J. Phys. Chem. 1989, 93, 8193-8197 Other Third Bodies The effect of other third bodies, M, aiding ionic recombination via Xe2+
+ SF6- + M
-
XeF*
8193
with unit efficiency upon the energy of the ionic system being collisionally reduced below zero. The experimental values at lower rare gas pressures are not accurately modeled by the low/medium-pressure Bates termolecular recombination theory. The predicted rate constants are far lower than observed experimentally and the calculated maxima occur at much higher gas pressures. Previous applicable extensions of the Bates theory fail to give any significant improvement in qualitative or quantitative agreement when applied to this gas system. The experimentally determined rate coefficient pressure profiles are shown to parallel the oxygen ionic recombination system when a large fraction of mutual neutralization enhancement assists the termolecular recombination process. In the Xe/SF6 system, this process corresponds to charge neutralization by an F ion abstraction from SF6- by Xe2+,to give the fluorescent exciplex. The fractional enhancement is estimated to be large, with up to -60% of ionic recombination proceeding by this pathway. Registry No. Xe2+,14066-95-6;SFC,25031-39-4.
+ products
was also investigated. It was found that there was no effect on the recombination rate constant when the SF6 gas pressure was varied from 0.01 to 5.00 Torr, at a constant xenon pressure of 20.0 Torr, or when helium (0-100 Torr) was added to a 100.0 Torr of Xe/0.50 Torr of SF6gas mixture. This indicates that both these gases are poor third bodies, relative to xenon, for recombination under these conditions.
Conclusion The agreement between experiment and theory for ionic recombination in pulse electron irradiated Xe/SF6 gas mixtures at very high rare gas pressures is excellent. The exciplex is seen to be formed by diffusion-controlled ionic recombination processes, as described by the Langevin-Harper model, and to recombine
Stationary and Pulsed Photolysis and Pyrolysis of 1,1-Dimethylsilacyclobutane+ Th. Brix, N. L. Arthur,$ and P. Potzinger*?$ Max- Planck- Institut fur Strahlenchemie. Stiftstrasse 34-36, 0-4330 Mulheim a.d. Ruhr. FRG (Received: March 2, 1989; In Final Form: June 20, 1989)
A study of the photolysis of 1,l-dimethylsilacyclobutaneat 147-214 nm shows that of the four primary processes identified
. -
the predominant mode of decomposition is to C2H4 and dimethylsilaethene. Evidence from experiments in the presence of SF6 suggests that the dimethylsilaethene is formed initially in a vibrationally excited state: Me2SiCHzCH2CH2+ hv Me2SiCH2"+ CH2 = CH2. Laser pulsed photolysis experiments at 193 nm have been carried out to measure the absorption spectrum of Me2SiCH2, its absorption cross section, and the rate constant for Me2SCH2combination: 2Me2SiCH2 (Me2SiCH2)2.The values obtained are u (240 nm, base e ) = (1.0 0.2) X cm2 and k7 = (3.3 0.8) X IO-'' cm3 s-I. The kinetics of the pyrolysis of Me2SiCH2CH2CH2have also been reexamined, yielding the following rate constant exp(-(31043 218)/7') and k-l/k71/2/(~m3/2 d2) = 10-7.0*0.3 exp(-(7850 f 3 O O ) l T ) . expressions: kl/(s-') = 1015.46*0.'3 From these results, the heat of formation, *-bond energy, and entropy, of Me2SiCH2,have been deduced: AH,8(g, 298 K) = 36 f 7 kJ mol-', B, = 157 11 kJ mol-', and SB(g,298 K) = 332 8 J mol-' K-I.
.
*
-
*
*
Introduction Silaethenes have been the subject of much interest in recent years, and a large amount of data, especially on dimethylsilaethene, Me2SiCH2,has accumulated.' Most of the data are spectroscopic and thermochemical in nature, and few quantitative kinetic data have been reported. Me2SiCH2dimerizes readily to tetramethyldisilacyclobutane, DSiCB, in pyrolysis experiments, and the combination rate constant has been measured by Guselnikov and co-workers.2 From data obtained in flow experiments, they extracted, in a rather indirect way, the value k7 = 6.6 X cm3 s-I, which they found to be independent of temperature. As has been shown by W a l ~ h , ~ this value, being so low, is incompatible with presently accepted thermochemistry and with the results of Flowers and Guselnikov4 on the pyrolysis of dimethylsilacyclobutane, SiCB. Walsh3 did not abandon this value of the rate constant, altogether, however, suggesting that it was loaded with an appreciable activation energy. This contradicts conclusions drawn from experiments carried out in this laboratory. In an investigation of the photolysis of tetrameth~lsilane,~ we concluded that both the 'In memoriam of 0.E. Polansky. *Permanent address: Chemistry Department, La Trobe University, Melbourne, Victoria 3083, Australia. 1 Permanent address: Max-Planck-Institut fur Stromungsforschung, Bunsenstrasse-10, D-4300 Gottingen, FRG.
0022-3654/89/2093-8193$01.50/0
*
combination of Me2SiCH2 and radical addition to Me2SiCH2 proceed with negligible activation energy. Carrying this result over to our studies of the Hg-sensitized photolysis of tetramethylsilane6 and he~amethyldisilane~ we obtained, in each case, a high value for the a-bond energy (190 f 20 kJ mol-') in Me2SiCH2,in agreement with theory.* If we had assumed that combination of, and addition to, Me2SiCH2involved an activation energy, we would have obtained a much higher value for the a-bond energy. In order to resolve these inconsistencies, we decided to remeasure the rate constant for Me2SiCH2 combination in a pulsed laser experiment with Me2SiCH2being generated by the long-wavelength photolysis of SiCB. It has been shown that SiCB decomposes thermally4 and p h o t ~ c h e m i c a l l y ,mainly ~ ~ ~ ~ ' ~to Me2SiCH2and C2H4. In pyrolysis (1) Raabe, G.; Michl, J. Chem. Reu. 1985, 85, 419. (2) Guselnikov, L. E.; Konobeyevsky, K. S.; Vdovin, V. M.; Nametkin, N., S. Dokl. Akad. Nauk.SSSR, 1977, 235, 1086. (3) Walsh, R. J . Phys. Chem. 1986, 90, 389. (4) Flowers, M. C.; Guselnikov, L. E. J . Chem. SOC.B. 1968, 419, 1396. (5) Bastian, E.; Potzinger, P.; Ritter, A,; Schuchmann, H.-P.; von Sonntag, C.; Weddle, G. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 56. (6) Potzinger, P.; Reimann, B.; Roy, R. S. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 1119. (7) Davidson, I. M. T.; Potzinger, P.; Reimann, B. Ber. Bunsen-Ges.Phys. Chem. 1982, 86, 13. ( 8 ) Ahlrichs, R.; Heinzmann, R. J . Am. Chem. SOC.1977, 99, 7452.
0 1989 American Chemical Society
8194
The Journal of Physical Chemistry, Vol. 93, No. 25, 1989
TABLE I: Relative no. C2H4 I 1.0 2 1.0 3 1.0 1.0 1.0 1.0 1.0 4 1.0 1.0 1.0 1.0 1.0 5 1.0 6 1.0 7 1.0 1.0 1.0 1.0 1.0 1.0
Brix et al.
Product Yields of SiCB Pbotolysisasb
DSiCB
SiOSi
ADMSi
TMVSi
c-C3H6
0.13 f 0.03 0.36 0.21 0.18 0.20 0.18 0.21 0.17 0.17 0.14 0.17 0.12 0.0 0.10 0.17 0.31 0.37 0.38 0.42 0.39
0.05 f 0.02 0.03
0.017 i 0.003 0.023
0.004
0.005
0.003
0.005
0.017 0.017 0.023 0.019 0.01 0.003 0.008 0.026 0.035 0.059 0.068
0.004 0.004 0.003 0.0
0.005 0.003 0.004
0.03 0.05 0.03 0.09 0.01 0.05 0.0 0.06 0.04 0.05 0.05 0.07
0.005 0.03 0.007
0.005 0.005 0.006 0.006 0.005 0.005
exptl conditn 185 nm. rt. 1 Torr of SiCB. 100 Torr of He 185 nm, rt, 1 Torr of SiCB; 100 Torr of N, 147 nm, rt, 1 Torr of SiCB, 100 Torr of He 175 nm 185 nm 206 nm 214 nm 0 Torr of He, rt, 1 Torr of SiCB, 185 nm 13 Torr of He 100 Torr of He 270 Torr of He 710 Torr of Xe 480 Torr of air, rt, 1 Torr of SiCB, 185 nm 330 OC, 185 nm, 1 Torr of SiCB, 100 Torr of He 0.0 Torr of SF6, 185 nm, rt, 1 Torr of SiCB 2.7 Torr of SF, 20.5 Torr of SF, 5 1.7 Torr of SF6 504 Torr of SF, 754 Torr of SF,
Conversion in all experiments 510%. *Abbreviations used: SiCB = (CH3)2SiCH2CH2CH2r DSiCB = (CH3),SiSi(CH,),, SiOSi = (CH3)3SiOSi(CH3)3, ADMSi = (CH3),HSiCH,CH=CH2, TMVSi = (CH3)3SiCH=CH2,rt = room temperature. experiments, the combination of Me2SiCH2to DSiCB appears to be quantitative, but in photolysis experiments, this is far from being the case. The conclusion to be drawn from this discussion is that the photochemical decomposition of SiCB is not yet fully understood. Since an understanding of the system is a conditio sine qua non if it is to be used for kinetic measurements, we have reinvestigated the stationary photolysis of SiCB. In order to make our results mutually consistent and to reduce error limits, we have also reinvestigated the pyrolysis of SiCB.
0.2
7
0
I
Experimental Section The laser pulsed photolysis apparatus was of conventional design and has been described elsewhere." The photolyzing light, 193-nm radiation from an excimer laser (Lambda Physik Model EMG IOO), enters a Pfund cellI2 at 90" to its long axis. Intermediates are monitored by absorption of light from a D2 lamp, which passes through the cell three times and is then focused onto the entrance slit of a monochromator (Minuteman Model V302 UV). The dispersed light is detected by a photomultiplier (EMR Model 541). The signal from this passes through a current to voltage converter (EG&G Model 184) and a preamplifier (EG&G Model 1 13) and is then fed to a digital oscilloscope (Gould Biomation Model 4500). The data are transferred to a microcomputer (Atari Model 520 ST) for further processing. Stationary photolyses were carried out at five different wavelengths in the range 147-214 nm. For the three wavelengths 147, 175, and 206 nm, microwave-powdered lamps of either the sealed or flow type were used, while for the two wavelengths 185 and 214 nm a low-pressure mercury arc (Grantzel Type 5) and a Zn hollow-cathode lamp (Philips Model 93106 E) were utilized. Pyrolysis experiments were performed in a cylindrical quartz vessel (1 30 cm3). The temperature used in the experiments ranged from 370-500 "C. The vessel was connected to a membrane manometer (MKS-310) and two separate vacuum lines that allowed fast charging and discharging of the reactor. Photolysis and pyrolysis products were analyzed by GC (Carlo Erba; glass capillary column, 64 m; OV1, FID). SiCB was of commercial origin and was purified by preparative GC; all the other gases (He, Xe, N2,SF6 (Messer-Griesheim 3.0)) were used as obtained. (9) Tokach, S.;Boudjouk, P.; Kwb, R. D.J . Phys. Chem. 1978.82, 1203. (IO) Low, H. C.; John, P. J . Organomet. Chem. 1980, 201, 363. ( I !) Brix, Th.; Bastian, E.; Potzinger, P.J . Photochem. Photobiol. A, submitted for publication. (12) Pfund, A. H. Science 1939,90, 326.
0.1
h. 02
[DSiCBI
[CZH~I
Figure 1. Dependence of the DSiCB and Me3SiOSiMe3yield on each
other.
Results and Discussion Stationary Photolyses. Our results for the stationary photolysis of SiCB under different experimental conditions are summarized in Table I. At 185 nm, which is close to the laser line employed in our pulsed photolysis experiments, decomposition according to (1) is by far the most important process. Three other minor
-
Me2SiCH2CH2CH2 Me2SiCH2 + C2H4
(1)
channels were also identified: decomposition to dimethylsilylene and C3H6,previously reported in ref 9, the isomerization either to allyldimethylsilane (ADMSi) or vinyltrimethylsilane (VTMSi). The formation of ADMSi and VTMSi can be thought of as occurring by S t C and C-C bond rupture respectively, followed by a 1,3-hydrogen shift. A similar process has been observed in the thermal decomposition of 1,1,3-trimethylsila~yclobutane.~~ In agreement with earlier s t u d i e ~ , the ~ J ~DSiCB does not account quantitatively for the Me2SiCH2formed. The values given in experiment 1 of Table I are average values over more than 10 runs. In some experiments, e.g., no. 2, we noticed that the DSiCB yield lies far outside the given error limits. There are strong indications that impurities are responsible for this behavior. As can be seen from Figure 1, the yields of DSiCB and Me,SiOSiMe,, SiOSi, are dependent on each other. We attribute this to the scavenging of MezSiCH2by varying amounts of water present as an impurity. We do not think, however, that impurities (or the presence of radicals as in other systems5) are the sole reason (13) Guselnikov, L. E.; Nametkin, N. S.; Dolgopolov, N. N. J. Organomet. Chem. 1979,169, 165.
Photolysis and Pyrolysis Study of Me2SiCH2CH2CH2 for the low DSiCB yields. Under no circumstances were we able to obtain a stoichiometric DSiCB yield under low-pressure conditions. Koob and co-workers9 observed only trace amounts of DSiCB in their 147-nm photolysis. In light of the results of Low and John,Io this could be interpreted in terms of an increasing vibrational excitation of Me2SiCH2 with decreasing photolysis wavelength. No such wavelength dependence was observed in our experiments, however, and we conclude that it is likely that low light intensities coupled with reactive impurities were responsible for the low DSiCB yield found in ref 9. We also found that there was no dependence of the DSiCB yield on the rare-gas pressure up to 1 bar (Table I, no. 4); only ADMSi seems to increase with increasing pressure. The presence of rather large amounts of O2 has no influence on the C2H4 yield; DSiCB disappeared completely, as expected, and c-C3H6increased in agreement with earlier r e s ~ l t s . ' ~It has a very peculiar effect on the isomerization reaction, however. The process producing ADMSi is not affected at all while the formation of VTMSi is completely quenched. This must mean that VTMSi is formed via a long-lived intermediate, either a triplet state or a long-lived biradical. The DSiCB yield is also not improved by an increase in temperature, in agreement with earlier findings2v5 and contrary to the speculation of Walsh3 alluded to in the Introduction. None of the experiments described so far give a conclusive answer as to the reasons for the different behavior of photochemically and thermally generated Me2SiCH2. We can, however, say that the existence of an activation barrier to the combination of Me2SiCH2can be definitely excluded and also that the case for the formation of Me2SiCH2with excess vibrational energy is, at first sight, not strong. In our experiments, the mean lifetime of Me2SiCH2is about 10 ms, which means that it undergoes lo8 collisions in the presence of 1 bar of inert gas. Under these conditions, even a weak collider should have ample time to remove the excess energy. This argument may not hold, however, in the event that a small excitation of one particular vibrational degree of freedom in Me2SiCH2occurs. Because, as is well-known, the probability of removing small vibrational quanta by collision is low, especially in the case of collision partners with no internal degrees of freedom, ring closure of the 1,Cdiradical initially formed may be prevented and polymerization would take place instead. This model would also account for why the DSiCB yield appeared to be independent of pressure and photolysis wavelength. To investigate this idea further, we therefore chose a larger polyatomic molecule as the moderator for the excited Me2SiCH2 with immediate success. As can be seen from Table I, in no. 7, even at very low pressures of SF,, the measurable Me2SiCH2products increased to 75%, and this figure rose to 92% in the presence of larger amounts of SF,. Experiment no. 7 also emphasizes once again the completely different formation modes of ADMSi and VTMSi. The rate of formation of the former is pressure dependent as already suspected from experiment no. 4 while the latter is not. Our results together with those of George and KoobI4 suggest the tentative mechanism shown in Scheme I. Photochemically activated SiCB undergoes predominately internal conversion to the ground state. The vibrationally highly excited SiCB molecules break apart either via Si-C or C-C bond rupture. The energetically favored C-C diradical decomposes further to CzH4 and vibrationally excited Me2SiCH2. In the case of the Si-C diradical, decomposition is slow enough that collisional interference is possible. For the deactivated diradical, two paths are open: ring closure back to the reactant or ADMSi formation by H transfer. To a minor degree, SiCB undergoes intersystem crossing to the triplet state where it decomposes again by either Si-C or C-C bond rupture. But contrary to the ground-state reaction, Si-C radicals decompose to cyclopropane or propene and dimethylsilylene while triplet C-C diradicals undergo H transfer to give VTMSi and are long lived enough to be scavenged by 02. (14) George,
C.;Koob, R. D.Organomet. 1983, 2, 39.
The Journal of Physical Chemistry, Vol. 93, No. 25, 1989 8195 SCHEME I. Primary Processes in the Photolysis of SiCB
1
I
I
I
I
I
I
1
220
230
240
250
260
270
280
290
h/nm
Figure 2. Absorption spectrum of Me2SiCH2.
--
The mechanism may be summarized in a less detailed manner by the following equations:
+
Me2SiCH2CH2CH2 hv Me2Si:
-+
--
-
Me2SiCH2'
+ C2H4
+ (CH2)3
(1')
(2)
Me2SiHCH2CH=CH2
(3)
Me3SiCH=CH2
(4)
--+
(5)
2Me2SiCH,'
Me2SiCH2SiMe2CH2
Me2SiCH2' + M
+
Me2SiCH2 M
2Me2SiCH2 DSiCB Me2SiCH2 H 2 0 Me3SiOH +
2Me3SiOH
Me3SiOSiMe3
+ H20
(6)
(7) (8) (9)
Absorption Spectrum of Me2SiCH2. An absorption spectrum of Me2SiCH2 in a matrix has been reported by Maier and co-
The Journal of Physical Chemistry, Vol. 93, No. 25, 1989
8196
Brix et al. of Flowers and Guselnikov on the thermal decomposition of SiCB, values for some important thermochemical parameters can be deduced. The thermal decomposition is described by the three reactions m Me2SiCH2CH2CH2s Me2SiCH2 C2H4 (1,-1)
L 15 m .w
-
C
3
-
fi .
2Me2SiCH2
w
3.98 1
2
3
L
5
6
7
8
9
t/ms
+
DSiCB
(7)
Flowers and Guselnikov deduced the following quantities from their experiments: k , / ( s - ] ) = 1015.64i0.30 exp(-(31465 f 403)/T)
Figure 3. Deuterium light intensity as a function of time.
workers.15 Our gas-phase spectrum, which is an average over three spectra, was obtained with a mixture of 1 Torr of SiCB and 55 Torr of He and is shown in Figure 2. It agrees well with those given in ref 15 except for the shoulder at 275 nm. The shoulder, which we consider to be due to unresolved vibrational fine structure, is more pronounced in individual spectra, but fluctuations in peak position lead to its being washed out in the averaging process. The absorption cross section was calculated from the C2H4 yield, which was determined by GC. The value obtained cm2. was u (240 nm, base e ) = (1.0 f 0.2) X Rate Constant of Me#iCHz Combination. A reaction mixture of 1 Torr of SiCB and 55 Torr of He was photolyzed by 193-nm laser pulses at room temperature. The time dependence of the 240-nm monitoring light intensity is shown in Figure 3. The curve, an average over 256 runs, was evaluated by a least-squares analysis on the assumption that the disappearance of Me2SiCH2 was second order. On this basis, we obtained 2 k 7 / d = 68500 f 1000 s-I, where k7 is the second-order rate constant, u is the absorption cross section at 240 nm, and I is the absorption light path. When combined with 1 = 97.5 cm, this gives k 7 / u = (3.34 f 0.05) X lo6 cm s-I, and if u is taken to be (1.0 f 0.2) X cm2, k7 = (3.3 f 0.8) X 10-l' cm3 s-'. This value is almost 4 orders of magnitude larger than that reported by Guselnikov et aL2 It ,~ agrees, however, with the A factor calculated by W a l ~ husing the data of Flowers and Guselnikov4 together with an estimate of the entropy change for reaction 1. The agreement of the calculated A factor and experimental rate constant supports our contention that the combination reaction has little or no activation energy. There remains the possibility that the vibrational excitation noticed in the stationary photolysis experiments is responsible for the large rate constant. We therefore added 7.5 Torr of SF6 to the reaction mixture, but this had no effect on the rate constant. Increasing the SF6 pressure to 57 Torr led to an increase in the rate constant by a factor of 2. This unexpected result was first thought to be due to the Occurrence of a pseudo-first-order reaction of Me2SiCH2. Reevaluation of the data on the basis of a mixed first-second-order reaction scheme revealed that only a small contribution was made by the first-order reaction, however. There is, of course, the possibility that in foregoing experiments the high-pressure limit was not attained. This seems unlikely to us. We have also considered the involvement of reactions with impurities, but our stationary photolysis results argue against this being a significant factor. At present, this matter remains unresolved. We interpret our results the following way: The combination of two Me2Si=CH2 molecules is not activated; a vibrational excitation inhibits ring closure but not the first step, the formation of the diradical. That the combination reaction has no activation energy barrier is also implied by matrix experimentsi6 in which it was found that dimerization sets in at the temperature 40 K, where translational motion becomes possible. Thermochemical Implications. When the combination rate constant, k7, determined in this work, is combined with the results (15) Maier, G.; Mihm, G.; Reisenauer, H. P.Chem. Eer. 1984, 117, 2351. (16) Maltsev, A. K.; Khabashesku. V.N.; Nefedov, 0. M. J. Organomet. Chem. 1984, 271, 5 5 .
L/(cm3/2 k7'l2
s-'/~) = 10-7.09*1,2 exp(-(7250 f 2014)/7')
If k7 is available from independent experiments, the equilibrium constant K( 1,-1) can be evaluated and from it the heat of formation, the a-bond energy, and the entropy of Me2SiCH2can be deduced. Unfortunately, the error limits associated with the given by the results activation energy difference EA(l)- 1/2EA(7) of Flowers and Guselnikov4 are so large that it was not possible to narrow down the presently known values of the abovementioned thermochemical quantities. We therefore decided to repeat the experiments of Flowers and Guselnikov4 over an extended temperature range. Pure SiCB was pyrolyzed in the temperature range 646-774 K. Our results deviated from those of ref 4 in that, besides the two main products, we also observed a small amount of allyldimethylsilane and a higher molecular weight product, to which we tentatively assign the structure of a cyclic trimer. The formation of this trimer is temperature dependent: at the lowest temperature employed, it accounts for about 20% of all the Me2SiCH2formed, while at the highest temperature its contribution is only a few tenths of a percent. The material balance is satisfactory over the whole temperature range if the trimer is taken into account. From the data we obtained kl/(s-]) = 10'5.46i0.'3 exp(-(31043 f 218)/7'), which is in good agreement with the previous value. Experiments with added C2H4 were done in the same temperature range. A mixture of 20:l C2H4/SiCB was used in the hightemperature range and a 1O:l mixture in the low-temperature range. The data were evaluated by computer simulation on the basis of the three-step mechanism, reactions 1, -1, and 7. We obtained the following expression for the relative rate constant: k/(cm3/2 k7II2
s-'/~) = 10-7.0i0,3exp(-(7850 f 300)/T)
Again, the agreement is good, but the error limits are considerably reduced. Taking EA(7) = 0 f 4 kJ mol-], we obtain AHRB((l),700 K) = EA(1) - EA(-l) + RT, = (258.1 f 1.8) - (65.3 f 2.5) (0 f 2.0) + 5.8 = 199 f 4 kJ mol-'
+
Assuming = 4.5 f 0.5 J mol-' K-' from the structural analogous reaction of 1 ,1-dimethylcyclobutane, we arrive at AHRB((l), 298 K) = 197 f 4 kJ mol-' With the known heat of formation of SiCB,I7 we get for AH:( Me2SiCH,)
AHf((CH3),SiCH2) = (197 f 4)
+ (-108.7
f 6.0) -52.3 =
36 f 7 kJ mol-' Our value agrees with most of the published values within their rather large error limits but is distinguished by much higher precision. The same is true for the s-bond energy, B,, which can be calculated by the thermochemical cycle
+
AHRB(lO)= B,(Me2SiCH2) DHB(H-H) DHB(Me2HSiCH2-H)- DHB(Me3Si-H) (17) Steele, W . C. Unpublished results.
J. Phys. Chem. 1989, 93, 8197-8203
-
where AHRB( IO) is the enthalpy of MezSiCHz
+ Hz
Me3SiH
(10)
which, in turn, can be calculated from known heats of formation A H R B ( I O ) = (-163 f 4)18 - (36 f 7) = -199 f 8 kJ mol-' All the bond dissociation energies are known with the exception of DHB(Me,HSiCHz-H), which is taken to be equal to DHe(Me,SiCH,-H) = 415 f 5 k J / m 0 1 . ~ ~Thus
+
+
B,((CH3),SiCH2) = (-199 f 8) - 436 (415 f 5) (377 f 5),O = 157 f 11 kJ mol-' This value lies in the lower part of the range of published values but agrees very well with a more recent value for the ?r-bond energy of H2Si=CH2.21-22 We can also make use of the A factors of the measured rate constants. With 4 7 ) = k,, we obtain log (A(-l)/(cm3 s-I)) = -12.2 f 0.3 (18) Doncaster, A. M.; Walsh, R. J . Chem. Soc., Faraday Trans. 2 1986, 82, 707. (19) Doncaster, A. M.; Walsh, R. J . Chem. Soc., Faraday Trans. 1 1976, 72, 2908. (20) Walsh, R. Acc. Chem. Res. 1981, 14, 246. (21) Shin, S.K.; Irikura, K. K.; Beauchamp, J. L.; Goddard, W. A., 111. J . Am. Chem. Soc. 1988, 110, 24. (22) Schmidt, M. W.; Truoung, P. N.; Gordon, M. S.J . Am. Chem. Soc. 1987, 109, 5217.
8197
and ASe((l), 700 K) = 175 f 7 J mol-' K-l (standard state, 1 atm) At 298 K, we get with = 4.5 f 0.5 J mol-' K-I ASe((l), 298 K) = 171 f 7 J mol-' K-' This value is in good agreement with the value of 168 J mol-' K-I for the structurally analogous reaction of 1,l-dimethylcyclobutane.23 Guseinov et al.24have obtained values for the entropy of SiCB, So(],298 K) = 279.6 f 1.6 J mol-' K-I, for the entropy of vaporization ASva, (355.9 K) = 90.3 f 2.1 J mol-' K-I, and for its f 1.0 J mol-I K-I. These heat capacity CJl, 298 K) = 197.6values together with an estimate of C,(g), allow the calculation of Se(g, 298 K). Taking CJg) = 132 f 4 J mol-' K-', we obtain Se(g,298 K) = 381 f 5 J mol-' K-I. This value, when combined with the reaction entropy ASe(l), gives P(g, 298 K) = 332 f 8 J mol-' K-' for MezSiCH2. Registry No. SiCB, 2295-12-7; DSiCB, 1627-98-1; SiOSi, 107-46-0; ADMSi, 3937-30-2; TMVSi, 754-05-2; Me2SiCH2, 41 12-23-6; CzH4, 74-85-1. (23) Benson, S.W.; Cruickshank, F. R.; Golden, D. M.; Haugen, G. R.; O'Neal, H. E.; Rodgers, A. S.; Shaw, R.; Walsh, R. Chem. Reu. 1969, 69, 279. (24) Guseinov, Z . A.; Karasharli, K. A.; Dzhafarov, 0. I.; Nurullaev, G. G.; Nametkin, N. S.;Guselnikov, L. E.; Volnina, E. A.; Burdasov, E. N.; Vdovin, V. M. Dokl. Akad. Nauk. SSSR., 1975, 222, 1369.
Magnetic Field Effects on the Decay Rates of Photogenerated Geminate Radical Pairs in Reversed Micelles' Akihiro Uehata, Hiroshi Nakamura, Satoshi Usui,and Taku Matsuo* Department of Organic Synthesis, Faculty of Engineering, Kyushu University, Fukuoka 81 2, Japan (Received: June 9, 1989)
Laser excitation (351 nm) of zinc tetraphenylporphinate-viologen pairs in reversed micelles (AOTIisooctane or n-hexane) afforded triplet radical pairs in a cage, and the decay was remarkably retarded in external magnetic fields (0-1 T). In the case of the porphyrin-viologen linked compound, the radical decay rate decreased by an order of magnitude to reach a plateau region at above 0.3 T. Exactly the same behavior was also observed with nonlinked porphyrin-viologen pairs, which consisted of amphiphilic or cationic porphyrin and a zwitterionic viologen, in a small water pool (ca. 10 A in radius) of the reversed micelles. The large external magnetic field effects on the linked (or pseudolinked) radical pairs were explained on the basis of Zeeman splitting of the triplet in combination with electron spin relaxation from the sublevels (relaxation mechanism), and supporting evidence was provided by the use of paramagnetic lanthanide ions. For larger water pools, the magnetic field effects on the decay rate of nonlinked radical pair rapidly decreased, and the diffusion-controlled process was suggested to be dominant.
Introduction Photoinduced electron transfer and charge separation steps have been one of the most important research subjects for the photochemistry and its related fields. Various surfactant molecular assemblies have been known to aid photoinduced charged separation., Particularly useful information has been obtained with (1) Contribution No. 904 from Department of Organic Synthesis, Faculty of Engineering, Kyushu University. (2) (a) Fendler, J. H. Chem. Reu. 1987,87, 877 and references therein. (b) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic: New York, 1986. (c) Ohsako, T.; Sakamoto, T.; Matsuo, T. J . Phys. Chem. 1985,89, 222. (d) Matsuo, T.; Nagamura, T.; Sakamoto, T. Mukromol. Chem. Suppl. 1985,14,119. (e) Thomas, J. K. J . Phys. Chem. 1987, 91, 267. (f) Hurst, J. K.; Thompson, D. H. P. J . Membr. Sci. 1986, 28, 3. (g) Hurst, J. K.; Lee, L. Y. C.; Gratzel, M. J . Am. Chem. SOC.1983, 105, 7048.
0022-365418912093-8197$01.50/0
reversed micelles, which provide small water droplets of various sizes suspended in organic bulk phase.3 Kinetic features of electron-transfer (ET) processes at the microscopic phase boundary of the reversed micelles still remained to be clarified. Recent studies of external magnetic field effects (EMFES) on photoinduced ET and the succeeding reactions have revealed that EMFES on the decay kinetics of the photogenerated triplet radical pairs provide a useful means to investigate kinetic behaviors of the radical pairs in relatively loose ~ o n t a c t . ~ (3) (a) Pileni, M. P. Chem. Phys. Lett. 1981,81,603. (b) Willner, I.; Ford, W. E.; Otvos, J. W.; Calvin, M. Nature 1979, 280, 823. (c) Backer, C. A.; Whitten, D. G. J . Phys. Chem. 1987, 91, 865. (d) Brochette, P.; Pileni, M. P. N o w . J . Chim. 1985, 9, 551. (e) Laane, C.; Verhaert, R. Isr. J. Chem. 1987188, 28, 17.
0 1989 American Chemical Society
