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Photolyses of ketene at 3130 A and 3340 A Connelly, Barry Thomas 1958

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PHOTOLYSES OP KETENE AT 3130 A AND 3340 A by BARRY THOMAS CONNELLY B.Sc, U n i v e r s i t y of Sydney, A u s t r a l i a , 1953 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in. the Department of CHEMISTRY We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1958 ABSTRACT Previous research on the photolyses of ketene at 2700 A and 3650 A has shown that considerably d i f f e r e n t mechanisms are necessary to explain the experimental r e s u l t s . Exactly how the process of deactivation of the excited ketene molecule was af f e c t e d by change i n wavelength was not f u l l y understood. I t was f e l t that i n v e s t i g a t i o n s of the primary quantum y i e l d s at intermediate wavelengths would be f r u i t f u l i n obtaining a better understanding of the v a r i a t i o n of react i o n mechanism with wavelength. At 2700 A, using pure ketene, the only primary process i s d i s s o c i a t i o n and therefore the primary quantum y i e l d i s unity, while at 3650 A d i s s o c i a t i o n occurs by way of an excited state which has a f i n i t e l i f e t i m e during which the excited ketene molecule may undergo c o l l i s i o n a l d eactivation and i n t e r n a l conversion. At 3650 A therefore the primary quantum y i e l d i s much l e s s than u n i t y even at low pressure, and decreases with incre a s i n g pressure.' This research has shown that i n the case of ketene at 3130 A the primary quantum y i e l d i s approximately u n i t y at low pressures, 20 mm., and decreases to 0.6 at one atmosphere. At 3340 A the primary quantum y i e l d i s approximately 0.7 at 26 mm. and 0.2 at 400 mm. and increases with increasing temperature. i v The dependence of primary quantum y i e l d on pressure at 3130 A and 3340 A was a n t i c i p a t e d and can be explained by almost the same mechanism as that proposed f o r 3650 A r a d i a t i o n . The amount of r a d i a t i o n absorbed during each run was very accurately measured and i t was therefore possible to determine quantum y i e l d s to within - 2% at 3130 A, and within - 10% at 3340 A. For the photolysis at 3650 A the r a t i o s of the rate constants of c o l l i s i o n a l deactivation and product formation and of i n t e r n a l conversion and product O A formation at 26; C are 4.6 x 10 l i t r e s / m o l e and 28 re s p e c t i v e l y . At 3130 A and 37°C t h i s research has shown these r a t i o s to be 16.8 li t r e s / m o l e and zero r e s p e c t i v e l y , while at 3340 A the r a t i o s are 1.64 x 10 l i t r e s / m o l e and 0.25 at 37°C and 0.99 x 10 2 and 0.1 at 100° C. I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C olumbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by t h e Head o f my Department o r by h i s r e p r e s e n t a t i v e . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed, w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f 7? The U n i v e r s i t y o f B r i t i s h C olumbia, Vancouver 8, Canada i ACKNOWLEDGMENT This i n v e s t i g a t i o n was c a r r i e d out under the supervision of Sr.G.B.Porter to whom the author i s great l y indebted. My appreciation i s due to the B r i t i s h Columbia Sugar Refining Company f o r a 1957/1958 Scholarship. I am g r a t e f u l to D r.J.Ferguson f o r performing the transmission determinations on the f i l t e r solutions used. My sincerest thanks go to my wife Margery R.Connelly f o r the encouragement and assistance given throughout t h i s research and f o r typing and proofreading t h i s t h e s i s . TABLE OP CONTENTS PAGE INTRODUCTION 1 EXPERIMENTAL 4 Lamp a 4 O p t i c a l System 4 Oven 4 F i l t e r Solutions 3130 A 5 3340 A 6 Gas Analyses 6 P u r i f i c a t i o n of D i e t h y l Ketone 7 Ketene 7 Actinometry 3130 A 8 3340 A 10 RESULTS 11 3130 A 11 3340 A 14 DISCUSSION ' . . 16 Reaction Mechanism 16 Theory 17 Steady State Equation 18 2700 A 19 3130 A 19 Ethylene Quantum Y i e l d 22 v i PAGE 3340 A 22 A c t i v a t i o n Energy . . . . 24 3650 A 24" k^/k^ as a function of wavelength 25 k 2/k^ as a function of wavelength 25 V a r i a t i o n of k^ with energy of e x c i t a t i o n . . 27 Energy of A c t i v a t i o n as a function of wavelength 27 BIBLIOGRAPHY 28 LIST OF TABLES TABLE PAGE I. Photolysis of Ketene at 3130 A 12 II. Photolysis of Ketene at 3130 A 13 III. Photolysis of Ketene at 3340 A 15 LIST OF FIGURES FIGURE PAGE 1. Dependence of Primary Quantum Yield on Pressure at 3130 A . . . 21 2. Dependence of Primary Quantum Yield on Pressure at 3340 A . . . 23 3. Variation of k^/k^ with Energy of Excitation . . . 26 INTRODUCTION Previous experiments have shown that the primary quantum y i e l d of ketene decomposition i s a function of the wavelength of the absorbed r a d i a t i o n . At 2700 A Strachan and Noyes ^ found the primary quantum y i e l d to be un i t y within experimental error, independent of pressure and temperature. At 3650 A the primary quantum y i e l d , extrapolated to zero pressure of ketene, i s much l e s s than unity, decreases as pressure i s increased and increases with increase i n temperature. Following the absorption of 2700 A r a d i a t i o n and the formation of an excited ketene molecule, d i s s o c i a t i o n occurs i n l e s s time than that required f o r c o l l i s i o n with another molecule, to give a methylene r a d i c a l and a molecule of carbon monoxide. The methylene r a d i c a l then attacks another ketene molecule to form ethylene and another carbon monoxide molecule, according to the following mechanism: K + hT> (2700 A) K m (Im) K m = CH 2 + CO ( ( 2 m ) CH 2 + K C 2H 4 + CO where K i s a ketene molecule and K m i s a ketene molecule excited by 2700 A r a d i a t i o n . For each photon absorbed, one molecule of ketene i s dissociated, two molecules of carbon monoxide and one molecule of ethylene are formed. Strachan and Noyes ^ found the quantum y i e l d of carbon monoxide formation to be 2.12 - 0.15. They also found the r a t i o of the quantum y i e l d of carbon monoxide formation to the quantum y i e l d of ethylene to be 2.2 instead of the 2.0 expected from reactions (lm) and (2m). It i s evident therefore that some methylene r a d i c a l s are involved i n another r e a c t i o n such as: CH 2 + y CH2C0 = polymer + CO At 3650 A, the low quantum y i e l d s can be a t t r i b u t e d to c o l l i s i o n a l deactivation and i n t e r n a l conversion of 2 the excited ketene molecules , according to the following mechanism: K + hi) (3650 A) = K q (lq) = products (2q) K 5 = K (3q) K q + K = 2K (4q) The energy i s diss i p a t e d i n processes (3q) and (4q) as heat, since no fluorescence has ever been observed. I t i s evident from the low quantum y i e l d that reaction .(3q) i s more rapid than re a c t i o n (2q). As the pressure i s increased the rate of r e a c t i o n (4q) increases from zero to become the dominant process. Extrapolation to zero pressure at 26°C gives a carbon monoxide quantum y i e l d of approximately 0.07. At 3650 A therefore d i s s o c i a t i o n occurs only a f t e r a f i n i t e time i n t e r v a l 3 during which the excited ketene molecule may he deactivated. Kistiakowsky and Mahan ^ found that the primary quantum y i e l d at 3130 A was independent of pressure and suggested a value of unity. However, they apparently investigated the photolysis over a narrow range of pressures and t h e i r r e s u l t s are therefore^of minor importance. Inv e s t i g a t i o n of the quantum y i e l d at a number of wavelengths provides the means of determining the r e l a t i o n between the rate of d i s s o c i a t i o n and the energy of the d i s s o c i a t i n g molecule. It was therefore decided to investigate the photolysis of ketene at 3130 A more c a r e f u l l y over a wide range of pressures using a technique whereby quantum y i e l d s could be determined to within 1 or 2$. This technique would reveal any small e f f e c t of pressure on the primary quantum y i e l d that may have escaped detection i n previous work^. I t was f e l t that since another mercury-arc l i n e , namely 3340 A, occurs between 2700 A and 3650 A, precise quantum y i e l d determinations at t h i s wavelength would s u b s t a n t i a l l y increase our understanding of the process of deactivation. 4 EXPERIMENTAL A medium pressure 100 watt Hanovia mercury-arc lamp was used as the l i g h t source f o r runs at 3130 A. The lamp was encased i n aluminium tubing with a diameter hole opposite the arc. I t was found best not to a i r or water cool the lamp housing. The lamp was run from a constant voltage regulator (115 v o l t s ) . For runs at 3340 A a B.T.H. medium pressure mercury-arc lamp was used. The divergent l i g h t beam passed through a combination of two quartz convex lenses of e f f e c t i v e f o c a l length 1.5 cm. A f i l t e r c e l l block c o n s i s t i n g of two compartments each 2.5 cm. long and with quartz windows, together with a 2 mm. Corning 9863 f i l t e r to exclude v i s i b l e r a d i a t i o n , were placed between the lenses and the c e l l . The quartz r e a c t i o n c e l l of length 14.8 cm. and diameter 3*6 cm. was housed i n an aluminium block. Fourteen turns of f i n e gauge wire, insulated with asbestos sheet from the aluminium, constituted the heating power of the oven. The ent i r e block was well i n s u l a t e d with asbestos, glass wool, and Alundum cement. A 0-110°C thermometer was mounted i n the oven. A thermo-regulator i n the aluminium block gave accurate temperature c o n t r o l to within - 0.1°C. The emergent beam was passed through 5 another convex lens and focused on the cathode of an RCA 935 photocell. The f i l t e r solutions used at 3130 A were s i m i l a r to 4 those of Hunt and Davis although the n i c k e l chloride was omitted. Solutions used were: Potassium hiphthalate ...0.08 gm/ 100 ml Potassium chromate ... 0.008 gm/ 100 ml These solutions i n 2.5 cm. c e l l s gave transmissions of: 0 at 2700 A 36.4% at 3130 A F i l t e r A 24.0% at 3340 A 3.5% at 3650 A A 2 mm. Corning 9863 f i l t e r was used to absorb v i s i b l e r a d i a t i o n . The r e l a t i v e i n t e n s i t i e s of the 3130, 3340 and 3650 A l i n e s of the mercury-arc and the absorption c o e f f i c i e n t s of ketene at each wavelength i n d i c a t e d that the r e s u l t s obtained with these f i l t e r solutions might be appreciably d i f f e r e n t from r e s u l t s using monochromatic 3130 A r a d i a t i o n . A second f i l t e r was prepared which contained: Potassium biphthalate... 0.08 gm/ 100ml Potassium chromate... 0.019 gm/ 100 ml 6 These solutions i n 2.5 cm. c e l l s gave transmissions of: 0 at 2700 A 16.0% at 3130 A F i l t e r B 0.4% at 3340 A les s than 0.1% at 3650 A The Corning 9863 f i l t e r was again used. The f i l t e r solutions used at 3340 A consisted of : Nic k e l sulphate hexahydrate... saturated s o l u t i o n Naphthalene... 0.44 gm/' 50 ml. methyl a l c o h o l each s o l u t i o n contained i n a 1 cm. c e l l . These solutions 5 were s i m i l a r to those of Kasha and gave transmissions of: 0 at 2700 A 0 at 3130 A F i l t e r C 26.0% at 3340 A 0.03% at 3650 A A Corning 9863 f i l t e r was used as f o r 3130 A. Gas Analyses were c a r r i e d out i n an a l l - g l a s s high vacuum system evacuated hy a standard type o i l pump and a single stage mercury d i f f u s i o n pump. In t h i s system mercury cut-offs replaced the conventional stopcocks, i n order to eliminate the use of stopcock grease wherever possible because of the absorption of ketene and products i n the grease. The carbon monoxide analysis was performed by allowing 7 the products to flow through successive baths cooled to -78°C, -196°C, -210°C (dry ice-acetone, l i q u i d nitrogen and s o l i d nitrogen respectively).- The s o l i d nitrogen trap was prepared hy pumping on fr e s h l i q u i d nitrogen f o r about f i f t e e n minutes. Ketene was condensed i n the f i r s t two baths and the ethylene was c o l l e c t e d i n the s o l i d nitrogen trap. Carbon monoxide was the only product not condensed at -210°C and was measured with a McLeod-Toepler gauge I f , a f t e r the carbon monoxide had been c o l l e c t e d , the s o l i d nitrogen trap was removed, a quantity of ethylene equal to 1/ 2.14 of the carbon monoxide y i e l d was obtained. P u r i f i c a t i o n of D i e t h y l ketone and Ketene  Diethyl ketone, used as the i n t e r n a l actinometer f o r runs at 3130 A, was the d i s t i l l a t i o n f r a c t i o n b o i l i n g at l O l ^ C . Once on the l i n e the d i e t h y l ketone was d i s t i l l e d under vacuum, the middle t h i r d being retained. P r i o r to each run, a quantity of d i e t h y l ketone was transferred from a blackened storage bulb to a dry i c e -acetone cooled trap and outgassed three times u n t i l a l l non-condensables and low b o i l i n g gases were removed. Ketene was prepared i n the vacuum system by pyrolyzing a c e t i c anhydride • A slow stream of a c e t i c anhydride vapour flowed through an oven heated to 504°C 8 The unused a c e t i c anhydride and a c e t i c a c i d produced were c o l l e c t e d i n a dry ice-acetone bath on the low pressure side of the oven and the ketene was c o l l e c t e d i n l i q u i d nitrogen; approximately 10 ml. was obtained i n one hour. Trap to trap d i s t i l l a t i o n was performed on the f r e s h l y prepared ketene and the middle t h i r d retained. P r i o r to each run, a quantity of ketene was thoroughly outgassed by three d i s t i l l a t i o n s s i m i l a r to those f o r d i e t h y l ketone* The ketene was stored i n a trap immersed i n l i q u i d nitrogen to prevent slow polymerization. Actinometry 3130 A D i e t h y l ketone was selected f o r experiments at 3130 A where the quantum y i e l d of carbon monoxide ,. formation i s 1.0 at 100 C . The use of an i n t e r n a l actinometer was found to be very convenient because i t i s devoid of the errors u s u a l l y associated with external actinometers. These errors a r i s e because the amount of r a d i a t i o n incident to the external c e l l i s considerably l e s s than that incident to the r e a c t i o n c e l l . . The measurement of the i n t e n s i t y absorbed during a run was performed by the conversion of the current from the photocell into a voltage which was continuously recorded on a 10 m i l l i v o l t recorder. The following scale readings were taken; 9 R^ = c e l l empty R-k = photolyte i n c e l l The recorder scale was c a l i b r a t e d using d i e t h y l ketone as an i n t e r n a l actinometer. Scale readings were taken before and a f t e r each run to determine the average R^. This was necessary because of the f l u c t u a t i o n and decay of the lamp, and polymer formation. R^ was measured continuously during each run. The difference R. - R + = R„ was c a l i b r a t e d i n terms X u c l of absolute i n t e n s i t y by the measurement of carbon monoxide y i e l d s from the photolysis of d i e t h y l ketone at 100°C. I t was possible to measure R_ to wit h i n i 0.5% Si and, on averaging the r e s u l t s of a number of d i e t h y l ketone runs, quantum y i e l d s of carbon monoxide formation from ketene were obtained within - 3$. At 3130 A, a second actinometer was used, but because?: of the large amount of r e f l e c t i o n from the g l a s s - a i r i n t e r f a c e s and because of the suspected polymer on the exi t wall of the re a c t i o n c e l l , the values f o r the absolute i n t e n s i t y of the l i g h t were considerably lower than those obtained using d i e t h y l ketone. Twelve runs g were performed using potassium f e r r i o x a l a t e actinometer ; consistent r e s u l t s indicated the need f o r a conversion f a c t o r to be applied when the external l i q u i d actinometer was used to estimate the energy absorbed during runs 10 at 3130 A. The i n t e n s i t y found from a f e r r i o x a l a t e run when m u l t i p l i e d "by t h i s conversion f a c t o r gave the i n t e n s i t y incident to the c e l l , reproducible to within - 10%. 3340 A The potassium f e r r i o x a l a t e actinometer was used f o r runs at 3340 A since no i n t e r n a l actinometer i s avai l a b l e f o r t h i s wavelength. The i n t e n s i t i e s obtained by t h i s actinometry, when m u l t i p l i e d by the conversion f a c t o r found previously, were used to c a l i b r a t e the recorder scale. Absorbed i n t e n s i t i e s were obtained from measurements of R. , R., and R as described previously. 11 RESULTS 3130 A Using f i l t e r s o l u t i o n combination A, at low pressures of ketene, a carbon monoxide quantum y i e l d of 1.82 was obtained whereas at high pressures around one atmosphere, the carbon monoxide quantum y i e l d decreased to 1.08 as shown i n Table I. A decrease i n the carbon monoxide quantum y i e l d at high pressures was expected. The r e s u l t s of f i v e d i e t h y l ketone actinometer runs, at pressures ranging from 60 to 155 mm. of d i e t h y l ketone, and interspersed between the ketene runs, gave a l i g h t i n t e n s i t y of approximately 5.4 x 10"^ quanta per second. Using f i l t e r s o l u t i o n combination B, a new s e r i e s of d i e t h y l ketone actinometer runs gave i n t e n s i t i e s of approximately 1.6 x 10"^ quanta per second. The carbon monoxide quantum y i e l d at low pressures of ketene was 2.0 within experimental error, and at one atmosphere a value of 1.18 was obtained as shown i n Table II.. Absorbed i n t e n s i t i e s v a r i e d from 0.442 x 10"^ quanta 14 per second at 17 mm. to 1.911 x 10 quanta per second at 751 mm. A l l ketene runs were conducted at 37°.0 0 I t was found that the transmissions at 3130 A of f i l t e r s A and B were appreciably lower a f t e r two weeks constant use. 12 TABLE I PHOTOLYSIS OP KETENE AT 3130 A C e l l Temperature 37°.0 C Run Pressure of Length of I n t e n s i t y Absorbed 4> - i $. Ketene Run mm. sees. quanta/sec. xc 1 0 - 1 4 1 35.8 3600 2.305 0.91 + 0.02 5 56.5 1800 2.855 0.93 + 0.02 201.6 2700 4.360 0.74 + 0.02 2 402.7 900 4.790 0.59 + 0.01 3 444..5 914 4.580 0.57 + 0.01 7 726.8 1800 4.290 0.54 + 0.01 13 TABLE II PHOTOLYSIS OF KETENE AT 3130 A C e l l Temperature 37°.0 C Run Pressure of Ketene mm. Length of Run sees. Intensity-Absorbed quanta/sec. x I O " 1 4 14 17.0 3600 0.442 1.03 - 0.04 13 40.8 2460 0.846 1.00 - 0.03 12 57.1 1800 1.029 II.-00 - 0.03 11 135.8 2407 1.428; 0.94 - 0.02 9 230.3 2400 1.710 0.83 i 0.02 10 679.1 1920 1.584 0.63 - 0.02 8 750.7 2400 1..911 0.59 - 0.01 14 3340 A The naphthalene i n methyl a l c o h o l s o l u t i o n darkened a f t e r only a few hours exposure to the lamp and i t s transmission at 3340 A decreased noticeably. Even when a pyrex window, which absorbs 50% of the l i g h t at 3130 A and 100% of l i g h t of shorter wave lengths was placed i n front of the naphthalene solution, darkening continued. This e f f e c t may have been caused by the attack of methyl alc o h o l on the adhesive used to bond the quartz windows to the pyrex c e l l . The i n t e n s i t y of the beam as measured using the f e r r i o x a l a t e actinometer, when m u l t i p l i e d by the conversion f a c t o r of 2.72 gave an incident i n t e n s i t y of 15 approximately 2.1 x 10 quanta/second. When re l a t e d to the recorder t h i s became 0.87 x: 10"^ quanta/second/scale d i v i s i o n . The intense B.T.H. lamp necessitated short runs of "between 3 and 10 minutes whereas 30 to 60 minute runs were needed with the Hanovia lamp at 3130 A. The carbon monoxide quantum y i e l d s were determined . at both 37°C and 100°C and the r e s u l t s are summarized i n Table I I I . 15 TABLE I I I PHOTOLYSIS OP KETENE AT 3340 A C e l l Temperature 37°.0 C Run 18 15 16 17 20 19 Pressure of Ketene mm. 26.0 39.7 47.0 105.3 177.8 384.6 Length of Run sees. 180 4200 1800 600 390 900 Int e n s i t y Absorbed quanta/sec, x I O " 1 4 1.221 0.771 2.545 2.825 1.841 2.023 <0*i 0.72 0.72 0.51 0.41 0.37 0.21 24 23 22 21 C e l l Temperature 26.1 180 56.7 240 121.8 255 299.5 600 100 .0 C 0.811 1.239 1.579 1.920 0.80 0.74 0.58 0.37 16 DISCUSSION The photolysis of ketene at 2700, 3130, 3340 and 3650 A can he explained hy the following processes: K + h^(2700 A) = E m (lm) K + h * (3130 A) = K n (In) K + h-0 (3340 A) = K P (IP) K + hV (3650 A) = K * (lq) = Products (2m) K n = Products (2n) K p = Products (2p) = Products (2q) K*1 E (3m) K n K (3n) K P K (3P ) K (3q) K* + K 2K (4m) K n + K = 2K (4n) K P + K = 2K (4p) K + K 2K (4q) where K i s a molecule of ketene and K21, K n, K p and K^ - are excited ketene molecules i n order of decreasing v i b r a t i o n a l energy. Reactions 1 represent a c t i v a t i o n . Reactions 2 represent product formation. Reactions 3 represent i n t e r n a l conversion. Reactions 4 represent c o l l i s i o n a l deactivation. 17 It i s not implied that the above mechanism which represents the a c t i v a t i o n and deactivation of excited molecules K m, K n, K^, and formed at wavelengths 2700 A , 3130 A , 3340 A , and 3650 A i s complete. I t i s obvious that a ketene molecule i n v i b r o n i c state m can lose i t s excess energy by a se r i e s of v i b r a t i o n a l cascades to states n, p, q etc. u n t i l i t eventually reaches the equilibrium v i b r a t i o n a l state. The number of v i b r o n i c states may be 100 or more f o r a molecule i n i t i a l l y i n state m. The k i n e t i c s of such a process are very complicated. Por s i m p l i c i t y the present discussion w i l l consider only reactions 1, 2, 3> and 4 to represent the activation,, deactivation and d i s s o c i a t i o n of excited ketene molecules i n states m, n, p and q. For the photolysis at 3650 A the relevant processes are l q , 2q, 3q and 4q. The rates of these reactions are l a , k 2 [K* 1], k ^ f l ^ ] , k 4 q [ K ] [ K q ] r e s p e c t i v e l y , and rate of formation of K 1 = l a -k 2 J K9] - IC J J K I ] - k 4 j K ] l > ] I f steady state conditions are applied, the r i g h t hand side of the above expression equals zero, therefore: 18 Rate of product formation = k ^ ^ E ^ J k 2 q l a k2q + k3q + k4qf K J Rate of product formation = q u a n t u m y i e l d l a and: 4> k 0 2q k2q + k 3 q + k 4 q M therefore I ^ Q k, [k] 7jT 1 + + _ _ L _ k2q k2q Equations of the same form can he derived f o r each of the other wavelengths. In general ^ . 1 + ! l + 4> k 2 k 2 where [EJ = concentration of ketene i n m o l e s / l i t r e <$> = primary quantum y i e l d k^/k 2 = intercept on '/^> axis i n the plot / j (j) against pressure. k^/k 2 = slope of plot i n l i t r e s / m o l e . 19 2700 A Strachan and Noyes found that the quantum y i e l d of carbon monoxide formation was approximately two. According to the proposed mechanism the primary quantum y i e l d i s thus unity. I t i s evident therefore that over the range of pressures and temperatures used, there i s 2 neither i n t e r n a l conversion nor c o l l i s i o n a l d e a ctivation of the excited ketene molecules. The only way i n which these molecules can lose t h e i r excess energy i s by d i s s o c i a t i o n . The observed quantum y i e l d s can be explained by the mechanism: (lm), (2m) That i n t e r n a l conversion and c o l l i s i o n a l deactivation are absent may be seen from the intercept of u n i t y and the zero slope of the plot of against pressure. 3130 A The primary quantum y i e l d of ketene decomposition at t h i s wavelength appears as an i s o l a t e d number i n several papers devoted to ketene photolyses at other wavelengths. ^» 9» 10, 11 j j o w e v e r only the recent work of Kistiakowsky and Mahan ^ gives experimental data. From the l i m i t e d amount of information given i t appears that photolyses were only performed at low pressures, i n the region of 20 mm. I t was found ^ that the primary quantum y i e l d was independent of 20 pressure and possibly equal to unity. This research has shown that when l i g h t which was not monochromatic but which contained quantities of both 3340 and 3650 A r a d i a t i o n was used, primary quantum y i e l d s of 0.91 at 35.8 mm. and 0.54 at 726.8 mm. were obtained as shown i n Table I. The plot of against pressure intercepts the '/<^> axis at approximately 1.1 and has a p o s i t i v e slope. I t i s l i k e l y therefore that i n t e r n a l conversion and c o l l i s i o n a l d e activation of the excited ketene molecule contribute s l i g h t l y to the deactivation process. Because of the known low quantum y i e l d s at 3650 A and the suspected low y i e l d s at 3340 A, a new se r i e s of experiments was performed using monochromatic 3130 A r a d i a t i o n . The r e s u l t s of these experiments are shown i n Table I I . The primary quantum y i e l d i s 1.0 at pressures between 17 and.60 mm., and 0.59 at 750.7 mm. The plot of '/<j> against pressure i s shown i n Figure 1; although the extrapolation to zero pressure i s unity, because of experimental error the presence of i n t e r n a l conversion i s not e n t i r e l y excluded from the mechanism. The p o s i t i v e slope of the plot i n Figure 1 i n d i c a t e s the presence of c o l l i s i o n a l deactivation, and hence the mechanism: (In), (2n), (3n), (4n) 2 1 i—r -L_l L _L _L 100 AO© fro© V O © 6oo PRESSURE (mm. Hg) FIGURE 1 . DEPENDENCE OF PRIMARY QUANTUM YIELD ON PRESSURE AT 3 1 3 0 A. 22 may be postulated i n which the extent of re a c t i o n (3n) i s very small. k^/k 2 i s approximately zero while k^/k^ = 16.8 l i t r e s / m o l e . The mechanism of ketene decomposition at 3130 A i s intermediate between those postulated f o r 2700 A and 3650 A. Prom Table II i t appears that at low pressures of ketene, each excited ketene molecule d i s s o c i a t e s to form products whereas at pressures greater than approximately 60 mm., a large f r a c t i o n of the excited molecules lose t h e i r energy hy c o l l i s i o n with unexcited . molecules of ketene. The photolysis at 3130 A was not extended to higher temperatures because i t was thought that l i t t l e or no s i g n i f i c a n t information would be obtained. In three experiments the ethylene quantum y i e l d was found to be 1/2.14 of the carbon monoxide quantum y i e l d . This i s approximately the same as that found at 2700 A and 3650 A. 1' 1 2  3340 A This research has indicated that f o r photolyses at 37°C the primary quantum y i e l d i s 0.72 at 26 mm. and 0.21 at 384 mm. The zero pressure intercept i n Figure 2 i s 1.25. The slope of the p l o t i s much steeper than f o r 3130 A and 2 i s 1.64 x 10 l i t r e s / m o l e . This i n d i c a t e s that considerable c o l l i s i o n a l d eactivation of K p occurs. 1 1 1 1 / -3 0 2 0 Q y ^ — 1 0 1 1 1 I I 1 1 c > so loo 1 0 0 3 0 0 <hOO PRESSURE (mm. Hg) FIGURE 2. DEPENDENCE OF PRIMARY QUANTUM YIELD ON PRESSURE AT 3340 A. 2 4 Pour runs were performed at 100°C and as expected, both the slope and the intercept were smaller than f o r runs at 37°C At 26.1 mm. the primary quantum y i e l d i s 0.80 and at 300 mm. equals 0.37. The zero pressure intercept at 100°C i s 1.1 and the slope of the s t r a i g h t 2 l i n e i s 0.99 x 10 l i t r e s / m o l e , both obtained from Figure 2. The r a t i o of the rates of i n t e r n a l conversion to product formation at 100°C i s 0.1. The mechanism f o r the a c t i v a t i o n and deactivation of K P i s : : ( l p ) , (2p), ( 3 P ) , ( 4 P ) , at both 37°C and 100°C. From the information i n Table III and Figure 2, i t was possible to estimate approximately the a c t i v a t i o n energy of r e a c t i o n (2p) from the slope of the plot of log^k^/kg) against l/T where T i s the absolute temperature. The slope i s (Eg - E^) / R, where R i s the gas constant i n c a l o r i e s degree mole."** I f E^ i s assumed to be zero, then Eg i s approximately 2 Kcal./mole. This value i s subject to a large error. 3650 A Strachan and Noyes ^ found a zero pressure intercept of 28 i n the plot of r e c i p r o c a l primary quantum y i e l d against pressure. This corresponds to a primary quantum y i e l d of approximately 0.036 at 26°C. The slope of the l i n e a r 25 plot i s 4.65 x 10^ l i t r e s / m o l e . Both the intercept and the slope decrease with increasing temperature. 4.5 Kcal./mole i s the a c t i v a t i o n energy of r e a c t i o n (2q)^. ^3/^4 a s a function of wavelength At 2700 A t h i s r a t i o i s indeterminant. Values of approximately 3 x 10" , 1.2 x 10 and 6 x 10" , a l l expressed i n m o l e s / l i t r e , are found f o r 3130 A, 3340 A and 3650 A r e s p e c t i v e l y . I t must he emphasized that the values found f o r 3340 A and p a r t i c u l a r l y 3130 A are subject to possible large e r r o r s . The value given f o r 3130 A may be i n error by as much as 100%. kg/k^ as a function of wavelength The slope of the plot of r e c i p r o c a l quantum y i e l d against pressure shows a marked dependence on wavelength, ^ 4 , increasing from zero at 2700 A to 4.65 x 10 l i t r e s / m o l e at 3650 A. The intermediate values are 16.8 l i t r e s / m o l e at 3130 A and 1 . 6 4 x 10 2 l i t r e s / m o l e at 3340 A. I t should again be pointed out that the r e s u l t at 3650 A was found at 26°C whereas the other values were obtained at 37°C k 2/k^ i s thus i n f i n i t y , 0.06, 0.61 x 10" 2 and 0.22 x l O - 4 m o l e s / l i t r e at 2700, 3130, 3340 and 3650 A r e s p e c t i v e l y . In Figure 3 these values are plot t e d against the energy of the e x c i t i n g r a d i a t i o n . The slope increases r a p i d l y as the energy of e x c i t a t i o n increases. 26 l 1 1 1 1 r i i U 1 1 I I L 7 8 81 8<(- 87 io <& ENERGY (Kcal./mole) FIGURE 3. VARIATION OF kg/k^WITH ENERGY OF EXCITATION 27 I f i t i s assumed that k^, r e l a t e d to c o l l i s i o n a l deactivation, i s v i r t u a l l y independent of the energy of the r a d i a t i o n , Figure 3 represents the v a r i a t i o n of k 2 with energy of the e x c i t i n g r a d i a t i o n . I t i s evident therefore that the d i s s o c i a t i o n rate constant increases r a p i d l y with increasing energy. A f u r t h e r assumption that k^ i s the c o l l i s i o n rate constant, 1.29 x 1 0 1 1 moles-"1' l i t r e s s e c . " 1 c a l c u l a t e d f o r a c o l l i s i o n diameter of 3.5 A, gives absolute values q « f o r k 2 of i n f i n i t y , 7.74 x 1 0 % 7.87 x 10 and 2.84 x 10 se c " 1 at 2700,3130,3340 and 3650 A r e s p e c t i v e l y . A more p l a u s i b l e value of k 2 at 2700 A appears to be approximately l O 1 ^ s e c . - 1 Energy of A c t i v a t i o n of Reaction 2 as a function of wavelength E^, the energy of a c t i v a t i o n , may be assumed to be zero at 2700 A, and has been found to be approximately 2 Kcal/mole at 3340 A and 4.6 Kcal/mole at 3650 A 1. These values have very large e r r o r s . The energy difference of 2.6 Kcal/mole calcul a t e d from above i s much smaller than the 8 Kcal/mole energy difference between 3340 A and 3650 A r a d i a t i o n . This i n d i c a t e s a mechanism i n v o l v i n g more v i b r o n i c states than shown i n reactions 1, 2, 3 and 4. 28 BIBLIOGRAPHY 1. A.N.Straciian and W.A.Noyes J r . J.Am.Chem.Soc. 76, 3258, (1954) 2. G.B.Porter J.Am.Chem.Soc. 79, 827 (1957) 3. G.B.Kistiakowsky and B.H.Mahan J.Am.Chem.Soc. 78 2412 (1957) 4. R.E.Hunt and W.Davis J.Am.Chem.Soc. 69, 1415 (1947) 5. M.Kasha J.Opt.Soc.Am. 38, 929 (1948) 6. G.J.Fisher, A.F.Maclean and A.W.Snizer J.Org.Chem. 18, 1055 (1953) 7. W.Davis J.Am.Chem.Soc. 70, 1868 (1948) 8. C.G.Hatchard and C.A.Parker Proc.Roy.Soc. A 235, 518 (1956) 9. W.F.Ross and G.B.Kistiakowsky J.Am.Soc. 5j5, 1112 (1934) 10. R.G.W.Norrish, H.G.Crone and O.D.Saltmarsh J.Am.Chem.Soc. 56, 1644 (1934) 11. W.A.Noyes J r . , G.B.Porter and J . E . J o l l e y Chem.Rev. 56, 90 (1956) 12. G.B.Kistiakowsky and N.W.Rosenberg J.Am.Chem.Soc. 73, 321 (1950) 

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