10563
J. Phys. Chem. 1994,98, 10563-10567
Product-Yield-Detected ESR Study on the Dynamic Behavior of Radical Pairs Generated in Photoreduction of Acetylenic Ketones in SDS Micellar Solution Nicolai E. PolyakovJ Masaharu Okazaki,* and Kazumi Toriyama National Industrial Research Institute of Nagoya, Hirate, Kita, Nagoya, 462, Japan
Tatiana V. Leshina Institute of Chemical Kinetics and Combustion, Novosibirsk 630090, Russia
Yoshihisa Fujiwara and Yoshifumi Tanimoto Department of Chemistry, Faculty of Science, Hiroshima University, Higashihiroshima, 724, Japan Received: June 11, 1994@
Large magnetic-field and microwave effects were detected on the spin-adduct yield in the photoreduction of 1,5-diphenyl- 1,4-pentadiyn-3-0ne (I) and 1,3-dipheny1-2-propyn-1-one (11) in SDS micellar solution. These phenomena were interpreted with the “relaxation mechanism” for the intermediate radical pair. The dynamic behavior of the intermediate radical pair was elucidated by the pulse-mode product-yield-detected ESR (PYESR), in which the effect of microwave pulse on the spin adduct yield is observed. The rate constants for hydrogen abstraction, escape process of radical pair, and so on were successfully determined through the simulation of these microwave effects as functions of the delay period which is brought into the pulse sequences. The reactivity of excited ketones in the hydrogen abstraction increases significantly in the sequence I >> I11 > 11, where I11 represents benzophenone studied as a reference system. The results were compared with those estimated from the transient absorption observations.
Introduction A large number of studies have been carried out to establish the mechanism of magnetic field effect in the photoreaction of carbonyl compounds, such as anthraquinone, benzophenone, and so on, in micellar solution^.^-^^ The large MFE observed in these systems were successfully explained with the radical pair model, which was first presented for the explanation of CIDNP. The identification of the radical pair and the observation of its dynamics are, however, not easy since it is usually very difficult to detect the component radicals of the pair and the interaction between them at the same time. We proposed the spin trapping technique as a method to probe the radical pair which should appear as the transient intermediate in the photochemical reaction of carbonyl compounds in micellar solutions.12 Subsequently, we observed the ESR spectrum of the intermediate radical pair through detecting the effect of ESR transition of the radical pair on the yield of spin adduct of one of the components of the radical pair.l39l4 We have named this method product-yield-detected ESR, or PYESR in short. This method is analogous to fluorescence detected ESR or RYDMR (reaction-yield-detected ESR), in which recombination fluorescence is detected to obtain the effect of ESR transitions on the recombination rate of radical The PYESR apparatus has been extended to the pulse-mode operation, which enabled us to control these reactions by the microwave pulse irradiated in correlation with the laser pulse. 19t20 Recently, we presented a new version of the pulse-mode PYESR technique for determination of the kinetic parameters of radical pair,21922where the spin adduct yield is detected as a function of incremental delay in three different pulse sequences shown in Figure 1. In the present paper we apply this latest method for the kinetic + O n leave from Institute of Chemical Kinetics and Combustion, Novosibirsk 630090,Russia. Abstract published in Advance ACS Abstracts, September 15, 1994. @
PS I
PS 2
krq ...... ...... ...... ...... ...... ...... ...... ...... ......
PS 3
Laser
m.w. Figure 1. Three timings between the laser pulse and the microwave pulse for pulse-mode PYESR observation. A 20 ps microwave pulse was used for PS1 and PS2 and 50 ns for PS3.
study of a new series of diary1 ketones containing one or two acetylenic groups attached to the carbonyl group. According to the literature, the reactivity of ketone changes dramatically upon introducing the acetylenic group from those of benzophenone (111) or acetophenone, which are representative systems in p h o t o c h e m i ~ t r y . ~ To ~ - ~elucidate ~ the characteristics of the photochemistry of these interesting systems, the MFE on the spin adduct yield as well as the radical pair kinetics by means of pulse-mode PYESR were observed for 1,5-diphenyl- 1,4pentadiyn-3-one (I),1,3-dipheny1-2-propyn-1-one (11), and I11 0
II
Ph - C ~ C - C - C ~ C - P h
0
I1
Ph -CGC-C-Ph
I
I1
0
II
Ph-C-Ph
I11
in a SDS micellar solution. The kinetic parameters of the radical pairs were compared with those obtained by laser flash photolysis study for acetylenic ketones I and I1 in SDS and Brij 35 micelles.
0022-3654/94/2098-10563$04.50/0 0 1994 American Chemical Society
Polyakov et al.
10564 J. Phys. Chem., Vol. 98, No. 41, 1994 400 r
2
350
E2 300 a
250
. . .
.L 200 rA
.-e
t 4
150 100 50
.
0
200
300 400 A /nm
100
300
200
500
Figure 2. Absorption spectrum of ketones 1-111 in 0.2 M SDS solution. Concentration of ketones is 0.08 mh4 and the light path is 1.0 cm.
Experimental Section Sodium dodecyl sulfate (SDS) was purchased from Nakarai Chemicals (Kyoto) as the purest grade and used without further purifications. Benzophenone was from Wako Pure Chemicals (Tokyo) as guaranteed grade reagent and used as supplied. Acetylenic ketones (I and 11) were kindly supplied by Prof. A. S. Zanina of the Institute of Chemical Kinetics and Combustion, Novosibirsk (Russia)26and recrystallized from ethanol. Dibromonitrosobenzenesulfonate (DBNBS), a spin trap, was synthesized following the 1iteratu1-e.~’For PYESR and MFE experiments we employed the following concentrations for the reagents: SDS 200 mM, DBNBS 1 mM, ketone 0.08 mM. The concentration of DBNBS and the ketones was adjusted by monitoring the optical absorption spectrum. The absorption spectra of ketones 1-111 are shown in Figure 2. PYESR and MFE measurements were made with the following sequences at ambient temperatures (19-21 “C). The reactant solution degassed by Ar bubbling for 1 h was let flow into a quartz flat cell (thickness = 0.25 mm, width = 10 mm, length = 40 mm) in the ESR cavity (JEOL RElX) through a flow system and was irradiated with YAG laser at 1 = 355 nm (Spectra Physics, GCR 150/10). Laser power (30 mJ) and irradiation time (30 s, 10 Hz) were set in such a way that the spin adduct yield increases linearly with the UV dose. Spin adduct concentration was measured as a function of the incremental delay between the laser and the odoff time of the microwave pulse in the three Pulse Sequences (PS 1, PS2, and PS3) depicted in Figure 1. The changes in the spin adduct yield as the functions of the delay period were simulated by numerical integration of a set of differential equations for a reaction scheme (( 1)-(7)) by the Runge-Kutta method to obtain the kinetic parameters (hydrogen abstraction rate, recombination, and escape rates of radical pair).21,22A short microwave pulse was used for PS3 (50 ns) and a long one (20 ps) for PS1 and PS2. As for the MFE measurement, spin adduct concentration was measured as a function of the magnetic field, which is applied during the UV irradiation, and the value of magnetic field effect (MFE) was calculated as M ( H ) = (Z(H) - Z(O))/Z(O), where Z(H) is spin adduct signal intensity at the field of H. All the apparatus used, i.e., laser, pulse programmer (Iwatsu SY8220), and ESR spectrometer, were controlled by a personal computer (NEC PC9801). Further details of the apparatus for MFE and pulse PYESR experiments have been described in previous papers.l9sZ0 Transient absorption measurements were made in the following manner at ambient temperatures (ca. 24 “C). Ketones (0.2 mM) in SDS (0.4 M) or Brij 35 [poly(oxyethylene(23) lauryl ether)] (0.1 M) micellar solutions were deaerated by several freeze-pump-thaw cycles before the laser flash pho-
400
500
600
B /mT
Figure 3. Magnetic field effect on the spin adduct yield observed for the photolysis of acetylenic ketone I in 0.2 M SDS micellar solution in the presence of 1.0 mM DBNBS. The effect of ESR transition of the ketyl radical induced by the microwave pulse at 4 W appears at the resonance magnetic field of 334 mT. The solid line is the simulation curve (see text).
tolysis experiments, which were made in high (150 mT) or low (0 mT) magnetic fields. The pump and probe light sources were the third harmonics (335 nm, fwhm 8 ns, 110 &/pulse) of a YAG laser (Spectra-Physics, GCR-11-1) and a Xe arc lamp (Ushio, 150 W), respectively. The probe light was led to a 20cm monochromator (Ritsu, MC-20L) and detected by a photomultiplier (Hamamatsu, R928) and a digital oscilloscope (Tectronix, 2440). The data were processed by a microcomputer (NEC, PC-9801). INDO-MO calculation was made with Pople’s program2*(the dimensions were expanded) using a HP-730 workstation.
Results and Discussion Magnetic Field and Microwave Effects on Spin Adduct Yield. Figure 3 shows an example of the magnetic field effect (MFE) and product yield detected ESR (PYESR) response on the spin adduct yield for ketone I. It is remarkable that most of the increase in the spin adduct yield due to MFE is almost canceled by the ESR transition of the ketyl radical of the acetylenic ketone, which appears at around 334 mT in Figure 3. These large MFE and high PYESR amplitudes were also observed on the spin adduct yields for the other ketones I1 and 111. The existence of an a-hydrogen, which is clearly proved by the six-line ESR pattern of the spin adduct (the spectrum, very similar to those in previous s t ~ d i e s , ’ ~is, ~ not~ ,shown ~~ here), clearly shows that the trapped radical is an SDS radical and not the ketyl radical. The former should be produced from an SDS molecule through hydrogen abstraction reaction by the excited triplet state of ketone. The spin state of the excited ketone is conserved as the spin state of radical pair, which is clearly determined as the triplet state because the spin adduct yield decreases upon inducing the ESR transitions of one of the component radicals, i.e., SDS radical or ketyl radical, of the radical pair. Since the spin adduct concentration increases gradually with increasing the field up to 6 kG, the “relaxation m e c h a n i ~ m ” ~is~the - ~ main ~ mechanism for this MFE. The same mechanism was suggested in the literatures for the photoreduction of benzophenone, anthraquinone and other carbonyl compound^^-^^ in micellar solutions. The common scheme for the photoreduction of carbonyl compounds in the presence of a spin trap, which is added to probe the intermediate radical pair,12-14J9is given as follows (Scheme 1):
K
+ hv
.+
‘K*
-
3K*
(1)
Dyna&c Behavior of Radical Pairs
3K*
+ RH - 3(KH*'R)
3(KH"R) '(KHO'R)
-
-
'(KIC'R)
cage product
-'KH + + + ST ST
1s3(KH"R)
[kH]
(2)
[kIsc]
(3)
400
I
350
300
d
Ig3(KH''R) R'
J. Phys. Chem., Vol. 98, No. 41, I994 10565
[kR]
(4)
[kEsc]
(5)
[kT]
(6)
250
.g 200
'R
spin adduct
VI
.e
k
150 100
L
spin adduct
[ki]
(7)
rp
50
0
Here, IK* and 3K* in (1) are respectively the excited singlet and triplet states of ketone (K), the latter of which abstracts a hydrogen atom from a hydrogen donor RH (=SDS in the present case) to form a triplet radical pair (2). Intersystem crossing between the two spin states of the radical pair occurs both in a coherent way by static interactions and in an incoherent way by spin relaxation (3).29-32 According to .the spin conservation principle, cage product formation occurs only from the singlet radical pair (4). The free radicals escaped from the original micelle ( 5 ) may be captured again into another micelle, since the solubilities of the component radicals in bulk phase are not high enough. Therefore we put k~ = k ~ ' the , former and the latter of which are the rate constants for spin trapping of SDS radical escaped from the radical pair (6) and that forming the radical pair (7), respectively. An evidence for spin trapping directly from radical pair is that a significant decrease in the lifetime of radical pair is observed upon increasing spin trap concentration.21s22From this experiments the spin trapping rates were estimated to be 2.0 x lo8 M-' s-' under the present experimental setup. ST and KH' are the spin trap and the ketyl radical, respectively. The other symbols are self-explanatory. According to the "relaxation mechanism" for the magnetic field effect on chemical reaction, transition rates between the singlet S and the two triplet states T* are governed by the spinlattice relaxation: UT, = y2t$:/(1
4a
-I- y 2Bo2 t,2 )
0
200
100
300
400
500
600
B /mT
Figure 4. Model calculations of the magnetic field effect on the spin
adduct yield for the acetylenic ketone I system on the rotational correlation time: tc = 2.2 ns (a); 0.44 ns (b); 0.09 ns (c). Other parameters: k~ = lo7 s-'; kmc = 2 x lo5 s-l; k~ = 2 x los M-l s-l; k~ = lo7 s'; HL= 1.2 mT.
r
0
2
4
6
8
t / (us
In this equation zc is the rotational correlation time, HL the anisotropic magnetic field, and Bo the extemal magnetic field (w = yBo; the angular frequency of Larmor precession). At a field lower than the hyperfine coupling in the radical pair the coherent mixing between S and T51 levels occurs in addition to that between S and To. The latter is operative for all the magnetic fields since the exchange interaction in the present system is much less than the hyperfine interaction which comes in the range 0- 10 mT. It is a clear evidence for the relaxation mechanism that the observed effect induced by the magnetic fields in the range 0- 10 mT is almost negligible compared with that due to the increase in magnetic field from 10 to 600 mT. Therefore, it is possible to estimate the relaxation parameters through the simulation of experimental MFE curves, which can be made by integrating the differential equations made for the reaction steps (1)-(7) and eq 8. In the previous study we obtained the following relaxation parameters for the anthraquinone system:21*22 HL= 1.2 mT and z, = 0.44 ns. Figure 4 shows model calculations of M E for keton I with different values of zc = 2.2 ns (a), 0.44 ns (b), and 0.09 ns (c). As we can see, it's impossible to obtain a complete agreement with the experimental curves (Figure 3) in both low- and high-field regions simultaneously by using a single tc.There may be two reasons: (1) distribution of zc and (2) distribution of recombination rate, due to the inhomogeneity of micelle. On the other
Figure 5. Reductions in the spin adduct yield for, upper: acetylenic ketone I, lower: acetylenic ketone II, due to the microwave pulse irradiated in three modes: a, PS1; b, PS2; c, PS3 (see Figure l), as functions of delay period t. Lines are the simulated dependences using the parameters listed in Table 1.
hand, we obtained a complete agreement with the experimental result by superposing the curves a and c of Figure 4 at the ratio of 0.3 and 0.7, respectively. However, kinetic calculations in the later sections were made with z, of 0.44 ns, which is the best single parameter in simulating the magnetic field dependence. Kinetic Measurements. Figure 5 shows the decrease in spin adduct yield due to the ESR transition of the ketyl radical of the transient radical pair as functions of the delay period in the three pulse sequences shown in Figure 1. The upper and the lower diagrams are those for acetylenic ketones I and 11, respectively. Curve a and closed triangles represent, respectively, the calculated and observed increase of microwave effect by extending the irradiation period from the point of laser pulse. A large difference between these grow-up curves for ketones I and 11is due to a large difference in the hydrogen abstraction rate. Curve b and open circles show the decrease of microwave effect by delaying the start of microwave irradiation from the point of laser pulse. Although this curve usually indicates the decay process of radical pair in the T+ states, the slow decay
10566
Polyakov et al.
J. Phys. Chem., Vol. 98, No. 41, 1994
TABLE 1: Kinetic Parameters of Transient Ketyl SDS Radical Pair Obtained by the Simulation of Pulse-Mode PYESR for Ketones I-III in SDS Solutiona ketones k~ kTb kH k ~ s c tc(ns) HL(mT) 0.44 1.2 10 0.2 I 10 200 I1 I11
6.5
200
1.0
7.0
200
3.7
0.2 0.2
0.44 0.25
1.2
1.5
All rate constants are given in the units of lo6 s-l; experimental error 4~5%. In lo6 M-' s-l, obtained from the dependence on spintrap concentration. a
.-*
I
5
.-C 40
JLU ',
',
1
'8
\\\
'\
*
',
'h
-.
'\
'\
A
in curve b for I1 reflects also a slow hydrogen abstraction. Roughly speaking, curve c and open rectangles reflect the concentration of radical pair as the function of time. Theoretical simulations of the results shown as curves a-c were made by integrating the differential equations made for the reaction steps (1)-(7) as mentioned above. The transition between T* and S levels due to microwave is added in step (3).22 Since the agreement between the experimental results and the calculated ones are very good, our model for the reaction scheme is essentially correct. The calculated kinetic patameters of the radical pairs for ketones 1-111 are presented in Table 1. The most interesting fact is that hydrogen abstraction rate for I is 10 times as large as that for 11, and that for benzophenone is between these two values. Only a small difference is detected for the rate of recombination of radical pair. This may indicate that recombination occurs between the common components, Le., SDS radical and the ketyl radical. The equal values of the escape rate ( 2 x 105 s-I) for ail systems lead us to conclude that the main process for step (5) is diffusion-out of the SDS radical from the micelle. It is reasonable because the solubility of these ketones is so low in the aqueous phase that it remains in the micelle during the lifetime of the radical pair. In fact, when we add xanthene (whose solubility in water is very low) as the hydrogen donor, the lifetime of radical pair increases to about 1.5 times of that without xanthene. This is due to the fact that a large part of SDS radical is replaced with xanthene radical in the radical pair. The large difference in the reactivities of I and 11 may be due to the difference in nonbonding character of the unpaired electron orbital in the triplet state. Although we did not make MO calculations for the excited triplet state due to difficulty of this kind of calculation using a semiempirical method, some insight can be obtained even from the closed shell calculation. In the calculation the twisting angle (in the same direction) of the phenyl groups and the C-C(0)-C angle are set 15" and 130°, respectively. The standard values were employed for the bond lengths. The direction of the C=O bond is taken as y axis, and x and z axes are defined in and out of the molecular plane, respectively. Introduction of two acetylenic groups decreases the mixing between the 2Px orbital on the oxygen nucleus and the a-orbital of the phenyl groups in the nonbonding molecular orbital (l6bl for I). As the result, nonbonding character of the highest occupied bl orbital (non-bonding orbital) are calculated as 0.390, 0.354, and 0.610 by introducing zero, one, and two acetylenic groups, respectively. On the other hand, the reactivity of ketone should increase if the energy of the transient ketyl radical is lowered due to the existence of many resonance structures through the acetylenic groups. This means that the n-orbital (probably 6b2 for I) to which the nonbonding electron is transferred in the triplet state spread over the molecular flame. Therefore introduction of two acetylenic groups into the ketone frame causes a large increase in kH due to above two reasons. We do not have any reason to suppose different mechanisms, at least in SDS micellar solution, for the photoreduction of I and 11, such as hydrogen abstraction by
550 nm
370 nm
"- \
:',
t /us t /,us Figure 6. Decay kinetics of the transient absorption signals for acetylenic ketone I in 0.4 M SDS micellar solution aft& laser Gadiation. Left I = 370 nm in the absence (A) and the presence (B) of magnetic field (150 mT); right: I = 550 nm, no magnetic field dependence is observed.
the triple bond, because PYESR spectra for both ketone systems are almost identical and do not contain components of other radicals besides SDS and ketyl radicals. In the simulation quantum mechanical coherent S-To mixing is replaced with a classical fist-order rate equation. The validity of this assumption for the system with many nuclei with considerable hyperfine couplings has been confirmed.22 According to the quantum mechanical calculation with full consideration of the nuclei, the triplet population of the radical pair, originally produced in the triplet state with equal sublevel populations, decays rapidly to and stays at the equilibrium value. Although periodical refocusing to the pure triplet state occurs for the model system without reaction, the duration of this period is very short and it dephases rapidly again to the equilibrium state.22 Transient Absorption Study. Upon laser excitation of acetylenic ketones I and I1 in 0.4 M SDS micellar solution at room temperature two intensive absorption bands appear in the transient absorption spectrum. The first band (Ams = 550 nm) does not exhibit magnetic field dependence and thus corresponds to the excited triplet state of ketone. We consider that another band (Amm = 370 nm) which exhibits significant MFE is due to the ketyl radical. Figure 6 shows transient absorption decays at 370 nm in zero and 150 mT (left) and at 550 nm (right) for acetylenic ketone I. The decay kinetic for the triplet state was well reproduced with a single-exponential function:
I(t) = I, exp(-r/tT)
(9)
where ZT represents the lifetime of the triplet state. Transient absorption for the ketyl radical could be analyzed with the following equation:
I(t) = I, exp(-rlt,)
+ I,
(10)
where t~ and 10 are the lifetime and the total production of ketyl radical, respectively, and 11 represents the relative absorption of long-lived species, such as escaped radical and reaction products. The parameters are summarized in Table 2. Since the inverse values of ZT'S for I and I1 are calculated as 1.1 x lo7 and 0.7 x lo6 s-l, respectively, and these values are very close to the k H values obtained by the pulse PYESR technique (see Table l), the triplet states of both acetylenic ketones should be quenched by hydrogen abstraction almost quantitatively. A large difference between the quenching rates for I and I1 was also observed for Brij 35 micellar solution, although the rates are much higher compared with those in SDS
Dynamic Behavior of Radical Pairs
J. Phys. Chem., Vol. 98, No. 41, 1994 10567
TABLE 2: Lifetimes of the Excited Triplet States and the Ketyl Radicals of I and II Obtained from Transient Absorption Decays at 550 and 370 W I 11 micelle SDS Brij 35
TT
t~ (H=0 mT)
ZD (H= 150 mT)
tT
t~ ( H =0 mT)
ZD (H = 150 mT)
91 < 5‘
440 950
2800 3800
1460 56
6
2420
1130
3100
triplet life time and decay time of ketyl radical, respectively, given in units of nanoseconds. * Not detectable due to overlapping of the product bands. Not detectable. t~and t~ represent
micellar solution. The great increase in the quenching rate in Brij micelle may merely due to the fact that hydrogen abstraction occurs much faster for the ether C-H bond of Briji-35 compared with that for the hydrocarbon C-H bond of SDS. t~ in (lo), the lifetime of the ketyl radical, relates with many rate constants, such as the rate of intersystem crossing, spin-lattice relaxation of the radicals, escape rate from the micelle as well as reaction rates of free ketyl radicals. Therefore, it is difficult to determine each of the rate constants by the transient absorption method, although the kinetics of a transient radical is very helpful for the discussion of radical pair kinetics.
Conclusion Pulse-mode PYESR was employed to study the kinetic behavior of transient radical pairs produced in photoreduction of acetylenic ketones (I and 11) in SDS micellar solution. From the simulation of the kinetics using a detailed classical model for the reaction scheme, the details of the reaction mechanism as well as the kinetic parameters for both the formation and the decay of the radical pair were obtained. It was suggested for both systems that (1) hydrogen abstraction by the excited triplet state of the ketones is the main mechanism of the photoreaction for these systems and (2) the reactivity of the carbonyl group changes significantly with the introduction of acetylenic groups into the benzophenone frame. We suppose that the main reason for the large difference in the reactivity is due to the fact that the interaction of the 2Px orbital of carbonyl oxygen with the Q electrons in the phenyl groups decreases upon introducing two acetylenic groups while the resonance for the carbonyl z electrons with those on the phenyl groups are maintained via acetylenic groups.
Acknowledgment. Sincere thanks should go to Professor A. S . Zanina of Institute of Chemical Kinetics and Combustion for presenting us the acetylenic ketones. The authors should also acknowledge for the technical assistance of Dr. Y. Konishi. This work was supported by Grand-in-Aid for Scientific Research on Priority Area “Molecular Magnetism” (Area No. 228/04242102) from the Ministry of Education, Science and Culture of Japan. References and Notes (1) Sagdeev, R. Z.; Molin, Yu, N.; Salikhov, K. M.; Leshina, T. V.; Kamha, M. A.; Shein, S. M. Org. Magn. Reson. 1973, 5, 603.
(2) Salikhov, K. M.; Molin, Yu. N.; Sagdeev, R. Z.; Buchachenko, A. L. Spin Polarization and Magnetic Effect in Radical Reactions; Elsevier: Amsterdam, 1984. (3) Hayashi, H. In Photochemistry and Photophysics;Rabek, J. F., Ed.; CRC Press: Boca Raton, FL,1990; Vol. 1, pp 59-136. (4) Steiner, U. E.; Wolff, H.-J. In Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1991; Vol. 4, pp 1-130. ( 5 ) T w o , N. J.; Kraeutter, B. J. Am. Chem. SOC.1978, 100, 7432. (6) Sakaguchi, Y.; Nagakura, S.; Hayashi, H. Chem. Phys. Lett. 1980, 72, 420. (7) Scaiano, J. C.; Abuin, E. B.; Stewart, L. C. J. Am. Chem. SOC. 1982, 104, 5673. (8) Tanimoto, Y.; Udagawa, H.; Itoh, M. J . Phys. Chem. 1983, 87, 724. (9) Tanimoto, Y.; Takahashi, M.; Itoh, M. Chem. Phys. Lett. 1983, 100, 442. (10) Sakaguchi, Y.; Hayashi, H. J . Phys. Chem. 1984, 88, 1437. (11) References in refs 1-3. (12) Okazaki, M.; Sakata, S.; Konaka, R.; Shiga, T. J . Am. Chem. SOC. 1985, 107, 7214. (13) Okazaki, M.; Shiga, T. Nature 1986, 323, 240. (14) Okazaki,M.; Sakata, S.; Konaka, R.; Shiga, T. J. Chem. Phys. 1987, 86, 6792. (15) Frankevich, E. L.; Pristupa, V. M.; Lesin, V. I. Chem. Phys. Lett. 1977, 47, 304. (16) Molin, Y. N.; Anisimov, 0. A.; Grigoryants, V. M.; Molchanov, V. K.; Salikhov, K. M. J. Phys. Chem. 1980, 84, 1853. (17) Smith, J. P.; Trifunac, A. D. J . Phys. Chem. 1981, 85, 1645. (18) McLauchlan, K. A.; Nattrass, S. R. Mol. Phys. 1988, 65, 1483. (19) Okazaki, M.; Toriyama, K. Bull. Chem. Soc. Jpn. 1993,66, 1892. (20) Okazaki, M.; Konishi, Y.; Toriyama, K. Chem. Lett. 1993, 737. (21) Polyakov, N. E.; Konishi, Y.; Okazaki, M.; Toriyama, K. J . Phys. Chem., preceding paper in this issue. (22) Okazaki, M.; Polyakov, N. E.; Konishi, Y.; Toriyama, K. Applied Magn. Reson., in press. (23) Gutrin, B.; Johnston, L. J. J. Org. Chem. 1989, 54, 3176. (24) Maryasova, V. I.; Zanina, A. S.; Kruppa, A. I.; Leshina, T. V. J. Photochem. Photobiol. A: Chem. 1991, 61, 201. (25) Nishio, T.; Nakata, H.; Omote, Y. J. Heterocycl. Chem. 1986, 23, 1011. (26) Zanina, A. S.; Shergina, S. I.; Sokolov, I. E.; Kotlyarevsky, I. L. lzv. A M . Nauk SSSR, Ser. Khim. 1981, 5, 1158 (in Russian). (27) Kaur, H.; Leung, K. H. W.; Perkins, M. J. J. Chem. SOC.,Chem. Commun. 1981, 142. (28) Pople, J. A.; Beveridge, D. L. Approximate Molecular Orbital Theory; McGraw-Hill: New York, 1970. (29) Brocklehurst, B. J . Chem. Soc., Faraday Trans. 2 1976, 72, 1869. (30) Hayashi, H.; Nagakura, S. Bull. Chem. SOC.Jpn. 1984, 57, 322. (31) Okazaki, M.; Tai, Y.; Nunome, K.; Toriyama, K.; Nagakura, S. Chem. Phys. 1992, 161, 177. (32) Tanimoto, Y.; Fujiwara, Y.; Takamatsu, S.; Kita, A.; Itoh, M.; Okazaki, M. J. Phys. Chem. 1992, 96, 9844.
