4632
J. Phys. Chem. 1988, 92, 4632-4636
in determining uwiP)values, and one should expect that a more complete treatment of substituent effects on IJkk values than the present work would take this issue into account. Similar arguments regarding substituent effects on the effective nuclear charge, and its effect on energy eigenvalues, could be made in terms of the Aejk values of eq 6. Of course, in a SCF-MO calculation with a suitably extended basis set, this issue is accounted for automatically in the calculation of the eigenvalues and eigenvectors. In any case, this issue is considered to be beyond the scope or level of sophistication of the interpretations of the present paper. As stated above, the interpretation of 13C a k k data in terms of simple chemical and physical concepts is increasingly difficult for carbon atoms closer to the substituent, as additional factors, such as inductive effects, field effects, and neighbor anisotropy effects, must be considered. For this reason specific discussion of the C2, C3, C4, C5, and C6 data is not included here.
nuclei, for which dipolar corrections are extremely difficult, at best. There is a clear need for new and improved techniques to supplement or replace the SSB and 2D FT flipper approaches, so that accurate Ukk determinations can be made on sufficiently varied types of samples in sufficiently varied contexts to permit a good assessment of the extent to which the determination of ell, uzz,and u33indeed provides a much more useful level of information than the measurement of simply qwvalues. The measured substituent effects on I5N and l3C IJkk values of para-substituted benzonitriles do not correlate with traditional reactivity parameters, probably reflecting the much different characteristics and dependences on fundamental structural properties of chemical shift and reactivity parameters. The gross structural dependences and patterns of substituent trends of Ukk values can be explained qualitatively, especially for I5N, in terms of Pople's MO perturbation theory of shielding.
Conclusions The principal elements of the chemical shift tensors obtained for "N and 13Cin a variety of para-substituted benzonitriles show a wide range of substituent-dependent values and present a variety of experimental difficulties. For one-line spectra (e.g., 15N in the present case) the static powder pattern provides the most reliable approach for determining ukkvalues. For more complex, multiline cases, either the 2D FT flipper technique or the spinning sideband (SSB) MAS approach can be used, each with its advantages and disadvantages. For a large number of closely spaced isotropicaverage peaks, it can be difficult or impossible to obtain clearly defined and nonoverlapping SSB patterns. The 2D FT flipper approach circumvents this problem but suffers from signal-to-noise difficulties. By extrapolation it appears that, for highly complex spectra, the 2D flipper approach may be the only currently available technique that is applicable. All of these methods can suffer from the effects of dipole-dipole interactions with nearby nuclei; this problem is especially serious for nearby quadrupolar
Acknowledgment. We gratefully acknowledge support of this research by National Science Foundation Grant CHE-861015 1 and use of the Colorado State University Regional N M R Center, funded by National Science Foundation Grant CHE-86 16437. We also are very grateful to Prof. Paul Ellis of the University of South Carolina for providing a copy of his computer program for analyzing spinning sideband data and to Prof. A. K. Rappe and J. Iwamiya for assistance with the molecular orbital calculations. Registry No. p-MeOC6H4CH0, 123-1 1-5;p-Me2NC6H4CH0,10010-7; P - O ~ N C ~ H ~ C H555-16-8; O, p-NCCsH&HO, 105-07-7; pClC6H4CH0, 104-88-1; p-BrC6H4CH0, 1122-91-4; p-FC6H4COCI, 403-43-0;p-MeC6H4COC1,874-60-2;p-(Me)3CC6H4COCl,1710-98-1; p-FC6H4CONH2, 824-75-9; p-MeC6H4CONH2, 619-55-6; p (Me)3CC6H4CONH2,56108-12-4; p-methoxybenzonitrile, 874-90-8; p-dimethylaminobenzonitrile,1197-19-9;p-nitrobenzonitrile,619-72-7; p-dicyanobenzene,623-26-7;p-chlorobenzonitrile,623-03-0;p-bromobenzonitrile, 623-00-7;p-fluorobenzonitrile, 1 194-02-1;p-methylbenzonitrile, 104-85-8;p-tert-butylbenzonitrile,4210-32-6;p-cyanophenyltrimethylammonium iodide, 17311-01-2.
Infrared Laser-Induced Reaction of Ethyl Isocyanide: Comparison with Methyl Isocyanide L. M. Yam, M. J. Shultz,* Elizabeth J. Rock,+ and Susanne Buchaut Department of Chemistry, Tufts University, Medford. Massachusetts 02155 (Received: December 29, 1986; In Final Form: March 4, 1988)
Results from the infrared laser-induced reaction of ethyl isocyanide indicate the overall behavior to be similar to that of methyl isocyanide with some significant differences. Similarities include the following: (1) both exhibit a dramatic dependence of nitrile yield on reactant pressure, including a threshold pressure above which massive isomerization occurs; (2) this threshold pressure is not due to a thermal explosion; (3) both contain a radical channel along with the isomerization channel. Major differences between the methyl isocyanide and ethyl isocyanide reactions are attributable to the nature of the radicals in the particular isocyanide system.
In the past decade, interest in infrared laser-induced chemical reactions centered on the quest for mode selectivity. This interest was sparked by work on laser isotope separation in the 1970s.'~* In much of this work it is common to find a strongly increasing yield with fluence. However, it is much less common to find an increase with pressure, particularly in a substrate that does not show evidence of saturation. The most dramatic example of an increasing yield with pressure is the IR laser-induced reaction of methyl i ~ o c y a n i d e . ~ - ~ This work extends the previous methyl isocyanide investigation to ethyl isocyanide, which contains nine additional vibrational 'Department of Chemistry, Wellesley College, Wellesley, MA 02181.
0022-365418812092-4632$01.50/0
degrees of freedom. These extra vibrations will illustrate the effect of increasing the density of states on the reaction. Note that the activation energy (38.24 kcal/mol)6 and enthalpy of reaction (1) Ambartzumian, R. V.; Letokhov, V. S.; Rayabov, E. A,; Chekalin, N.
V. JETP Lett. (Engl. Transl.) 1974, 20, 273.
(2) Lyman, J. L.; Jensen, R. J.; Rink, J.; Robinson, C . P.; Rockwood, S.
D.Appl. Phys. Lett. 1975, 27, 87.
(3) Shultz, M. J.; Rock, E. J.; Tricca, R. E.; Yam,
L. M. J. Phys. Chem.
1984,88, 5157. (4) Shultz, M. J.; Tricca, R. E.; Yam, L. M . J. Phys. Chem. 1985,89, 58. (5) Shultz, M. J.; Tricca, R. E.; Berets, S. L.; Kostas, C.; Yam, L. M . J. Phys. Chem. 1985, 89, 3113. ( 6 ) Maloney, K. M.; Rabinovitch, B. S. J . Phys. Chem. 1969, 73, 1652.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 16, 1988 4633
Reaction of Ethyl Isocyanide (-21.5 kcal/mol)' for the thermal isomerization of ethyl isocyanide are similar to those for methyl isocyanide (38.4 and -23.70 kcal/mol, respectively)?*8 Since the additional CH2 group in ethyl isocyanide has little effect on these thermal parameters, the two systems can be directly compared. As in the methyl isocyanide r e a c t i ~ n , the ~ - ~laser-induced reaction of ethyl isocyanide is strongly pressure dependent; above a critical pressure of ethyl isocyanide, over 85% of the reactant is consumed following irradiation with a single pulse. Below this threshold, less than 0.1% conversion per pulse is observed. For comparison of this reaction to the laser-induced reaction of methyl isocyanide, experiments that were conducted for methyl i ~ o c y a n i d e ~were - ~ also conducted for ethyl isocyanide, with as little change in the reaction conditions as possible. Thus, the characterization of the ethyl isocyanide reaction includes the following: (1) determination of the nature of the threshold (that is, is it a thermal explosion?); (2) determination of the relationship between the threshold and the average level of excitation of the reactant; (3) examination of the products. Results show that a radical channel exists in conjunction with the isomerization channel and that it becomes more significant with increasing pressure and fluence. The propagating step for the radical channel C2H5.
+ C2HSNC
+
C2HsCN
+ C2H5.
Experimental Section The experimental setup described previously3 was used with minor modifications. A 9-mm CaFz attenuator was used to prevent damage to the Scientech energy meter. The C 0 2 laser was operated on the 9P(30), 1037.43-cm-' transition. As in our earlier work, the laser was operated with a lean mixture, giving a fwhm of 60 ns with little tail. This wavelength excites a vibration that has been assigned as the v9 C-C-N skeletal-in-phase stretch of ethyl isocyanide." The cylindrical Pyrex cells used for most of the experiments are 2.5-cm long by 2.5-cm i.d. For the Beer's law determination, the cell dimensions are 25-cm long by 2.5-cm i.d. The following were used for product analysis: a HewlettPackard Model 5750 gas chromatograph interfaced to an IBM Instruments Model CS9000 microcomputer; an IBM Instruments Series 85 FTIR spectrometer; a Hitachi-Perkin Elmer Model RMU-6 mass spectrometer. Ethyl isocyanide was synthesized according to the method of Casanova et a1.I2 with the modifications that were reported for (7) Baghal-Vayjooee, M. H.; Collister, J. L.; Pritchard, H. 0. Can.J . Chem. 1977,55, 2634. (8) Schneider. F. W.: Rabinovitch, B. S. J . Am. Chem. SOC.1962, 84,
(9) Shaw, D. H.;Dunning, B.; Pritchard, H. 0. Can. J . Chem. 1969,47, 669. (10) Shaw, D. H.; Pritchard, H. 0. Can. J . Chem. 1967, 45, 2749. (11) Bolton, K.;Owen, N. L.; Sheridan, J. Spectrochim. Acto, Port A 1969, 25A, 1. (12) Casanova, J., Jr.; Schuster, R. E.; Werner, N. D. J. Chem. Soc. 1963, 4280.
a
01
7
11
9
13
15
17
PRESSURE (torr) Figure 1. Subthreshold yield of ethyl cyanide versus ethyl isocyanide pressure (100 pulses per point). The indent fluence is 1.56 f 0.13 J/cm2, and the threshold pressure is 19.3 f 0.7 Torr. A
1
0.25
A
0.15 -
++
A
Massive lsometlzatlon Yield > 85% per wlse
0.10 -
p'
0.05 -
-
'
?A
Yield c 0.1% per pulse
(1)
was previously postulated in work conducted by Pritchard and co-worker~.~~'~ Comparison with the laser-induced reaction of methyl isocyanide reveals important differences, e.g.: (1) Under similar excitation conditions, the threshold pressure for ethyl isocyanide is lower than that for methyl isocyanide. (2) The threshold for ethyl isocyanide is more sensitive to the average level of excitation than is the case for methyl isocyanide. These differences can be attributed to the greater stability of ethyl radicals in the ethyl isocyanide isomerization relative to the stability of methyl radicals in the methyl isocyanide reaction. This paper is set out as follows: the first section deals with the experimental setup; the second section describes the results; the third section discusses the results in relation to the laser-induced reaction of methyl isocyanide; the fourth section is a summary.
421's.'
a
0
'
2 I
I
1
1.5
,
2
3 I
2.5
Fluence (J/crn2) Figure 2. Inverse pressure versus average number of photons absorbed
per pulse for ethyl isocyanide. The solid line represents the boundary between the region of low yield and the region of massive isomerization. The ordinant is also given as the incident fluence to facilitate comparison of the data in this figure to that of the other figures. TABLE I: Threshold Behavior of Ethyl Isocyanide at Constant Incident Fluence for Different Beam Areas area of beam, cm2 threshold, Torr fluence, J/cm2 0.667
0.412
* 0.005 * 0.005
13.6 f 0.3 13.4 f 0.3
1.68 f 0.10 1.68 f 0.10
the preparation of methyl i~ocyanide.~ From gas chromatographic analysis, the product obtained was better than 99.0% pure.
Experimental Results Threshold Behavior. The laser-induced reaction of ethyl isocyanide exhibits a dramatic dependence of ethyl cyanide yield on pressure. For an incident fluence of 1.56 f 0.13 J/cm2, the threshold for massive conversion is 19.3 f 0.7 Torr. That is, below 19.3 Torr, the ethyl cyanide yield is extremely small, less than 0.1% per laser pulse, while above 19.3 Torr, over 85% of the ethyl isocyanide isomerizes. Figure 1 shows that the subthreshold yield rises slowly before massive isomerization takes place. To determine if the threshold is due to a thermal explosion, we examined the threshold behavior as a function of beam diameter. The theoretical background for this experiment has been documented el~ewhere.~Briefly, if the threshold is due to a thermal explosion, the threshold pressure should depend strongly on the beam diameter. Table I indicates that, for an incident fluence of 1.68 J/cm2, the threshold is 13.5 Torr, regardless of the size of the irradiating beam. Hence, as with methyl isocyanide, the threshold for ethyl isocyanide is not due to a thermal explosion. Figure 2 illustrates the relationship between the threshold pressure and the average level of excitation. Increasing the average
The Journal of Physical Chemistry, Vol. 92, No. 16, 1988
Yam et al. +
A
methane
b
ethane
1.50 r A
I
A
A
+ methane
1 -
ethane
A A
0.50
1
0 00
ethylene
A H C N
8
49 I O l 1.50
I
* +
-
L 2
2.50
3
G! 30
25
20
15
10
35
INCIDENT FLUENCE (J/crn2) INITIAL WESSURE (torr) 0
ethylene
0
Em, CYande
*I 25 1.50
0
1.20 -
0
0
0
0.80-
0
e
2
0
2.50
0
3
0.40
b
INCIDENT FLUENCE (J/crn2)
Figure 3. Product partial pressure versus fluence. The initial pressure of ethyl isocyanide is 30.2 f 0.1 Torr: (a) methane, ethane, ethylene, and HCN; (b) ethyl cyanide.
level of excitation (incident fluence) results in a lower threshold pressure. Radical Channel. As with methyl i ~ o c y a n i d e , ~the J ~ laser-ind u d reaction of ethyl isocyanide gives small quantities of products other than its isomer. The nature of these other products indicates the presence of a radical channel. They make up, collectively, about 5 7 % of the initial pressure of ethyl isocyanide and consist of, in decreasing amounts, hydrogen cyanide, ethylene, ethane, and methane. In addition, there is a nonvolatile product that is observable when a high-pressure sample (>30Torr) is irradiated. Upon irradiation, a faint white cloud, similar to that observed in the reaction of methyl i ~ o c y a n i d e , ~appears -'~ at the center of the cell and spreads outward. The results of the analysis of all the products as a function of incident fluence are shown in Figure 3, where the initial reactant pressure is 30.2 f 0.1 Torr. As the incident fluence increases, the amounts of HCN, CH4, C2H4,and C2Hsincrease, while the amount of ethyl cyanide decreases. The amount of unreacted ethyl isocyanide is less than 3% of its initial pressure. In Figure 4 the effect of initial reactant pressure on the amount of several products is shown for an average incident fluence is 2.22 f 0.10 J/cm2. As the reactant pressure increases, the relative yields of HCN, CH,, and C2H4 increase, while that of C2H6 decreases. The ratios of the pressures of unreacted ethyl isocyanide and of ethyl cyanide to initial reactant pressure remain constant.
Discussion The overall reaction behavior of ethyl isocyanide is very similar to that of methyl i~ocyanide.~-~ Both systems exhibit a threshold pressure that is not due to a thermal explosion, and both have a radical channel in addition to the isomerization channel. However, a closer examination of the two isocyanide reactions, particularly the conditions at the threshold, reveals important differences. With the same average level of excitation, the threshold pressure for ethyl isocyanide is lower than that for methyl isckyanide. For example, an average level of excitation of 2.5 photons/molecule (13) Tricca, R. E. Ph.D. Thesis, Tufts University, 1985.
01
10
20
15
30
25
35
INITIAL PRESSURE (torr)
+
HCk
A
ethyl isocyanide
10 i 1
61 + + +
2 10
+
*
+ c
+ 15
20
25
30
35
INITIAL PRESSURE (torr) 0
ethyl cyantoe
90.
0
85
0
0
0
0
I
d
1
80, 10
'5
20
25
30
35
INITIAL PRESSURE (torr)
Figure 4. Yield as the ratio of product pressure to initial reactant pressure versus initial reactant pressure. The average incident fluence is 2.22 f 0.10 J/cm2. The reactant pressure is known to &l%: (a) methane and ethane, (b) ethylene, (c) HCN and ethyl isocyanide, (d) ethyl cyanide.
The Journal of Physical Chemistry, Vol. 92, No. 16, 1988 4635
Reaction of Ethyl Isocyanide
r
I
a
o.201 0'40
-; .
0.30
I
7
J
2.50-
K
1
IU
. 5 aJ
21.50 0
I
I i ' o
0.50 -
Q 10
15
20
K
O
PRESSURE (torr)
Figure 5. -In (J/Jo)versus ethyl isocyanide pressure. The incident fluence is 1.78 0.10 J/cm2.
*
results in a threshold pressure of 18.9 Torr for methyl isocyanide! Ethyl isocyanide, on the other hand, exhibits a threshold pressure of 5.3 Torr for the same average level of excitation. This finding is unexpected and argues against a thermal explosion. If the threshold was a result of a thermal explosion, then a lower initial temperature would require a higher pressure to reach the critical condition. (See ref 3 for further discussion of a thermal explosion.) Since the heat capacity of ethyl isocyanide is greater than that of methyl isocyanide (from 300 to 800 K, the heat capacity for ethyl isocyanide ranges from 17.3 to 32.7 cal/(mol K), while that for methyl isocyanide ranges from 12.83 to 21.34 cal/(mol K),I4 ethyl isocyanide would require a greater absorption of laser energy to produce the same initial temperature rise. Thus, for the same initial reactant pressure and for the same level of excitation, the temperature of the ethyl isocyanide would be lower. The threshold for ethyl isocyanide then should be higher than that for methyl isocyanide. From a purely unimolecular point of view, the density of states of ethyl isocyanide is greater than that of methyl isocyanide for the same level of excitation, so that the length of time the energy stays within a particular mode is shorter. Again, this would require ethyl isocyanide to absorb a larger amount of laser energy to attain sufficient excitation in the reaction mode. Hence, for the same pressure, the threshold fluence should be higher for ethyl isocyanide. Since threshold pressure and threshold fluence are inversely related, for the same average level of excitation, this reasoning predicts a higher threshold pressure for ethyl isocyanide. Further comparison of the threshold behavior of methyl isocyanide and ethyl isocyanide indicates that the ethyl isocyanide threshold is much more sensitive to the average level of excitation than the methyl isocyanide threshold is. This sensitivity is reflected in the slope of a graph of l/threshold pressure versus the average level of excitation ( n ) (see Figure 2 and ref 4). An equation that describes the behavior in Figure 2 is 1/Pth = (0.137)(n) - 0.160 where P t h is the threshold pressure. Since the slope for the analogous graph for methyl isocyanide4 is 0.026, it is evident that the ethyl isocyanide threshold varies much more with a change in photons absorbed than does the methyl isocyanide threshold. Because the absorption cross section of methyl isocyanide is not enhanced with p r e s s ~ r e , ~a ,rotational l~ hole-filling mechanism for ethyl isocyanide was investigated as a possible explanation for the above-mentioned differences between the two isocyanide reactions. Figure 5 shows that the greater reaction sensitivity of ethyl isocyanide cannot be attributed to saturation since under laser excitation ethyl isocyanide follows Beer's law with an extinction coefficient of 7.33 X lo4 cm-' Torr-', in agreement with the linear extinction coefficient. The adherence to Beer's law and agreement with the linear extinction coefficient indicate that an increase in the number of collisions neither aids nor inhibits energy absorption.
(3) increases with pressure. The results from the ethyl isocyanide reaction are consistent with this: (1) there is an increase in both ethane and ethylene concentrations with fluence (Figure 3a); (2) ~
(14) Bcnson, S.W.; Cruickshank, F. R.; Golden, D. M.; Haugen, G.R.; ONeal, H. E.;Rodgers, A. S.;Shaw, R.; Walsh, R. Chem. Reu. 1969, 69,
279.
(15) Gray, P.; Herod, A. A. Faraday SOC.Trans. 1968, 64, 2723. (16) Shultz, M.J.; Yam, L. M.; Berets, S. L., unpublished results. (17) Pacey, P. D.;Wimalasena, J. H. J . Phys. Chem. 1984, 88, 5657.
J. Phys. Chem. 1988, 92, 4636-4640
4636
threshold to changes in the level of excitation. These differences are attributable to the nature of the radicals involved in the particular isocyanide reaction. Further work on the ethyl isocyanide system involves a more extensive characterization of the radical channel. Experiments include the addition of a known ethyl radical generator, as well as the monitoring of the reaction in real time to obtain kinetic information on product formation. The addition of appropriate radical scavengers to the system is also being undertaken.
there is an increase in the ratio of ethylene to ethane with reactant pressure (Figure 6). Ethylene and ethane may also be produced and consumed in secondary reactions. Therefore until the radical channel is further characterized, the production of ethylene and ethane, as well as of methane and HCN, cannot be solely attributed to any particular reaction( s).
Summary The laser-induced reactions of ethyl isocyanide and methyl isocyanide are very similar. In both reactions, there is a pressure threshold above which nearly complete isomerization occurs in a single pulse. Furthermore, in neither case is this threshold the result of a thermal explosion. Both reactions contain a radical component that increases with reactant pressure and with incident fluence. The main differences between the two isocyanide reactions are the lower threshold at a given level of excitation for ethyl isocyanide and the greater sensitivity of the ethyl isocyanide
Acknowledgment for partial support of this work is made to the National Science Foundation (Grant No. Rll-8503811, M.J.S. and L.M.Y.), to the Research Corp., and to the Brachman Hoffman fund, Wellesley College (S.B.). We also thank Mike Crimmins for valuable assistance and discussions. Registry No. C2H5NC,624-79-3; C2H,CN, 107-12-0;C2H5*,202556-1.
Luminescence Decays and Spectra of Ru(bpy):+ the Presence of Water Vapor
Adsorbed on TiO, in Vacuo and in
K. Hashirnoto,t M. Hirarnoto,t T. Kajiwara,f§ and T. Sakata*>' Institute for Molecular Science, Myodaiji, Okazaki 444, Japan, and Department of Chemistry, Faculty of Science, Toho University, Miyama, Funabashi 274, Japan (Received: June 15, 1987)
The luminescence of Ru(bpy)?+ (bpy = bipyridyl) was quenched strongly on Ti02,and its decay rate was increased remarkably due to electron transfer from the excited Ru(bpy)?+ to Ti02. The decay curve was nonexponential and was fitted well by using the sum of four exponentials. The time-resolved spectra of the faster decay components were shifted to shorter wavelength by about 500 cm-' compared to those of the slower ones. On introduction of water vapor into the system, the decay rate of the fastest component became slower, indicating that the electron-transfer interaction of the Ru complex interacting most strongly with Ti02 was weakened by the solvation effect of water. When the vapor pressure exceeded the saturated value, the blue shift of the faster decay components disappeared and the time-resolved spectra of the faster components became almost the same as those of the slower ones.
was purchased from Furuuchi Chemical Corp. and was used as received. The particles were allowed to stand in contact with a water solution of Ru(bpy),2+, where the ratio of Ti02 particle to Ru complex weights was controlled, and then dried by evacuation ( Torr) at 50 O C for a few days. The surface coverage was calculated approximately by assuming that Ru(bpy)$+ is a sphere with a radius of 5 A and is adsorbed uniformly on the Ti02surface. The dried surface modified particles were set in an optical cryostat (Oxford Instruments Ltd., type CF1104) at room temperature in vacuo (lo+ Torr). Water vapor was introduced into the cryostat from a glass vacuum line in which degassed liquid water was trapped. The pressure was controlled with a needle valve, and pressures higher than the saturated water vapor pressure were obtained by heating the liquid water trap. R ~ ( b p y ) adsorbed ~~+ on T i 0 2 in liquid water was prepared as follows. The Ru-
Introduction Electron-transfer processes between molecules in homogeneous solution have been widely studied, and remarkable advances have recently been achieved both theoretically' and experimentally.2 On the other hand, the electron transfer between molecules and semiconductors has been less well studied, although it is an essential process in photoelectrochemistry, photocatalytic reactions, photography, etc. To get information on its dynamic process, we have measured the decays and spectra of the luminescence from photoexcited semiconductor3 or photoexcited adsorbed We previously reported that the luminescence of Ru(bpy)$+ and its derivatives adsorbed on various oxide semiconductors is quenched by electron transfer from the excited state of Ru(bpy)? to Ti02 both in vacuo4 and in various solvent^,^ and the electron-transfer rate in solvents is much slower than in vacuo. In the present study, detailed analyses of decay curves of R ~ ( b p y ) , ~ + in vacuo and effects of water vapor on the luminescence decay curves and time-resolved spectra were studied.
(1) For example: (a) Kestner, N. R.; Logan, J.; Jortner, J. J. Phys. Chem.
1974, 78,2148. (b) Marcus, R. A.; Siders, P.Ibid. 1982,86,622. (c) Miller, J. R.; Beitz, J. V.; Huddleston, K. J. Am. Chem. SOC.1984, 106, 5057. (d) Kakitani, T.; Mataga, N. Chem. Phys. 1985, 93, 381, and references therein. (2) For example: (a) Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J. Am. Chem. SOC.1984, 106, 3047. (b) Closs, G. L.; Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R. J. Phys. Chem. 1986,90, 3673, and references
Experimental Section R ~ ( b p y ) , ~was + purchased from Strem Chemicals Inc. and was sometimes purified by several recrystallizations from water, but no difference in the results was observed between purified and nonpurified samples. Ti02powder (rutile, ca.0.5 fim, ca. 10 m2/g)
therein. (3) Hiramoto, M.; Hashimoto, K.; Sakata, T. Chem. Phys. Lett. 1987,133, 440. (4) Kajiwara, T.; Hashimoto, K.; Kawai, T.; Sakata, T. J. Phys. Chem. 1982, 86, 4516. ( 5 ) (a) Takemura, H.; Saji, T.; Fujihira, M.; Aoyagui, S.; Hashimoto, K.; Sakata, T. Chem. Phys. Letf. 1985, 122, 496. (b) Hashimoto, K.; Hiramoto, M.; Sakata, T.; Muraki, H.; Takemura, H.; Fujihira, M. J . Phys. Chem. 1987, 91, 6198.
Institute for Molecular Science. *Toho University. f Deceased.
0022-3654188 , ,12092-4636$01.50/0 0 1988 American Chemical Societv I
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