of the w i t and general good humour of my labmates, Messrs. C o l i n R. B y f l e e t and Dave F. Kennedy and of Mr. Ralph Moses,who made some of the ESR measurements. The e f f o r t s of Mr. J . S a l l o s and Mr. I . T. Markus who c a j o l e d and threatened the ESR spectrometers i n t o continuous o p e r a t i o n are appreciated, Acknowledgement must a l s o be expressed to P r o f e s s o r L. W. Reeves who suggested the research t o p i c f o r HMR s t u d i e s . P r o f e s s o r H. C. C l a r k , now at the U n i v e r s i t y of Western Onta r i o , i s to be acknowledged f o r h i s good humour, and e x c e l l e n t chemistry i n p r o v i d i n g the most important organotin compounds and M. & T. chemicals Co., L t d . i s to be thanked f o r p r o v i d i n g , f r e e , the other organotin compounds. The f i n a n c i a l a s s i s t a n c e of the N a t i o n a l Research C o u n c i l of Canada studentship through the e n t i r e years of the p r o j e c t i s a l s o acknowledged. The f i n a l d r a f t of the t h e s i s was typed by Mrs. J . Cable who brought t h i s work i n t o acceptable form. H e a r t f e l t a p p r e c i a t i o n i s to be expressed to my husband, who gave v a l u a b l e d i s c u s s i o n , a s s i s t a n c e i n w r i t i n g coraputor programmes and encouragement throughout the years of study. ix LIST OF TABLES (PART,I), Page I S n 1 1 9 NMR Cheraical S h i f t s '29 I I ( C H 3 ) 3 S n C l 31 I I I (CH-^SnBr 32 IV ( C H 3 ) 2 S n C l 2 33 V (CH ) SnCl 36 VI Change of J i n ( C H ^ S n C ^ by a d d i t i o n 0 f NaCl 37 VII S n 1 1 9 -H Coupling Constant 38 Y l l l f f e s u l t s of Chemical S h i f t C a l c u l a t i o n s 50 of M e t h y l t i n h a l i d e s IX R e s u l t s of Chemical S h i f t C a l c u l a t i o n s of T i n Cations 53 X Chemical S h i f t of some f i v e coordinated Organotin Compounds 56 XI E f f e c t of V i n y l and Phenyl Groups 5 0 SnXi-y NMR Spectra of (CH 3)/jSn a) without decoupling protons b) with decoupling protons 119 Concentration Dependence of Sn 7 Chemical Shift and JsnHS- CH^  Concentration Dependence of J s n 1 1 ^ , CB> in ( C H o) ?SnCl xi LIST OF TABLES (PART II) I Proton Kyperfine Coupling Tensor for the radical CH(C0NH2)2 86 II Radical -CONH in MalonamMe 8? III Radical CNCHCONH2 in Cyanoacetamide 88 IV • Radical -CONH in Cyacoacetaraide 89 V Anisotropic Coupling of NH Proton 100 VI Unpaired Electron Spin Densities 101 VII Configuration Interaction Calculation 108 VIII Nitrogen Transitions 114 IX Calculated Unpaired Electron Densities of CNCHCONH2 118 x i i LIST OF FIGURES (PART II) ££££ Fig. 1 Apparatus for ESR measurements on single crystals 77 Fig. 2a ESR spectrum of X-ray irradiated single crystal of maionamide (H//x) 80 2b ESR spectrum of X-ray irradiated single crystal of deuterated maionamide (H/^ x) 81 F i g . 3 ESR spectrum of X-ray irradiated single crystal of maionamide (H//y) 82 Fig. k ESR spectrum of deuterated maionamide (H in y-z. plane and 15° from z axis) Sh F i g . 5 Nitrogen splittings for -CONH in maionamide (H in x-z plane) 91 Fig. 6 Reference axes of cyanoacetamide single crystal 93 Fig. 7 ESR spectrum of X-ray irradiated cyanoacetamide single crystal Fig. 8 ESFt spectrum of deuterated cyanoacetamide 96 a) f i r s t derivative spectrum b) computer drawn absorption spectrum and. i t s integration curve Fig. 9 Nitrogen splittings for -CONH i n cyanoacetamide gQ (H i n y-z plane) Fig. 10 Results of MO Calculation of -CONH Radical 109 Fig. 11 Calculated ESR Line Shapes 115 a) observed b) (0, 0, 10) c) (^.5, i|-.5, 10) PART I* HIGH RESOLUTION NMR OF ORGANOTIN COMPOUNDS * Part of this thesis was presented at the 7th Experimental NOT. Conference held at Mellon Institute, Pittsburgh, in February, 1966. 2 CHAPTER I INTRODUCTION Since the general theory of nuclear magnetic resonance (NMR) is 1-3 familiar to most readers and also described elsewhere ~ , only the theories of particular interest will be reviewed in this chapter, 1.1 Theory of NMR Chemical Shift The resonance condition for a magnetic nucleus: ^ = Y H 0 (1.1) is actually applied only to the free nucleus. In this equation is the resonant frequency, y is tho raagnetogyric ratio of the nucleus ARID EQ is the steady magnetic field. However, the field experienced by a nucleus in an 3 atom or molecule differs from the externally applied steady magnetic field by a factor, (1-9.-), i.e., the local magnetic field at the nucleus will be no longer Hp but H^ ( 1 - a ) where a is a non-dimensional constant independent of Jig and is called the "screening constant", "shielding constant", or "chemical shift". The chemical shift is due to the motions of the atomic or molecular electrons which induce & secondary magnetic field at the nucleus. The magnitude of a varies from about 10"-? to 10 '. As a result, resonance will occur in a different part of the spectrum for each chemically (electronically) non-equivalent atom.. In 1950» Ramseyderived an equation for the averaged chemical shift for averaged orientations of electrons with respect to a nucleus. The total vector potential of the system due to the joint action of the external field, J l p , and a parallel magnetic dipole moment,^ / , at retaking the origin at the nucleus under investigation is written as: A, = * + yksgj (1.2) where the origin is at the nucleus. The Hamiltonian of this perturbation is divided into two parts, Hi « d jlz : H - Hi + Hz > (1.3) where P. is the momentum of the electron j . The change of energy is obtained by applying first-order perturbation theory to j£ second-order perturbation theory tojt,, T h e resulting average chemical shift i s : - 2 where 2mc '*3*}fe and Av^ _ indicates the averaging over a l l orientation X of the electrons. The f i r s t term of equation (1.4) i s i d e n t i c a l to that derived previously by Lamb^ for the diamagnetism of the single atoms. The induced magnetic f i e l d , H^O) due to the spherically distributed atomic electrons at the nucleus, i n the same d i r e c t i o n , z, as the external f i e l d , i s written: . ^ i - ^ r p f f O / r ' . - i^V-co) (i.5) o where J 5 ( r ' ) i s the r a d i a l electron charge density at distance r* from the nucleus and. \T(0) i s the electronic potential produced, at the nucleus by the electrons. This expression, rj - ' - " j ^ r— l / t o ) i s called the diaraagnetic s h i f t . Dickinson^, using Kartree-type wave functions, calculated numerical values of for free atoms. The second term i s only s i g n i f i c a n t when there i s a non-. spherical d i s t r i b u t i o n of electronic charge,J^(r, c), and when the energy difference between the ground and excited state i s small. That i s — i s large. Although the numerical evaluation of the second term n "* o would be extremely d i f f i c u l t since one has to know a l l about the excited states, the calculation can be somewhat simpler i f one replaces the (En-Eo) term within the sums by an average excitation energy,/JE which may be treated as a constant for the system considered. Then tho equation (1.4) becomes: (1.6) where Now the expression only depends on the ground state ofthe molecule. The magnetic shieldings i n nuclear magnetic resonance observed in different molecules i s one of the important interests i n high resolution nuclear magnetic resonance experiments. For many atoms other than hydrogen, the valence electrons occupy orbitals with non-zero int r i n s i c angular momentum (orbitals with ,£>.!). These may give rise to large deviation from spherical symmetry on molecule formation and the chemical shift i s usually dominated by the second-order term. The assumption that the second-order paramagnetic term i s dominant has been used satisfactorily to explain the chemical shifts of the F'9 7 ' 8 , P 3 ! 9 , 1 0 and X e ' * * 1 1 nuclei. Karplus and Das 6 and Gutowsky 12 et a l derived explicit expressions for this second-order paramagnetic shielding term i n terms of localized, bond parameters including p and d orbitals. In LCAO-MO (linear combination of atomic orbital) frame-work, a typicri molecular orbital formed by a linear combination of s, p, and d. orbitals on the nucleus i n question i s : + orbitals on other atoms (1.?) where and C^ , are the coefficients of . each p and d orbital.,. If one defines the orbital population, p^; asj where n a is the number of electrons in the ath molecular orbital, usually two, Zjx and are the coefficients of the atomic orbitals^cand )J in and the summation extends over a l l molecular orbitals a, then " - • Ramsey's second-order paramagnetic shielding tensor in equation (1.6) in terms of p ^ t y becomes: 2 r « * ( W ^ I <?((*)><&C*) \U \ %S*>>\ a- 9) where the superscript2 on^"implies "second-order term". When the system experiences rapid reorientation such as the molecular tumbling of liquids c (2) gases,OH ' is replaced by its average value: ( T E ) ^ 2 ) =lr|) + ^ 2 ) + ( r , < 2 ) ) u.io) The equation (1.9) is essentially of the form: where Pu and Du are the "imbalance" of the valence electrons in the p and d orbitals centred on the atom in question, For example, Pu takes the following form: P U = PJOC + pyy + pzz " a^axPzz + PyyPxx + PyyPzs) + ^(pxyFyx + PxaPzx + PzyPya^ ^ 1' 1 2) when applied to p-electrons in terms of the orbital populations, • The numerical values of Pu and. Du depend, largely on the coordination number of the atom, the hybridization of its bonding orbitals, and the ionicity of its bond. Also d E, and y / / ^ a r e important parameters to 12 determine the chemical shift. Gutowsky et al point out that the cause of increase in the range of chemical shift with atomic number of the elements in a given grouo could be due to the increase in the terms (y)\ and /-!—\ . By this method, Gutowsky calculated the chemical shift of Xe-^9 ^ n different ?1 9 coordination numbers and VJ in trivalent phosphorus and. obtained good agreement with the experimentally observed values. However, this agreement seems to be largely dependent on the choice of the average excitation energy value, A E. 1.2 Theory of Nuclear Spin-Nuclear Spin Coupling Constant. 13 Ramsey pointed out that the Hamiltonian for the molecular system in a magnetic field, JB, , can be divided into four terms,JC-^ jK-g, a + ^ (1.13) •k N W = -'#2JC K/&'\ ^^^N'-WJ and is the magnetogyric ratio of Nth nucleus, I M is the nuclear spin angular momentum in units of ^ is the distance vector between kth electron and the Nth nucleus, S. is the electron spin angular momentum in units of "ft , and other symbols are in usual meanings. The first term of <fffj9 represents the kinetic energy of the electrons and. their interaction as moving particles in the magnetic field. V is the electrostatic potential,^ T , t f andtf are the interactions between electron LL LS SS b d orbitals, electron orbital and spins, electron spins, and. electron spin and external field. is the magnetic moment interaction between the nuclear spin and the electron spins. ^ - s also magnetic interaction between ths nuclear spins and electron spins and. is called "hyperfine 14 interaction" or "Fermi interaction". This term is non-zero only when the electron has &• non-aero probability density at the nucleus. It has been shown in many cases that this is the largest contribution to the nuclear spin-spin interaction."^i'C^ is the direct magnetic interaction of the nuclei with each other and averages to zero to the fi r s t order when frequent collision occurs such as in liquids. Using the second-order perturbation calculation i n v o l v i n g a n d choosing those terms giving spin coupling between a pair of nuclei, the perturbation energy for a pair of nuclei, I// and I// i s : (E - E .) n o The interaction energy is proportional to the scalar product when rapid molecular tumbling occurs: = h J w ^LA/ * J-*/' (1.15) where JN$P is the contribution o f ^ to the spin coupling constant. If we make tho approximation of a l l the excitation energies, (En-Eo), by a mean value, A > 3 M (1.16) If. the orbital and spin x-;ave functions are separable in the ground state: J3\ 2 /16 rtPA \ 2 y 10 •whereto'! is the coordinate part of the ground state wave function and 0^"J'. is the spin part, end <0 | S« .£ |0V < 0 * | f { ^ - S 1 f - ^ j l o ' > = (1.18) where 0 / i s the probability density for the kth electron to be on the Uth nucleus while the jth electron is on the N'th nucleus. The energy for^ v.,, to the second order perturbation i s : E - E n o (1.20) and the contribution from this term is much less compared to that from Also the contributions from^, ft^ and the cross terms are negligible in most cases. If we assume that^C^, the Fermi contact term, dominates in the coupling constant, then the coupling constant between the nuclei N and N* 2r i s proportional to the probability density, |^ pl [N(l), KV(2)J , v?hen the firs t electron is on N while the second, electron i s on N*. Therefore; JNN« -<^3 l+'l2 [N(1), N« (2)J (1.21) The cases which we consider ere when N i s a proton and. N* is some other nucleus such as C"^  or Sn^"^. By introducing a two centre approximation and constructing the 11 X~R bond orbital vrith linear combinations of the atomic orbitals on X and H, the molecular orbital will look like: . . C - ^ (1 ) + C Cp¥. (2) 2 „ 2 + C 2 where <fix (l) is the hybrid atomic orbital of X, <fix (l) = S(l) + A i p d ) 9 1 +A1^ (2) 2 r andYV ' is the Is orbital of hydro gen Then I VI (_X(l),H(2)j is proportional to the product of the square of the coefficients of s orbitals. Because the s character of the ground state hydrogen orbital is unity, Jjrj, i s essentially proportional to the scharacter of tho bonding atomic orbital of X. The average excitation energy, A^> is also another factor which could vary J v u . In the case of direct C^ -^ -H coupling constants, i f one assumes An to be constant a direct proportionality between and s character of - 15-1? X bonding atomic orbitals has been established experimentally. P'or example, in methane where s character of carbon is ^ r, J -i-s is 125 Hz:, in (CHo)-?-C J-n -> c 13* 1 1? ]'t C=C=C -IrU where s character is 4, J TO is 166 Hz , and ih CH_C=C ''h £. j> c >"H 3 where s character i s •§-, J -> o is 2h& Hz . C^-H However, in certain cases such as in hale-methanes, where the 19 molecular structures are known quite accurately, i t has been shown that changes in Z)^  are equally important as the s character in determining JC^-H. Taking 5°° ^ z for 100$ s character of the carbon atomic orbital in the carbon-hydrogen bond, tho equation: ABK 50Q | c / v \ ~ _ _ | S ( 0 ) < | ( 1 - 2 3 ) leads to the estimation of average excitation energy of the halomethanes. In equation (1.23),/dS^ and ^ E M are the average excitation energy of substit-uted methane and methane, respectively, and |S(0) J is the character of the X carbon bonding orbital of tho substituted, methane to the hydrogen. 12 1.3 Nuclear Double Resonance Generally speaking, double resonance i s a spectroscopic experiment i n "which a system i s resonating simxiltaneously at two dif f e r e n t frequencies. One can investigate the spin-spin coupling of two nuclei by a double resonance technique. Suppose that two o s c i l l a t i n g magnetic f i e l d s , and H , are applied so that one has the frequency,60^, required for nuclear resonance from one of the two nucl e i and the other has the frequency,Cc> , required for X nuclear resonance from the second nucleus. When the nuclear spin resonance of the second nucleus i s saturated by increasing the radio frequency ( r . f . ) input power to the nuclear system then the resonance signal of the f i r s t nucleus w i l l be seriously affected. I f the second r . f . input power i s very much stronger than the spin-spin coupling energy, {yiH^> |j{) then the multiplet structure i n the resonance of the f i r s t nucleus due to the second nucleus w i l l be observed to collapse. We then say that the two-spin system i s "decoupled". The effect of double resonance on a general spin system v&s 20 described by Bloch. In t h i s thesis we are interested i n the study of the two sets of n u c l e i , both with spin •§. The case of two nuclei with spin -§ 21 and some other three-spin systems have been treated by Bloom and Shoolery ' , 22 23 2k 2*5 Baldeschwieler and Randall , Freeman , Corio , and Abragam. -1 I f two o s c i l l a t i n g magnetic f i e l d s , H-^  and K^ ', with frequencies, COl andO^, are applied perpendicular to the steady magnetic f i e l d , JJp, the nuclear spin system w i l l experience a t o t a l magnetic f i e l d : H = K„k + (H cos k i t + H 0 cos<c>0t) i (1.24) (H , since) t + H six\LO t ) j 1 1 2 2 Xy For a magnetic nucleus i n a l i q u i d system, the Hamiltonian describing that system may be divided into two parts, time dependent and time independent, as follows: a- K° + tf'ct) (1.25) wheref't contains the nuclear Zeeman and nuclear spin-spin interactions: 11° = ? {- Y£o_ I (i) j -i- £ Jij I (i)-I (j) (1.26) 1 1 2rc Z J i<j ~ ~ The time dependent interactions,^ (t) may be written: ft1 (*) =2 - ^ i H 2 [ l x ( i ) coso^t - I y ( i ) s±noo2t] - £ / j H l r I (i) cos£j, t - I (i) s i n o A t l i 2 L x 1 y 1 J simplified i f we make the usual transformation to the rotating coordinate system, rotating with frequency^g' The Hamiltonian becomes: K-R^l +ftjf(t) (1.28) where${° i s the time - independent part: •V* = £ ( . - ^ 1 H ° ^ 2 ) I (i) i —TiT""" B + 2 J I ( i ) - l ( j ) + Z (nKiH2) I (i) (1.29) i<-i J - J ^ 1 — . x and 1 < J " 2 TL ^ • ( t ) = £ - H x (i) cos CuJ^  -*Uz)t - I y ( i ) sinfc^ ~^ )"J (1.30) 2 L ^ (t) may be treated as a perturbation onj^f since H„<<CH and H . R * ' lR 1 2 o We apply first-order perturbation theory to the AX system where the chemical shift of A and X i s raxich larger than the coupling constant between A and X,. (| J [<<: IV" - • |Jj> ). The f i r s t term i n equation (1.28) • A 0 X| r) - j -i s : '2 TC ito* ( "fA*o+*>Z) I (A) + CKV'o+CV2 ) I (X) R 27C 2 27C + J I (A)-I(X) - <^ AH2 I (A) - f_x2 I (X) ( l . ^ l ) A X ~ «^ / 27C 27C It i s assumed t h a t 6 0 ^ i s very close to the resonant frequency of nucleus X, This means that x^o "!'&2 i s almost zero. 27C Therefore, from the complete matrix ofK. for the AX spin system, R. off-diagonal elements i n and J.y cab be neglected, the reason being that those off-diagonal elements connect diagonal elements that dif f e r in energy by about which i s large. 27C With this approximation, the secular determinant becomes: ^ R 11-% '2x E 22- R 0 0 R33"ER ^ 0 -E ^ R l * R = 0 (1.32) where the diagonal matrix elements of a r e : * " A x * JAx 1 27C end the states 1,2,3 and 4 correspond toJ.c^cK^, }o<p^» 1(3°^ ^ d [ p ^ ^ f o r the AX spin system. 15 ]h = - y>c ^ 2 and. Ep i s the eigen value for the unperturbed Hamiltonian, 27C o 22 . The t r a n s i t i o n p r o b a b i l i t i e s and energy differences are: R Transition Frequency Relative Intensity where i _ [ f t ^ - W 2 « ~ ~ ) + £ * H 2 * J * ^ A + \ (ra 2 cos A - - A n 2 JL(m - 2 -X) 2 cos & -#m 2 i/w> a ; A + +/) 2 sin 2 h*p> ^ A - 2 + Z ) . 2 son < & - A n 2 When H g = 0, c o s ^ = c o s $ m ~ 1 s o that$£ ~ ^ m ~ °* 1 - 1 f 0 1 1 0 " " 5 t h a t when H 2 - 0, we observe only two l i n e s with the same intensty for A nucleus separated by just as we expect from the r e s u l t of f i r s t 2 order spectrum of AX system. 15 "L^ x = " %JL~JL and is the eigen value for the unperturbed. Hamiltonian, o 27C . 22 ^ R . The transition probabilities and energy differences are: m Transition Frequency Relative Intensity fc\ cos 2 & " #m , V l A AN i i J COS 2 where T 2 7 m = 2 <*4- ft = T " - ^ 2 + 4" I WhenH = 0, c o s ^ = c o s ^ - 1 so that$£ - $ m = 0. It follows that when Hg = 0, we observe only two lines with the same intensity for A nucleus separated by Jy^ just as we expect from the result of first 2 order spectrum of AX system. When the second frequency, tc^,* is set to the resonant frequency of X nucleus and. the second r.f. field, H^ , is strong enough to satisfy \Yy. H 2 | » | J | , cos &£= cos$ *z 0 and / = n ^y^2' T h erefore the A resonance will consist of a single line &tUj. = CO -i• l.k Sn 1 1 9 High Resolution NMR Tin 119 nucleus has a spin of •§, i t s natural abuncance is 8.68$, and the raagnetogyric ratio, Y , is 1.58? x 10^ Hz/gauss while that of the proton is 4.258 x 10^ Hz/gauss. The low isotopic abundance, long relaxation times, and low MR sensitivity (0.052 relative to proton i f observed at the same, main magnetic field, strength) of the SnH9 nuc3.eus have made normal 119 high resolution absorption mode magnetic resonance of Sn containing compounds extremely difficult. There is only one literature report of Sn~^ high resolution resonance to date. Burke and Lauterbur2^ have 119 studied Sn 7 magnetic resonance at 8.5 MHz and found i t necessary to observe 27 the dispersion mode rapid, passage singal. In this manner they were able to study compounds such as SnClg^HgO in solutions of 5 g in 3 sil of HC1. High resolution NMR studies of heavy nuclei such as Sr?^^ are of interest to physical chemists, and in this thesis, because the electronic structures of molecules containing tin can be approximately elucidated and because the second-order paramagnetic terra in the chemical shift may possibly be the dominant term in determining the large chemical shifts. This may be compared with the perhaps more complicated calculations of proton resonance where diamagnetic shielding is also important. It is of interest, also, to note that the 5d orbitals of tin may participate in bond formation thus allowing tin to form five- and six-28 29 coordinated, stable molecules. ' The participation of the 5d. orbitals in a vT -bond to a ligand has been established by X-ray crystallographic studies of (CH^)^SnCltpyridine.^0 The large changes in the Sr?^ M R chemical shifts of these compounds to high field, from the original four-coordianted. compounds can be explained satisfactorily by this model. Similarly a model, in which the 5d orbitals of .Sn participate in "die- Pit" bonding with the 18 7t or lone pair electrons of substituents such as vinyl-, phenyl-, halogens, etc., can be used to explain satisfactorily the chemical shifts of these compounds. 19 *1 NMR sensitivity The centre height of the absorption curve, Vmax, given by the 2) steady-state solution of the Bloch equations l v - - MO _ _ y J i i j r z _ _ . . ( 1 .33) 1 + T 2 (U. . O »CO) 2 - . -YV T 1 T 2 is Vmax = -*fcYHlT2 (1 . 3 * 0 when we assume that the saturation factor Y^Hi T-jT2 ^ £ v e r y much less than unity. By substituting: M 0 = ^ H 0 - ( i + 1 ) NU. H 0 3IkT (I +1) Ny ^ I H I 31 kT' then V max is proportional to N Y ^ H ^ H Q T 2 . If we compare the maximum height of absorption signal of one nucleus X, to that of a proton at the same steady magnetic field strength, H , and using the same r.f. power, H - ^ , the ratio i s : V max 2 S 2 L M = ( N Z ) C&N3 ( 1 4 4 ) • / ,s ^ax(H) NH YH; T ? ( H V ( 1 .36) V max where N is the number of nuclei per unit volume, Tg is the spin-spin relaxation time. For example, the ratio, expressed by (I . 3 6 ) for the tin and proton resonances of (CH )^ Sn ,, is approximately 0.004 i f Tg(Sn) and T^(H) are assumed to be equal and the affects of spin-spin coupling are neglected. 20 CHAPTER I I EXPERIMENTAL 2.1 Instrumentation A Varian Associate's R.F. u n i t series V 4-310, which was modified with the corresponding probe to operate at 22.3822 MHz, was used to observe nuclear magnetic resonance absorption s i g n a l . The receiver output f i l t e r i n g network was removed to permit recording of signals through a Varian V3521 i n t e g r a t o r used as a base l i n e s t a b i l i z e r , which applies an audio modulation frequency of 199& Hz to the main magnetic f i e l d v i a a u x i l i a r y f i e l d sweep c o i l s on the Varian probe. With t h i s modulation technique the NMR signal was observed as a modulation on the r . f . output of the receiver c o i l . This use of the phase s e n s i t i v e detection system of the V3521 u n i t eliminated the long term r . f . f l u c t u a t i o n s due to spinning of the l a r g e samples, etc., and permitted observation of the weak t i n resonance. 31 Anderson has given the theory f o r sideband operation of a 21 modulation system. This theory shows that the f i r s t side band resonance under the above condition i s given by: m = r(^H<)M 0"^ ; -r4 2/, ,-2 (2.1) where the modulation index, ft = yH Jul , H and n) are the modulation I " m m ro i n amplitude and frequency respectively, and ^ was usually set less than one for side band operation. This side bend resonance, centred A^L00 ~ rtOm* i s of the same form as the simple absorption mode resonance except that has been replaced everywhere by ^-j^H^. The saturation factor, normally 2 2 2 taken as Y H^T1 i s reduced by the factor \ p and the saturation does not occur with higher r . f . power and signal can be amplified i n a subsequent audio amplifier. 119 Using this technique, i t was possible to observe Sn resonance absorption of organotin compounds, i n some cases in solution of 10 mole $ in acetone. The f i r s t side bands of modulation frequency 1996Hz were used to calibrate the signal separations. The proton spins coupled to the S n ^ 9 nucleus were decoupled via an NMR Specialties SD60 decoupler. Although this instrument contains a variable frequency oscillator which i s quite unstable, the frequency locking 32 procedure suggested by Anet was not applied because i t was necessary to change the decoupling frequency during the recording of the spectrum. For example, the typical f i e l d sweeps between the t i n resonances which corresponded to 100 ppm at Ih k gauss, required the irradiating frequency of the proton spin system to be changed by approximately }f ^  H o x 10"^ = 6000 Hz. Samples were measured in 8 mm O.D. sample tubes. The sample tubes were constricted, to a capillary dimension about 3 cm and h cm from the bottom to avoid vortexing i n the li q u i d during spinning. The spinning 22 assembly was home built for the 8 mm tube and is a scaled up version of the Varian spinning assembly supplied with the HR 60. It was possible to spin the 8 mm sample tubes at a rate up to 60 revolutions per second. Corrections for bulk susceptibilities of cylindrical samples' were not made since such corrections wovild be negligable in comparison to the observed chemical ,shifts. The field sweeps and the recorder chart speed were assumed to be linear over the whole range of about 5 gauss. A typical sweep rate was 0.01 gauss/sec. Repeated measurements of the chemical shift in a given sample were reproducible to 1$. In most measurements, the sample replacement method was used to determine the chemical shifts. Tetramethyltin and. trimethyltinchloride were used as exter?ial standards. Proton resonance studies of tin compounds were made using Varian A60 and HA100 spectrometers to obtain the proton-tin coupling constants. On the HA100, the transmitter frequency was locked on to the centre peak of the methyl proton resonance and the Sn^ -9 side bands were counted relative to the locking signal. 23 2.2 Material Most of the organotin compounds were supplied by M and T Chemicals. They were not further purified. No impurity signals were observable in the proton resonances of any of the samples. Impurities containing more than 1$ of the sample protons should be readily visible in the proton resonance. Solvents were purified by distillations .a nd dried, over suitable drying reagents. The solutions of organotin compounds were prepared, by weighing the constituent amounts. Trimethyltiniodide was prepared by refluxing a 3*1 mole mixture of the tetramethyltin and tetraiodotin for twenty hours. The unreacted. tetramethyltin and tetraiodotin (solid, at room temperature) were removed by distillation and filtration, respectively. o The trimethyltiniodide was purified, by distillation at 1?0 C and 1 atra. pressure. The dimethyltindiiodidfeby-product was obtained, by slow crystallization at ~5 ° C. The purify of both products was ascertained, by proton magnetic resonance studies at 60 MHz to be better than 99%. 24 CHAPTER III RESULTS H Q 3.1 Sn 7 Nuclear Magnetic Resonance In tetramethyltin without decoupling, nine of the thirteen 119 Sn resonance lines due to twelve equivalent methyl protons (intensity ratio It 12: 66: 220: 495". 792: 924: 792 : 495: 220: 66: 12:1) were observed, and. the half line width was about 5 Hz. Typical spectra of tetramethyltin with and. without strong irradiation at the proton resonance are shown in Fig. 1. With other compounds such as trimethyltinbromide, 119 six out of ten Sn resonance lines were observed, but with most of the compounds the signal was observed as a single broad peak without the resolution of any fine structure. This must be due to the inhomogeneity of the magnetic field, over the large sample and. some saturation broadening due to the relatively high r.f. power and a long tin spin-lattice relaxation time. When the alkylprotons in the organotin compounds were decoupled by a strong second r.f. magnetic field, sharper and. more intense single.lines 25 of about JOllz line width were observed. However, vrith the compounds x-rhich has methyl and vinyl groups where the proton chemical shifts ranges about 7 ppm with strong proton-tin coupling comstants from 90.6 Ha to I83.I Hz,'"^  the complete decoupling vrith a single proton frequency was impossible and even the tin resonances where the decoupling was attempted, were quite broad. 1 1 9 Sn chemical shifts of some organotin compounds measured are listed, in table I in ppm where tetreraethyl tin was used as the reference standard. The experimental error was estimated to be _ 0.5 ppm.' Also chemical shifts of some methyltinhalides ( (CH^)^ SnCl, (CH^)^ SnBr, (CH^Jg SnClg, and CH^  SnCl^) in various solvents and various concentrations are given in tables II, III, IV and V and Fig. 2. Tin 119-methylproton coupling constants, J g n i l 9 ^ t are also tabulated. The carbon 13-proton coupling constants, JQ13_H> ^ n ^ e m e ,thyl groups were always found to be 132.0 _ 0.2 Hz as measured from the proton resonance. As seen from the tables II, III, IV and V and Fig. 2, the tin chemical shifts of methyl tin halides are strongly solvent and concentration dependent. In the inert solvents such as carbontetrachloride, chloroform and benzene, the changes in the tin chemical shifts upon dilution were very small. In electron donor solvents or ionizing solvents such as water, acetone, acetonitrile, resonances shifted to high field upon dilution. Tin-methylproton coupling constants were also found to be concentration and solvent dependent. The concentration dependence of J 1 1 9 ra in dimethyltindichloride Sn —Wi ^ as "a result of adding NaCl to a 0.239 mole/l aqueous solution is shown in table VI. In the mixtures of (CH-p^SnCl and (CH^)^ SnBr, only one exchange averaged signal of Sn"'"''"9 was observed. The observed chemical shift was linearly dependent on the mole ratio of the mixture. Since (CH^)^SnCl is solid at room temperature and is soluble in (CH ) SnBr, the chemical shift 3 3 26 of the pure compound of trimethyltinchloride was obtained from the usual linear extrapolation of the averaged chemical shift of the mixture to 100$ trimethyltinchloride. It was found to be -153.5 PPm from tetramethyltin. J of methyltinhalides at infinite dilution in inert Sni:L9_CH3 solvents and in water are tabulated in table VII together with the reported values and their literature sources. The values were obtained by extrapolation to the infinite dilution using the data such as shown in tables II, III and IV. ( a ) L o Fig. 1 Sn 1 1 9 NMR Spectra of (CH ) Sn • 3 4 (a) without decoupling protons (b) with decoupling protons i _ j (b) 500 Hz. 66 65 64 63 62 61 60 i r ••66-° Me SnBr in Acetone K=3.00 ~ 6 7 2 : o = chemical shift o = coupling constant J L -70 -80 0 -120 ^xi-^30 !0 20 30 40 50 60 70 80 90 10 ' 6 6 h 65 k65-' i i i i Me 3SnBr in CH 3 CN i i r K=3.48 •73.8-64 63 62h 61 60 '\. o _I 1_ 90 100 -f-IIO 120 30 67 66 65 -65.4 -90 64 -100 63 -110 6 2 6 I h 60 i r "I r Me SnCl in Acetone K=7.0 -106.6-o -90 +100 no 120 -130 -140 -150 0 10 20 30 40 50 60 70 80 90 100 67 F -70 66 -66.7 -80 6 5 64 63 62 61 60 -i r i r Me SnCl in CH CN K=2.7 -86.5-0 10 20 30 40 50 60 '70 80 90 100' Concentration, Mole % Fig. 2. Concentration Dependence of Sn 1 1 9 Chemical Shift and. ^ Sn119CH J I 1 L -90 100 --II0 --I20 --130 -140 -150 0 10 20 30 40 50 60 70 80 90 100 Table I Sn 1 1 9 NMR Chemical Shifts 29 Compounds Chemical Shift from (CHo^Sn (ppm) (CH2 = CH)^ Sn Triphenyltin 2-Ethy3h0Xoate in CHCl^ (C'6Hn) Sn (C 6H 5) 3 in CHC13 (CH 3) 3 Sn-Sn ( C H ^ . (C6H11^2 S n ( C 6 H 5 ) 2 i n C K C 1 3 (C 6H n) Sn (C6H5) in CHC13 Diphenyltin bis-thiobenzoate in CHCl^ Triphenyltin SS* dimethyl-dithiocarbamate in CHCl^ (CH2=CH)2 Sn ( n - C ^ g bis-Triphenyltinoxide in CHCl^ (n-C 4H 9) 3 Sn-Sn (n-C^) (CH 3) 2 Sn (CH = CH 2) 2 (C 6H 5) 2 Sn (n-C 4H 9) 2 (C 6H 5) 2 Sn (CR ) 2 (C 6H 5) 3 Sn - S-S-Sn (CgH^ Triph enyltinl au rylmerc apti de (C,H J SnCl in CHC1_. b 5 3 3 ( i - C 3H ?) 4 Sn (C6H5) Sn (n-C^H9)3 (CH2 - CH)2 SnCl 2 (CH3) Sn (CH = CH2) (CH 3) 3 Sn (C 6 H5) 165.1 115.3 113.7 109.0 106.5 102.5 101.2 94.5 86.4 80.6 79.5 79.4 65.9 59.8 50.9 48.7 46.0 43.9 41.7 40.9 35.4 30.3 16.8 30 Table I continued: Compounds Chemical Shift from (CH3)/.fin (ppm) C2H5)Z)Sn 6.7 CH 3) 3 Sn (n - C3H7) 2.9 CH^ Sn (n - Cfy) 2.0 CH 3) 3 Sn (C6H ) 1.7 CH3)^ Sn 0 CH 3) 2 Sn (n - C^ H ) - 0.4 C H 3 ) 3 S n ( C 2 H 5 ) " 5-9 CH 3) 3 Sn ( i - C H?) - 9.9 CH 3) 3 Sn (t - C^ H ) - 17.5 C 6H 5) ? Sn = S in CHC1 - 19.5 CH3) SnCl:pyridine in CHCI3 - 25.4 CH3)3 SnOH in methanol - 51.8 ^ 3 ) 3 SnMn (C0)^ in cyclohexane - 66.3 CH3/2 SnBrp in benzene - 74.3 CHj„ Sn = S in CS. -125.6 ^ 3 ) 3 SnBr ..130.7 CH 3) 3 SnCl: aniline in C$:13 -149.9 C 2 H 5 ) 3 S n C 1 -155.9 CHj„ SnCl 3 3 -158.6 31 Table II ( C H ^ Sn CI Solvent Conc'n v r(l) vT(2) (mole $) (ppm) (ppm) SnXJ-<HB>. Cyclohexane 46.0 - 155.7 2.9 60.0 27.1 - 153.2 5.6 59.4 H20 14.0 - 47.9 110.7 67.8 9.9 - 37.9 120.7 68.6 7.2 - 36.5 122.1 68.3 ,5.2 - 33.6 125.0 68.7 2.8 - 33.0 125.6 69.O Methanol 21.8 - 50.5 108.1 66.6 18.9 - 49.3 109.3 66.9 14.2 - 43.I 115.5 67.I 10.1 40.2 118.4 67.2 3.3 - - 67.7 Acetone 27.6 - 48.6 110.0 67.9 -H?0 c 22.8 - 39.8 118.8 68.3 50 mole $ mixture 15.9 - 32.3 126.3 68.7 10.6 - 24.1 134.5 69.I 1.0 _ SB 69.5 vT (1) from Me^  Sn qr (2) from (CH ) Sn CI 32 Table III Solvent Conc'n <T~(l) — (mole j>) , _ i £ P m l CHCI3 50.0 - 133.4 •• 2.7 -31.8 - 132.9 2.2 -18.8 - 134.9 - 2.2 -45.0 - 129.4 1.3 -37.8 - 128.5 2.2 -29.5 - 125.8 4.9 -Benzene 6I.3 - 131.4 - 0.7 58.1 39.6 - 131.1 - 0.4 58.5 31.8 - 130.5 - 0.2 58.5 29.0 — — 58.2 0.8 — . — 57.6 Dioxane 45.6 - 102.3 28.4 • — 30.2 - 99.6 31.1 -25.0 - 94.0 36.7 • -H 2 6 4.7 - 39.9 90.8 68.8 3-9 - ,37.0 93.7 68.8 3.2 - ' 33.2 97.5 69.O 0.8 — — 70.0 Acetone:- 34.0 - 52.6 78.1 66.0 - H2° 50 mole <fo 24.2 - 47.5 87.2 66.9 mixture 14.2 - 27.7 103.0 67.9 9.7 = 22.5 108.2 68.7 2.9 • — - 68.0 0.4 69.5 0"(1) from Mo^ Sn g~(2) from -(CHJ, Sn Br (CHj_ Sn Br 3 3 < r ( 2 ) J ! E E » L L JSnll9 CH 3 Acetone 37.9 - 52.8 30.2 - 50.6 28.8 - 44.0 25.0 - 43.0 4.6 3.0 2 Jt "JL *= CH^ CN 30.9 - 53.7 80.2 25.4 _ 51.2 81.0 16.7 - 35.3 82.6 3.2 ' ' - 87.6 1.9 - 84.4 H20 9.2 186.4 98.4 6.9 210.1 100.8 5.8 226.5 102.0 5.7 228.7 102.0 4.1 243.5 104.0 1.4 105.5 0.4 - 108.4 Table IV (CRy 2 Sn C l 2 Solvent Conc*n CT (mole (ppm) 81.0 83.O 82.6 82.9 89.6 85.9 85.I Table IV continued: JSnll9CB> Solvent Conc'n (mole i) (ppm) cn-—^ 3 ^ Methanol 31.5 158.6 84.6 26.6 172.1 86.1 21.9 I83.3 91.1 11.8 208.3 93.8 1.0 - 90.3 Acetone- 25.7 72.6 88.4 H20 50 mole $ 21.8 89.7 90.2 mixture 15.8 120.4 93.1 9.8 145.4 96.3 5.7 - 99.3 1.1 r- 97.6 \ J from (CH ) Sn 3 4 M e u S n C L 2 2 mixture acetone-H^O CH CN acetone 70' 0 20 40 60 80 Mole % Me^SnCL 2 Fig. 3 Concentration Dependence of ^ Sn^'CH" ^ n (CE3)2 SnCl2 Table V (CH„) Sn CI Solvent conc'n ^ J no (mole j) (ppm) Sn^CH 3 ( H J Acetone 44.0 103.6 109.3 21.1 141.0 117.6 15.2 151.5 119.0 2.0 ~ 120.8 H20 7.1 481 . 3.6 475 • <J" from ( C H ^ Sn 37 Table VI Change of J in (CH ) 2 Sn C l 2 by addition of Na CI added NaCl (mole/l) J S n 1 1 9 ~ C H 3 0.0 108.4 t 0.05 0.229 108.8 0.371 10.7.0 1.223 106*4 2.64 105.0 0.239 mole/l aqueous solution of (CH ) Sn CI was used Compound Table VII 119 Sn — H Coupling Constant Solvent JSn"-9 C H, CHz.O ' it-JSn n9cH 3 (Hz) :CH3)4 Sn [CH3)3 Sn CI [ C E J ) J Sn Br :CH3)3 Sn I ;CH3)2 s n ci 2 ;CH3)2 S nBr 2 :CH3)2 S n I 2 [CH3) s n c i 3 neat Cyclohexane Benzene CH CI, CHC1 CHC1, 59.4 57.6 58.0 63.6 90.0 54.0 a) 59.7 a ) 58.6 b ) 71.01 0 100.0 a) :CH3)3 S nCl H20 :CH3)2 S n B r :CK 3) 3 S nI CH3)2 S n C l 2  : C H 3 } 2 Sn B r2 :CH3)2 s n i 2 :CH3) s n c i 3 (CH3)2 S n(C10 4) 4« 69.5 70.5 108.4 126.5 106. c) * From the literature a) - 37) b) - 38) c) - 36) 39 CHAPTER IV" DISCUSSION 4.1 Dilution Study of Kethyltinhalides In this section we attempt to interpret the results of the dilution studies of methyltinhalides, (CH-^SnXj^ in various solvents. It i s known that these halides form solid addition compounds with amines.2^ Hulme30 has suggested, that (CH^SnCl: pyridine, has the trigonal bipyramidal. structure with the three methyl groups and tin in a plane. From conductivity measurements, Rochow et. a l . find that these halides ionize in aqueous solutions 2 9, but they do not reach any definite conclusion concerning the nature of the ions produced. Although i t has been sugg ested?^ that tetrahedral coordination is maintained in the aquo ions by the mere replacement of two halogens by water molecules, proton magnetic resonance studies-^'3?»38 g ^ V Q strong evidence suggesting a change in the hybridization of the tin atomic orbitals on forming these addition compounds or on ionization. It i s also suggested that the trimethyl cation has a planar Sn-C^ skeleton and. that the dimethyltin dication has a linear Sn-C2 rkeI#tonin aqeous solution on the basis 40 of Raman and IR studies.J° The cations are probably hydrated by highly polar bonds. ^  26 Lauterbur and Burke observed a very strong concentration 119 dependence of the Sn chemical shift of Sn Clg • 2HgO in HC1 and they suggest that this change is due to the complex formation. Likewise, they observed concentration dependences of the Sn^^ chemical shift of the same compound in acetone and ethanol solutions. The equilibrium constant of formation of the addition compound in the system: R^ Sn CI + TJ-!SO^±R3Sn CI : TMSO (4.1) where TMSO is tetramethylene sulfoxide has been obatined from the intensity 39 -1 measurements of several infra-red absorptions to be 5«6 mole . Aqueous solutions of (CH^^Sn c i ^ have also been studied by Raman spectroscopy, EMF, and NMR and the stability constants, |3|, andftg* *"or the 4 0 formation of mono- and di- chlorocomplexes were obtained. i Trimethyltinhalides 119 We suggest that small changes in the Sn chemical shifts of these compound upon dilution in inert solvents such as chloroform, carbontetrachloride, or benzene, may be due to the breaking of the Sn... X - Sn weak bonds between neighbouring molecules. Infra-red, Raman, and 41 42 X-ray crystallographic analyses * of the solid trimethyltinhalides suggest that the tin atom is five coordinated and that the solid is really a polymer. In polar organic solvents such as acetone or acetonitrile, Rochow et. a l . found no evidence for extensive ionization. 29 i t i s therefore reasonable to formulate only the following equilibrium: 41 (CKJ)J SnX + :D ;==± ( C H ^ SnX:D • (4.2) C (1 -oi.) 1- c (/+&) cot where :D stands for the polar or electron donor solvent, C is the concen-tration in mole f> and <Xis the degree of association. The equilibrium constant of the above system i s : r(CHo)oSnX : D "j OC K « -p—^ — = (4.3) [ (CH^SnX J [:D J (/~oi)(/~C-C°<) The observed chemical shift, cSQ^s, is the average of the chemical shift of trimethyltinhalide,S , and that of the complex, due to the fast exchange: S obs= (/-©()<§+ <X$C (4.4) If vre measure the chemical shift relative to pure (CH.^ SnX, the equation (4.4) becomes: Since the observed coupling constant, Jgn119^^, i s also the average of coupling constants, exactly the similar equation holds: Jobs " J = °< (KG)* where <X is the same parameter OC in equation (4.5), J is the observed obs coupling constant,J is the coupling constant of free trimethyltinhalide and .4J~c is the difference of coupling constants between that of the free trimethyltinhalide and that of the complex. From equations (4.3) and. (4.5) , the relations between S , or«X and K are: ODS 42 5c Sots ' K= — — — (4.?) and K = (43L)X/-C) - Ale (Jcbs-J) + t(j0y-J¥ (4,8) Using the equations (4.7) and (4.8) on pairs of points on the curve concentration v.s. chemical shift, the equilibrium constant, K, and the chemical shift of the complex,^) , and theCT .-^ in the complex, were calculated. The equation used to calculate £ i s : £ = / £oi>S/ <Sg& S 3 . (C-'SohS/ C2%obS2 ) (4.9) v 0-c2)$ats, - (/-cyScte and»T i s obtained by replacing^ by (JT -T) and.T by C7"-J") c obs obs c c o r Re-using the average values of K and S thus obtained. S , were recalculated c o t ) s and shown by dotted lines in Fig. 2. The values for K, , exidj" are c c also shown in the same figure. In water, as mentioned above, both trircethyltinchloride.and -bromide are very likely to ionise into hydrated trimethyltin cation, (CH 3) 3Sn +. nHgO and the halide anions. Uncertainty in the activity coefficients of the ions and the limited solubility of these two compounds in water precluded the computation of equilibrium constant and chemical shift of but the trimethyltin cation^using the values shown in tables II and III . one may obtain by extrapolation of chemical shifts in the two aqueous solvents the chemical shift at 4 3 i n f i n i t e d i l u t i o n was + 15 ppm from t h a t of t e t r a m e t h y l t i n . The t r i m e t h y l t i n c h l o r i d e - m e t h a n o l system y i e l d e d approximately the same chemical s h i f t . . T his suggests t h a t t h i s compound i s a l s o i o n i z e d i n methanol. In (CH-p-^SnX - (50-50) mole per cent acetone-water system, the a d d i t i o n of acetone should decrease the d i e l e c t r i c constant and a l s o the degree of i o n i z a t i o n . Thus one should expect , l e s s high f i e l d s h i f t r e l a t i v e t o t h a t which would be observed f o r the (CH^-^SnX .»• water system at the same co n c e n t r a t i o n . However, w i t h t r i m e t h y l t i n c h l o r i d e the opposite was observed, although both systems appear t o e x t r a p o l a t e to the same chemical s h i f t v a l u e s , + 15 ppm, at i n f i n i t e d i l u t i o n . Thus we suggest t h a t complete i o n i z a t i o n a l s o occurs i n t h i s system. i i D i m e t h y l t i n d i c h l o r i d e ( C H 3 ) 2 S n C l 2 has been known to give 1:2 a d d i t i o n compounds w i t h 24 amines and w i t h p y r i d i n e . I t i s reasonable to assume t h a t i n e l e c t r o n donor organic s o l v e n t s , the a s s o c i a t i o n takes p l a c e i n two steps: ( C H 3 ) 2 S n C l 2 + : D ^ ( C H 3 ) 2 S n C l 2 : D (4.10) (CH3) 2SnCl 2:D + :D •?=--> (CH 3) 2SnCl 2:2D (4.11) The v J i n acetone, a c e t o n i t r i l e and methanol s o l u t i o n s i n c r e a s e s as Sn 1 1 9-CH3 c o n c e n t r a t i o n i n c r e a s e s , then reaches maximum value and s t a r t s to decrease a f t e r c e r t a i n c o n c e n t r a t i o n s as shown i n F i g . 3. This behavior i s explained by the f o l l o w i n g argument: 44 We assume the structures of the free dimethyltindi chloride, the 1:1 complex with electron donor solvent and the 1:2 complex to be approximately tetrahedral, trigonal bipyramidal, with two methyl groups in the plane, and octahedral, respectively. The resulting hybrid orbitals can be described as being sp^, sp^ d., and sp^d2. The methyl groups in the 1:1 complex will be occupy-ing very probably equatorial positions. The chlorine ligands will be in axial positions, bonded to tin mainly by the d22 and p z orbitals. The hybridization for an. equatorial ligand can be approximately described as sp 2, resulting in ">-'33»3$ 108 s character in the tin orbital. Then i f one assumes that the average excitation energies in equation (1.22) remain unchanged so the ^ sr?^-C&^ *"s directly proportional to the percent s character of the tin atomic orbitals bonding to methyl groups, Jgn119..cH3 in 1:1 complex, i s the largest among the three species. Therefore, as the concentration of the 1:1 complex increses the observed coupling constant becomes larger until the concentration of the 1;2 complex starts to affect the observed coupling constant. In aqueous solutions, (CH^gSnCX, is known to dissociate in* two steps : ^ (CH3)2SnCl2 1 s (CH3)2SnCl+nH20 + CI" (4.12) K 2 ^ (CH3)2SnCl+nH20 y s (CH^Sn n'H"20 + Cl" (4.13) Addition of Cl" ion to this system should move the equilibrium shown in equations (4.12) and (4.13) to the left and we should observe the change in the Sn"*"^  chemical shift as well as in J Sn^ -^ -CH-^ * U s i n £ the stability constants, ^ and |^2, obtained by Tobias et al the equilibrium constants, K, and K are: 1 2 K 2 = — = 0.417 • (4.14) 45 Xh^ coupling constant, ^ sn^^-CH^ °^ the intermediate cation, (OT^gSnCr^nHgO, was calculated from the data shown in table VI. The following assumptions were made: 1. The activity coefficients are: (CH^SnClg unity (CH3)2Sn++. nH20 — unity (the degree of dissociation into this species i s small) (CH^gSnCl+.n H20 between 0.5 and 1.0^ 3 CT* same as in NaCl.^ 2. J S n119_cH 3 of (CH^gSn^.n'HgO was taken to be 108.0 Hz, the value which corresponds to 50$ s character in the Sn-C bond. With these assumptions, J ^ U ^ Q ^ o f (CH^gSnCl4* was calculated to be approximately 108 Ez. These calculations suggest that the tin orbitals bonded to methyl groups are the same kind of sp hybrid orbitals as in •H-(CH3)2Sn . One can surmise that this dication probably exists as an ion pair with CI" ion like (CH^gSn^Cl" and this experiment . indicates that the CI or solvation does not appear to involve bonding using the tin 5s orbital. ** I would like to thank Dr. F Aubke of this department for the helpful suggestions particularly concerning the structural organotin chemistry. *2 Essentially the same equation was used, later by Drago^ to find the equilibrium constants and the coupling constants of the trimethyltinchloride complexes in several solvents. 46 4.2 Calculation of Chemical Shifts in Some Organotin Compounds Using Gutowsky1s expression^-2 derived from Ramsey* s theory^ for magnetic shielding of nuclei in molecules, the second-order paramagnetic term in the tin NMR chemical shifts of some methyltinhalides, methyltin cations, and five-coordinated compounds were calculated. The expression for the second-order paramagnetic chemical shift, in terms of orbital population, Vjjoi > ^ d the average triplet excitation energy,4E, i s given by equation ( l . l l ) . Gutowsky^0 has calculated the necessary matrix elements for p and d orbitals. His final expression for the average chemical shift including only p electrons i s : where P«. is the "unbalance" of the valence electrons in the p orbitals, and is given by equation (1.12). The orbital population, p ^ , is defined by equation (1.8). i Methyltinhalides • Formulation of tin hybrid orbitals In methyltinhalides, (CH^SnXj^ (where X i s CI, Br, or I), precise 4? bond angles are not known accurately. Therefore the tin bonding angles were assumed to be the tetrahedral angles, 109°28». A right handed corrodinate system i s defined, with the z axis along one of the four bonds, a second bond, in the x-z plane, and. the origin at the tin nucleus. Thus the four sp^ hybrid atomic orbitals of tin are: = a.?/6fic - o. XZffe (4.16) where sdenotes the 5 s orbital and p. the 5p. orbitals on tin. The 3 methyl carbon atomic orbitals were also considered to be pure sp hybrid orbitals. Only p orbitals of the halogens were considered to form 0" bonds with tin. The bonding orbital between tin and methyl carbon is? where ^  and^are the atomic orbitals of tin and carbon and i is the ionic character of the Sn - C bond. This ionic character of the bond was obtained from the electronegativity of Sn and C according to Pauling's formula:^ i - / - e T i M ("-is) where^^is "the difference in the electronegativities of the atoms which form the bond. Pauling's electronegativity scale J for atoms was used in the calculations. S^is the overlap integral between the tin and the carbon orbitals and is calculated according to Mulliken's formula.^ The bonding orbitals between tin and the halogen atoms are obtained in a 43 similar manner. Orbital Populations If we define: I* = / / C4.19) for each bond, then, by equation (1.22), the orbital populations are: 9 z z Pxx = 2 Z 1^ C z pyy 2 Z l I* ^ (4,20) it-/ 2 2 2 where C„ . C , C are the square of the p orbital coefficients in tin x y » 7, atomic orbitals. In the case of tetramethyltin or tetrahalogenotin, o p ^ r = p = p =21 because a l l the four orbitals are equivalent. pxy* pyz ^ pxz a r e a l ^ a v s z e r o except for p ^ in (CH-^g SnX2 type compounds. If we take^^ and (pz in equations (4.16) to be the tin orbitals 2 o bonding to two methyl groups, then p x g is -0.472 ( l x - IQ ) ^or (CH^gSnXg. <-£A and 4 E (2) Other tvio values needed for calculation of were the expectation -3 value of r for 5p electrons for tin and the average excitation energies, 4E. ^"jjrj^pobtained from the Hatree-Fock calculation is. 6.748 atomic units.^ The average excitation energy, ids, is the most difficult value 9 11 to choose. For a l l the previous calculations ' the results of u.v. spectroscopy have been used. However, these values of A E estimated from u.v. spectroscopy are probably quite uncertain due to poor definition of 48 the absorption bands. For example Thompson states that tetramethyltin o "absorbs continuously from a long wave limit at ca 2200A". We may calculate relative values of 4E by a novel technique. If we assume that the change of the tin-methylproton coupling constants, J -,,Q is due to the change of the average triplet excitation energy Sn±iy-CH3, in equation (1.31) rather than due to the change of s character in Sn - C bond^ (because the bond angles are almost pure tetrahedral angles), then the average excitation energies for other compounds can be calculated from the relation similar to the equation (1.23):^ A£K 2/6.0X (s ch. meter) ' __ where 4 E x and 4 E^ S n are the average excitation energies of any methyl tinhalide and tetramethyltin, respectively and J x is ^ sn119_CH3 *"or the same methyltinhalide. The s character of tin bonding orbitals in tetramethyltin is O.25. The coupling constant which corresponds to 100$ s: character is four times the coupling constant observed with tetramethyltin i.e. 54.0 Hz x 4 = 216.0 Hz. The excitation energies obtained this way may be used to calculate CT^where the coupling constants are known. An alternative way is to assume that does not change very much from compound to compound and use one value a l l the way through the calculations. Results from both of these calculations and some of the parameters used are tabulated in table VIII, In the series of chlorine substituted methyltin compounds, the calculated chemical shifts from tetramethyltin show the same trend as the observed chemical shifts; as the number of chlorine substituents increased, the chemical shift moved to high field. This i s a strong indication that the assumption that the second-order paramagnetic term is dominant in the Table VIII Results of Chemical Shift < compound bond S 2 I 2 (CR^Sn Sn - C 2.18 0.0071 0.8342 (CH3)3SnCl Sn - C 2.19 0.0071 0.8342 Sn - CI 2.37 -0.1501 0.8776 (CH 3) 2SnCl 2 Sn - C 2.17 0.0071 0.8342 Sn - CI 2.34 -O.I525 0.8800 (CH3) SnCl 3 Sn - c 2.19 0.0071 0.8342 Sn - CI 2.32 -0.0405 0.7806 Sn Clh Sn - CI 2.30 -O.O398 0.7800 (CH3)3SnBr Sn - C 2.17 0.0 071 0.8342 Sn - Br 2.49 -0.6285 0.2005 (CH 3) 2SnBr 2 Sn - C 2.17 0.0071 0.8342 Sn - Br 2.48 -O.6332 0.2030 (CH3) SnBr3 Sn - C 2.17 0.0071 0.8342 Sn - Br 2.45 -0.6482 0.2110 Sn Br u Sn -• Br 2.44 -O.65O8 0.2125 Pu ^E(eV) 1.4587 5.7 1.4644 1.4696 I.436I 1.4274 5.2 5.6 6.9 1.3799 5.3 1.2021 0.9350 0.5697 07 (ppm) 01 (ppm) -1668 -1668 (0) (0) -I836 -1675 (-168) (-7) -1711 -1681 (-43) (-13) -1357 -1642 (311) (26) -1697 (-29) 1632 (36) -1578 (90) -1362 (306) -1069 (499) -651 (1017) Opfts (ppm) -158.6 - 52.8'a^ 103.6 a ) 150^ -130.7 -74.3 638 b) Table VIII. cont'd compound bond o c ) r(A) S 2 2T Pu A E(eV) <JT(ppm) (T* (ppm) ^Q)S(ppm) (CH3)3SnI Sn -Sn -C I .2.17 2.72 0.0071 0.2173 0.8342 0.6916 1.4435 5.3 -.1775 (107) - 1651 (17) (CH3)2SnI2 Sn -Sn -C I 2.17 2.69 0.0071 0.2624 0.8342 0.6672 1.4102 6.4 - 1436 (232) , - 1612 (56) (CH3) Snl 3 . Sn -Sn -C I 2.17 2.68 0.0071 0.2635 0.8342 0.6666 1.3752 - 1573 (95) ; Sn - I 2.64 0.2288 O.6853 1.3513 - 1545 (123) l698b> a) measured in acetone h) ref. 22 c) ref.44 * Electronegativities for atoms used in calculations are taken from re. 45 Sn 1.8, C 2.5, CI 2.95, Br 2.8, I 2.5 •^were calculated using^E in previous column andCT^ were calculated using^dE = 5*7 ©V. * The values in ( ) are from (CR^)^ Sn. * E a 5.7 eV ( = 2200 A ) for ( O L ^ S n was taken from the result of u.v. measurements.^ 52 chemical s h i f t changes i n t h i s m e t h y l t i n c h l o r i d e group i s v a l i d . U n f o r t u n a t e l y a comparison of one halogen s e r i e s t o another i s not p o s s i b l e because of a p a u c i t y of observed & values. I t i s evident from the equations ("4.15), (4.19) and (4.20) t h a t the f a c t o r s which vary chemical s h i f t s are l ) e l e c t r o n e g a t i v i t y of the s u b s t i t u e n t s , 2.) the- overlap i n t e g r a l s i n the bonding o r b i t a l s , 3) the average e x c i t a t i o n energies, and 4) the h y b r i d i z a t i o n of the t i n o r b i t a l s . Another f a c t o r which should be remembered i n the chemical s h i f t s of halogen s u b s t i t u t e d t i n compounds i s the p o s s i b l e e l e c t r o n donation from the lone p a i r e l e c t r o n on halogens to the empty 5d o r b i t a l s of t i n . Extended Hlickel M.O. calculations''9 (see P a r t I I ) were t r i e d without i t e r a t i o n i n the 7C system of t r i m e t h y l t i n c h l o r i d e . The o r b i t a l s used i n the c a l c u l a t i o n were 3 Fx and, 3 Fy o r b i t a l s on c h l o r i n e and 5 &xz and 5 dy ? j o r b i t a l s t a k i n g the d i r e c t i o n of the Sn - Clbonding as z d i r e c t i o n . The Coulomb i n t e g r a l of the CO t i n 5d o r b i t a l s was obtained from the atomic s p e c t r a l data-^ according t o Cusach's m e t h o d ^ and c a l c u l a t e d t o be =2,5 eV. K u l l i k i n ' s overlap i n t e g r a l s * * ^ were used. The t o t a l e l e c t r o n p o p u l a t i o n i n the t i n 5d o r b i t a l was calculated, to be approximately equal to 0.8$. I t i s very d i f f i c u l t t o estimate the change of magnetic s h i e l d i n g a t the t i n nucleus due to t h i s e l e c t r o n donation. In comparison, we need not consider t h i s f a c t o r i n CXJ NMR because of non-a v a i l a b i l i t y of empty d jtf r b i t a l s on the carbon atom. i i ) K e t h y l t i n c a t i o n s I n the s e r i e s of m e t h y l t i n c a t i o n s , (CH^^Sn*", (CH^JpSn + +,' (CH^) Sn , and Sn , the second-order paramagnetic chemical s h i f t s were a l s o c a l c u l a t e d . The p r i n c i p a l assumptions made i n these c a l c u l a t i o n s were: 53 1) The structures of trimethyltin cation and dimethyltin 35 36 dication are planar and linear respectively. * 2) Tin-carbon bond length is the same as in tetramethyltin, which is 2.18A°. 3) The effect from the hydrated. water is negligible. Other parameters used were the same as the ones used in the calculation of methyltinhalides. The results are shown in table IX, Table IX Results of Chemical Shift Calculations of Tin Cations compound calculated ..(ppm) observed, (ppm) ( C H ^ S n 0 0 ( C H 3 ) 3 S n + +86 - 1 5 a ) ( C H ^ g S n * * +??6 3OO^.350a) "(CH 3) Sn + 1668 > 500 a ; Sn**** t 1668 > 600 b ) a) Taken from the extrapolated values at infinite dilution in aqueous solutions. b) Taken from the value of Na Sn(OH)^ in H20 in reference 22) -H-l- -H-H-The second-order paramagnetic chemical shift in (CH^) Sn and Sn is zero because the former does not have valence p electron and the latter is spherically symmetric. From the table IX, the calculated values of CT^agree fairly well with the relatively small chemical shift of (CH-^^Sn and (CH^) Sn , and the small chemical shift difference between (CH ) Sn4"1"1" and Sn*t'!"H". 6 The calculation of diamagnetic shielding constant by Dickinson shows that the difference in shieldings between the neutral atom and the 5h positive ion decreases as the atomic number increases. For example, the difference is 0.8$ for sodium (Z=ll) and is 0.2$ in Arsenic (Z=33). If the difference in neutral tin atom and tin monocation is assumed to be 0.1$ at the most, the difference in diamagnetic shielding constant would be only 5 ppm. Also the change in chemical shift due to the change of the di -amagnetic term with the loss of outer electrons would only change the total chemical shift to low field. This should give better agreement with the observed chemical shifts. i i i Five-coordinated Compounds Approximate calculations of the second-order paramagnetic chemical shift were carried out for the typical five-coordinated compound where nitrogen is-the donor atom to the trimethyltinchloride molecule. The model vised in these calculations is a trigonal bipyramidal structure with three methyl groups in the equatorial plane, as shown in the illustration below: ^ ck The Cl-Sn-N axis was chosen to coincide with the z axis of the model, one of Sn - CH^  bonds with x axis, and. the origin at the Sn nucleus. Sn - CH and Sn - CI bond lengths of the five coordinated compounds were assumed to be the same as those in triraethyltinchloride which are shown in table VIII, Some difficulty was encountered, in choosing an appropriate Sn - N bond length. To date no estimates of this distance have been published in the literature.3° A Sn - N bond length of 2$, appropriate for most tin compounds, was chosen for these calculations. The hybrid atomic orbitals of the tin atom were chosen to be: / VJ $ — s / + Pa ) Is: ( + a ) fa* (4.22) Pu and Du were calculated in a manner similar to that discussed in the previous section, methyltinhalides. New overlap integrals, appropriate for the hybridised orbital scheme in equation (4.22), were calculated, Pauling's electronegativity for nitrogen, 3<,0, was used. The Sn-N bond. was assumed to be an ordinary<T bond, to simplify the calculation. The average excitation energy, 6.1 eV, was estimated from equation ( 4.21 ) where J = 67.4 Hz., which was observed in (CH«)^SnCl: pyridine Snn9-CH3 5 -> in CHCIQ, and the scharacter = i . The calculated value of the p orbital 3 contribution to the second-order paramagnetic chemical shift (first term of equation (l.ll)), was -1529 ppm-. To obtaiii the d orbital contribution (second term of eqviation ( l . l l ) ) , ^5^^must be known. The values of ^T^SsL ^ 0 T ^ ^ 9 ^ a v e b e e n estimated to be 0.036"*"2 and 2^°, respectively. If we assume a rough linear relation between <^-py>^and atomic number as has been shown between (~f}')g c'-nd atomic number"^ , ^-^-^^for tin is approximately 0.4. The value of Du was calculated to be 0.7526. This is approximately one-half of the value of Pu (1.4303). Even i f we estimate the contribution from electrons in d orbital to be 5$- of the contribution from p electrons, the total chemical shift is -1605 ppm at the most. Comparing this value with that of trimethyltinchloride in table VIII (= -I836 ppm), the chemical shift of this five-coordinated compound with respect to the original trimethyltinchloride was + 231 ppm. This positive shift agrees with the experimentally observed chemical shift for (CH^^SnCl: pyridine, or any five-coordinated compound of trimethyltinhalide as seen in the table X. Table X Chemical Shift of some five coordinated Organotin Compounds compound Chemical Jfoift (CH3)3SnCl : He ( 2 3 l ) a (CH^SnCl : pyridine in CHCl^ 133.2 b (CH3)3SnCl : CH3CN 7 2 . l b (CH3)3SnCl : acetone 52.0 b (ChT3)3SnCl : aniline 8.7b (CH3)3SnBr : acetone 63,5° (CH3)3SnBr % CH^ CN 55*9° a calculated value b from (CK ) SnCl c from (CH ) SnBr 4.3 Role of 5d orbitals of tin •>( The high field shift observed in the compounds with substituents with 7C electron systems, such as vinyl or phenyl groups, is possibly due to a TL electron donation into the empty tin 5d orbitals. Table XI shows some chemical shift changes by replacing saturated alkyl substituents with vinyl and phenyl groups. Since the change of the methyl substituent to the n-butyl substituent does not seem to affect the tin chemical shift by more than a few ppm, the comparison in table XI is reasonable. Donation of TC-electrons into the tin 5d orbitals can affect the chemical shift in two ways; l ) a low field. second»order paramagnetic shift and 2) a high field diamagnetic shift since there is an extra electron density near the tin nucleus. It appears that the diamagnetic shift, which is usually assumed to be constant from one molecule to the next, does vary significantly when there is a donation of TC-electrons into empty 5^  orbitals of tin, that is, "d-^ -PT^  bonding." Table XI Effect of "Vinyl and Phenyl Groups Vinyl Group (CH3)4Sn (CH2 = CH)^ Sn + I65.I ppm (CH3)4Sn (CK3)2Sn (CE = CH 2) 2 79.4 t) (CH^Sn (CH^)2Sn (CH = CH2) 35.4 tt (CH3)2Sn (n-But)2 (CH - CH)„Sn (n-But)_ 2 2 I 87.0 tt Phenyl Groups (CH3)4Sn (CH^ SnPh t 30.3 ppm (CH^Sn (CH3)2SrJPh2 (n - But) SnPha? 59.8 t; (CH3)4Sn 41.? 11 (CH3)2Sn(n-•But) (n - But)2SnPh2a^ 44.4 It (n - But) 3 SnPh (n - But) 2 SnPh^ 24.2 It a) Taken from ref. 22 This -pir_ bonding between the Tt electrons in vinyl or phenyl substituents and empty d orbitals on silicon . has been deduced from proton magnetic resonance studies.^^'-^' According to Kulliken's formula^, the 2pz orbital of the vinyl or phenyl carbon which is bonded to tin has an overlap integral of 0.2688 with the tin 4dxz orbital in trimethylvinyltin or trimethylphenyltin. The direction of the bond was taken as z and the vinyl or phenyl molecular planes were as in y-z plane. The tin-carbon distance was set equal to 2,18 A, an average value for organotin compounds,^4' From table XI, one substituted vinyl or phenyl group seems to cause a shift of ^  40 and-—'30 ppm respectively, to the high field. A larger stabilization energy in the system of the phenyl group than of the vinyl group-^ may decrease electron donation into the 5d orbitals of tin from the 7Csystems of the phenyl group, relative to donation from the vinyl group. We do not calculate this diamagnetic shielding change, resulting from p-^ -d^ bonding, since present theory is inadequate for such calculations. FART I I ESR STUDY OF X-RAY IRRADIATED ORGANIC SINGLE CRYSTALS 60 CHAPTER I INTRODUCTION Since the basic theory of electron spin resonance (ESR) is described elsewhere,"^ only the theories of particular interest "will be discussed in this chapter. 1.1 Theory of Electron-Nuclear Hyperfine Interaction The interaction between the magnetic moment of an unpaired electron and that of a nucleus gives rise to the hyperfine splitting commonly encountered, in ESR spectra. The hyperfine interaction energy results in a splitting of the electronic Zeeinan levels into sub-levels. Selection rules for the first-order approximation is A M„ = 1 and. A M - 0 S I where M5 and Mj are the electron spin and. the nuclear spin quantum numbers, respectively. Therefore, (21 + l ) lines are observed as a result of the interaction. The most commonly encountered nuclei which give '(• rise 61 to hyperfine splittings in organic radicals are proton, deuteron, nitrogen 14, carbon 1 3 , and fluorine. The energy of a system involving a spin placed in an external magnetic field, Hp , |S. usually described by the so called spin Hamiltonian in which a l l operators involved are spin operators. In this application, 56 the spin Hamiltonian consists of three terras : K - K s * + # x s + f < i B (1 .1 ) where The symbols which have not been previously defined are: J ^ , the^-tensor; A , the hyperfine coupling tensor and the summation is taken over the nuclei, j . For a system involving only one magnetic nucleus and for •^ nearly isotropic such as in the case of most organic radicals, the interaction between the electronic spin, , and the nuclear spin, I , in the second term in equation ( l . l ) can be divided into two parts: KlS = + Ux (1.2) where „ tofflfefrL- i f ' ( e ) £ • I 3 ~ ~ and if » -mm&i- [ 3(I ; £ ) C £ - r ) _ j . s The f i r s t term in equation (1.2) has non-zero values only i f there is a finite probability of finding the unpaired electron at the interacting nucleus since H^ °)is the probability density of electrons at the nucleus. This term is called the "Fermi contact term". In a large external magnetic field, the electronic and nuclear spins precess independently about the magnetic field, Ho , and the energy levels are defined by the quantum numbers Mg and Mj t'6 the first-order approximation (Paschen-Back region). Then the Fermi contact hamiltonian, l^ .^ * i s : ^ y>YO S . I a (1.3) A S& 1B where z is taken along the direction of external magnetic field. This part of the hyperfine coupling constant shows the s character of the unpaired electron orbital. The isotropic hyperfine coupling constants, &b s for the unpaired electron which has 100$ s character have been calculated for several nuclei^? using self consistent field (SCF) orbitals. Comparing the experimentally observed isotropic hyperfine coupling constant CL t with those of the theoretical values, tZ0 , the s character of the unpaired electron orbital can be estimated. • The second term in equation (1.2) is the electron spin-nuclear spin dipole-dipole interaction term and is direction dependent. It is called the anisotropic part find the hyperfine coupling is expressed in the form of a tensor. For an axially symmetric tensor, the hamiltonian can be written as:-^ where r i s the distance from the nucleus to the unpaired electrons, Oi is the angle between this direction and the principal axis of the hyperfine tensor, and $ i s the angle between the external magnetic field, jlp, and the principal axis. /Oc&dU-/)\ The expectation value, \ —^ / , is non-zero only when tne unpaired electron has p- or d- character. Similar to the isotropic term, i t is possible to estimate the p character of the orbital of the unpaired, electron orbital by comparing the experimental value with the theoretical value. Perhaps i t is appropriate to mention here that this term averages out to zero when rapid tumbling of the molecules occurs as in liquid. 1.2 Theory of Isotropic Hyperfine Interaction in 7C-Electron Radicals In the so called. 7C-electron radicals such as aromatic ions, the protons are in plane rath the aromatic rings and the unpaired electron is usually located in the 7C-orbital system. Therefore one would expect no unpaired electron density at the proton hence no isotropic hyperfine splitting. However, one usually finds iso^topic components of the ' " " ~ \ splitting due to the protons amounting to about! 20 ^ gauss. This corresponds V.. s to about kfo unpaired electron density in the Is hydrogen orbitals. This fact has been explained by KoConnell et al by the second-order process such that some excited states are admixed with the ground state wave function of the impaired electron orbital wave function."^'^ The outline of KcConnell's calculation by molecular orbital theory is as follows: Consider a hypothetical -CH- fragment abstracted from a TC-orbital system trith three electrons, two 0" bonding electrons between carbon and hydrogen and an unpaired electron. The unpaired electron is considered to be mainly in the 2p7 atomic orbital of the carbon and the <T bonding orbital is made up of^sp^ hybrid orbital of carbon and the Is atomic orbital of hydrogen. The ground state of this fragment can be represented by the normalized Slater determinant:^ O t t C O V O L (2) va(3) p c * ( i ) p 0 ^ ) pc<C3) (1.5) where^ is the antisymmetrization and normalization operator, is the bonding orbital, and p is the 2p^ atomic orbital on carbon. TheO" orbital between carbon and hydrogen i s : / 6 5 cr - fs + O (1-6) where S0 is the overlap integral between the hydrogen lis. orbitals and the carbon hybrid orbital, h. The ground state wave function in equation (1.5) gives no proton hyperfine splitting because the unpaired electron is exclusively in the 2pz orbital. Excited states can be formed by promoting one of the two bonding electrons to the antibonding orbital, CT. Two excited state wave functions which are the eigen functions of Sz and 5 2 are: n and % = 4=&Wp ( o < p o ( - p o « X ) ( 1 # 8 ) The wave function including the configuration interaction i s : $ = lfc t % f ^ % (1.9) where | ? » f « / and I / In accordance with equation (1.3), we define the isotropic hyperfine coupling constant for an n-electron system by: A = JZ-£\p\frpN c?,y (1.10) with k = (1 .11) 66 In equation ( l . l l ) Sfrf, ^ s the electron spin operator for kth electron, is the Dirac delta function and S z is the z coraponent of the total electron spin ^(=i-|-).- By substituting the wave function in equation (1.9) into equation ( l . l l ) : + - 2 ^ < K | Z ^ ) S ^ / f t > (1.12) In this equation the only non-zero term is the second term and i s : 1* \S (o)\ 2 I ( 1 , 1 3 ) By substituting this equation into equation (1.10): A •-- - -4r , / -- (1.1*0 where is the isotropic hyperfine interaction constant for the hydrogen atom, i.e. 506.8 gauss. The coefficient, Y^x , may be estimated by the perturbation method as: wherej\ is the complete Hamiltonian for the three electron system. With a reasonable estimate for ^ 2 7 McConnell evaluated A in equation (1,14) to be of the correct order of magnitude compared with the experimental value. 1 .3 The Semi-empirical Molecular Orbital Theory-It is very simple and useful to express the molecular orbitals as linear combinations of atomic orbitals. These molecular orbitals are presumed to be the solutions of the single electron Schrbdinger equation? ( 1 . xO where ^  is some kind of an effective Hamiltonian. The f i r s t step is to find, a set of normalized molecular orbitals: if -- 2 ccXi (1.17) where C * s are properly normalized coefficients of atomic orbitals, i The energy of the molecular orbital wave function, f^ , is given by: E -^a^ctz d.i8) We are interested in finding the values of atomic orbital coefficients, in equation (1.17) which will minimize the orbital energy, E. This is done by solving the secular determinant: | - E | = o (1.19) where {-flj = ^ 0Ci^%j.d-X s- ry<^  Sj_j ^ s the overlap integral defined as j^i %j. The method of molecular orbital calculations used in this thesis is often called "Extended Httckel Molecular Orbital Calculation" a recent version 4 9 of which is described by Carrol et al. y An outline of their treatment is as follows, There exist several equations for the estimation of the resonance 6 2 integral, H^j. For example or H i f = k'/Hx s t (1'21) where k and k' are arbitrary constants usually between 1.5 and 2, and H^ are called "Coulomb Integrals" and were approximated by the negative of atomic valence state ionizationi potentials (VSIP). The value of H^'s were obtained from the atomic spectral data by the method, suggested by Cusachs. They are dependent on the charge and the configuration so that some kind of self-consistency can be effected since the charge can be calculated from the molecular orbitals obtained. However, using the resonance integrals, H^, given in equations (1.20) or ( l . 2 l ) , the agreement with experiment in most cases was produced by the arbitrary choice of the proportionality constants k and. k*. To overcome this problem, Cusachl^ suggested the approximation? ' 2 independent of any arbitrary constant. To solve the secular equation, overlap integrals and Coulomb integrals with arbitrary original atomic charge are computed beforehand. Explicit formulas and numerical tables for the overlap integrals between atomic orbitals ^^and have been given by Mulliken and. et a l . ^ They used the Slater-type atomic orbitalss &l fa) Y t , ( 1 . 2 3 ) where /?M£(V)is the radial part of the wave function and they used: 77-/ -.M*r RnlCrhIZ d*r*e. (1.24) . -k-l when there is more than one electron, d^'s and JJ-k$ depend on the principal quantum number, n, the orbital quanttim number,/, and on the particular atom and electronic state. When they consider atom-pairs, they subclassified atomic orbitals with different set of quantum numbers under the influence of the cylindrically symmetrical field of its partner. Using the original parameter, electron population and the charge of the nucleus are computed.. If the molecular orbital is expressed, in the form of equation (1.17), the population is calculated as: Sxnce the calculation usually diverges i f the output charge distribution from one iteration is used as the input charge for the next, the second input charge is taken as the value of the previous input charge corrected using the relation: Input II = Input I - ^ ( Input I - Output I) (1.26) where /"\, the damping parameter, Is chosen for the most rapid convergence of the charge, for example, /\_~ 0.1 is used. 70 1.4 C a l c u l a t i o n of the Hyperfine Coupling Tensor from the Observed S p l i t t i n g s The s p i n Hamiltonian ( l . l ) f o r the case of s i n g l e n u c l e a r spin system can be w r i t t e n as: vrhere ^ i s the symmetric ^~ -te n s o r , i s the hy p e r f i n e c o u p l i n g tensor. Using/jstrong f i e l d approximation t h a t the e l e c t r o n s p i n , _S^ i s quantized along the d i r e c t i o n of the vector Jf^?, the nuc l e a r s p i n i s quantized along the r e s u l t a n t f i e l d , (S_JV-^ ;^H), which depends on the s p i n s t a t e s of the e l e c t r o n . I n most cases, the ^  -tensor i s n e a r l y i s o t r o p i c and the e l e c t r o n s p i n , S, can be regarded as t o be quantized along R, Then . + 2f*bl™ + ZfaiVt + -2$bc_rY)Y) ) (1.28) where ^ ( i , j = a, b, c) are the tenser component of and. 1, m, n are the d i r e c t i o n cosines of^H^. The second term (S. A - ^ j ^ H j . I ^ c a n be expressed as where A (lis) being a f u n c t i o n of the e l e c t r o n spin quantum number, can be w r i t t e n as: A (Ms) = j£[JMs(AaAli-/lu»> pp« HI I* + j Ms (A«bl.+ Ati m t Ad n.) ~ f«p*, I* + j f i s (/lac I t AU™ 1- An*. ) - fofa {-J*] *]~* - (1.30) The energies of the system are expectation values of the s p i n H a m i l t o n i a n , ^ , w i t h the eigen s t a t e s , | Mg, K ^. The strong or allowed t r a n s i t i o n s occur according t o the s e l e c t i o n r u l e , ^ K g ~ 1 andZfMj = 0 when the a x i s of q u a n t i z a t i o n i s f i x e d i n space.^5 The observed s p l i t t i n g s can be c a l c u l a t e d from the d i f f e r e n c e of the field where the transitions occur. ,14 For example, in case of hyperfine splitting due to K , the eigen states are |Mg, M ^ where Kg = -'-§ and = 1, 0, or -1. Energies calculated from the spin Hamiltonian in equation (1.27) are: E 1 = ^ H + f A ( l ) E2=X?P E3 = -f?P - i A <i> E4 = -5-?PH - i A (~* ) (1. E 5 = -f^H 3D <-E6 = -f?/JH + f A (_|) and the allowed transitions occur between Ej and E^ (at H-}), E? and E^ (at Hg), and E^ and E^ (at E-^). The separation of these lines will be: and i i ^ - J A ^ ( i ) + A H , (1.32) in units of gauss. The superscripts, H^  and H^ , on A (Mg) indicate the magnetic fields where transitions are observed. When the hyperfine splittings are much smaller than the external field, then H . 72 1.5 ESR of Radiation Damaged Organic Single Crystal It is well known that radicals produced, by radiation damage of the host crystals can be trapped in a crystal matrix. The trapped radicals are almost always oriented, in the crystal and. their alignments are closely related, to the crystal symmetry. The concentrations of radicals produced by irradiation are usually dilute enough so that the electron dipole-dipole interactions can be neglected. Therefore those radicals in single crystals can be easily studied by the technique of ESR. Many data have been published, on the ESR of radiation damaged crystals. Most of the works have been done on crystals such as dicarboxylic 66-76 acid and amino acids. One type of radical which is commonly detected, at room temperature in radiation damaged organic crystal containing a carboxylic group is a7(_- electron radical centred, on a carbon atom. In such radicals, the unpaired. e3.ectron occupies what is predominantly a carbon 2p orbital directed perpendicular to the plane trigonal skelton of the radical. The hyperfine interaction tensors of ot -protons were fo\ind to be remarkably constant in. this type of 57 radical with different nature of the various substituents. Theoretical explanations for the constancy of the isotropic hyperfine 59,60 coupling constant have been given by McConnell et al taking the -CH-fragment of an aromatic molecule. This was briefly reviewed, in the section (1.2) previously. Very common organic 7C-eleetron radicals, which are produced by eliminating one of the methylene protons next to a carboxylic group, have principal, values of hyperfine coupling tensor of OL-proton of about -30, -20, and. -10 gauss with the external magnetic field in the plane of the radical and perpendicular to the C-H bond, perpendicular to the radical plane, and parallel to the C-H bond, respectively (see illustration). I - 20 gauss 73 r C ( 3 E S H 3 H > _ 10 gauss - 30 gauss The isotropic part of this coupling tensor, a , has been found to be proportional to the spin density on the cetral atom according to the equation?*7 : <Zn= Qj> (1.33) where Q i s a constant equal to approximately -22.5 gauss and. y0 is the spin density at the carbon. Another type of radicals are the0~_electron radicals:, which received far less attention. TheCT-electron radicals are characterized by the molecule having a delocalised 7L~electron system but with the unpaired electron being in a CT-molecular orbital. Examples of this type of radicals are the formyl"'radical?'7, ethynyl and vinyl radicals?^, the phenyl radicals?^, CO^ - 80,81^  M02 8 l»82, a radical formed, by the irradiation of dimethyl glyoxime^3»84} a n ( j iu^inoxy radicals.85 One of the striking feature observed in the formyl radical??, which is a 0"-electron radical, was the exceptionally large proton hyperfine splitting of 13? gauss. Although the unpaired electron orbital in 0~-electron radical does not vanish at the proton, rough estimates of this direct contribution 2 made by assuming that the unpaired, electron orbital is a carbon sp hybrid. 78 gave a hyperfine coupling constant of the order of only 10 gauss. Cochran et al gave an explanation of this large proton hyperfine coupling constant by mixing a low lying excited states of formyl radical. Such an excited state may be described roughly as an unbonded hydrogen atom interacting with a carbon m on-oxide molecule. The ground state and. this excited state may be represented pictorially as: 74 ; C = 0 H-Excited state This excited structure is also stabilized by the resonance energy of the CO molecule. This mechanism leads to the proton hyperfine coupling constant largely dependent on the bond angles and they concluded the bond angle, Z.OCH, in formyl radical being approximately 120, which gives the hyperfine coupling constant of-^13? gauss. In both X-ray damaged malonamide and cyanoacetamide, another kind of 0" -electron radicals were found besides the usual TC -electron radicals which are similar to the radical obtained by the irradiation of malonic acid.^® TheO" -electron radical was produced by losing one of the protons on amino nitrogen by X-ray irradiation. This radical also gives a very large isotropic proton hyperfine coupling constant of more than 80 gauss, which is unreasonable for theTC -electron radicals and has to be explained by assuming i t to be a r j - -electron radical. H Ground state 75 CHAPTER II EXPERIMENTAL 2.1 ESR Instrumentation Two spectrometers were utilized in the ESR experiments. The first, a Varian E3 ESR' spectrometer, was used without field or frequency calibration to measure some of the hyperfine splitting constants. It was felt that this .spectrometer was reasonably accurate, for measurements on a relative scale, for these experiments. Critical measurements were performed on the second, easily calibrated, spectrometer. The second spectrometer was a home built spectrometer, built almost identical to the Varian 4500 lOOKHz ESR spectrometer, operating.at about 9200 MHz. A glycerol proton resonance magnetometer was used, in conjunction with a Standard Signal Generator (General Radio Co.,)? for field measurements. Signal positions were determined by least square fitting, assuming a quadratic relation between the position on the chart and the magnetic field. The microwave frequency was measured with a Hewlett Packard model 5245 L frequency counter with a plug in 5255 A high frequency 76 converters The crystals were mounted on a lucite apparatus shown in Fig. 1. The crystal mountings were arranged so that crystal orientations with respect to the magnetic field could be easily accomplished by visual adjustment. First derivatives of the absorption spectra were recorded at 15° intervals, or less, as dictated by the hyperfine anisotropy, in each of the planes xy, yz and zx. The crystals were irradiated, with a Machlett Lab. Type OFG 60 X-ray tube at -40 KV for one hour to two hours at room temperature. n 2.2 Single Cyrstals Maionamide and cyanoacetamide were obtained from Eastman Organic Chemicals. Maionamide was vised, with no further purifications end cyano-acetamide was treated with charcoal in aqueous solution and. filtered, before single crystals were made. Single crystals of maionamide and cyanoacetamide were prepared by slow evaporation from aqueous solutions. Deuteration of these compounds were carried out by repeated, treatment with 99.?^ D?0 and by crystalization in a desiccator. 79 CHAPTER III RESULTS 3.1 ESR of X-ray irradiated rnalonamide single crystal. The unit-cell dimensions and crystal type of this crystal is tabulated by Groth. O D The crystal belongs to the monoclinic system and the unit cell dimensions are a. b; c = 1.3859 -1 : O.8505 A, and 0 p- 107.02. The ESR of one of the radicals produced by -irradiation of rnalonamide (CH(C0NH )„) has already been investigated by Rexroad et al 2 <~ and the same orthogonal axis system, (x, y, z), was used in this study for convenience. The y and z axes coincide with the crystalline b and c axes, respectively. Figures 2a and 3 show spectra of undeuterated rnalonamide single crystal corresponding to the Ho^x and He// y orientations, respectively. The two strong lines in the centre in Fig. 3 were assigned to the radical, CHCCONHjp^ . A triplet cn each side of the strong central doublet was assigned to a second radical 0 0, HgNOCCK^ COM, which has been observed, but not 3.H S P L I T T I N G FOR - C H - . Fig. 2a ESR spectrum of X~ra.y irradiated single crystal of maionamide (H/x) 1.. H SFLITTIKG FOR -CH~ 2o D SPLITTING FOR -CD-3. H SPLITTING FOR -COHH k. D SPLITTING FOR -COND 5. N SPLITTING J • I ' I 81 V ^ 50 GAUSS _ ! Wis i-H 5 _ J Fig. 2b ESR spectrum of X-ray irradiated single crystal of deuterated maionamide ( K //x) 82 i d e n t i f i e d b efore. The h y p e r f i n e s p l i t t i n g s due to the amide proton are i n d i c a t e d i n the f i g u r e s . Each of the s i x l i n e s are p o s s i b l y broadened by the unresolved h y p e r f i n e s p l i t t i n g s from the methylene protons. However, vdth some o r i e n t a t i o n s (Hff i n y-z p l a n e ) , each component of the t r i p l e t s , due t o the n i t r o g e n , s p l i t f u r t h e r i n t o three or f o u r d i s t i n c t l i n e s separated by as much as about 5 gauss. o A f t e r annealing the c r y s t a l a t about 100 C f o r 20 hours, the centre doublet s i g n a l (assigned t o the CH(C0MH 2) 2 r a d i c a l ) d i d not change markedly i n the i n t e n s i t y . However, the weaker s i g n a l s (assigned t o the -CONH r a d i c a l ) almost completely disappeared. Hoping to ob t a i n b e t t e r r e s o l u t i o n of the spectra due t o the second r a d i c a l , s p e c t r a of deuterated. rnalonamide were taken and F i g . 2b shows the one corresponding to the same o r i e n t a t i o n as F i g , 2a. A proton magnetic resonance study of the rnalonamide deuterated i n DpO showed t h a t the methylene protons ( f =» 6.?) were almost completely deuterated w h i l e the amino-protons ('C ~ 5-'-0 were p a r t l y deuterated. I t was concluded t h a t the X-ray i r r a d i a t e d s i n g l e c r y s t a l of deuterated. rnalonamide had two types of the "second" r a d i c a l s besides the main r a d i c a l species $ -CD-** They were -COKH and -C0ND. As i s seen from equation (1.3), the s p l i t t i n g by deuterium i s about j of t h a t by proton. (Note t h a t deuterium has a magnetic moment of 0.85'?38 and 1 = 1 which may be compared vdth the proton which has a magnetic moment of 2.7927p*/ and I = -|. Then i f the e l e c t r o n i c f u n c t i o n s remain unchanged when deuterium i s s u b s t i t u t e d f o r the proton, the new s p l i t t i n g f a c t o r should be reduced, by the f a c t o r , 2*7927 X" 2 " ^ shows another ESR spectrum o f the deuterated rnalonamide vdth the o r i e n t a t i o n o f H i n y-z plane and 15 degrees from z a x i s . This shows a p a t t e r n t h a t i s due t o two non-equivalent s i t e s , In both f i g u r e s 2b and h the c e n t r a l strong l i n e s were due to the r a d i c a l , -CD-, where the deuterium h y p e r f i n e 5 0 Gauss H K S P L I T T I N G F O R -NH ( S I T S l ) N S P L I T T I N G F O R - K I ! ( S I T E 2) K S P L I T T I N G F O R - E H D S P L I T T I N G F O R I 1 1 | i! ii ni isi.II II 85 splittings are too small to be resolved. The remaining lines were attributed to the two types of radicals, -CONH (indicated by solid lines) and -COND (indicated by dotted lines). Although the more stable radical (CH^CNH^p) has been investigated , the proton hyperfine splittings were measured and its coupling tensor was deduced using the equation (1.32) by the method of least square fittings to a l l the experimental points. The tensor is listed in table I together with Rexroad1s values in brackets for comparison. It was found that although the maionamide crystal belonged, to the monoclinic system, only one non-equivalent site was observed for this radical. This showed that the C-H bonds of the radical for the two magnetically non-equivalent sites, supposed to be present in a monoclinic crystal, must happen to be parallel or at least approximately parallel to each other. The direction cosines of the last two principal 8? axes differ somewhat from those obtained by Rexroad et al. This slight disagreement is most likely due to some slight misalignment of the crystalin either .or both of this and Rexroad's studies. The general agreement in the direction cosines confirms that our reference axes are the same as theirs. In the second radical, two coupling tensors, due to the amide proton and nitrogen, were observed, Although the deuterium coupling tensor and the nitrogen coupling tensor of -COND were not determined, the corresponding lines always appeared at the expected positions whereever observable and resolvable. A tensor and Q. tensor of this radical are tabulated, in table II together with the corresponding direction cosines, g-tensor was determined, from the equation: <T where is the experimentally observed ^ xralue, ^^and are ikth and kjth elements of the g tensor? and Jii and Jlj< are direction cosines of JBo. 86 Table I Proton Hyperfine Ccmpling tensor for the radical• ClKCOh^^ Principal values (gauss) -31.7 (-31.9) -20.2 (-21.1) - 7.0 (-10.2) Direction cosines w.r.t. reference axes x z 0.999 (0.997) -0.03 (-0.08) -0.02 (0.02) 0.03 (-0.08) 0.999 (-0.94) 0.04 (0.33) 0.20 (-0.01) -0.04 (-0.33) 0.999 (-0.94) Radical Table II * -CONK i n l i a l o n a n i i d e g - tensor Principal values Direction X cosines w.r.t. y , reference axes z 2.0043 - 0 . 0 3 0.97 0 . 2 3 2.0026 - 0 . 9 5 - 0 . 1 0 - 0 . 3 1 2.0016 0.33 - 0 . 2 1 - 0 . 9 2 Proton Hyperfine Coupling Tensor Principal values (gauss) Direction X cosines w.r.t. y reference axes z 82.7 - 0 . 0 9 0.07 0.994 9 9 . 6 0.90 - 0 . 4 3 0.11 7 8 . 3 - 0 . 4 4 - 0 . 9 0 0.02 Nitrogen Hyperfine Coupling Tensor Principal values Direction X cosines w.r.t. 7 reference z 36.6 0.92 t 0.31 - 0 . 2 5 0 .35 ± 0.89 0.21 3.9 0.19 -f- 0.25 0.92 88 Table III Radical CNCHCONH,, g - tensor Principal values Direction X cosines i r.r.t. - , ~, y , reference axes z 2.0066 -0.18 -0.29 0.94 2.0030 0.82 -0.58 -0.02 2.0016 0.58 -0.42 -0.49 Proton Hyperfine Coupling Tensor Principal values (r?;auss) Direction X cosines w.r.t. ,y reference axes z -^9 »5 0.09 -0.994 -0.06 -20.0 0.03 -0.06 0.998 -7.9 0.995 • 0.09 -0.03 Table IV Radical -CONH in Cyanoacetamide g - tensor Principal values Direction X cosines w.r.t. reference axes 2.0056 0.67 - 0 . 0 3 0.74 2.0030 0.73 0.19 - 0 . 6 5 1.9997 0 .90 0.31 -0.14 Proton Hyperfine Coupling Tensor Principal (gauss) values Direction X cosines w.r.t. reference axes 85.O 0.18 - 0 . 3 3 0.93 84.5 0.97 - 0 . 1 2 - 0 , 2 3 8b.b 0.19 0 . 9 4 0.30 Nitrogen Hyperfin e Coupling Tensor Principal values (gauss) Direction X cosines w.r.t. reference axes y a 2 5 . 4 0 .30 ± 0.13 0 .95 5*1 O.83 + 0.53 - 0 . 1 9 5-1 -0.47 + 0.84 0.26 90 # The proton coupling tensor for the amide radical, -CONH, was found to be nearly isotropic but slightly axially symmetric. Due to the isotropy, slightly non-equivalent sites would probably not be distinguishable. However, the nitrogen coupling tensor was seen to represent two non-equivalent sites as predicted, by the monoclinic crystal symmetry. The experimentally determined, nitrogen coupling tensor was used to recalculate the nitrogen hyperfine splittings as a function of the magnetic field orientation and. comparison of these calculated and. measured values is show in Fig. 'j. 92 3.2 ESR of X-ray irradiated cyanoacetamide single crystal The crystal structure of cyanoacetamide has not been previously reported. Rotation, Precision and Weissenberg X-ray diffraction photographs taken in this department showed that the cyanoacetamide crystal is Q 0 O monoclinic. The unit cell dimensions ares a = 7.7A, b = 13«5A and • o . c = 7.05A with p~ 112 , and the space group is Pp-^ /c. The reference axes assigned to the cyanoacetamide single crystal are shown in Fig. 6. The x and y axes l i e in the cleaveage plane of the single crystal. These x, y and z axes correspond to the three orthogonal optical extinction axes. Therefore one of them should match the crystal b axis. From the observation of ESR spectra and. from X-ray diffraction, the y axis was found, to be the unique b axis of the monoclinic crystal. Fig. ?'. shows an ESR spectrum of X-ray irradiated cyanoacetamide o with the magnetic field, Hp, in y-z plane and 30 from y axis. There are two sets of triplet and more signals on both sides of the priplet. By annealing o this irradiated crystal at 75 C for 18 hours, the ratio under the integrated signals of the triplets and the rest of the signals changed from approximately 1:1 to 0„5:1« This indicates that there are at least two different radical • species with different therrao-stabilities existing in the X-ray irradiated cyanoacetamide crystal. The structural formulas of the radicals which are consistent with the ESR observations are: NCCHCONHg (I) and NCCH CONH (II) where dots indicate unpaired electrons resulting from the loss of hydrogen atoms. Triplets in Fig. ?;. are due to the radical (I), and with this radical the OC-proton and cyano-iritrcgen hyperfine splittings were observed.. 93 9k 1. ri SPLITTING FOR .-CC£!H • ( s i t e 2 not r e s o l v e d ) 4 i . g . 7 L'SR spectrum o f X - r a y i r r a d i a t e d cysnoacotarrdde s i n g l e c r y s t a l (H i n y~s p lane 30" from, y a x i s ) 95 The principal values and direction cosines of hyperfine coupling tensor of the proton are shown in table III together with the ^ -tensor. These tensors indicate that this radical is very similar to the ones observed in many radiation damaged dicarboxilic acids. Since the cyano-nitrogen hyperfine splittings, with H* orientations in x-y plane, were small and. overlapped those of the proton, the splittings with Ho in x~y plane could not be obtained accurately. Therefore, the tensor for the cyano-nitrogen hyperfine coupling was not calculated completely. However, only one site was found and the principal axes were found to l i e approximately parallel to x, y and. z axes. The splitting in z direction was about 11 gauss and. those in x and y directions were some small values. The radical II is a radical similar to ths CT-electron radical observed in rnalonamide. The amido proton coupling was again found to be very large and almost isotropic, and the nitrogen coupling was very anisotropic. Probably due to the coupling from methylene protons, signals for the radical II were quite broad, and accurate measurements were difficult with undeuterated cyanoacetard.de. An ESR spectrum for an irradiated, deuterated cyanoacetamide with its computed absorption and intensity integrated curves is shown in Fig. 8 . Although i t is often difficult to estimate the relative intensities of the signals from the derivatives, Fig. 8 shows approximately 50$ of the methylene protons were replaced, by deuterium. The overlapping nitrogen triplets from the second radical giving a quintet with intensity ratio of 1; 1: 2: i t 1, are not always symmetric. This is attributed to a slight anisotropy. The principal values and the direction cosines of the amide proton and nitrogen are tabulated with the ^  -tensor in table IV. The experimentally determined, nitrogen coupling tensor was used to recalculate the nitrogen hyperfine splittings as a function of the magnetic field orientations in y~z plane i s shown in Fig. 9, which shows two non-equivalent sites. As seen in figures ? and 8 , the peaks which are marked * above 'ig. 8 ESR spectrum of deuterated cyanoacetamide a) fi r s t derivative spectrum b) computer drawn absorption spectrum and. its integration curve 97 were not assigned to either of the radical I or II. As the microwave power was increased the signals due to radical I saturated fi r s t , the signals with * saturated, next, and. the ones due to radical II saturated finally. Although the third radical was not identified, the different relaxation times of the signals with * is a good indication that they do not belong to either of the two radicals. *1 The author would like to thank Mr. P. C. Chieh of this department for the crystallographical measurements of cyanoacetamide. 99 CHAPTER IV DISCUSSION 4.1 -CONH radical 4 Since the second radicals, obtained i n maionamide, HpNCOCHpCONH, and i n cyanoacetamide, CNCI^CONH, seem to have very similar hyperfine coupling tensors, both radicals w i l l be discussed together i n this section. As already mentioned in chapter III, the amide-proton coupling tensor was almost isotropic and i t s magnitude was quite large i n either the radical produced in maionamide and that produced i n cyanoacetamide, being 82.0 and 84.6 gauss, respectively- This i s to be oqoected for a <T-electron radical. In fact, the formyl radical which i s a prototype of 77 this radical was found to have a proton hyperfine coupling of 13? gauss.' Cohran et a l ? 8 carried out a valence bond (VB) calculation for the formyl radical by introducing a resonance structure which involved a non-bonded hydrogen atom. They found that the proton splitting of this QT-electron 100 radical varied over a vide range of values depending on the 0-C-H bond angle. o They found that the experimental value predicted an angle of about 120 . Recently Hinchliffe and Cook^ published an ab i n i t i o calculation of this isotropic proton hyperfine coupling constants. They obtained 134 gauss which i s a very good agreement with the experimental value. If the unpaired electron i n the -CONH radical i s i n the nitrogen 2p orbital perpendicular to the sp" hybrid bonding orbitals the interaction of the unpaired electron with the oL hydrogen should be similar to that found in R2CH. The expected anisotropic component, b^, for RNH type radical has been calculated by Rowlands^ using the theory of McConnell and Strathdee?'1' The values obtained, for several combinations of effective nuclear charge on nitrogen, Z, and the N-H bond distance, R, i s reproduced in the table V in units of gauss. Every case shows much larger anisotropy than the observed values i n both radicals, CNCH CONH and. NH C0NHoC0NH. 2 2 2 Table V Anisotropic Coupling of NH Proton P.(A) . Z _.. // N-H //2pN ±_ , 1.0 3.7 22.6 -3.9 -18.8 1.0 4.0 25.9 -5.9 -20.0 1.05 3.7 20.8 -4.2 -16.6 1.05 4.0 24.2 -6.4 -17.8 1.1 3.7 20.0 -4.9 -15.1 1.1 4.0 22.4 -6.6 -I5..9 Although the radical, -CONH, found, i n X-ray irradiated, rnalonamide and. cyanoacetamide i s a CT-electron radical, there have been a few examples of -CONH type radical which were TC-electron radicals. These were radicals °2 93 found i n irradiated Hydroxy urea / , Pentafluoro-propionamide , and zn 101 94 formamide in aqueous flow technique. By comparing the experimental results given in tables II and IV with the theoretical values corresponding to 100 % unpaired electron density on 57 the particular atomic orbitals , the nitrogen 2s $the nitrogen 2p, and the proton Is character of the unpaired electron were calculated and tabulated in table VI. The remaining spin densities of 0.178 and. 0.418 could be regarded as due to delocalization to the carbon and. the oxygen orbitals through the <Y electron system. Experimental Value (gau ss) Table VI Unpaired. Electron Spin Densities Theoretical Value Maionamide N isotropic 14.8 550 N anisotropic 10 .9 17 .1 H isotropic 82.0 506.8 Cyanoacetamide N isotropic 11.8 550 N anisotropic 6.8 l ? . l H isotropic 84.6 506.8 total total Unpaired. Electron Density 0.027 O.637 0.822 0.021 0.39S _Ojl62 0.582 Perhaps i t would be interesting to consider •fee -CONH type radical from the M0 theory point of view, The calculation was carried out using the semi-empirical approach which was outlined, in chapter I. Although the structures of maionamide and cyanoacetamide have not 102 9 S 0 6 97 98 been reported, those of formamide , ac et amide'' , oxamide and succinaraide were a l l known. These crystals a l l have a planar amide group with NH bonds being in the 0-C-N plane. It was assumed that the;amide group of both rnalonamide and cyanoacetamide also had a planar structure (though the planes of the two amide groups of rnalonamide need not be co-planar). I f was also assumed that the radical, -CONH, retained the same planar structure as that of the undamaged -CONHg group, except for the fact that the 0-N-H angle might be different. If the hydrogen atom in -CONH were allowed to assume an out of plane position, than even a -electron radical might have a large proton coupling. This case was, however, not considered for i t was thought to be unlikely. In carrying out the MO calculation, only the- part of the radical in the following formula: was used and the methylene carbon was assumed to contribute an • sp-^  hybrid to form a 0" bond with the amide carbon. Atomic distances, bond, angles, and coordinate axes used in the calculation are shown in Fig. 10 Only the valence electrons were considered, hence the problem was reduced to that of a five atom -,17 electron and lb orbital-problem. The orbitals and the number of electrons in each orbital are: 103 atom H K orbital number of electrons 2s 2Px y 2p. 2s 2Pa 2s 2Px 2r> 2Pz sp^ 1 1 1 2 1 1 1 1 2 1 2 1 total 17 h9 As was done by Carrol et al**'7, Cusach's method.-^  (equation (1.22)) and parameters^ (from VSIP) were used to estimate the matrix elements of the effective Hamiltonian, Hj^ and H^ ^ (i ^  j ) . For the overlap integrals, S i j , only those between neighbouring' atoms were considered. A damping parameter, X= 0 , 1 , for adjusting the input charge was used as was suggested ho by Carrol et al . y The iteration was carried out until the difference *2 between two consecutive input charges on the nitrogen atom was 0 , 0 1 . The calculation was carried out with the angle $ (see Fig. 10) varying between 0 and 75 degrees. There was no convergence after 50 iterations for $ <. 10 n 0 and (y = 75 • Fortunately, these were not the angles of interest because wnen (9 % 0 , the unpaired electron orbital becomes almost pure 2py orbital (Kradical) and a Q- more than 60 was thought to be unlikely. For G between 15 and 60 degrees, i t was found that the calculation converged either to a a~~ type radical or a TC -type radical. In fact t i t oscillated between these, two types. By a judicious choice of the limit for convergence, i t could be made to converge to the desired, type, in this case, a CT-type,' in order to be 104 c o n s i s t e n t w i t h the experiments. The unpaired e l e c t r o n d e n s i t y i n n i t r o g e n 2s o r b i t a l , i n n i t r o g e n 2p o r b i t a l , and i n hydrogen I s o r b i t a l c a l c u l a t e d f o r $ v a r y i n g between 15 and 60 degrees are shown i n F i g . 10. The r e s u l t shows t h a t there was an over-estimation of the n i t r o g e n 2p character and an under-estimation of the n i t r o g e n 2s c h a r a c t e r as compared with the experimental r e s u l t which was shovm i n t a b l e VI. F i g . 10 a l s o shows t h a t , without c o n f i g u r a t i o n i n t e r a c t i o n , the spin d e n s i t y on the hydrogen I s o r b i t a l i s i n the order of 0.014 which i s an order of magnitude smaller than the experimental value of O.I58. This c l e a r l y i n d i c a t e s t h a t c o n f i g u r a t i o n i n t e r a c t i o n must be taken i n t o account. Lefebvre^*? discussed the c o n f i g u r a t i o n i n t e r a c t i o n c a l c u l a t i o n f o r a s p i n doublet s t a t e with one non-closed s h e l l . H i s d i s c u s s i o n was, however, based on s e l f c o n s i s t e n t f i e l d (SCF) o r b i t a l s . Since only non-SCF molecular o r b i t a l s were used t o s t a r t w i t h , the ground s t a t e wave f u n c t i o n with c o n f i g u r a t i o n i n t e r a c t i o n should i n c l u d e more terms. The ground s t a t e of the r a d i c a l fragment, -CONH, without c o n f i g u r a t i o n i n t e r a c t i o n i s ? where (pi — X*Cio>. %^ , are the atomic o r b i t a l s , (fa i s the e s s e n t i a l l y N-H bonding o r b i t a l , and $ 1 i s the unpaired e l e c t r o n o r b i t a l . The bar i n d i c a t e s t h a t the s p i n p a r t i s |3 . A l s o t o s i m p l i f y the n o t a t i o n , a l l doubly occupied o r b i t a l s other than ^ have been dropped. Doublet e x c i t e d s t a t e s corresponding to i - * k, i—> n, and n k e x c i t a t i o n s are: 105 where </^is the essentially NH antibonding molecular orbital. Therefore the first-order ground state with configuration interaction can be expressed as follows: where and \> are mixing coefficients a l l of which are much smaller than unity. For SCF orbitals, only the first term is non-vanishing. All other configurations resulting from one electron excitations have been neglected since the contribution to the hydrogen Is character would be small. The spin density, which is the expectation value of: (4.6) where r and r^ c are the position vectors of proton, and an electron, respectively, in the ground state, ^ ' ^ o , was calculated to be: The mixing coefficients,X , JL , and I) are, from the fir s t order perturbation theory: V = Hoi /IB, H02 Ha (4.8) where and j ^ , is the complete many electron Hamiltonian: (4.9) (4.10) (4.11) 106 In this last equation, h(jA,) is the one electron core Hamiltonian, and Y^jis the distance between electrons^Cand V . 4 E^'s were approximated as simply the difference between the molecular orbital energies: 4E2*~ j (4.12) The H . «s O l were found to be: 7 6 7° in 2. (4.13) • tl + Z(2 5 J " n-, t 0' ) + L l n (4.14) " 0 3 • (4.15) V. C where ^ ^ a r e the core integrals: Erf <Ps dv (4.16) YS and are- the two electron interaction integrals: 5fu = foMQt^lfa 0sO)<pu.(*) civ-, cC-ui ( 4 a 7 ) The set of equations (4.14) and. (4.15) were further approximated, by considering the fact that contributions to the matrix elements would be appreciable only when the coefficients were.simultaneously large. With this assumption, the only term in the summation over j needed to be included was that with j = i . Hence the approximate expressions for H ^*s are: H o 2 '= (£f, + Cn + C £ " ) (4.18) / >. c. . r. j- >i-/f _ r ni \ ' /• \ 10? The core integrals, and the Coulomb integrals were obtained by the method of Roothaan"'"^ , the Exchange and the Hybrid integrals by the Mulliken approximation.^^1 jo obtain Ers, the kinetic energy integrals, J^%^ )PC^d.V^ were estimated as such and the potential energy integrals, yr) /Xb<£v~J were calculated using the nuclear charge, Z, of hydrogen as unity and that of nitrogen as two. The reason for choosing the nuclear charge for nitrogen as two is that in the approximation of this case only two valance electrons (one for NH bonding and the other for the unpaired electron) were considered. The values of A,^C, and ~P in equation (4.8) and the unpaired electron density, J^(o)(see equation (4,7) ), at the hydrogen nucleus calculated using the results of the extended Htlckel MO calculation are tabulated in table VII. The unpaired electron density, J^C0)^ is also shown as a function of the angle (9 in Fig, 10. Although the calculation did not give the required amount of hydrogen Is character (^ 16-16), i t nevertheless gave an order of magnitude estimation of the value and represented the theoretical interpretation of the mechanism involved. 108 Table VII Configuration Interaction Calculation (degree) A. V _f>(0> 15 •-0.0069 0.0010 -0.3249 0.0710 20 -0.0060 -0.0095 -0.3580 0.0916 25 -0.0055 -0.0208 -0.3783 0.104? 30 -0.0051 -0.0346 -0.3885 0.1103 35 -0.0048 -0.0476 -0.3917 0.1 o?4 r 40 -O.OO.43 -O.O638 -0.3'859 0.1018 45' -0,0036 -0.0725 -0.370? O.O89O 60 -O.OP95 0.1522 0.416? 0.0826 109 110 4.2 CMCHC0NH2 r a d i c a l The nature of the proton h y p e r f i n e c o u p l i n g f o r t h i s type of r a d i c a l i n which the unpaired e l e c t r o n i s confined t o the 2p^ o r b i t a l of the sp^ h y b r i d i z e d carbon atom i s w e l l established.60,68,76,91 p rom the p r i n c i p a l a x i s of the hydrogen h y p e r f i n e c o u p l i n g tensor ( t a b l e I I I ) corresponding to the inte r m e d i a t e c o u p l i n g constant, the symmetry a x i s of the carbon 2p„-orbital i s deduced t o be n e a r l y p a r a l l e l t o the z a x i s . I n the r a d i c a l w i t h the s t r u c t u r e I , there are two p o s s i b l e n i t r o g e n atoms, the cyano- and the amide-, which might i n t e r a c t w i t h the unpaired e l e c t r o n . However, i n the s i m i l a r type of r a d i c a l produced i n the X-ray i r r a d i a t e d maionamide, the hy p e r f i n e s p l i t t i n g due t o the amide-nitrogen was not observed. I t was t h e r e f o r e not unreasonable t o assume t h a t the observed n i t r o g e n h y p e r f i n e s p l i t t i n g f o r CHCHCOKHg must be due to the cyano-nitrogen. Smith e t a l - ^ ^ p r e d i c t e d t h a t the i n t e r a c t i o n of an unpaired e l e c t r o n i n a 2p o r b i t a l of n i t r o g e n i s c y l i n d r i c a l l y symmetric. When Ho i s along the symmetry a x i s of the n i t r o g e n 2p o r b i t a l , the n i t r o g e n h y p e r f i n e c o u p l i n g constant i s given by (a^ + 2bjj) where a^ i s the i.sotropic component and 2bvj i s the a n i s o t r o p i c component of the hyp e r f i n e c o u p l i n g t e n s o r . The observed cyano-nitrogen c o u p l i n g constants show t h a t the d i r e c t i o n of the 2p7c.orbital i s a l s o approximately p a r a l l e l to the z a x i s . This i s i n good agreement w i t h the common concept t h a t there i s an extended 7C system i n the H s t r u c t u r e K = C - C As was mentioned b e f o r e , the n i t r o g e n h y p e r f i n e coupling tensor components which are p a r a l l e l t o the x and the y axes were d i f f i c u l t t o determine because of the f a i r l y l a r g e l i n e - w i d t h (-^ -2.5 gauss) and overla p p i n g . A l s o when the e l e c t r o n - n u c l e a r i n t e r a c t i o n term (second term i n equation (1*27) ) becomes s m a l l e r , the nucl e a r Zeeman term ( t h i r d term i n equation I l l (1 .27) ) could affect the structure of the spectrum appreciably. For this reason some theoretical spectra were calculated for comparison with' the observed •.spectra to see i f some conclusion could thus be reached. The spin Hamiltonian for a paramagnetic system with one nuclear spin, I, is written as If the external field, Ho, is large such as in the case of an ESR experiment of organic radicals, the electron spin, __S, is generally assumed to be quantized, in the direction of JIo. However, the nuclear spin, I, is not quantized in this direction but rather in the resultant field, ( 5 * ^ ~~ §«/prJ The vector representation of this situation i s : where tc and are the unit vectors along the direction of quantization of the nuclear spin. The eigen values of the Hamiltonian for the states: tp = |Hs>|Mi„> where 14 = . - •? and K T is the component of I along the unit vector ^ or IX."* depending on Yi^ i s : E = HB $S + R(Ms)Sz ZVL (4 .21) Here the ^  -tensor is assumed to be isotropic. In equation ( 4 . 2 l ) 5 R(Ms) = [\(/!xx- <rCris))£ + A^cTn ~t~ Azxn$* 112 0<H. ) - -$0&lL. (4. 2 3) Ms ' and 1^ are the eigen values of the nuclear spin angular momentum operator, 1^ . , for the component in the direction of unit vector, LL . Mien the interacting nucleus is nitrogen 14, with nuclear spin quantum number 1, the eigen. value equations are: Ia'f<>= ]>>'=» 0 Iu'H>'= -I-if ) (^ .24) ana L / l - > = - H > " where a state with a prime corresponds to S = -i and that with a double ori^e corresponds to S? -. ~-|. The six different energy levels of the system with one unpaired electron and. one nitrogen atom in an external magnetic field., H , ares E 2 = I ft H 1 ? P " (4.25) 3 = ^ f P n 113 E5 = -* n H When the transition of the system from one electronic state to the other ( ^Ms = - l) occurs the nuclear spin quantum number in general changes. The ESR transition probabilities are proportional to the absolute square of the matrix element of the electron spin operator perpendicular to the direction of the external magnetic field, S , say: (4.26) The set of nuclear spin states, (/'> , ) c a n ke expressed by the linear combination of the set of states, (/'>, \oy using the rotation "I matrix for spin 1*.~> / f (/r coop) 7s A^(b \iro-(4.27) ( l i - c*op) ) where >^ is the angle between U. and and cos |3 =J£,'J£** Then: !0>"= -g/u«>pit>' t cjopio/- -jf^pi-i/ ( ^ 2 8 ) H > " = -£(/-C*sp )//>' + -£/U~,p I0> + ~~ (It COOp) I-:// The ESR transition frequencies and the relative intensities of the signals are tabulated in table VIII. Assuming that the principal axes of the nitrogen hyperfine coupling tensor l i e along the orthognal axes, x, y, and E, the theoretical ESR lines 114 with respect to the centre signal (transition 5 in table VIII) and their intensities were calculated for various orientations of _Hp in the y-z plane. In Fig. 11 an experimentally observed spectrum (a) with the external magnetic field orientation almost parallel to the y axis in the y-z plane (9 90°) is shown together with the computor plotted signals*^ with two sets of nitrogen hyperfine coupling principal values, (0 gauss, 0 gauss, 10 gauss) in (b) and (4,5 gauss, 4.5 gauss5 10 gauss), in (c). Table VIII Nitrogen Transitions Transition , Frequmcies fi Relatiyg^ Intensities^ 1 fr/SH+i CffC-r) - M-i)] * (i+cos/3) 2 2 fpH ) -I ( i - cos/3)2 3 <ffr\ t± R Or) 1 (1- Cos 2 /3) , tfH + ± W ± ) - * ( l - c c s ^ ) 2 5 ^ptf Cos2p 6 ^ H - - H ^ ) - £ C - i ) j * ( 1 - c o s ^ ) 2 7. I ( 1 - cos^) 8 ^(3 H I ( 1 - c o s ^ ) 2 9 $pH - i-[R(i)+ R(-k)l -I (i+cos/j) 2 115 136, The nitrogen splitting in the x-y plane could, not be resolved, i t could, be any value less than 4.5 gauss. This was the reason why the above tw6 s*ets of principal values were used. The line width used to produce the spectra was 2 gauss. From the similarity of figures (11 - a) and. (11 - b), th© nitrogen hyperfine coupling in the y - direction and in the x - direction must be near to zero or at least very much less than 4.5 gauss. From the observed hyperfine splittings and assuming the principal values along the x and the y directions to be zero, a^t :isotropic Nsoteporierit, aaad;- b^ ,? the anisotropic components -s of the cyano-nitrogen coupling tensor were calculated to be 3»7 gauss and 3«? gauss, respectively. The anisotropic component of the nitrogen hyperfine coupling tensor, i f the unpaired electron 104 density is entirely in the nitrogen 2p orbital has been calculated... The value of 48 MHz (= 17.1 gauss) was obtained using ("y}^ = 3«1 a.u, for the nitrogen 2p orbital. The observed value of 3«7 gauss for cyano-nitrogen implies the spin density in the nitrogen 2p orbital to be 0.22. An extended. Hlickel MO calculation and. a simple Htickel MO calculation"^ were carried out on the TO system of the radical, CMCHCONH^ , to see how the theory predicts the unpaired, electron densities on various atoms. For the extended Httckel MO calculation, the atomic distances used for the system is shown below: 117 The hybrid, carbon orbitals on the two carbon atoms were assumed to be pure sp 2 hybrid orbitals. Only the TL electron system consisting of 2pz orbitals perpendicular to the radical plane were taken into consideration. The problem was therefore that of a six orbital -, seven electron- and six atom- one. The detailed procedure for the calculation was already described in chapter I. For the simple Hlickel MO calculation, the Coulomb integrals, and the resonance integrals, |3cx, for heteroatoms, X, (nitrogen or oxygen) were estimated from the equations given by Streitweiser Ux = °<o f- hxfio r (4.29) and. parameters, h x and k c x» given in the same reference were used. .tV0and. p o were the standard values for benzene. The results of both calculations are shown in the table IX. Using the relation in equation (1.33) (a^ =Qf>)* the observed imparled electron density on methylene carbon was found to be O.85. The results of both calculations predicted a much smaller density on methylene-carbon and a much larger density on the cyano-nitrogen. It is not very clear why experimentally, there was such l i t t l e derealization of the unpaired electron to the cyano-nitrogen. 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