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Molecular motion in urea and thiourea adducts Gilson, Denis Frank Robert 1959

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MOLECULAR MOTION IN UREA AND THIOUREA ADDUCTS by DENIS FRANK ROBERT GTLSON B . S c , U n i v e r s i t y C o l l e g e L o n d o n , 1957 A THESIS SUBMITTED I N PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE I n t h e D e p a r t m e n t o f C h e m i s t r y We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF B R I T I S H COLUMBIA O c t o b e r , 1959 ( i i ) ABSTRACT The p r o t o n m a g n e t i c r e s o n a n c e o f t r i d e c a n e a n d h e xadeeane and t h e i r u r e a a d d u c t s and o f some t h i o u r e a a d d u c t s h a s been s t u d i e d a s a f u n c t i o n o f t e m p e r a t u r e . I t h a s b e e n shown t h a t q u i t e c o n s i d e r a b l e m o l e c u l a r m o t i o n c a n o c c u r i n t h e a d d u c t s e v e n a t l o w t e m p e r a t u r e s . The r e s u l t s s u p p o r t p r e v i o u s c o n c l u s i o n s o b t a i n e d by o t h e r methods. In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make i t freely available for 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 representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. The University of British Columbia, Vancouver Canada, Department of ( i i i ) ACKNOWLEDGEMENT I wish t o express my thanks t o Professor C. A. McDowell f o r h i s guidance and encouragement. The discussions I have had with Dr. B. A. Dunell have been of great b e n e f i t . I should a l s o l i k e t o thank Messrs. Hawkins, Meulchen, and Sawford of the Departmental Workshops f o r t h e i r able assistance i n the design and construction of apparatus. (iv) CONTENTS Introduction Page 1 Chapter 1 Nuclear Magnetic Resonance 2 (i) Introduction 2 ( i i ) Relaxation 3 ( i i i ) Line widths and shapes 7 (iv) The effect of motion on line shape 10 and second moment Chapter II The Structure of Urea and Thiourea Adducts 13 Chapter III Apparatus and Experimental Procedure 17 (i) The Spectrometer 17 ( i i ) Variable temperature systems 19 ( i i i ) Calibration 19 (iv) Preparation of Samples 20 Chapter IV Results and Discussion 21 (i) ti-Paraffins 21 ( i i ) % e a Adducts 25 ( i i i ) Thiourea Adducts 30 Appendix Bibliography (v) DIAGRAMS Fig. (1) Precession of p~ about H . to follow page 2. (2) Local Field due to prec§ssing nucleus. 2. (3) Nuclear Suseeptibilites. 7. (4) Line Shape for static doublet. 7. (5) Line Shape for static t r i p l e t . 9. -^  (6) Motion of internuelear vector. 9. (7) lane Shape for rotating doublet. 11. (8) Line Shape for rotating t r i p l e t . 11. (9) Structure of urea hydrocarbon adduct. 14. (10) Block diagram of spectrometer. 17. (11) Dewar Insert Design I. 19. (12) » " Design II. 19. (13) Line Shape derivatives for tridecane. 21. (14) Line Shape derivatives for Hexadecane. 21. (15) Line width and second moment vs temperature for Tridecane. 22. (16) Line width and second moment vs temperature for Hexadecane. 22. (17) Line shape derivatives for tridecane adduct. 25. (18) Second Moment vs temperature for tridecane adduct. 26. (19) Second Moment vs temperature for Hexadecane adduct. 26. (20) Line Shape derivatives for thiourea adducts. 30. (21) Line Shape derivatives for thiourea adducts. 30. TABLE Line Widths of thiourea adducts. 31 1 INTRODUCTION > "The term clathrate i s derived from 'clathratus 1 (Latin, cage) and i s used to describe c e r t a i n compounds i n which one component i s trapped within the l a t t i c e of another without apparently being chemically bonded to i t . Compounds of t h i s type include the gas hydrates :(1), urea adducts (2), benzene n i c k e l ammonium cyanide clathrates (3), and the clathrates of quinol (4)« The preparation, structure and properties of clathrates have been reviewed by Powell (5) (6). Structural studies indicate that the distance between the cage and the enclosed component i s too great f o r any chemical bond-ing to occur hence the material must be held only by physical forces. These comparatively s l i g h t forces may allow the enclosed molecule a ce r t a i n freedom of motion, thus i n the sulphur dioxide-quinol clathrate the S0 2 molecule has been shown d i r e c t l y by Xray Fourier analysis to reorient about one axis i n i t s quinol cage (7). The d i e l e c t r i c absorption at microwave frequencies of quinol clathrates (8) and of urea adducts of long chain ketones and a l k y l halides (9), and the poor Xray scattering of gas hydrates i n i c e (5) lead to s i m i l a r conclusions. This i n v e s t i g a t i o n was concerned with the motion of the enclosed molecules i n the hydrocarbon adducts of urea and thiourea. "Wide l i n e " Nuclear Magnetic Resonance techniques provide an excellent means of studying molecular motion i n s o l i d s and are thus well suited to a problem such as t h i s . The application of Nuclear Resonance theory to s o l i d s i s b r i e f l y discussed. CHAPTER 1 2 NUCLEAR MAGNETIC RESONANCE (i) Introduction and theory. The existence of nuclear spin was postulated in 1924 by Pauli (10) to account for the hyperfine structure of atomic spectra, and although molecular beam methods of observing magnetic moments (11) have been used for some time, attempts to obtain nuclear resonance signals from a condensed phase were not successful u n t i l 1946 when Bloch, Hansen and Packard (12) at Stanford, and Purcell Torrey and Pound (13) at Harvard, independently observed nuclear magnetic resonance. The i n i t i a l application of the method was to the precise determination of magnetic moments and spin, but the work of Bloembergen, Purcell, and Pound (14), and of Gutowsky and Pake (15), indicated the possi b i l i t i e s of NMR as a tool in the investigation of the structure and properties of solids and liquids. The phenomenon of nuclear magnetic resonance can be explained both from a classical viewpoint and by a quantum mechanical treatment. A nucleus possessing spin must also have a magnetic dipole moment associated with the non-zero spin angular momentum. Since both angular momentum and magnetic dipole moment are vectors, they are linearly related by a scalar,-y, termed the magnetogyric ratio, such that A magnetic dipole j*. placed in an external magnetic f i e l d H Q w i l l experience a torque tending to turn the dipole about the direction of the f i e l d . The dipole w i l l precess about the ROTATING COMPONENT Fig.2. LOCAL FIELD DUE TO PRECESSING NUCLEUS. •3 3. d i r e c t i o n o f H Q a t t h e L a r m o r f r e q u e n c y w Q g i v e n b y wQ= ^ H 0 I f a s m a l l a d d i t i o n a l f i e l d i s now a p p l i e d a t r i g h t - a n g l e s t o H Q a n d i n t h e p l a n e o f a n d H Q , a f u r t h e r t o r q u e i s e x p e r i e n c e d t e n d i n g t o i n c r e a s e 0 , t h e a n g l e b e t w e e n ^ and H Q , a n d i f i s r o t a t i n g a t t h e same f r e q u e n c y a s ^ , t h e n 9 w i l l s t e a d i l y i n c r e a s e . I f , however, t h e frequency o f H-^  i s n o t t h e same a s t h e L a r m o r f r e q a e n c y o n l y s l i g h t p e r t u r b a t i o n s o f t h e p r e c e s s i o n w i l l o c c u r . I n p r a c t i c e i t i s e a s i e r t o p r o d u c e a l i n e a r l y o s c i l l a t i n g f i e l d t h a n a r o t a t i n g f i e l d a n d so i s r e p l a c e d b y a l i n e a r l y p o l a r i s e d f i e l d 2H-^ w h i c h may be c o n s i d e r e d a s two c i r c u l a r l y p o l a r i s e d f i e l d s r o t a t i n g w i t h t h e same f r e q u e n c y i n o p p o s i t e d i r e c t i o n s . O n l y t h e f i e l d r o t a t i n g i n t h e same s e n s e a s t h e L a r m o r f r e q u e n c y w i l l i n d u c e t r a n s i t i o n s ; t h e o t h e r h a v i n g n e g l i g i b l e e f f e c t . A c c o r d i n g t o quantum m e c h a n i c s t h e a n g u l a r momentum o f t h e n u c l e u s must be q u a n t i s e d i n u n i t s o f , and may o n l y assume c e r t a i n o r i e n t a t i o n s when p l a c e d i n a m a g n e t i c f i e l d . Hence o n l y c e r t a i n d i s c r e t e e n e r g y l e v e l s a r e p e r m i s s i b l e . The number o f s u c h l e v e l s i s g i v e n b y ( 2 1 + I), and t h e e n e r g y d i f f e r e n c e between a d j a c e n t l e v e l s i s A-E "jjMjj = «{k>W0. Now & £ = fvO , h ence 2u^ ,-y|40 o r i n t e r m s o f a n g u l a r f r e q u e n c y uv=^H0 , w h i c h i s t h e same c o n d i t i o n f o r r e s o n a n c e a s t h a t o b t a i n e d f r o m t h e c l a s s i c a l a p p r o a c h . ( i i ) R e l a x a t i o n . I n t h e a b s e n c e o f some r e l a x a t i o n p r o c e s s b y w h i c h , s p i n s may l o s e e n e r g y , t h e c o n t i n u e d a b s o r b t i o n o f e n e r g y b y t h e l o w e r l e v e l s w o u l d teiad t o e q u a l i s e t h e p o p u l a t i o n s o f t h e l e v e l s so t h a t no f u r t h e r a b s o r b t i o n c o u l d be d e t e c t e d . T h e r e must 4. therefore be some mechanism by which the Boltzmann d i s t r i b u t i o n between the l e v e l s can be maintained during resonance absorption. Two d i f f e r e n t relaxation processes ex i s t , the spin l a t t i c e i n t e r -action and the spin-spin i n t e r a c t i o n . S p i n - l a t t i c e relaxation. A s p i n - l a t t i c e relaxation time may be considered as the time constant f o r the system with equal populations of l e v e l s to exponentially approach to the e q u i l i b -rium state, i . e . to at t a i n thermal e q i l i b r i u m with the l a t t i c e . The population of the l e v e l s at eqilibrium i s governed by the Boltzmann factor N+^N-^  fcxp^A/ij) where N- 3 3 3 ( 1 N +,are the number of n u c l e i i n the upper and lower states respectively. I f the pr o b a b i l i t y per unit time of downward t r a n s i t i o n s i s W_and f o r upward t r a n s i t i o n s i s W+ then at eqil i b r i u m W +I +» W/:JI3_. Therefore W- /VJ + « « exp(2 A A/kT) « 1 + 2 / f c A/lcT. I f W i s the mean of W+and W_ then W. - W(w2 A o r UkT) ; W + - W(< - Z/*X/&). I f the spin temperature i s now altered, a change i n the popula-tions must r e s u l t , i f the excess of n u c l e i i n the lower state i s n, then dn/dt 2W(n_ - n), where nn=N/iJI /tCT. Hence -2Wt 0 0 / 0 ( n Q - n) * (n 0-nj)e , where i s the i n i t i a l difference i n population. Thus the equilibrium i s approached with a character-i s t i c time constant T-^  = 1/2W. The relaxation process has also been treated from the phenomenological viewpoint by Bloch (16). For an assembly of weakly i n t e r a c t i n g spins, the magnetisation vector, M, i s the vector sum of a l l the nuclear magnetic moments i n unit volume. In the absence of the ro t a t i n g magnetic f i e l d and with the spin system and l a t t i c e i n thermal e q u i l i b r i u m then ffiL 't the component 5. parallel to the steady f i e l d HQ, i s given by M0 = 'X0Ho. When the system i s not in equilibrium, then dMz/<tfc * (M0-Hz) l~Tx . From this formulation Bloch termed T-^the longitudinal relaxation time. from experiment i s found to range from microseconds to several hours.(14). A thorough theoretical analysis of spin la t t i c e relaxa-tion has been made by Bloembergen Purcell and Pound (14).* In liquids the relaxation i s due to the time varying dipolar interaction resulting from translational and rotational motion. In general a continuous spectrum of frequencies of the dipolar fields w i l l be produced but only a narrow band of frequencies near the resonant frequency and. twice the resonant frequency, w i l l be effective in producing spin-lattice relaxation. A similar process occurs in solids when molecular rotation and translation occur, in the absence of such motion however some other relaxation process must be present. The mechanism was shown by Bloembergen (17) to be due to paramagnetic centres present in the solid. Electron magnetic moments being of the order of 10 times larger than the nuclear moments produce very large fluctuating magnetic fields in their immediate vi c i n i t y and cause rapid nuclear spin relaxation for those nuclei close by. Spin-Spin Interaction. Apart from the interaction between the spins and the l a t t i c e , an I interaction occurs between the spins themselves. According to the Bloch formulation the relaxation time T2 i s termed the transversal relaxation time since i t i s the time constant for the exponential decay of the deferred to as BPP. transverse x and y components,of the nuclear magnetisation vector cLMx-a/cIT*-Mx. y/j 2 . The physical significance of T 2 i s the time constant f o r the i n d i v i d u a l spins to lose r o t a t i o n a l phase coherence with one another. The nuclear spins w i l l be i n s l i g h t l y d i f f e r e n t magnetic f i e l d s due to the magnetic dipoles of t h e i r neighbours, they w i l l therefore precess at d i f f e r e n t Larmor frequencies and eventually become phase incoherent. The l o c a l f i e l d E L due to the neighbouring dipoles consists of two l o c components, the resultant of the s t a t i c f i e l d s and the resultant , of the r o t a t i n g components -of a l l l o c a l f i e l d s (Fig.2.) The magnitude of the f i e l d due to a dipole JUL at distance r i s of the order of yur"3 • This dependence on the inverse cube i n f e r s that only nearest neighbours make any appreciable contribution. The resonance i s no longer sharp but w i l l be broadened about.H Q, the l i n e shape depending on the s p a t i a l configuration of the l a t t i c e and the given d i r e c t i o n of H 0. For completely random d i s t r i b u t i o n of a large number of near neighbours a Gaussian l i n e shape w i l l r e s u l t . In general the l i n e shape deviates from the pure Gaussian shape due to the ordered arrangement of dipoles i n the l a t t i c e . A second process leading to the broadening i s the simultaneous exchange of energy by two a n t i p a r a l l e l spins. One nucleus may produce a f i e l d at a second nucleus, o s c i l l a t i n g at i t s Larmor frequency, r e s u l t i n g i n r e c i p r o c a l t r a n s i t i o n s . This exchange of spin energy does not r e s u l t i n emission or absorption of energy, but does l i m i t the l i f e t i m e i n given state. The resonance l i n e width i s thus dependent on the spin-spin i n t e r -action time or spin phase-memory time. In the notation of BPP, 7 0?2 i s r e s e r v e d f o r t h e s p e c i f i c a t i o n o f t h e l i n e w i d t h i n t h e g e n e r a l c a s e t o w h i c h T-j. a l s o c o n t r i b u t e s , T 2 " d e f i n e s t h e l i n e w i d t h i n t h e l i m i t i n g c a s e o f a r i g i d l a t t i c e , a n d T 2 , L f o r t h e ea s e where l a t t i c e m o t i o n o c c u r s . T h e r e a r e two components o f t h e t r a n s v e r s e n u c l e a r m a g n e t i s a t i o n , t h a t i n p h a s e w i t h t h e r a d i o - f r e q u e n c y f i e l d a n d t h a t Hj> o u t o f pha s e w i t h H^. These a r e known a s t h e d i s p e r s i o n mode o r u-mode a n d a b s o r p t i o n o r V-mode. The m a g n e t i c s u s c e p t i b i l i t i e s a r e d e f i n e d b y M-(%/-^ y-*)H, where %f i s t h e r e a l p a r t and X" "foe i m a g i n a r y p a r t o f t h e complex s u s c e p t i -b i l i t y , X r i s a s s o c i a t e d w i t h t h e d i s p e r s i o n mode a n d X" w i t h a b s o r p t i o n . The d e p e n d e n c e o f X* a n d X" on (H -H0) i s shown i n P i g . (3). ( i i i ) L i n e W i d t h s and S h a p e s . The s i m p l e s t a s s u m p t i o n t h a t may be made r e g a r d i n g l i n e s h a p e s i s t h a t t h e a b s o r p t i o n c u r v e c o r r e s p o n d s t o e i t h e r a G a u s s i a n o r a L o r e n f c z i a n d i s t r i b u t i o n . I n p r a c t i c e t h e s e s i m p l e l i n e s h a p e s a r e n o t f o u n d , i t i s i n g e n e r a l i m p o s s i b l e t o c a l c u l a t e a n e x p e c t e d l i n e shape e x c e p t i n c e r t a i n s i m p l e g r o u p s . R e s o n a n c e l i n e w i d t h s may be c l a s s i f i e d i n t o two t y p e s :-(a) Homogeneous b r o a d e n e d , i n c l u d i n g d i p o l a r i n t e r a c t i o n between l i k e s p i n s , m o t i o n a l n a r r o w i n g , s u f f i c i e n t l y r a p i d s p i n -l a t t i c e r e l a x a t i o n s u c h t h a t T^ • T 2 . (b) Inhomogeneous b r o a d e n e d due t o inhomogeneous s t a t i c f i e l d , a n i s o t r o p i c c h e m i c a l s h i f t , K n i g h t s h i f t , ( 1 8 ) Q u a d r u p o l e b r o a d e n i n g , ( 1 9 ) d i p o l a r i n t e r a c t i o n between u n l i k e s p i n s . (i) absorption Vnode. (2) dispersion mode. Fig.3. NUCLEAR SUSCEPTIBILITIES. Fig.4. THEORETICAL LINE SHAPE FOR STATIC DOUBLET. 8. Dipolar broadening, (a) Two spin groups The line shape of such a system was studied by Pake (20) in a single crystal of gypsum CaS04.2H20 at room temperature, when the system i s effectively r i g i d . Since the dipolar interaction i s dependent on the inverse cube of distance, by far the largest effect i s due to the neighbouring proton in the water molecule. The component of this local f i e l d parallel to the applied f i e l d H Q i s given by i j^v"^ {Zco^-Q-\). Thus, neglecting local fields of more distant neighbours, the resultant f i e l d i s H " R0-/AX^ CZcofQ-ty. This results in two lines equally spaced about H Q. In gypsum two proton pairs occur with different values of 0 , a single crystal w i l l therefore give two pairs of symmetrically disposed resonance lines which w i l l be broadened by the small local fields of more distant neighbours. Pake however found that resonance should occur at H * W0± ^ ^ ( " S c o ^ S - t ) , the term \ arising from the spin exchange process discussed under spin-spin relaxation processes. I f the sample i s polycrystalline a random distribution of individual crystals occurs and the resultant line shape i s the sum of a l l individual spectra. The calculated line shape i s shown by the broken line in f i g . (4) the f u l l line shows the broadening by other neighbours. Prom this the interproton distance may be calculated but the use of polycrystalline material precludes the measurement of angular disposition. Three spin system. The resonance line shape for a triangular configuration of nuclei has been derived by Andrew and Bersohn (21). The case i s considerably more complicated than the two spin system. Por a single crystal, neglecting 9 broadening by neighbours, the spectrum consists of a central line and three pairs of lines symmetrically placed about the centre. For polycrystalline material the line shape i s ob-tained by summation over a l l crystals, Fig. (5). When sbroaden-ing i s taken into account the line structure i s smoothed out. The confirmation of the existence of HjO in e.g. the hydrates of HN03.H20 .(22) and HClO^.HgO (23) at 90°K was obtained from the characteristic resonance line shape. More complicated systems. For groups with more than three nuclei only rough predictions; can be made regarding the line shape, although certain cases such as CH^ and NH^ have met with some success (24) (25). Van Vleck (26) has derived i n a rigorous manner an expression for the second moment of the absorption curve in terms of the magnetic moments, spins, and internuclear distances of the nuclei. The second moment <AHj.> of the shape function g(H-HQ) normalised to unit area i s <AHV> = p3(R-Ho)(H-H0)\*H. Van Vleck showed that for a single crystal containing N nuclei of only one magnetic species, the second moment i s given Strictly, N should be taken for the total number of resonating nuclei in the sample but due to the dependence on the inverse sixth power of distance N may be reduced to the number i n the unit c e l l and their nearest neighbours. M may be reduced s t i l l further i f the unit c e l l possesses symmetry such that two or more (a) (b) Fig.5. THEORETICAL LINE SHAPES FOR (a) STATIC A WITHOUT BROADENING, (b) BROADENED BY NEIGHBOURS. Fig.6. MOTION OF INTERNUCLEAR VECTOR O P ABOUT AXIS O N . 10. nuclei have the same environment. I f the sample i s polycrystalline the angular factor may be averaged over a sphere, and i f the crystal contains other magnetic species a second term must be included. The second moment now becomes The validity of the second moment has been amply supported and has proved extremely valuable in structural determinations. (iv) The effect of motion on line shape and second moment. If the motion of molecules or groups i s sufficiently rapid the line width and line shape may be profoundly altered. Hindered rotation of groups of nuclei in solids often occur about one direction or axis and only a partial reduction in line width may result. Isotropic rotation i s necessary for complete averaging of the dipolar f i e l d s . The limiting line width for a particular mode of motion may be calculated by averaging the appropriate angular factors. The effect of rotation on the line shape for a two spin system has been given by Gustowsky and Pake (15) > Pig.(6). OP i s the vector r]k joining nuclei j and k at an angle Gjk to the applied f i e l d H q. ON i s the axis of reorientation making an angle 91 with H G and angle y j lc with . The reorientation causes to vary with time-and i t becomes necessary to take the average value of which oecomes • I f the axis of reorientation ON i s perpendicular to the internuclear vector as i s commonly 11. the case, the absorption l i n e has two components at W - W0 t | A r - 3 ( 1 - 3 c o S i 0 / ) . The maximum s p l i t t i n g i s only h a l f that of the r i g i d system., see F i g . (7). A three spin system reorien t i n g about any given axis (21) leads i n the general case to a central l i n e and three pairs of lines, as i n the case of the r i g i d l a t t i c e . However fo r the spe c i a l case i n which the axis of rota t i o n i s perpendicular to the plane of the tr i a n g l e two pairs of l i n e s disappear, Fig.(8). This has been observed f o r methyl groups i n a number of compounds (15). For a single c r y s t a l the angular term i n the second moment must be averaged over the motion. For a p o l y c r y s t a l l i n e sample with axis of rotation randomly d i s t r i b u t e d i t i s necessary to average over a l l values of 0', which leads to an expression fo r the second moment of I f the r o t a t i o n a l axis i s perpendicular to a l l i n t e r -nuclear vectors the above expression reduces to a value one fourth of the r i g i d l a t t i c e value. In the more general case the f a c t o r ^C^CO8>LV$jc-0 must be applied. This factor decreases from unity at 0° to zero at •=. 54°44' and increases to ^  at Y J J c . 9o° The e f f e c t of r o t a t i o n a l o s c i l l a t i o n has been considered by Andrew (27). For small angular amplitude of Fig.7. THEORETICAL LINESHAPES FOR A ROTATING DOUBLET. (a) (b) Fig.8. LINESHAPES FOR A ROTATING A (a) isolated A rotating about normal to plane. (b) broadened by neighbours. 12. o s c i l l a t i o n the reduction factor a of the second moment i s given by >^ =. 1 - ^ S u a y -13 CHAPTER 11 THE STRUCTURE OP UREA AND THIOUREA ADDUCTS The o c c l u s i o n compounds o f u r e a a n d t h i o u r e a , v a r i o u s l y r e f e r r e d t o a s i n c l u s i o n compounds, addufcts, c h a n n e l c o m p l e x e s and u r e a c o m p l e x e s , can be p r e p a r e d f r o m a v a r i e t y o f o r g a n i c m o l e c u l e s b e a r i n g v a r i o u s f u n c t i o n a l g r o u p s . The u r e a a d d u c t s were a c c i d e n t a l l y d i s c o v e r e d b y B engen (2) d u r i n g r e s e a r c h on t h e f a t c o n t e n t o f m i l k . S u b s e q u e n t s t u d i e s have c o n f i r m e d a n d e x t e n d e d Bengen's r e s u l t s (28) (29) (30). I n g e n e r a l t h e e a s e o f f o r m a t i o n and s t a b i l i t y o f u r e a a d d u c t s a r e d e p e n d e n t on t h e c h a i n l e n g t h and v a p o u r p r e s s u r e o f t h e o c c l u d e d com-pound. C e r t a i n b r a n c h e d c h a i n a n d c y c l i c compounds w i l l a d d u c t w i t h u r e a p r o v i d e d t h e r e i s a s u f f i c i e n t l y l o n g c h a i n i n t h e s t r u c t u r e (31) (32). The a d d u c t s o f t h i o u r e a were d i s c o v e r e d i n d e p e n d e n t l y b y A n g l a (33) and b y F e t t e r l y (34). The c o m p l e x e s a r e s i m i l a r t o t h o s e o f u r e a b u t f o r m w i t h b r a n c h e d c h a i n and c y c l i c s t r u c t u r e s (35) (36^ a l t h o u g h r a t h e r u n s t a b l e c o m p l e x e s w i t h s t r a i g h t c h a i n h y d r o c a r b o n s c a n be p r e p a r e d (37). The a n a l y s i s o f u r e a c o m p l e x e s showed a n o n -s t o i c h i o m e t r i c c o m p o s i t i o n b u t t h a t a d e f i n i t e r a t i o e x i s t e d between t h e amounts o f e a c h component, and, f o r s t r a i g h t c h a i n a d d u c t s , t h a t t h i s r a t i o was d e p e n d e n t on c h a i n l e n g t h . Debye d i f f r a c t i o n p a t t e r n s e x h i b i t e d t h e same p a t t e r n f o r n e a r l y a l l c o m p l e x e s (38) i n f e r r i n g t h a t t h e s t r u c t u r e o f t h e c o m p l e x e s was t h e same, i n d e p e n d e n t o f t h e h y d r o c a r b o n . 14. A precise X-ray determination of the structure of urea adducts was made by Smith (39) (40). In urea i t s e l f the unit c e l l i s tetragonal whereas in the complex i t i s hexagonal 6 ' o with six urea molecules per unit c e l l . a 0- 8.23A, C 0- 11.0OA., Fig. (9). The urea molecules form a hollow channel in which the hydrocarbon i s enclosed. The hydrocarbon chains are in an extended planar zig-zag configuration with their long axes parallel to the c axis. The time average position of the plane of the hydrocarbon molecule i s randomly oriented perpendi-cular to the a axis and at multiples of 60° to this position. Laue and rotation photographs show oontinuous layer lines attributed to the hydrocarbon molecules, the distance between these lines can be used to compute the molecular length of the enclosed hydrocarbon (41). In the complex each oxygen i s hydrogen bonded to four nitrogen atoms and each nitrogen to two oxygen atoms. o Two.different hydrogen bonds occur (i) short, c. 2.93A- ( i i ) long o c. 3.04A. The bonds are essentially co-planar with the urea molecules to which the nitrogen atoms are attached. C-N-H—0 o o angles are about 136 and 116 for the strong and weak bonds respectively. The hydrogen bonds account largely for the sta b i l i t y of the complex. The heat of formation of the adduct arises from (a) differences in hydrogen bond energy in urea and in the complex, (b) differences in van-der Waals forces in tetragonal urea and hydrocarbon and in the complex. The energy associated with the observed shortening of the hydrogen bonds i s of the order of magnitude of the observed heat of formation. The van der Waals forces between the oxygen of the urea and the Fig 9 STRUCTURE OF UREA HYDROCARBON ADDUCT. Showing position of hydrogen bonds and hydrocarbon chain. A.E.Smith. J.Chem.Phys. j§, 150,1950. 15 atoms of the hydrocarbon chain, although small for each atom pair^ contribute to the st a b i l i t y of the adduct due to the large number of such interactions. 0 o Taking values of 1.54A and 109 28' for the bond length o and angle of the carbon chain and 2*0A as the van der Waals radius of the methyl groups, the chain length for the extended o planar zig-zag configuration i s given by 1.256(n-1) *• 4.OA. Prom the dimensions of the urea unit c e l l the mole ratio of urea to hydrocarbon i s 0.6848(n-l) * 2.181,which i s in agree-ment with experiment. The structure of thiourea adducts has been mentioned briefly by Smith (39) and also investigated by Schlenk (42). The unit c e l l i s rhombohedral and the molecules arranged i n a similar fashion to urea in urea adducts. The larger size o of the sulphur alom results in a larger channel 5.8 - 6.8A as o opposed to 4.1 - 4.8A in urea adducts. The length of the unit o c e l l i s 12.6A . Ho simple relationship linking the dimensions of the enclosed component and the mole ratiox exists for thiourea adducts. It i s found however, for ag.-w-w' dicyclohexyl alkanes, that constant mole ratios of about 9 :1 and 6 :1 are obtained for certain members of homologous series. This i s explained as resulting from compression of the paraffinift portion of the molecule. The interest shown in urea and thiourea adducts has largely been confined to their technological applications i n the petroleum industry (43). Apart from the structural studies of Smith and Schlenk, the only physical measurements which have been made are those of Fischer and McDowell (44),Stewart (45) i. 16. and Barlow and Corish (46) on infra red spectra, Meakins (9) dielectric measurements, and Topchiev etal. (47) diffe r e n t i a l thermal studies. The infrared studi.es show shifts to longer wavelengths in the N - H stretching region corresponding to increases in hydrogen bond strength. The absorptions due to the enclosed molecule show l i t t l e or no change in frequency. Meakins (9) examined the dielectric absorption at microwave frequencies of the adducts of long chain polar compounds. In a l l cases a large dielectric absorption was obtained and interpreted in terms of the reorientation of the long chain dipoles i n the urea l a t t i c e . The presence of dielectric absorption in 1,10, dibromodecane adduct,in which the chain i s known to be in an extended planar configuration, (40) and should therefore possess zero dipole moment, was assumed to result from reorientation of the C-Br dipolesdndepen-dently of the main chain. The high frequency of the absorption maximum also inferred that the long chain molecules were loosely held i n the urea l a t t i c e . The thermal studies of Topchiev etal. (47) show evidence for various phase changes in the adducts of several n-paraffins in the range C21 - • Three or four points were observed (i) A polymorphic reversible phase change. ( i i ) The melting point of the hydrocarbon. ( i i i ) Decomposition of the adduct. (iv) The melting point of urea. The existence of definite decomposition points has also been reported by Sweria «ifoal.(30) for adducts of fatty acids. 17 CHAPTER 111 APPARATUS AND EXPERIMENTAL PROCEDURES (i) The Spectrometer. The methods employed by Purcell, Torrey and Pound (4) and by Bloch, Hansen and Packard (3) differed in the instrumen-t a l technique used to observe resonance. In the spectrometer of Purcell et. a l . a single c o i l , which acted as both trans-mitter and receiver, was placed in one arm of an R.P. bridge. The bridge could be balanced to obtain either the absorption or the dispersion mode. At resonance a signal appeared at the balance point of the bridge as a result of the change in the impedance of the arm containing the sample. The nuclear induction method of Bloch employed a second c o i l , acting as a receiver, which was placed orthogonally to the transmitting c o i l and to the steady magnetic f i e l d . On passage through resonance an e.m.f. was induced into the receiver c o i l . The' magnetic coupling (leakage) between the coils was controlled by the geometry of the system and by movable paddles, which caused the flux lines to deviate from axial symmetry and thus compensated for slight departures from orthogonality. The spectrometer used i n these investigations was a Yarian V4200/4300B Dual purpose spectrometer (48) operating on the Bloch or crossed c o i l principle. A block diagram i s given in Pig. (10). Por proton resonance the spectrometer was operated at 40/Mc/s and 9400 gauss. The R.P. power was supplied MAGNET FIELD POWER SUPPLY SCANNING UNIT ELECTRO-PROBE -MAGNET PRE-AMP SWEEP AMPLIFIER TRANSMITTER RECEIVER OUTPUT CONTROL - , AUDIO OSCILLATOR I SELECTOR UNIT SWEEP UNIT RECORDER 1 POWER SUPPLY UNITS Fig. 1 0 . BLOCK DIAGRAM OF VARIAN "WIDE" LINE" SPECTROMETER. 18 by a 10 Mc/s crystal oscillator followed by two frequency doublers. The induced signal in the receiver c o i l appeared as a modulation of the c®&Jtpling between the c o i l s . This signal was fed to a low noise preamplifier in the probe and then, after further amplification,was demodulated to audio frequencies by a tuned R.F. amplifier-detector. In order to calibrate signals, provision was made for audio frequency modulation of the R.F. power. The polarising magnetic f i e l d was modulated at audio frequencies by sweep coils in the probe,,enabling A.C amplifiers to be used in the spectrometer. The output from the detector was converted to direct current by a phase sensitive detector and recorded as the derivative of the absorption signal. The signal could be presented on a Dumont persistent screen oscilloscope by driving the X-axis by the sweep oscillator and applying the signal to the Y-axis. In recording the signal, f i l t e r s placed after the phase sensitive detector allowed much of the noise to be integrated. The sweep modulation was reduced to a value low compared with the line width i n order to avoid spurious broadening of the signal. Certain minor modifications were made which improved the performai ce of some components of the spectrometer but did not substantially alter any aspects of i t s operation. ( i ) The V2100 Magnet Power Supply was modified to exclude the V2101B Voltage Regulator (as per instructions from Varian Associates). ( i i ) The drive shafts from the synchronous motors i n the V4280 A Field Scanning Unit were replaced by a single spindle. 19 This prevented the drive slipping at an Allen nut connection in the original meehanism. ( i i i ) To prevent lateral movement of the scanning motors, which allowed the gears to disengage, the springs holding the unit were replaced by a clamp. ( i i ) Variable temperature systems. Two designs of variable temperature equipment were used. In design I, f i g . (11), cooling was achieved by a stream of dry nitrogen controlled by a reduction valve, needle valve and flowmeter. The gas was cooled by passing through a copper c o i l inmersed in refrigerant (liquid nitrogen or CC^/acetone). Por temperatures above room temperature a Uichrome wire heater wound on a quartz rod was inserted i n the gas stream. The system labelled Design I I , shown in f i g . (12) and described by Connor (49) was also used for some measurements. Although of limited temperature range (-60°C to + 120°G) this apparatus had the advantage of an improved f i l l i n g factor and the sample could be easily changed. I t was possible to examine different samples at the sample temperature without dismantling the entire apparatus. Temperatures were measured with a copper constantan thermocouple connected to a "Rubicon" potentiometer. ( i i i ) Calibration of signals. TheiBsonanee line derivatives were recorded on either a Varian G-10 or a Leeds Northrup "Speedomax". These recorders were calibrated in terms of /gauss per cm of chart paper. The method o f calibration entailed the measurement o f the distance r i h Silvered to here / B24 \ Probe Insert BIO Fig-DETAILS OF DEWAR INSERT. to fit Varian 15mm insert. LT B29 Aluminium ^ support Design I Sample -I Dewar r Sample Coil Insert base Glass bellows /Thermometer Thermocouple entry /Heating element Support Fig 12. DEWAR INSERT AND DEWAR LEAD. Design II 20. "between the sidebands on a signal from water. The sidebands were produced by modulating the R.P.power with the output i from a Hewlett Packard Audio Oscillator. Experimental second moments were obtained from the derivative curves in the usual manner. The correction for f i n i t e modulation was applied (50). Line widths were measured in terms of the width between points of maximum slope. (iv) Preparation of Samples. Samples were contained in 5mm pyrex tubes of which the bottom 3cm were thinned to improve the f i l l i n g factor,. (a) Urea adducts. According to Smith's X ray data (40) the mole ratio of urea to hydrocarbon i s given by which i s in agreement with the observed composition. In pre-paring urea adducts the theoretical amount of hydrocarbon was added to a saturated solution of urea in methanol or isopropanol. The solution was warmed u n t i l the precipitated adduct had dissolved and then allowed to cool in a preheated lagged Dewar. Deuterated urea and thiourea were prepared by repeated recrystallisation from D 20. Most solvents polar enough to dissolve urea also exchange with i t , therefore deuterated methanol was prepared by decomposing magnesium methylate in D2O. (b) Thiourea Adducts. The values given by Eedlich et.al. (35) were used and the same experimental procedure followed as for urea adducts. A l i s t of reagents, their source and purity i s given in the Appendix. 21. CHAPTER IV RESULTS AND DISCUSSION (i) Normal Paraffins. Results. (a) Tridecane n-C H . At 77°K, the lowest 1 j 28 temperature of measurement, the line width was 15.6 gauss and the second moment 21.5 gauss . At about 200 Z a narrow line of 5 gauss appeared and fine structure was present i n the centre of the resonance li n e . This fine structure become more pronounced as the temperature increased. The second moment 2 o decreased smoothly to 16.3 gauss at 255 K when i t f e l l to 4.0 gauss 2. The wide line narrowed to 12.7 gauss at 255°K and then disappeared. The line width and second moment then remained constant from 255°K to the melting point 268°K. Resonance line shapes are shown in f i g . (13), the variation of second moment and line width with temperature i n f i g . (15 a,b.). (b) Hexadecane nC^gH,^. At 77°K the second moment 2 2 was 23.5 gauss and decreased smoothly to 15.4 gauss at the melting point 289°K. The line width f e l l from 15.4 gauss at 77°K to 12.3 at 289°K. The fine structure observed in tridecane was also present i n hexadecane. No discontinuities in second moment or line width occurred. Resonance line shapes at different temperatures are shown i n f i g . (14) and the temperature variation of second moment and line width in f i g . (16 a,b.) Fig. 13. LINE SHAPE DERIVATIVES OF TRIDECANE. 14. LINE SHAPE DERIVATIVES OF HEXADECANE. 22 ( i i ) Discussion. The behaviour of tridecane and hexadecane follows a similar pattern as that discussed by Andrew (27) for octadecane, oetacosane and dotriacontane. The theoretical intra- and inter-molecular second moments were calculated by Andrew on the basis of Miiller's Xray structures for the n-paraffins (51). Por the general member, c n H2nt2' ^ B intra-molecular r i g i d l a t t i c e second moment (26) i s given by (18.5 + 19.1/n-rl). The inter-molecular contribution was based on the structure of nonacosanejC^^o, for which the structure was completely determined and calculated to be 2 7.8 gauss . Andrew's experimental second moments for oetaco-sane and dotriacontane at 95°K agreed with the theoretical r i g i d lattice values and this was taken as evidence for the absence of molecular motion at this temperature. In octade-cane the calculated second moment was much higher than the experimental value at 95°K. suggesting that some form of motion was present. In tridecane and hexadecane the experimental second moments of 21.5 gauss 2 and 23.5 gauss 2 at 77°K are 2 well below the calculated r i g i d l a t t i c e values of 27.7 gauss 2 and 27.3 gauss respectively. The type of motion most l i k e l y to occur i s that of rotation or at least rotational oscillation. Using the formula derived by Gutowsky and Pake (15) for the reduction in second moment due to rotation the intra-molecular contri-bution becomes (6.8 - 11.6/n+l) gauss 2 and the inter-molecular contribution reduces to 2.6 gauss 2. I f a l l the molecules AH gauss 10 F ig . I 5 ( a ) LINE WIDTH vs TEMPERATURE TRIDECANE. ° 5 — ° — s — n °°e T° K 100 200 300 L 2 0 Fig.15 (b). 10 SECOND MOMENT vs TEMPERATURE TRIDECANE. A H | gauss2 T° K 100 2 0 0 —I / 1 3 0 0 _ J AH. _ o ° 5 c — » gauss. -10 Fig. 16(a). ! LINE WIDTH vs TEMPERATURE HEXADECANE. 1 - 5 ; 100 T°K . 200 1 1 300 1 i - 20 AHj ^ 2 gauss• Fig. 16(b). 1 -10 SECOND MOMENT vs TEMPERATURE 1 HEXADECANE. 1 T°K 100 200 i i i 1 1 3 0 0 23. rotate the theoretical second moments for tridecane and hexa-decane would be 8.5 gauss^ and 8.7 gauss 2 respectively. The experimental val;ue for hexadecane just below the melting point ' 2 i s 15,*4 gauss , which i s too high for complete rotation of a l l the molecules. I f however, rotational oscillation occurred, the reduction factor for the second moment would be much less, and dependent on the angular amplitude. Andrew (27) derived an expression for this reduction factor in terms of the angular amplitude of oscillation and the orientation of the internuclear vector to the axis of oscillation. Applying Andrew's treatment, an angular amplitude of 30° at 77°K and of 45°at 250°K would be necessary to reduce the second moment by the required amount. An alternative explanation, suggested by Andrew, involved a form of co-operative motion in which the hydrocarbon chains behaved as a set of meshing gears such that some molecules rotated while others remained stationary. The large change in second moment in tridecane at 255°K corresponds to the phase transition reported by Finke et.al. (52) from heat capacity measurements. The phase changes which occur i n n-paraffins a few degrees below the melting points are attributed to the onset of rotation about the long axis of the molecule (51) and a change in crystal structure from monoclinie or t r i c l i n i c to orthorhombic (53). In tridecane 0 9 the second moment above 255 K -bf 4.0 gauss*1 i s lower than the calculated 8.5 gauss for complete rotation. A similar behaviour was found by Andrew in octacosane and dotriaeontane, Andrew suggested that other forms of motion such as torsional 2 4 . o s c i l l a t i o n , l o n g i t u d i n a l a nd l a t e r a l m o t i o n and f l e x i n g were r e s p o n s i b l e . A l t h o u g h t h e change i n s e c o n d moment a t 255°K i s f a i r l y s h a r p , t h e c h a n g e s i n l i n e shape a r e d e f i n i t e l y n o t , t h e n a r r o w 5 g a u s s component i s e v i d e n t n e a r l y 5o° b e l o w t h e t r a n s i t i o n p o i n t a nd i n c r e a s e s i n i n t e n s i t y a s t h e b r o a d l i n e d e c r e a s e s b o t h i n w i d t h and i n t e n s i t y . T h i s s u g g e s t s t h a t some f o r m o f m o t i o n must be p r e s e n t a t a b o u t 200°K a n d i s i n -c r e a s i n g a s t h e t e m p e r a t u r e r i s e s . From t h e r m a l and d i e l e c t r i c d a t a on l o n g c h a i n compounds Hoffmann (54) assumed t h a t t h e o n s e t o f r o t a t i o n i s a c o - o p e r a t i v e e f f e c t b u t t h a t t h e c h a i n s do n o t t w i s t s e v e r e l y d u r i n g r o t a t i o n i n t h e s o l i d s t a t e . A c c o r d i n g t o S m i t h (55) t h e a d d i t i o n o f a few p e r c e n t o f n e i g h b o u r i n g homologues i s s u f f i c i e n t t o a l t e r t h e t r i c l i n i c a nd m o n o c l i n i c f o r m s t o r h o m b o h e d r a l . The p r e s e n c e o f i m p u r i t i e s c a p a b l e o f m o t i o n t h e m s e l v e s o r p r o d u c i n g c e n t r e s i n t h e l a t t i c e w h i c h a l l o w t h e h y d r o c a r b o n i t s e l f a h i g h e r d e g r e e o f m o t i o n , c o u l d be r e s p o n s i b l e f o r t h e l o w v a l u e o f t h e s e c o n d moment. A l t h o u g h t h e h y d r o c a r b o n s were c l a i m e d tobe99 mole p e r c e n t minimum p u r i t y t h e i n f l u e n c e o f s m a l l amounts o f i m p u r i t y on t h e r e s o n a n c e l i n e shape h a s be e n a d e q u a t e l y d e m o n s t r a t e d (56). I n c a l c u l a t i n g t h e i n t e r - m o l e c u l a r s e c o n d moment, Andrew u s e d t h e s t r u c t u r e o f n o n a c o s a n e . The d i f f e r e n c e s i n p a c k i n g and c h a n g e s i n l a t t i c e d i m e n s i o n s w i t h t e m p e r a t u r e between n o n a c o s a n e a n d he x a d e c a n e a n d t r i d e c a n e w i l l l e a d t o d i f f e r e n t v a l u e s o f s e c o n d moment. However t h i s s h o u l d n o t be o f s u f f i c i e n t m a g n i t u d e t o a c c o u n t f o r t h e d i s c r e p a n c i e s between t h e e x p e r i m e n t a l and t h e o r e t i c a l s e c o n d moments. 25. The fine structure occuring in the centre of the resonance signal was discussed by Andrew in terms of the rotation of the terminal methyl groups independently of the main chain. Andrew suggested that the central component of the resonance line shape arising from a triangular three spin system (22) becomes observable when rotation occurs since the broadening due to neighbours i s partly removed. A similar narrow line i n methyl chloroform was shown by Powles and Gutowsky (57) to be due to an impurity capable of rotation at f a i r l y low temperatures. No adequate experimental evidence exists to prove impurities are responsible for a l l or part of the fine structure, or for the low second moment in tridecane, but the possibility cannot be entirely ruled out at the present moment • ( i i ) Urea-d^ Adducts of Tridecane and Hexadecane. Urea adducts of nrrparaffins C^-C^and G]_g were studied. A l l showed a narrow line due to the hydrocarbon and a broad line of 7.1 - 7.3 gauss due to the protons of the urea l a t t i c e . The temperature '<.: dependence of the line width and second moment of tridecane and hexadecane - urea d4 adducts was examined. (a) Tridecane Adduct. The resonance line shape of the tridecane adduct showed a complicated temperature dependence. At 77°K the resonance derivative curve, fig( 17<U, consisted of a broad line of 9.8 gauss and a narrower line of J ^ Q g a U S S . The two lines narrowed and appeared to merge as the temperature increased u n t i l , at 119°K, a single line was obtained, fig.(17c). At 150°K the signal separated into two components of widths 6.8 and 1.8 gauss respectively. As the temperature increased the 26 broader line decreased in intensity, and a narrow spike appeared in the centre of the resonance signal, Pig. (17b). Above about 225°K the broad component vanished leaving the narrow li n e , which decreased slowly in width to 1 gauss at 268°K, when i t disappeared. The resonance now consisted of a sharp line of about 30 milligauss wide. The variation with temperature of the second moment i s shown in Pig. (18). The second moment decreased from 12r6 gauss 2 at 77°K to 2.8 gauss 2 at 160°E and then decreased only p o 2 slowly 2.11 gauss^at 225 K,when i t f e l l to 0.7 gauss . At 268°K the second moment f e l l from 0.4 gauss 2 to zero. (b) Hexadecane Adduct. The behaviour of the hexadecane adduct was very similar. The second moment at 77°K was 1 5 . 5 gauss decreasing to 2 .6 gauss 2 at 160°K and to 1 .3 gauss 2 at 220°K. In the range 220°K - 230°K the second moment decreased 2 o to 0 .7 gauss and at 289 K f e l l to zero. The temperature variation of the second moment i s shown in f i g . (19). Two components were observed in the line shape, a narrow line of 2.2 gauss at 77°K decreasing to 0 .9 gauss at 289°K and a broader line of 11 .9 gauss at 77°K which decreased to 5.5 gauss at 217°K and^ vanished i n the range 220-230°K. Discussion. The second moment of the resonance line i n the urea adducts should consist of the following contributions (i) intramolecular, from the hydrocarbon chain. (^ .i) intermolecular, from terminal methyl groups at the end of chains in the same channel. 16 gauss T° K 100 \ 150 Rg. 18. SECOND MOMENT vs TEMPERATURE. TRIDECANE UREA-d 4 ADDUCT. 200 250 300 -20 Fig. 19. SECOND MOMENT vs TEMPERATURE. <> HEXADECANE UREA - ADDUCT. 10 \ \ 2 X AH| \ \ gauss2 \ \ s o • • ° T°K 9 — 1 <— L . i 100 150 200 250 300 27. ( i i i ) intermolecular, from chains in different channels. (iv) contribution from unexchanged protons i n the urea l a t t i c e . (v) contribution due to other atoms possessing spin eg H2, O 1 3, B 1 4, B 1 5. The second moment i s dependent on the inverse sixth power of distance thus ( i i i ) may be neglected since the minimum o possible distance between chains i s at least 5 A. The contribution from (Iv) i s also sufficiently small to be neglected The second moment contribution due to other nuclei w i l l certain-ly be less than 0.2 gauss . The distance apart of the terminal methyl groups i s of the order of 2.0$, the van der Waals radius. The second moment for two r i g i d methyl and methylene groups p at this distance i s 1.2 gauss . The total calculated second 2 moments for the r i g i d hydrocarbon adducts w i l l be 21.1 gauss for the tridecane adduct, and 20.8 gauss 2 for the hexadecane adduct, which are much greater than the experimental values. The graphs of second moment vs temperature show that the second moment i s s t i l l increasing at 77°K and obviously the hydrocarbon cannot be r i g i d at this temperature. Free rotation of the terminal methyl groups about their 0^ axes w i l l reduce the 2 intramolecular second moment by 3.8 gauss , and the inter-molecular contribution to zero. The calculated values now 2 2 become 16.1 gauss for tridecane adduct and 15.8 gauss for the hexadecane adduct. The experimental value for hexadecane i s 2 15.5 gauss and, from the temperature dependence of the second moment, appears to be approaching a limiting value. The chain o length of hexadecane i s 22.84 A which i s slightly greater than 28. •twice the urea repeat distance of 11.01A. According to Smith (40) the chain i s shortened slightly in order to f i t into two unit c e l l s . This shortening w i l l be either a twisting of the chain or a flexing. The distance between certain protons w i l l thus be decreased while others w i l l be further apart, however, since the second moment i s dependent on the inverse sixth power of distance, the second moment w i l l be increased slightly by 2 about 0.3 gauss . The result of the observed shortening w i l l be a restriction of rotational or torsional oscillation but w i l l s t i l l presumably allow the methyl groups to rotate. I f the methyl group rotation alone i s responsible for the reduction in second moment, then Andrew's (27) explanation of the narrow spike i n the resonance line of hydrocarbons in terms of methyl rotation must be incorrect. The reduction of second moment can be explained alternatively in terms of rotational oscillation, but in the case of the hexadecane adduct at this temperature, this does not seem to be the most probable mechanism. It has already been mentioned that the dielectric absorption studies of Meakins favour the rotation of the terminal groups i n 1.10 dibromodecane. In hydrocarbons the removal of the bulk effect of the bromine atom would leave the methyl groups even less hindered. In the tridecane adduct the experimental second 2 moment of 12.6 gauss at 77°K i s too low to be explained in terms of methyl group rotation alone. The graph of second moment vs temperature does not cover a sufficient range to justify extrapolation to what might be a limiting value. The second moment of both adducts decreased over the range 77°K - I 6 0 K t o between 2.5 and 3.0 gauss2. Por complete 29. rotation the calculated intramolecular second moments for 2 tridecane and hexadecane are 6.3 and 6.6 gauss respectively. There i s no break in the second moment versus temperature curves to suggest that only pure rotation occurs. Prom the temperature variation from 197°K to 293°K of the frequency maximum of dielectric absorption, Meakins (9) has obtained estimates of the barrier heights involved in dipolar reorienta-tion. In octadecyl bromide and 1.10 dibromodecane the energy barriers are 2.4 kcal/mole and 2.3 kcal/mole respectively. In pure crystalline long chain compounds of similar molecular weight the barriers would be of the order of 10 - 30 kcal/mole (9). The energy barriers are thus very low and together with the high frequency of the absorption indicate that the molecules are very loosely held in the l a t t i c e . The decrease in the second moment can at least he qualitatively accounted for by assuming that the chains undergo oscillations of increasing amplitude as the temperature rises u n t i l complete rotation occurs. At the same time the hydrocarbon chain becomes capable of lateral and longitudinal motion within the channel. The fact that the narrow spike in the centre of the resonance line seems to increase with the disappearance of the wide line may be significant i n this respect. The line of about 3 gauss wide present in the signal from both adducts at 77°K and narrows to about 1 gauss at the melting points of the hydrocarbons may be due to free or only partially adducted hydrocarbon. The samples were prepared by crushing selected single crystals of adduct into the sample tube, decomposition due to abrasion i s possible but not l i k e l y . 30. A l t e r n a t i v e l y t h i s l i n e may be due t o o c c l u d e d s o l v e n t m o l e c u l e s o f m e t h a n o l o r i s o p r o p a n o l w h i c h have been d r a g g e d i n t o t h e l a t t i c e d u r i n g a d d u c t f o r m a t i o n . T h a t t h e l i n e i s due t o h y d r o c a r b o n however, i s s u p p o r t e d b y t h e change i n l i n e w i d t h and s e c o n d moment a t t h e m e l t i n g p o i n t s o f t h e h y d r o c a r b o n s . The d e c r e a s e i n s e c o n d moments o f b o t h a d d u c t s i n t h e r e g i o n o f 225°K a r e u n l i k e l y t o be o f a s i m i l a r n a t u r e t o t h e p o l y m o r p h i c p h a s e c h a n g e s r e p o r t e d by T o p c h i e v e t . a l . (47). The h y d r o c a r b o n s examined were i n t h e r a n g e C2l"C32 p h a s e c h a n g e s o c c u r r e d w i t h i n 10 - 20° o f t h e i r m e l t i n g p o i n t s . No r e s u l t s on l o w t e m p e r a t u r e s h a s b e e n r e p o r t e d . I n v i e w o f t h e c o i n c i d e n c e i n t e m p e r a t u r e o f t h e s e t r a n s i t i o n p o i n t s i t i s p o s s i b l e t h a t t h e u r e a l a t t i c e i t s e l f h a s a l t e r e d . D e f i n i t e c o n c l u s i o n s c a n n o t be drawn on t h e b a s i s o f o n l y two a d d u c t s . An e x a m i n a t i o n o f o t h e r a d d u c t s and p o s s i b l y a d i l a t o m e t r i c s t u d y w o u l d be u s e f u l . ( i i i ) T h i o u r e a A d d u c t s . R e s u l t s . The r e s o n a n c e l i n e d e r i v a t i v e s o f some o f t h e t h i o u r e a a d d u c t s i n v e s t i g a t e d a r e shown i n f i g s (20) (21). The c u r v e s have been r e d u c e d t o t h e same s c a l e . I n a l l c a s e s t h e w i d t h o f t h e b r o a d l i n e , a t t r i b u t e d t o t h e p r o t o n s o f t h e t h i o u r e a l a t t i c e , was 8.5 - 8.7 g a u s s . The l i n e w i d t h s o f t h e h y d r o c a r b o n r e s o n a n c e s a r e g i v e n i n t a b l e 1. The a d d u c t s o f d u r e n e (1,2,4*5^ t e t r a m e t h y l b e n z e n e ) and d e c a l i n ( d e c a h y d r o n a p h t h a l e n e ) showed a c o m p o s i t e l i n e shape o f two b r o a d l i n e s and sometimes a n a r r o w s p i k e . C y c l o p e n t a n e a n d Fig. 20. LINE SHAPE DERIVATIVES OF THIOUREA ADDUCTS. Fig. 21. LINE SHAPE DERIVATIVES OF THIOUREA ADDUCTS 31. d i c y c l o h e x y l showed o n l y n a r r o w l i n e s o f m o d u l a t i o n w i d t h . C y c l o h e x a n e C y c l o h e x e n e C y c l o p e n t a n e M e . c y c l o h e x a n e M e . c y c l o p e n t a n e D i c y c l o h e x y l D u r e n e D e c a l i n T a b l e 1. L I N E WIDTHS OP THIOUREA ADDUCTS. A b r i e f t e m p e r a t u r e d ependence s t u d y o f t h e a d d u c t s o f c y c l o h e x a n e and c y c l o p e n t a n e was made. The l i n e w i d t h a n d s e c o n d moment o f t h e c y c l o h e x a n e t h i o u r e a - d ^ a d d u c t a t 77°K were 4.6 g a u s s and 2.7 g a u s s 2 r e s p e c t i v e l y . The l i n e w i d t h a t 298°K was 1.6 g a u s s and t h e s e c o n d moment 0.3 g a u s s . Prom 177°iC t o 400°K t h e l i n e w i d t h r e m a i n s c o n s t a n t b u t b e g i n s t o d e c r e a s e i r r e v e r s i b l y i n i n t e n s i t y a b o v e 350°K. The c y c l o p e n t a n e a d d u c t showed t h e same d e c r e a s e i n i n t e n s i t y a bove a b o u t 310°K. D i s c u s s i o n . W i t h t h e e x c e p t i o n o f d u r e n e a l l t h e compounds s t u d i e d were l i q u i d a t room t e m p e r a t u r e i n t h e p u r e s t a t e . The o c c u r r e n c e o f a f i n i t e l i n e w i d t h and s e c o n d moment w o u l d t h e r e f o r e i n d i c a t e a r e s t r i c t i o n o f m o t i o n i n t h e a d d u c t . S c h l e n k (42) has p o i n t e d o u t t h a t t h e l a r g e r s i z e o f t h e s u l p h u r atom and t h e g r e a t e r d i m e n s i o n s o f t h e c h a n n e l r e s u l t s e f f e c t i v e l y i n a p e r i o d i c i t y o f t h e v a n d e r W a a l s f o r c e s a l o n g t h e l e n g t h o f t h e c h a n n e l . The N.M.R. e v i d e n c e s u p p o r t s t h e i d e a t h a t t h e a d d u c t e d 1.6 Gauss 0.65 " 0.95 " 1.4 " tt 2.2, 6.0 » 1.7, 5.5 " 32. compounds i s trapped in some potential minimum such that, while rotational or oscillatory motion may he quite unrestricted, transnational motion i s hindered. Andrew and Eades (59) have examined the proton magnetic resonance spectrum of cyclohexane from 95°K to the melting point. Below 150°K the lattice was effectively r i g i d with a second 2 2 o moment of 26.0 gauss , f a l l i n g to 6.4 gauss in the range 150 to 186°E. This reduction i s quantitatively accounted for by reorientation of the molecule about i t s triad axis. The discontinuous change in second moment at 186°K to 1.4 gauss 2 i s caused by almost isotropic reorientation which reduces the intra-molecular second moment to zero. Since the centres of mass remain fixed the intermolecular contribution does not average to zero. In the cyclohexane thiourea adduct at 77°K the second 2 moment of 2.7 gauss i s too low to be anything except inter-molecular in origin. The'motion must be isotropic reorientation. If , as was suggested for the urea adducts, there i s a change i n the dimensions of the thiourea channel at some temperature, then the adducted molecule would be allowed more freedom of transna-tional motion which would reduce the second moment s t i l l further. The lack of sufficient experimental data again makes any definite. deductions impossible. At higher temperatures the irreversible decrease in the intensity of the resonance signal i s obviously due to decomposi-tion. The composite line shapes of durene and decalin and the narrow lines of cyclopentane and dicyclohexyl adducts are d i f f i c u l t to explain. The smaller molecular size of cyclopentane 33. may a l l o w i t c o n s i d e r a b l e f r e e d o m o f m o t i o n i n t h e c h a n n e l , t h i s e x p l a n a t i o n w i l l n o t s u f f i c e f o r t h e d i c y c l o h e x y l a d d u c t . An i n v e s t i g a t i o n o f t h e s e r i e s o f W-W1 d i c y c l o h e x y l a l k a n e s m i g h t c l e a r up t h i s p o i n t . I t i s p o s s i b l e t h a t t h e l a r g e r more complex m o l e c u l e s o f d u r e n e a n d d e c a l i n have t r a p p e d a p p r e c i a b l e numbers o f s o l v e n t m o l e c u l e s i n t h e c h a n n e l a n d t h a t t h i s i s r e s p o n s i b l e f o r t h e l i n e s h a p e . The change i n h y d r o g e n bond e n e r g y on i s o t o p i c s u b s t i t u t i o n may s i g n i f i c a n t l y a l t e r t h e s t r u c t u r e and p r b p e r t i e s o f t h e l a t t i c e . I t i s n o t s u g g e s t e d t h a t t h e p r o t o n a t e d a d d u c t s w i l l n e c e s s a r i l y e x h i b i t t h e same b e h a v i o u r a s t h e d e u t e r a t e d s p e c i e s . I t i s d i f f i c u l t a t t h e p r e s e n t s t a g e t o a r r i v e a t any d e f i n i t e g e n e r a l c o n c l u s i o n s r e g a r d i n g t h e b e h a v i o u r o f t h e a d d u c t s . The mechanism by w h i c h t h e l o n g c h a i n m o l e c u l e s a r e r e t a i n e d i n t h e u r e a c h a n n e l o b v i o u s l y d i f f e r s f r o m t h e s i t u a t i o n i n t h e t h i o u r e a a d d u c t s . Some s u g g e s t i o n s have been made a s t o f u t u r e work w h i c h may l e a d t o a b e t t e r u n d e r s t a n d i n g o f t h e i n t e r a c t i o n between t h e cage a n d t h e e n c l o s e d m a t e r i a l . APPENDIX SOURCE AND PURITY OP COMPOUNDS U r e a T h i o u r e a T r i d e c a n e Hexadecane C y c l o h e x a n e C y c l o h e x e n e M e t h y l c y c l o h e x a n e M e t h y l c y c l o p e n t a n e C y c l o p e n t a n e Durene D i c y c l o h e x y l Eastman Kodak, W h i t e L a b e l May and B a k e r , " A n a l a r " P h i l l i p s P e t r o l e u m Co. Humphrey and W i l k i n s o n Eastman Kodak, S p e c t r o g r a d e P h i l l i p s P e t r o l e u m Co. Eastman Kodak, S p e c t r o g r a d e P h i l l i p s P e t r o l e u m Co. n tt I I Eastman Kodak, W h i t e L a b e l L i g h t a n d Co. 99 mole P e r c e n t 99 mole P e r c e n t . 99 mole P e r c e n t . 99.98 » BIBLIOGRAPHY van Stackelberg M., Muller H.R. Z.Elektrochem. 58 25 1954. 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