J . Phys. Chem. 1991, 95, 1228-1232
1228
Direct Kinetic Study of Radical Transformation Reaction Me,COH 4- Ph,CO 4- PhPCOH
-+
Me,CO
Attila Demeter and Tibor B6rces* Central Research Institute for Chemistry, Hungarian Academy of Sciences, 1525 Budapest, P.O.B. 17, Hungary (Received: February 9, 1990; In Final Form: August 21, 1990)
+
-
Reaction Me2COH Ph2C0 Me2C0+ Ph2C0H ( 5 ) was studied by laser flash photolysis under such experimental conditions where the changes in the concentrations of ketyl radicals with reaction time were controlled by this radical transformation process. Diphenylketyl radical concentration profiles were obtained by monitoring transient absorption at 540 nm and the ratc coefficient k S was extracted from that part of the concentration trajectory which was determined solely by reaction 5 . Thus, k 5 = (3.6 & 0.6) X IO5 dm3 mol-' s-I was determined at 298 K in acetonitrile, which is higher than the two recently reported values derived from quantum yields measured under steady-state Conditions. A reaction mechanism for the radical transformation process ( 5 ) is proposed in which hydrogen-bonded species formed from ketyl radical and benzophenone participate.
Introduction In the photoreduction of benzophenone by an aliphatic alcohol, diphenylketyl and hydroxyalkyl radicals are formed with equal rates. The ketyl radicals then disappear from the system mainly by self- and cross-reactions, forming reaction products. However, it was realized relatively early's2 that a radical transformation reaction may convert the aliphatic ketyls into aromatic ketyl radicals and as a consequence this may change the product distribution considerably. In the photoreduction by isopropyl alcohol, the radical transformation reaction is Me2COH PhzCO M e 2 C 0 + Ph2COH
-
+
whcrc Mc2COH and Ph2COH designate the dimethylketyl and diphenylketyl radicals, respectively. Two recent determinations of the rate coefficient for this radical transformation reaction have been carried o ~ t . ~The . ~reported room temperature values in acetonitrile were k = 3.5 X lo4 dm3 mol-' s-I (in ref 3) and k = 7.3 X IO4 dm3 mol-' s-I (in ref 4). Both rate coefficients were lower by orders of magnitude than the earlier determinations and estimations (for a survey of these see refs 3 and 4). These recent values were obtained from quantum yield measurements under steady-state conditions. The rate coefficient for the radical transformation reaction was extracted from steady-state kinetic equations derived from an assumed reaction mechanism. The rate coefficients obtained by such procedures are naturally dependent (i) on the values assumed for thc rest of the kinetic parameters (as can be judged by considering the simplified photoreduction mechanism shown in Scheme I ) , and (ii) on any othcr factors (c.g., internal filter effect, product quenching, etc.) that may affect the experimental value of the quantum yields. Therefore, we decided to choose a direct method for the determination of the rate coefficient k , in order to compare with results based upon previous published kinetic data. SCHEME I
-hu
PhzCO -'Ph2C0 ' P h 2 C 0 + Me2CHOH Ph2COH + Me2COH Me2COH MezCOH products Me2COH + Ph2COH products Me2COH P h 2 C 0 MezCO + Ph2C0H Ph2COH Ph2COH products
+
+
+
-- -
(1)
(2) (3) (4) (5)
(6)
( I ) Pitts, Jr., J . N.; Letsinger, R. L.; Taylor, R. P.; Patterson, J . M.; Recktenwald. 6.; Martin, R. B. J . Am. Chrm. SOC.1959, R I , 1068-1077. ( 2 ) Beckett, A.: Porter, G. Trans. Faraday Soc. 1963; 59, 2038-2050. ( 3 ) Naguib, Y . M. A.; Steel, C.: Cohen, S. G . J . Phys. Chem. 1988, 92. 6574-6579. (4) Demctcr, A.; LPszl6. B.; BErces. T. Ber. Bunsen-Ges. Phys. Chem. 1988. 92, 1478-1485.
0022-365419 112095- I228$02.50/0
Experimental Section Materials. Benzophenone (Fluka, purum) was purified by recrystallization from ethanol and subsequent vacuum sublimation. Isopropyl alcohol (Merck, Uvasol) was used as received. (CH,),CHOD was prepared from isopropyl acetate with NaOD, dried with CaCI2 and finally purified on a charcoal column (isotopic purity >98%). (CD3),CHOH was prepared from (CD3),C0 (Aldrich, >99.5% purity) by LiAIHl reduction in ether, dried with CaH2, and purified on a charcoal column (isotopic purity >99%). Acetonitrile (Merck, Uvasol), benzene (Carlo Erba, for spectroscopy), and cyclohexane (Carlo Erba, for spectroscopy) were distilled before use. Laser Flash Photolysis Transient Absorption Technique. The apparatus used here was similar to that described e l s e ~ h e r ewith ~.~ some modifications. Degassed samples were irradiated in an 1 X I X 4 cm Suprasil cuvette by the 308-nm laser light of an EMG 101 excimer laser (15-ns pulse duration). The flash energy was reduced by a factor of 102-104by using Pyrex disks. The analysis line was arranged perpendicular to excitation. A 250-W halogen lamp (with excellent long-term stability) served as the light source, while a lens system, a monochromator, and a specially wiredh RCA 1 P28 photomultiplier (ensuring good signal/noise ratio) was used to detect and monitor the diphenylketyl radicals at 540 nm. The signal was acquired on a DL 905 type transient recorder and after averaging 1000-5OoO profiles, data processing was done on a Tulip System I personal computer. Determination of Absorption Spectra. The spectrum of the p-LAT isomer (identified in the Disqussion) was taken in the 210-400-nm range. LAT was produced by photolyzing deaerated solution of 0.003 mol dm" benzophenone and 2.62 mol d m 3 isopropyl alcohol in acetonitrile with 308-nm laser flashes. After 20-30% conversion, the cuvettes were opened under argon atmosphere and p-LAT was separated by HPLC on a HP-1090 instrument using a 25 cm X 4 mm Hypersil ODS ( 5 pm) column and 50% acetonitrile-50% water mobile phase (freed from air by argon bubbling) with 0.5 cm'/min flow ratc. A diode ray detector was used to take the p-LAT spectrum from the emerging peak. The stability constant for the hydrogen-bonded complex formed from benzophenone and isopropyl alcohol was derived from the infrared spectra' of 0.05 mol d m 3 benzophenone solutions with various amounts of isopropyl alcohol added. Spectra were taken on a Nicolct 170 SX infrared spectromctcr using a KBr cell with 0.1-mm path Icngth. ( 5 ) Demeter. A.: BErces. T. J . Photochem. Photobiol. A: Chem. 1989, 46,
77-411 -
(6) Fenstcr. A,; LeBlanc, J . C.; Taylor, W . B.; Johns, H . E. Recr. Sei. Instrum. 1973, 44, 689-694. (7) Symons. M. C . R.; Eaton. G . J . Chem. Soc.. Faraday Trans. I 1985, 8 / . 1963-1977.
0 1991 American Chemical Society
Photoreduction of Benzophenone
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 I
I
I
I
I
1229
-I
100.00
0
2.0
1.0
3.0
Tim, / m
Figure 1. Time profile of diphenylketyl radical absorption at 540 nm, [ Ph2CO] = 1.O X mol dm-) and [ Me2CHOH] = 2.62 mol dm-) in acetonitrile at 298 K . Laser energy: 0.02 mJ/flash.
Results Radical transformation reaction 5 was investigated under experimental conditions where this reaction prevailed over the competitive radical-radical processes. These conditions were fulfilled at low light intcnsities and relatively high benzophenone conccntrations. The flash energy varied from 0.01 to 1 mJ/flash to 1.2 X mol and benzophenone concentration was 1.O X dm-). Isopropyl alcohol concentration was fixed at 2.62 mol d m 3 ; the solvent was generally acetonitrile, and in some cases benzene or cyclohcxanc. Time Profile of Ph2COH Radical Concentration. A typical diphenylkctyl timc profile is shown in Figure I . Fast formation of Ph2COHcan be seen in the initial stage, in the first microsecond, which is the result of photoreduction,step 2. This is followed by a much slowcr accumulation of Ph2COH which we interpret as dircct evidcncc for the Occurrence of radical-transfer reaction 5. Finylly thc decay observed at longer delay times must be due to Ph2COH radical consuming processes, first of all to reaction 6 . The "slow accumulation period" extending from the end of the fast growth to the maximum carries most of the kinetic information on reaction 5. A closer analysis of the results shows that the increase of absorption in the greatest part of the slow accumulation period can be well described by a single-exponentialrate law. This mcans that thc accumulation kinetics of the Ph2COH radicals in this time interval are determined practically only by radical transformation reaction 5. The fact that under favorable conditions of triplet benzophenone photoreduction the radical transformation reaction and the Ph2COH consuming processes are well resolved in time makes possible the direct determination of kS in a way that is hardly influenced by the possible errors of the other kinetic parameters. This is evidenced by the good agreement between the exponential growth parameter (of the single-exponential rate expression) and the ks value derived by the more sophisticated treatment described below. A systematic study of k, and the Arrhenius parameters under wider experimental conditions requires kinetic model simulation, i.e., the numerical solution of a system of kinetic differential equations. I n such computer-modeling studies, twice the length of the slow accumulation period has been analyzed, which covers more or less the time interval in which Me,COH radicals completely reacted. Two parameters, namely kSand [Ph2COH]o, were optimized. (The latter designates the initial diphenylketyl radical concentration which is attained as the result of the fast photoreduction process 2.) The rest of the parameters were fixed at their literature v a l ~ e ;i.e., ~ . ~in acetonitrile at room temperature k 3 = I .28 X IO9, k4 = 8.0 X IO8, and kh = 5.9 X IO7 dm3 mol-' s-I, and c(Ph,COH) = 3500 dm3 mol-' cm-' was used. In the case of the experiment presented in Figure I , computer modeling gave an excellent fit to the experimental data in a wide time range with adjusted parameters [Ph2COH]o = 1 . 1 X IO-' mol dm-'3and kS = 3.7 X IO5 dm3 mol-' s-l. (See calculated and experimental data in Figure 1 .) This kc value agrees within the error limits with the single-exponential growth parameter evaluated
0.005
I
0 010
[Ph$Ol/mol. dm-3
Figure 2. Plot of the pseudo-first-order rate parameter k5' = k5[Ph,CO] vs benzophenone concentration in acetonitrile ( 0 )and benzene (0). T = 296 K. Laser energy: 0.01-0.1 mJ/flash. logilP@OHI,/md~dm-~)
6000
I
-7.0 I
- 6.5
- 6.0 I
I
-5.5 I
T L I
t1
2wo
O -2
-1
0
log IEImJ)
Figure 3. Plot of the pseudo-first-orderrate parameter k < = k,[Ph,CO] vs laser flash energy in acetonitrile at 298 K . [Ph,CO] = 1 . I 7 X
mol dm-'.
in the slow accumulation range, as well as with the room temperature average rate coefficient determined in the series of experiments described below. Variation of Benzophenone Concentration and Laser Flash Intensity. In order to examine the reliability of the k5 values determined, benzophenone concentration and flash energy were varied over a broad range. In Figure 2, the pseudo-first-order rate parameter k,' = ks[Ph2CO] derived by model simulation is plotted as a function of benzophenone concentration. The rate coefficient k5 is seen to be independent of benzophenone concentration over a wide concentration range, and from the slope one obtains k, = (3.7 f 0.6) X 10, dm3 mol-' s-I at room temperature in acetonitrile. (The error limits represent 2 standard deviations.) A few experiments were performed in benzene and in cyclohexane. The rate coefficient obtained for room temperature in benzene was k5 = ( I .6f 0.4) X IO6 dm3 mol-' s-I and the value estimated in cyclohexane was k, E 3 X IO6 dm' mol-' SKI. Both are considerably higher than in acetonitrile. A further test of the reliability of the derived k5 rate coefficient should result from the investigation of the effect of laser flash energy. At the same time, this may be considered as a test for the assumed mechanism and kinetic parameters, since the relative contribution of second-order free-radical reactions for dimethylketyl radical consumption is expected to increase with increasing light intensity. The pseudo-first-order rate coefficient k,' = k,[Ph2CO] is plotted as a function of laser flash energy in Figure 3. The k,' values are given in the graph together with error bars which represent the estimated errors derived from model
1230 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 0
I
I
I
Demeter and Birces
1
I t
I
? --
Wavelength /nm
Figure 5. Absorption spectrum of p-LAT
11 I
I
1
3.0
3.5
4.0
4.5
1000T -11 K
Figure 4. Arrhenius plot of the rate coefficient k5 in acetonitrile.
calculation, including error propagation. Considering the fate of the diphenylketyl radicals, the results of kinetic model calculations demonstrate the increasing role of radical-radical reactions. At the four lowest flash energies, more than 95% of the Me,COH radicals reacted by radical-transfer process 5 , while this percentage dropped below 85% in the three experiments made at the highest light intensities. These theoretical conclusions are supported also by direct experimental observations: The [Ph,COH],,,/[Ph2COH]o ratio (where [Ph2COHl0and [Ph,COH],,, are the initial and the maximum diphenylketyl radical concentrations, respectively, in the slow accumulation period) changes from a value close to 2 at low intensities to a value just above I at the higher intensities. These extremes correspond to a dominant and a minor role, respectively, of reaction 5 among Me2COH radical reactions. Although the results given in Figure 3 show some scatter, it appears that the derived kinetic parameters are independent of the laser flash intensity. This series of experiments yields k5 = (3.5 f 0.6) X IO5 dm3 mol-' s-I in acetonitrile, in good agreement with the above derived value. Thus, considering the results of variation of benzophenone concentration and flash energy, we suggcst for 298 K in acetonitrile
k 5 = (3.6 f 0.6)
X
IO5 dm3 mol-'
s-I
(7)
where the 2u error is quoted. Temperature Dependence of k 5 . The temperature dependence of k c has been studied in the temperature range of 231-336 K in acetonitrile. The concentrations were [Ph,CO] = 5.0 X mol dm-' and [Me,CHOH] = 2.62 mol dm-', while the laser flash energy was 0.1 mJ/flash. The Arrhenius plot is shown in Figure 4. Least-squares treatment yielded log (k,/dm3 mol-' s-') = (8.20 f 0.26) - (3600 f 400) cal mol-l/(ln 1O)RT (8)
Experiments with Deuterated Isopropyl Alcohol. Deuterated isopropyl alcohol was used in a few experiments in order to identify thc hydrogcn atom abstracted in reaction 5. The concentrations uscd wcrc 2.62 mol dm-) alcohol and 0.01 mol d d benzophenone in acctonitrilc, and thc laser flash energy was a few times mJ / flash. With (CD,),CHOH, a valuc for k 5 of (4.1 f 0.7) X I O 5 dm3 mol-' SKIwas obtained at 298 K which agreed within the limits of cxperimcntal crror with the value determined for the undeuterated compound. Replacement of the alcoholic hydrogen by deutcrium using (CH3)+2HOD, however, changed the reaction rate, and a valuc for k 5 of (0.4 f 0.1) X IO5 dm' mol-' s-I was detcrmincd. Although only a few experiments were made with deutcratcd alcohols, ncvcrtheless the results indicate that the alcoholic hydrogen atom is transferred in reaction 5. Discussion
The room tcmpcrature ratc coefficient k , was obtained by laser flash photolysis by monitoring the slow accumulation of the di-
phenylketyl radicals under conditions where ketyl radical reactions other than radical transformation reaction 5 were of negligible importance. The rate coefficient k5 = (3.6 f 0.6) X IO5 dm3 mol-' S C I derived from this experiment at room temperature in acetonitrile is, however, higher than the recently reported values obtained from quantum yield measurements in steady-state photolysis The cause of this difference needs to be accounted for. Rate Coeffieats and Arrhenius Parameters. In a recent pap# we dealt with the kinetics of the ketyl radical reactions in benzophenone photoreduction by isopropyl alcohol. From the investigation of the effect of benzophenone concentration change mol dm-') on the quantum yields of to 3 X (from benzophenone consumption and benzopinacol formation, a room temperature rate coefficient of k5 = 7.3 X IO4 dm' mol-' s-I was derived in acetonitrile. In this determination, the internal filter effect caused by an intermediate.which was formed in the addition of Me,COH to the ring of PhzCOH (and is generally known as the "light absorbing transient", LAT) was neglected. Neglect of the internal filter effect at 250-nm wavelength and 15-2576 benzophenone conversion appeared justified on the basis of the LAT spectra available in the 300-430-nmrange.' However, the determination of the p-LAT spectrum over a wider wavelength range (see below) showed that absorption by the LAT isomers at 250 nm (the photolysis wavelength in our quantum yield determinations) is not at all negligible. The p-LAT spectrum presented in Figure 5 (which was taken as described in the Experimental Section) shows two peaks in the 210-400-nm range. The long-wavelength 'B 'A band has a maximum at 3 I7 and the 300-400-nm part of the spectrum agrees reasonably well with the earlier reported' one. The other band, ' A transition, has a maximum at 251 nm identified as the IC which is comparable in intensity with the long-wavelength absorption. These new spectroscopic results allow us to reevaluate the internal filter effect in our steady-state quantum yield determinations presented in Figure 6 of ref 4. The decrease of light intensity along the excitation light path due to LAT absorption can be estimated to be 15% at the four lowest benzophenone concentrations. At the next three concentrations, LAT absorption caused about 7-1 3% intensity decrease and in addition a 1-10% error in the quantum yields as a result of triplet-state quenching by LAT. The greatest internal filter effect of 1 1 5 % and quenching effect of >20% occurred at the highest benzophenone concentration. Neglecting the result at the highest benzophenone concentration and correcting the rest of the data for the internal filter effect and quenching by LAT, we calculate a value of k5 = 2 X IO' dm' mol-' SKI from the steady-state quantum yields, in reasonably good agreement with the present laser flash photolysistransient absorption results. In another report of a similar study by Naguib et al.,' a value for k , of 3.5 X IO4 dm3 mol-' s-I was obtained. Benzopinacol and mixed pinacol yields were measured at 366 nm as a function of benzophenone concentration (varied from I X to 1 X IO-' mol dm-)). The kinetic parameters used by Naguib et al.' in data processing and k5 evaluation differed considerably from the parameters used in this work. With our parameter^.^ the best fit to the benzopinacol quantum yields reported by Naguib et al. gives
-
-
Photoreduction of Benzophenone k 5 = 1.5 X IO' mol-' s-'. Most of the 5-fold increase in k5 is the result of an increase of k 4 from 1 X 1 Ox to the more reasonable value of 8 X IOx din3 mol" s-I. [Note that the higher value (i) is obtained4 in a rclatively direct way from experimental time profiles; (ii) is based on a large number of measurements; (iii) is between k 3 and k , as expected: (iv) is determined in acetonitrile as rcquired. On the other hand, the lower value (i) is derived from product ratios by several assumptions: (ii) is based on only two product ratios: (iii) is obtained from a k,,q/(k,,,,kBB)'/2 ratio which is far from the expected value of 2; (iv) is determined in acetone rather than in acetonitrile.] Although LAT quantum yields are not given by Naguib et al.,3 a rough estimate of these can be obtained by assuming +(LAT) N +(mixed pinacol). With these, the combined internal filter effect and triplet quenching correction is around 4-1 5%. (Note that the benzophenone concentrations are high and the excitation wavelength coincides with the longwavelength c-LAT absorption maximum.) Thus, the final k, value in acctonitrilc at room tempcrature is not far from 3 x IO5 dm' mol-' s-I, in agreement with the present result. Two previous impulse excitation studies of the benzophenone-isopropyl alcohol system have been reported in which diphcnylkctyl radicals were m ~ n i t o r e d . ~In . ~an initial stage of this rescarch, among other kinetic parameters a high k, value was givcmx This high value was extracted from a laser flash photolysis experiment carried out under experimental conditions unfavorable with rcgard to radical transformation reaction 5 and the rate coefficient was evaluated assuming unrealistic values for some kinctic paramctcrs, especially for k3 (see ref 4). Reinterpretation of the same experiment but by use of recent and more reliable ratc cocfficicnts4 yields k , C 5 X 10' cm3 mol-' S K I at room temperature. The other transient absorption study9 dealt with the low-temperature pulse radiolysis of 1 X IO-* mol dm-3 isopropyl alcohol solution of benzophenone containing I .2 mol dm-' hydrogen chloride. The rate coefficient reported at 183 K, k 5 = 1.06 X 1 O4 dm' mol-' s-', agrees within 25% with the extrapolated value obtaincd by using the Arrhenius parameters determined in this work. There is relatively little information available in the literature on the Arrhenius parameters of reaction 5. Hoshino et al.9 obtained A = 2.6 X IO'" dm3 mol-' s-' and E = 5.3 kcal mol-' in thcir low-temperature pulse radiolysis study in an isopropyl alcohol solution of benzophcnonc containing I .2 mol dm-] hydrogen chloride. The Arrhenius parameters, derived from temperaturc-dcpcndcncc mcasurcmcnts in an unspecified temperature rangc, arc ccrtainly too high since they yield an extrapolated room temperature rate Coefficient which is an order of magnitude higher than the experimental value. Naguib ct al.3 dcrivcd the rclation E4;,- ( 1 / 2 ) E 6 - E , = -1.8 kcal mol-' from steady-state quantum yield measurements as a function of tcmpcraturc and obtained with assumed values for E, and Ed;,(E4;, instead of E4 which appeared mistakenly in ref 3) a n activation cncrgy of E, = 2.3 kcal mol-'. By use of the recvaluated rate coefficient of k , = 3 X IO5 dm3 mol-' s-I, a value of A, = 2 X IO7 dm3 mol-' s-I is obtained (instead of 2 X IO6 given in rcf 3). Thcsc Arrhcnius parameters, while definitely smaller, are nevertheless not in disagreement with those obtained in this study. This becomes even more obvious if experimental values4 to obtain E , and instead of assumed ones are used for E(, and E4;1 in consequence A , (see Appendix). Rcwtion Mechanisnt. Thc rcsults of the cxpcrimcnts with deuterated isopropyl alcohols clearly indicate that the hydrogen atom of thc H O group of thc dimethylketyl radical is involved in the H-atom transfer from MezCOH to benzophenone. In the following discussion we shall consider the mechanism of this hydrogen atom transfer. Knowledge of the Arrhenius parameters contributes little to thc understanding of the details of the mechanism. Thc kinetic (8) LBsz16, B.; Demeter, A. ZJ-Mirreilungen 1984, 97, 147-152. (9) Hoshino. M.: Arai. S.: Imamura. M. J . Phys. Chem. 1980, 84, 2576-2579.
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1231 SCHEME I1 M&OH
Me&O
phzco
+
+ PhZtOH
P, M&-o-H---o=c~ 4
(A)
& -c
(B)
Ib
Me~C=0.--H-O-&2
parameters determined in this work and obtained by reinterpretation of the results of Naguib et aL3 are close to, or not much less than, the Arrhenius parameters for the exothermic metathesis reactions of alkoxy free radicals.'" (Consider that reaction 5 is exothermic by 15-20 kcal mol-' mainly as a result of the formation of the resonance stabilized PhzCOH radical.) However, in case of reaction 5, since the attacking species is a carbonyl compound and not a free radical, one expects a mechanism other than simple hydrogen atom transfer. Naguib et aL3 suggested that reaction 5 may occur by simultaneous, spatially separated, electron and proton transfer in a hydrogen-bounded complex, as visualized below:
\ +
>C
H
-.A 0:-
e
x - 0
Such a process would involve a highly polar transition state. This suggests that polar solvents which stabilize this transition state should enhance the reaction. However, we find the reaction to proceed faster in nonpolar media, which appears to indicate little charge separation in the transition state. A tentative explanation of the experimental findings, including the observations made on the effect of media on the reaction rate, can be given by Scheme I I which is analogous to the mechanism first suggested by Schenck et al." to explain the interaction between diphenylketyl radical and benzophenone. In order to support the proposed mechanism Schenck et al." identified the assumed hydrogen-bonded ketone-ketyl radical complex by its ESR spectrum. The esscntial feature of Scheme I 1 is the participation of complexes A and B in reaction 5. Species A and B are not resonance structures; rather they are hydrogen-bonded complexes (with strong hydrogen bonds) formed by association of a ketyl radical and a carbonyl molecule. The rate-determining step is the hydrogen atom transfer b in which a hydrogen-bonded aliphatic ketyl radical is converted into a hydrogen-bonded aromatic ketyl radical. In case of a thermoneutral reaction, Schenck et al." estimated the activation energy for step b to be 2 7 kcal mol-', while a definitely lower value is expected here in the case of the exothermic reaction 5. From this mechanism, the rate coefficient of radical transformation reaction 5 is given by
(9) where K , = k,/k_, designates the equilibrium constant for the formation of the hydrogen-bonding complex A from dimethylketyl radical and benzophenone. Thus, the somewhat lower Arrhenius paramctcrs of reaction 5 compared to "normal" metathesis reactions can be explained by the small negative reaction entropy of complex formation and the negative temperature dependence of K,, rcspcctively. In order to find some support for this mechanistic hypothesis, the cquilibrium constants for hydrogen bonding between benzophenone and isopropyl alcohol were investigated in various solvents. In the infrared spectra of benzophenone-isopropyl alcohol solutions, shown in Figure 6, the band at 1662 cm-' corresponds to
J . Phys. Chem. 1991, 95, 1232-1 240
1232
of isopropyl alcohol, equilibrium constants of 0.038 f 0.005 mol dm-3 and 0. I8 f 0.02 mol dm-' were obtained in acetonitrile and benzene solvents, respectively. These results arc in line with the association constant reported' for the Me2CO-MeOH system in acetonitrile ( K = 0.08 mol d m 3 ) and with that estimated'* for the Ph2CO-EtOH system in hexane ( K = 0.4 mol d ~ n - ~ ) . The infrared results show that the benzophenone-isopropyl alcohol complex is about 4.5 times more stable in benzene than in acetonitrile. I f roughly the same is true for the stability of the benzophenone-dimethylketyl radical complexes, then the agreement between the ratio of the K, values in the two solvents and the ratio of the appropriate rate coefficients (Le., k5(in benzene)/k5(in acetonitrile) = 4.4 f 1 . I ) may be considered as support for the mechanism presented in Scheme 11.
bl c
a
. i
P
b
8
Appendix 1700
1650
1600 1700
1650
1600
w n e ~ l b ricm-1 r Figure 6. I R spectra of solutions of 0.05 mol dm-' benzophenone and
isopropyl alcohol: (a) iicctonitrilc solvcnt, [Me,CHOH] = 0 ( I ) , 2.62 (2). 4.58 (3). 6.55 ( 4 ) mol d ~ n - (b) ~ : benzene solvent, [Me2CHOH]= 0 ( I ) %2.62 (2). 5.24 (3) mol dm-'. the C=O stretch vibration of benzophenone, while the new band which appears at 1653 cm-' in the presence of isopropyl alcohol and increases with increasing alcohol concentration may be assigned to thc hydrogcn-bonding complex. From the decrease of the 1662-cm-' absorption of the nonbonded molecule on addition
Reevaluation of the Arrhenius Parameters A5 and E5from the Results of Naguib et al.3 (i) E4;,- E6/2 - E5 = -1.8 kcal mol-' (a result of Naguibet aL3). (ii) Eh = 3.13 kcal mol-' (a literature value4 which is in agreement with unpublished results obtained in the Ph2CO-Ph2CHOH system.) (iii) E4a= Eh (a reasonable assumption for two combination reactions. (iv) E5 = 3.35 kcal mol-' (derived from the data given above). (v) k5 = 3 X IO5 dm3 mol-' s-' (obtained from the reevaluation of the results of Naguib et aL3, see text). (vi) A5 = 9 X IO7 dm3 mol-' SKI (derived with k 5 and E5 given above). (12) Brealey, G. J.; Kasha, M. J . Am. Chem. Soc. 1955, 77,4462-4468.
Kinetics and Mechanism of the Reaction of Hydroxyl Radicals with Acetonitrile under Atmospheric Conditions A. J. Hynes* and P. H. Wine Physical Sciences Laboratory, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, Georgia 30332 (Receiord: March 16, 1990; In Final Form: August 21, 1990)
The pulsed laser photolysis-pulsed laser induced fluorescence technique has been employed to determine absolute rate coefficients for the reaction OH + CH3CN ( I ) and its isotopic variants, OH + CD3CN (2). OD + CH3CN (3). and OD + CD3CN (4). Rcactions 1 and 2 were studied as a function of prcssurc and tcmpcraturc in N,, N 2 / 0 2 , and He buffer gases. In the absence of O2 all four reactions displayed well-behavcd kinetics w i t h cxponcntial OH decays and pscudo-first-order rate constants which were proportional to substrate concentration. Data obtained in N, over the range 50-700 Torr at 298 K are consistent with k , showing a small pressure dependcncc. The hrrhcnius cxprcssion oblaincd by averaging data at all pressures is k , ( T ) =) :( I: .! I X exp[(-l I30 h 90)/7'l cm3 molecule-' s-I. The kinetics of reaction 2 are found to be to (2.16 f 0.1 I ) X cm3 molecule-' s-I over prcssurc dcpcndcnt with k 2 (298 K ) increasing from ( I .21 f 0.1 2) X the prcssurc rangc 50-700 Torr of N 2 at 298 K. Data a t prcssurcs >600 Torr givc k 2 ( T ) = (9.4!i,:4) X IO-'' cxp[(-l 180 f 2 5 0 ) / q cm3 molcculc-' s-l. The rates of reactions 3 and 4 are found to be independent of pressure over the range 50-700 Torr of N 2 with 298 K rate coefficients given by k 3 = (3.18 f 0.40) X mi3 molecule-' s-l and k4 = (2.25 f 0.28) X cm3 molcculc-' s-I. I n the presence of O2each reaction shows complex (non-pseudo-first-order) kinetic behavior and/or an apparcnt dccrcasc in the observcd rate constant with incrcasing [ 0 2 ] ,indicating thc prcscncc of significant OH or OD regeneration. Observation of regeneration of OH in (2) and OD in (3) is indicative of a reaction channel which proceeds via addition followed by reaction of the adduct, or one of its decomposition products, with 0,. The observed OH and OD decay profiles havc been modeled by using a simplc mechanistic scheme to cxtract kinetic information about the adduct reactions with O2 and branching ratios for OH regeneration, A plausible mechanism for OH regeneration in ( 2 ) involves OH addition to thc nitrogen atom followed by 0, addition to the cyano carbon atom, isomcrixation. and decomposition to D2C0 + DOCN + OH. Our rcsults suggest that the OH + CH3CN reaction occurs via a complcx mechanism involving both biniolecular and tcrmolecular pathways, analogous to the mechanisms Tor thc important atmospheric reactions of OH with CO and HN03.
Introduction I t is now generally accepted that acetonitrile (CH3CN) is
present at ppt levels in the stratosphere.' Attempts to understand the role of acetonitrile in stratospheric positive ion chemistry2 and its contribution to the stratospheric NO, budget3 require a detailed *Author to whom correspondence should be
addressed.
0022-3654191/2095-1232$02.50/0
understanding of its atmospheric sources, emission rates, and oxidation mechanism. Acetonitrile was first proposed as a com( I ) Schlager, H.; Arnold, F. Planer. Space Sci. 1986, 34, 245. (2) Arijs, E.; Ncvcjans, D.; Ingcls, Inr. J . Mass Specrrom. Ion Processes 1987, 81, 15. ( 3 ) *Almospheric Ozone 1985." WMO-Report No. 16, Vol. I, World Mctcorological Organization, Gcncva, 1985.
0 I99 I American Chemical Society
