J O U R N A L
T H E
O F
PHYSICAL CHEMISTRY Registered in U.S. Patent Office
0 Copyright, 1978, by the American Chemical Society
VOLUME 82, NUMBER 13
J U N E 29,1978
Energy Partitioning in the Photodissociation of C3H40Near 200 nm M. E. Umstead, R. G. Shortridge,+ and M. C. Lin” Chemistty Division, Naval Research Laboratory, Washington,D.G. 20375 (Received November 2 1, 1977; Revised Manuscript Received April 13, 1978) Publication costs assisted by the Naval Research Laboratory
The photodissociation of methylketene and acrolein near 200 nm has been investigated using a CO laser probing and conventional gas analysis methods. The dissociation of methylketene in a Vycor tube (A e 2 2 6 nm) produces CO with a Boltzmann vibrational temperature of about 2300 K and with an average vibrational energy of 2.0 f 0.2 kcal/mol. These results are consistent with the mechanism proposed by Kistiakowsky and eo-workers CH3CHt (-+ in which the ethylidene diradical is formed in the first step of the reaction CH3CHC0 + hv CZHJ)+ CO and subsequently isomerizes to ethylene. The CO was found to have a vibrational energy distribution similar to that found previously in the O(3P)+ CH3C2Hreaction which is believed to occur via a methylketene intermediate. The dissociation of both methylketene and acrolein below 220 nm produced CO with similar, but nonstatistical vibrational energy distributions. A mechanism involving H-atom migration in both systems is postulated in which the direct production of CzH4 + CO takes place in addition to the major, but less exothermic CH3CH + CO channel.
-
Introduction The photodissociation of methylketene (CH3CHCO)in the range of 240-370 nm has been studied by Kistiakowsky and co-workerslJ in a static system. The major photolytic products were found to be CO, C2H2,CzH4,and C4H,. The results obtained from the photolysis of -2-50 Torr of methylketene were found to be consistent with the following mechanism: CH,CHCO t hv + C H , C H ~+
co
C H , C H ~-+ c , H , ~ C,H,t -+ C,H, t H,
C,H,t t M C,H, t M C H , C H ~t CH,CHCO -+ C,H, t -+
co
where the dagger represents vibrational excitation. The above mechanism is analogous to the one proposed earlier by Frey3 to account for his CH3CHNzphotolysis results. We have recently studied the dynamics of the O(3P)+ C H 3 C r C H reaction, which is believed to take place via NRC/NRL Postdoctoral Research Associate (Aug 1973-Jun 1975). Bell Aerospace Laboratory (Textron), Buffalo, N.Y. 14240 This article is not subject to U.S. Copyright.
a methylketene intermediate.4-6 The observed CO vibrational energy distribution, measured by a CO laser resonance absorption method, was found to be consistent with the initial production of CH,CH and C0.7,8 A similar study of the O(3P) 1- and 2-butyne reactions also indicates the production of diradicals (i.e., CH3CH2CHand CH3CCH3)as primary initial productsag In this work we have investigated the dynamics of CO production from the photodissociation of CH3CHC0 near 200 nm using a quartz (A 2190 nm) or a Vycor (A 2220 nm) flash tube. The nascent CO formed in the early stages of the reaction was measured by means of a continuous-wave (cw) CO laser. In order to confirm the results of earlier photolytic studies, the reaction products were also analyzed with a gas chromatograph. A parallel study of the photodissociation of acrolein (CH&HCHO), another member of the three known C3H40isomers, was also carried out in a quartz reaction cell. Detailed results of these studies are reported herein.
+
Experimental Section The flash photolysis and laser probing systems have been described previously in detail.7J0 In brief, a stabilized Published 1978 by the American Chemical Society
1456
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
M. E. Umstead, R. G. Shortridge, and
M. C. Lin
I
I
I
\ \
.
'---20
IO
0
30
TIME/,lJS
-
Figure 1. The solid curve shows the time-resolved CO absorption for 2 1 transition, 5 Torr of 2 % CH,CHCO in SF, flashed at an energy of 1 kJ (Vycor). The dashed curve is the flash profile.
cw CO laser which could be preset a t various vibrational-rotational lines accessible to the reaction was directed through the axis of either a quartz or a Vycor flash reaction tube. Mixtures of CH3CHC0 or CH2CHCH0 in He or SF6 were flash photolyzed, and time-resolved absorption curves were obtained for all vibrational levels populated by the reaction. Figure 1 shows a typical CO absorption curve for the 2 1 transition, as well as the flash profile. The CO vibrational populations were determined by analyzing the initial portions of the absorption curves, from about 4 to 10 p s , by means of the gain equation (given in ref 10) with the aid of a computer. The initial population distribution was evaluated by extrapolating the Nv/Noratios to the earliest appearance time of absorption in order to eliminate both the effects of CO vibrational relaxation and any possible contributions from secondary reactions that might generate CO. The validity of this procedure for obtaining the true initial distribution has been demonstrated previously.ll Stable products of the photolyses were measured by gas chromatography, using the flash photolysis and chromatographic systems described in ref 7. The CH2CHCH0 was purchased from the Aldrich Chemical Co. and was stated to be 97% pure with the remainder being water. It was distilled from a trap at 228 K before use. CH3CHC0 was prepared by the pyrolysis of propionic anhydride.'J2 The pyrolysis products were passed through a trap at 195 K to condense higher boiling products, and the CH3CHC0 was collected in a subsequent trap at 156 K which passed most of the CH2C0 formed as a byproduct. Chong and Kistiakowsky' found that some of their earlier experiments on CH3CHC0 photolysis were seriously influenced by the presence of small amounts of CH2C0 as an impurity. When quantities of CH3CHC0 were stored in a cold trap and small aliquots vaporized into the reaction vessel, the concentration of CHzCO in the gaseous mixture became greatly enriched because of its higher vapor pressure. In each batch of CH3CHC0 that we prepared, the amount of CH'CO in the vapor phase over the liquid CH3CHC0 was determined by condensing some of the vapors in ca. 1 mL of cold CH30H, and an-
-
A/nm Figure 2. UV absorption spectrum of methylketene: (I) 2.0 Torr (10% in He) in quartz cell; (11) spectrum I corrected for Vycor cutoff; dashed curve, Vycor cutoff.
TABLE I: Products Found from the Flash Photolysis of CH,CHCHO and CH3CHC0, 10 Torr of 10% Reactant in He Flashed at an Energy of 1.6 kJ in Quartz Yield, % Product
co CH.4 C*H, CZH, CZH, C3H6 + C,H, CH,CCH, CH,C,H
CH,CHCHO
CH,CHCO
90.9 2.1 58.0 11.6 0.43 0.41 0.28 0.49
0.55 13.6 5.83 0.69 1.25 0.17 0.25
alyzing the CH30H for methyl acetate and methyl propionate by gas chromatography. The amount found, as calculated from the methyl acetate to methyl propionate ratio, was generally 0.15-0.30'30, and in no case exceeded 0.44%. These values should be indicative of the upper limit of CHzCO present in the gas mixtures photolyzed. A fresh batch of CH3CHC0 was prepared for use each day. The SFs (Matheson Gas Products Co.) was thoroughly degassed a t 77 K before use. The UV spectra were obtained by means of a Cary 118C UV-visible recording spectrometer; they are shown in Figures 2 and 3 for CH3CHC0 and CH2CHCH0, respectively. Results The products found from the photolysis of CH3CHC0 were generally in agreement with those reported by Chong and Kistiakowsky2 and were not investigated extensively. Table I lists those found from the photolysis of 1 Torr of 10% CH3CHC0 flashed at energy of 1.6 kJ in quartz, as well as those from CH2CHCH0 under similar conditions.
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978 1457
Photodissociation of C3H,0
t z 0 c
P
w
0
v)
m
a
0
2
6
4
0
10
v Figure 5. Vibrational energy distribution of the CO formed in the acrolein photodissociation in quartz. The dashed curve shows methylketene data from Figure 3.
a/ nm Flgure 3. UV absorption spectrum of acrolein, 0.5 Torr (10% in He). 1.0
Compound CH,CHCO CH,CHCO CH,CHCHO
0
z
.Ol
';.
z
.oo I
.0001
TABLE 11: Average Vibrational Energy of CO Formed in the Photolysis of CH,CHCO and CH,CHCHO at Different Wavelengths
1
0
1
\ \
1
\
2
4
6
8
IO
V Figure 4. Vibrational energy distribution of the 120 formed in the methylketene photodissociation in quartz. The dashed curve show methylketene data (Vycor) from Figure 5.
Lesser amounts of various C4 hydrocarbons were also detected, but were not identified. The CO laser resonance absorption measurements showed that the CO formed in the photodissociation of CH3CHC0 in quartz was vibrationally excited to u = 9 and had a Boltzmann vibrational temperature of -33800 K. The results of two sets of CH3CHC0 measurements are plotted in Figure 4. One set was obtained from 5 Torr of a 1%mixture of CH3CHC0 in He, flashed at an energy of 0.5 kJ, and the other from a 1.5% mixture flashed at 1.0 kJ. The fact that the two sets of data led to the same
Flash tube Quartz ( h >190 nm) Vycor ( h 226 i 5 nm) Quartz
(E,), kcal mol-' 3.7 f 0.3 2.0 f 0.2 3.3 f 0.2
population distribution, even though the CH3CHC0 concentrations and the flash energies differed significantly, provides evidence that the excited CO formed in the primary reaction is free from relaxation and is not influenced by secondary reactions. This is also supported by an experiment in which O2 was added as a free-radical scavenger. A mixture containing 1%CH3CHC0 and 2% O2 in He was flashed a t an energy of 1.0 kJ. The results of the CO excitation measurements were the same as those from the 02-free mixtures within experimental error. The results from the photolysis of 5 Torr of a 2% CH2CHCHO-He mixture flashed in quartz a t 1.0 kJ are shown in Figure 5. The CO was found to be vibrationally excited to u = 9 with a Boltzmann vibrational temperature of -3000 K. Figure 6 shows the results obtained from the photolysis of 5 Torr of a 2% and a 5% mixture of CH,CHCO in SF6 flashed in Vycor at an energy of 1.0 kJ. The CO was found to be excited to u = 6 with a Boltzmann temperature of about 2300 K. Similar experiments with CH2CHCH0 in Vycor did not yield enough CO to permit measurements to be made. The amount of vibrational energy channeled into CO was calculated from the expression
( E , ) = C f,E, v> 0
where f, = N v / C u 2 , N vis the normalized vibrational population distribution and E , is the vibrational energy of CO at the uth level with the zero-point energy excluded. The results obtained are summarized in Table 11.
1458
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
STAT. D l S T
M. E. Umstead, R. G. Shortridge, and M. C. Lin
"1
I Etot = 122 It E t o t = 5 4 PRIOR
.IC
,001
't \
1
1 , ,\
\
t
.OOOl
s2
,
,
,
I
\
,
,
,
I
O t
CO + CH2CH2
C H J C H C O , So
-501
,
Figure 7. Schematic energy diagram for the CH,CHCO photodissociation and the O(3P) CH3C,H systems.
+
\
\
0
2
6
4
8
IO
V Figure 6. Vibrational energy distribution of the CO formed in the methylketene photodissociation in Vycor: triangles, 2 % CH,CHCO in SF,; open circles, 5 % CH,CHCO in SF6; solid circles, 0 -t CH&H reaction (from ref 8).
Discussion (I) Methylketene Dissociation at 226 nm. Although the region photochemistry of CH3CHC0 in the 200-nm (m*) has not been investigated before, the general reaction scheme of Kistiakowsky and co-workers1s2given in the Introduction is probably still operable. Since the extinction coefficient of CH3CHC0 increases by more than two orders of magnitude as the wavelength decreases from 400 to 220 nm, the photochemistry of the 350-nm (np*) band is not expected to be important. The vibrationally excited CO observed in the present study is believed to result exclusively from the direct photodissociation reaction 1. The production of CO from reaction 5, under our experimental conditions, can be readily shown to be unimportant. The rate constant ratio k2/k5 has been estimated by Chong and Kistiakowsky2 at different wavelengths between 260 and 365 nm and was found to increase with increasing dissociation energy as would be expected theoretically. A t 260 nm they found k2/k5 = 1.5 X l o 3 Torr. Accordingly, the relative rate of reactions 2 and 5 , under our typical experimental conditions (PCHBCHCO = 0.05 Torr), becomes R z / R 5 = 1.5 X 103/0.05 = 3 X lo4. At wavelengths of our present interest, h -230 nm, Rz/R6is expected to be even higher. CH3CH would, therefore, disappear exclusively via unimolecular isomerization (reaction 2) and reaction 5 should be of negligible importance. The objective of the present work is to elucidate the mechanism of the initial photodissociation reaction 1 by means of the detailed CO product vibrational energy distribution measurements. Since we are concerned with the partioning of reaction energy, the energetics of the CH3CHCO=CH3CH + CO system should be discussed first. The heats of formation of both CH,CHCO and CH3CH have not been determined experimentally. Theoretical values for the latter differ greatly.13 On the basis of a simple bond energy additivity argument, we estimated the energy released in reaction 2, CH3CH C2H4,to be approximately 68 kcal/m01.~ This value leads to AH?(CH,CH) = 81 kcal/mol, which compares closely with the -+
value estimated by assuming AH?(CH3CH) N AH?(CH,) + AHfo(CH3CrCH)- AH,"(C2Hz)= 82 kcal/mol, taking A&" (CHZ) = 92 f 2 kcal/mo114J5for the ground electronic (,E$) state. The heat of formation of CH,CHCO can also be estimated by a similar group additivity method. Assuming AHfo(CH3CH=C=O) - AH?(CH,=C=O) = AHfO(CH3CH=CHz) - AHf' (CH2=CH2), we obtain AHfo(CH3CHCO) = -19.0 kcal/mol, taking AHfo(CH2CO)= -11.4 kcal/mol16 and the known values for CzH4 and C3H6. Thus, adopting AH?(CH,CH) = 81 and AHfo(CH,CHCO) = -19 kcal/mol, the enthalpy change in reaction 1is AH: D(CH,CH=CO) = 74 kcal/mol. This compares reasonably with D(CH,=CO) = 77 kcal/mol derived from the values of CH2 and CH2C0 given above. The total available energy for product excitation is given by Etot=hv - D(CH,CH=CO) Eth
-
+
where E t h is the thermal (internal and translational) energy of the dissociating molecule. For the dissociation reaction in a Vycor tube, the absorption centers around 226 nm (see Figure 2), Etot= 126.5 - 74 + 3RT = 54 kcal/mol, which is almost the same as the energy released in the O(3P) CH3CzHr e a ~ t i o n : ~ , ~
+
CH,C,H
o(3~ t)
.+
CH,CH=CO?
-+
co t
CH,CH
(6)
In this reaction the CO was found to be vibrationally excited up to u = 5 with a vibrational temperature of about 2400 K which is very close to that of the CO produced in the present dissociation reaction (Figure 6). The energy diagram for the photodissociation at 226 nm as well as that for the 0 CH3CzHreaction is shown in Figure 7. The energetics of the ground states of both CH,CHCO and CH&H (triplet) are based on the above estimates and those of other low-lying states (given by the dashed lines) are qualitatively assigned according to Basch's recent theoretical estimates for CH2C0.17 The excited states of CH3CH are not shown because they are not known. Theoretical estimates for the heats of formation of the two lowest electronic states of the CH3CH radical, as in the case of CH2,vary considerably. Some of the calculated values have been briefly summarized in our recent paper on the O(3P) 1-and 2-butyne reaction^.^ The most interesting fact revealed in the present study thus far is the close similarity between the CO vibrational energy distribution observed in the photodissociation (at 226 nm) and that previously determined for the 0 +
+
+
Photodissociation of C3H40
The Journal of Physical Chemisrty, Vol. 82, No. 13, 1978 1459
CH3C2Hreaction. The latter reaction probably takes place via a triplet intermediate (if the spin conservation rule holds at all for this rather complex system) giving rise to the triplet CH&H product. Since the available energies for product excitation in both reactions are the same, the present results seem to indicate either that the ground triplet CH3CH is formed directly in the photodissociation reaction resulting from a rapid intersystem crossing from the excited singlet (labeled as S2 in Figure 7) to the neighboring triplet states or that the singlet-triplet energy difference (or splitting) is small, e.g., -5 kcal or less, so that the CO formed in both reactions carries about the same amount of vibrational energy. The results of several theoretical calculations12~18J9 do indicate that the splitting might be small. The observed CO vibrational energy distributions in both reactions 1 and 6 compare closely with those predicted by simple statistical calculations. The solid curves I and I1 given in Figure 6 were calculated from the following expression:'J'
N , / N o = WEto, - E,)/=(Eto3 where Nu and E, are, respectively, the population and vibrational energy of CO at the vth level, and ZP(E),the total energy level sum of the dissociating complex at energy E. This was evaluated by means of the simple Whitten-Rabinovitch approximation.20 Curves I and I1 were computed with Etot= 122 and 54 kcal/mol, respectively, using the tight model as described before in ref 7. Clearly the excitation with the full amount of energy (Eb,= 126.5 - 74 68 3RT = 122 kcal/mol), assuming C2H4 instead of CH3CH is formed, predicts a much greater extent of CO vibrational excitation. Etot= 54 kcal/mol, however, gives rise to a very close fit with the experimental distribution. The result of a prior statistical calculation based on the method of Levine and Bernstein,21 as extended by Bogan and SetserZ2(shown by the dotted curve), also agrees reasonably with the experimental data. This model allows the reaction energy to be statistically distributed among the translational, rotational, and vibrational degrees of freedom of both CO and CH3CH. In this calculation, similar to the above one, the possible effect of the conservation of angular momentum on energy disposal is ignored. The fact that the experimental and statistically predicted CO distributions closely agree makes it most likely that in both systems the available reaction energy has effectively been randomized within various degrees of freedom of the dissociating complex during the course of its decomposition. The possibility cannot be excluded, however, that the agreement is only fortuitous, as could happen if the total available energy differed from 54 kcal/mol and complete energy randomization did not take place. (11)Methylketene and Acrolein Dissociation Near 200 nm. The photochemistry of both methylketene and acrolein below 220 nm is also not known. The absorption of methylketene in this region increases significantly as the wavelength decreases as indicated in Figure 2. The spectrum has more distinct peaks and structure than that of acrolein in the same spectral region. Acrolein begins to absorb strongly near 220 nm and peaks at about 193 nm. The products of photodissociation of both compounds are very similar, they are principally CO, CzH4, and C2H2with lesser amounts of C3 and C4 hydrocarbons. More CzH2as compared with C2H4 was obtained from CH3CHCH0 as might be expected because of its strong absorption a t shorter wavelengths.
+ +
In this spectral region, the CO's produced from the dissociation of both isomers have approximately the same vibrational temperature and are distinctly hotter than that formed in the dissociation of methylketene at the longer wavelength (226 nm). In the case of methylketene, the observed CO distribution was not affected by the addition of O2as well as by the variation in flash energy and initial reactant concentration. Again, this indicates that the distribution given in Figure 4 is free from secondary reactions. The relative rate, R2/R5, is expected to be even larger in the present higher energy region. The vibrational energy distribution of the CO formed in both reactions below 220 nm is significantly different from the statistical one that predicts the same average vibrational energy (E,) as given in Table 11. Take acrolein for example. A statistical calculation based on Ebt = 75 kcal/mol, derived by fitting (E,) = 3.3 kcal/mol, predicts a much colder CO distribution above u = 5. The nonstatistically of the observed CO distributions in these two reactions may result from the occurrence of a new reaction channel which generates vibrationally hotter CO in this spectral region. In view of the similarity between the two systems, the hotter CO may be formed via the following more exothermic reaction routes. CH,CH=C=O
+ hv ( h ~ 2 2 n0m )
N -+ ----+
-+
C,H, t COT
acrolein
CH,=CHCHO t hv ( h < 2 2 0 nm)+
n
CH,-CH-CYO* F. ---+
.+
C,H,
+ COT
methylketene
In this higher energy region, hydrogen atom migration may become more rapid and thus might compete more favorably with the C=C bond breaking described earlier. The occurrence of these more exothermic reactions, although only taking place to a rather small extent, may affect the CO distribution observed at higher vibrational levels which accounts for less than a few percent of the total population in both cases. Other rearrangement processes involving the migration of the oxygen atom and/or the methyl group (in the case of methylketene), although possible, may require higher activation energies.23 The above mechanisms can explain the similarities in the CO vibrational energy as well as in the product distributions observed in both systems. This hypothesis could be tested by an experiment employing a short, monochromatic laser pulse (such as an ArF laser at 193 nm) as a light source for both product and CO vibrational distribution measurements. Further experiments along this line are planned.
Conclusion The photodissociation of methylketene and acrolein near 200 nm has been investigated using a CO laser probing and conventional gas analysis methods. The dissociation of methylketene in a Vycor tube (A -226 nm) generates CO with a Boltzmann vibrational temperature of about 2300 K. The CO was found to have a vibrational energy distribution similar to that detected in the O(3P)+ CHBC2H reaction which could be accounted for by simple statistical models, assuming the production of CH3CH instead of C2H4 in the initial reaction. The dissociation of both isomers below 220 nm produced CO with similar, but nonstatistical vibrational energy distributions. A mechanism involving H-atom migration in both systems is postulated; this mechanism invokes the production of C2H4
1460
D. Raiem and W. H. Hamill
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
+
CO, in addition to the major, but less exothermic CH3CH + CO channel.
Acknowledgment. The authors are grateful to Dr. John C. Cooper for making the absorption measurements for methylketene and acrolein. References and Notes (1) G. 8. Kistiakowsky and B. H. Mahan, J. Am. Chem. SOC.,79, 2412 (1957). (2) D. P. Chong and G. B. Kistikowsky, J. Phys. Chem., 68, 1793 (1964). (3) H. M. Frev, J . Chem. Soc.. 2293 (1962). (4) C. A. Arrihgton, W. Brennen, G. P. Glass, J. V. Michael, and H. Nlkl, J. Chem. Phys., 43, 525 (1965). (5) J. M. Brown and B. A. Thrush, Trans. Faraday Soc., 63, 630 (1967). (6) P. Herbrechtsmeier and H. G. Wagner, 2.Phys. Chem. (Frankfurt am Main), 93, 143 (1974). (7) M. C. Lin, R. G. Shortridge, and M. E. Umstead, Chem. Phys. Lett., 37, 279 (1976).
(8) M. E. Umstead, R. G. Shortridge, and M. C. Lin, Chem. Phys., 20, 271 (1977). (9) M. E. Umstead and M. C. Lin, Chem. Phys., 25, 353 (1977). (10) M. C. Lin and R. G. Shortridge, Chem. Phys. Lett., 29, 42 (1974). (11) R. G. Shortridge and M. C. Lin, J . Chem. Phys., 64, 4076 (1976). (12) A. D. Jenkins, J . Chem. SOC.,2563 (1952). (13) See references quoted in ref 9. (14) V. H. Dibeler, M. Krauss, R. M. Reese, and F. N. Harllee, J . Chem. Phys., 42, 3791 (1965). (15) W. A. Chupka, J . Chem. Phys., 48, 2337 (1966). (16) R. L. Nuttall, A. H. Laufer, and M. V. Kilday, J. Chem. Thermodn., 3, 167 (1971). (17) H. Basch, Theor. Chim. Acta, 28, 151 (1973). (16) J. A. Altman, I. G. Csizmadia, and K. Yates, J . Am. Chem. Soc., 96, 4196 (1974). (19) V. Staemmler, Theor. Chim. Acta, 35, 309 (1974). (20) G. 2. Whitten and B. S. Rabinovkch, J. Chem. Phys., 41, 1883 (1964). (21) R. D. Levine and R. B. Bernstein, Acc. Chem. Res., 7, 393 (1974). (22) D. J. Bogan and D. W. Setser, J . Chem. Phys., 64, 586 (1976). (23) I. G. Csizmadia, H. E. Gunning, R. K. Ctosavi, and 0. P.Strausz, J. Am. Chem. Soc., 95, 133 (1973).
Activated and Activationless Localization and Impurity Trapping of the Dry Electron in Methanol and Propanol. Rate Constants for Solvated Electrons Dusan Raiem’ and William H. Hamill” Department of Chemistry and Radiation Laboratory,* University of Notre Dame, Notre Dame, Indiana 46556 (Received January 16, 1978) Publication costs assisted by the United States Department of Energy
Dry-electron trapping by impurity competes with localization by the medium. Observations in the range 80-300 K for methanol and 1-propanol provide evidence that each of these processes has activated and activationless components, most clearly shown for methanol. The activationless processes dominate below 150 K. They are considered to resemble electron resonances in the gas phase, the energy being supplied by the electron, as for benzene. The disordered medium converts potential energy of electronic polarization to kinetic energy by scattering. The rate constants for scavenging solvated electrons do not correlate with dry-electron scavenging efficiencies over the range - 5 X 106-5 X lo8 M1 s-l, and only correlate poorly with the enthalpy of activation. For each scavenger the rate constants change systematically from solvent to solvent.
-
Introduction An adequate examination of the dry electron, e-, in molecular systems must include consideration of its localization by the medium. This event is the beginning of the relaxation process which terminates with a solvated electron, e;, and provides an inescapable intersection for theories of the two regimes. The problem is classic, beginning with Landau’s mechanism (the electron digs its own hole) of which Mott and Gurney3 observed that, despite its sound theoretical basis, there was no experimental evidence that it occurred. (That statement is still valid.) Instead, an electron in the conduction level of an ionic crystal is captured a t a preexisting anion vacancy, of which there are very few. In the literature of radiation chemistry, the solvated electron, as described for polar media, bears considerable resemblance to the F center. The mechanism of dry electron localization a t polarized vacancies is common to theories of the solvated electron. Clearly, more information is required concerning the delocalized, or dry, electron in disordered polar solids and liquids and its localization mechanism. Yields of e; in undoped polar media decrease rather little from -300 to -77 K. Since localization competes with recombination and the latter is expected to be ac0022-365417812082-1460$01.OO/O
tivationless, the activation energy of the former cannot be large, but it is not zero. It is now established that for several undoped media there is little or no temperature dependence below 100-150 K. For example, the yield of e, in ethanol is the same at 4 K as at 77 K.4 Consequently, there must also be an activationless mechanism. Electron trapping by impurities also changes only fourfold between 4 and 77 KV5 Both doped and undoped systems were discussed r e ~ e n t l y . ~ , ~ Hunt and his collaborators8have found that scavenging e- and e; for some systems in water and alcohols are related by eq 1where C37is the concentration of scavenger hes-C37 = Q (1) required to reduce the population of e- t o 37% of its original value. In terms of rate constants for localization and scavenging of e- and assuming simple competition, eq C37 = klo,[solvent]/he(2) 2 is ~ b t a i n e d .Combining ~ eq 1 and 2 gives eq 3.
he- = hes-kl,, [solvent]/&
(3)
Hunt’s studies were almost entirely limited to systems for which he; > lo9 M-l s-l. For the systems examined in 0 1978 American Chemical Society
