J. Phys. Chem. B 2000, 104, 703-708
703
Secondary Isotope Effect on Photoinduced Isomerization of Alkyl Radicals in Low-Temperature Solids Tomoya Takada, Hitoshi Koizumi,* and Tsuneki Ichikawa DiVision of Molecular Chemistry, Graduate School of Engineering, Hokkaido UniVersity, Kita-ku, Sapporo 060-8628, Japan ReceiVed: September 1, 1999; In Final Form: NoVember 22, 1999
Isotope effects on photoinduced isomerization of alkyl radicals in 77 K solid alkanes were studied by ESR spectroscopy. Quantum yield of the isomerization from secondary to primary radicals was compared for 2-hexyl-h13, 2-hexyl-1,6-d2, 2-hexyl-1,1,6,6-d4, 2-hexyl-1,1,1,6,6,6-d6, and 2-hexyl-2,5,5-d3 radicals. The quantum yield for 2-hexyl-1,6-d2, 2-hexyl-1,1,6,6-d4, and 2-hexyl-1,1,1,6,6,6-d6 is less than 1/10 of that for 2-hexyl-h13. The low quantum yield for 2-hexyl-1,6-d2 and 2-hexyl-1,1,6,6-d4 is due to a secondary isotope effect, which is an isotope effect on atoms not participating in bond breaking or forming in the reaction. In contrast, the deuteration of the 2,5-hydrogens causes a much smaller effect than the deuteration of the end methyl groups; the quantum yield for 2-hexyl-2,5,5-d3 radical is 4/5 of that for 2-hexyl-h13. These results support our mechanism of the photoinduced isomerization: a photoexcited alkyl radical is converted to a valence excited state in which two hydrogen atoms of the methyl groups are positively charged and the radical site is negatively charged, and one of the two hydrogen atoms transfers to the radical site. ESR spectra indicate that deuterium atoms of the CH2D groups of 2-hexyl-1,6-d2 radical and of the CHD2 groups of 2-hexyl-1,1,6,6-d4 radicals preferentially occupies the positions in the plane of the C-C-C bonds, where the two hydrogen atoms in the valence excited state are positively charged. Deuteration of the atom of the transfer causes a large isotope effect, and the partial deuteration of the end methyl groups hence causes the large isotope effect.
Introduction
In this paper, we have examined the photoinduced isomerization of hexyl radicals with partially deuterated methyl groups, 2-hexyl-1,1,6,6-d4 and 2-hexyl-1,6-d2 radicals. The quantum yields for the isomerization are compared with the quantum yields for 2-hexyl-h13 and for 2-hexyl-1,1,1,6,6,6-d6. This comparison will verify the validity of our mechanism of the photoinduced isomerization. Our mechanism predicts that quantum yield for the photoinduced isomerization of 2-hexyl-1,1,6,6-d4 and of 2-hexyl-1,6d2 radicals is smaller than quantum yield estimated with the H/D ratio in the methyl groups of the radicals and with the quantum yield for 2-hexyl-1,1,1,6,6,6-d6 and 2-hexyl-h13. The internal rotations of β-CH2D and of β-CHD2 in 2-alkyl radicals are different from those of β-CH3 protons and of β-CD3. The deuterons of the CHD2 group and of the CH2D group of 2-alkyl radicals preferentially occupy the position in the plane perpendicular to the singly occupied 2p orbital.7,8 Two hydrogen atoms at the positions are positively charged in the valence excited state of alkyl radicals, and one of the two hydrogen atoms transfers to the radical site according to our mechanism.6 The deuteration of the atom of the transfer much decrease the quantum yield.5 The quantum yield for 2-hexyl-1,1,6,6-d4 and for 2-hexyl-1,6-d2 radicals is hence expected to be smaller than the value estimated with the H/D ratio in the methyl groups of the radicals and with the quantum yield for 2-hexyl-1,1,1,6,6,6d6 and 2-hexyl-h13.
Electronically excited radicals may cause reactions different from those of ground state radicals. There has, however, been much less information on the reactions of excited radicals than those of ground state radicals.1 Secondary alkyl radicals in low-temperature solids isomerize to primary alkyl radicals with irradiation of ultraviolet light.1-6 We have demonstrated that this isomerization is an intramolecular hydrogen transfer reaction: a hydrogen atom of a methyl group of an alkyl radical directly transfers to the radical site.4-6 We have proposed a mechanism for the isomerization.6 It is that through a valence excited state of alkyl radicals. An alkyl radical is at first excited to a Rydberg state by ultraviolet light, and it is converted to the lowest valence excited state. The valence excited state is a state where two hydrogens of the methyl groups are positively charged, and the radical site is negatively charged. One of the two hydrogen atoms then transfers to the radical site. Secondary alkyl radicals thereby isomerize to primary alkyl radicals. However, there is little information about potential surfaces of excited alkyl radicals. Further experimental studies to evaluate the mechanism are needed. Isotope effects on chemical reactions give information on their mechanism. We have already observed a large isotope effect on the quantum yield of the isomerization:5 the quantum yield for sec-hexyl-1,1,1,6,6,6-d6 radicals is less than 1/10 of the yield for protiated hexyl radicals. The large isotope effect indicates that the isomerization will proceed through a tunneling mechanism.
Experimental Section
* Corresponding author. Fax: +81-11-706-7897. E-mail: koizumih@eng. hokudai.ac.jp.
Samples used were n-hexane (n-hexane-h14, stated purity >97%) supplied by Kanto chemicals and n-hexane-d14 supplied
10.1021/jp9931049 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/07/2000
704 J. Phys. Chem. B, Vol. 104, No. 4, 2000 by Aldrich (99 atom % D). n-Hexane-1,6-d2 was synthesized from 1,6-hexanediol. The starting material was brominated using phosphorus tribromide; the brominated compound was converted to hexane-1,6-d2 using lithium triethylborodeuteride. n-Hexane1,1,6,6-d4 and n-hexane-1,1,1,6,6,6-d6 were synthesized from dimethyl adipate. The starting material was converted to 1,6hexanediol-1,1,6,6-d4 using lithium aluminum deuteride; the hexanediol was brominated using phosphorus tribromide. The brominated compound was converted to n-hexane-1,1,6,6-d4 with lithium triethylborohydride, while it was converted to n-hexane-1,1,1,6,6,6-d6 with lithium triethylborodeuteride. The samples were purified by passage through a column containing activated alumina and silica gel. Absorbance of the purified samples at 254 nm was less than 0.2 with light path length of 1 cm. They were degassed by freeze-pump-thaw cycles and sealed in high-purity quartz tubes whose end has the form of a thin capillary. Single crystals of the samples were prepared to observe the hyperfine splitting of deuterons. Crystal growth was made by slowly lowering one of the tubes into liquid nitrogen.9 The samples were γ-irradiated at 77 K with a dose of about 10 kGy. UV-irradiation with a low-pressure mercury lamp was also made at 77 K. ESR spectra were measured with an X-band ESR spectrometer (JEOL JES-TE200). The orientation of the single crystals was chosen to show better resolution in the spectra. For the partially deuterated radicals, the spectra in which the hyperfine splitting of the deuterons are most clearly observed were chosen. Results Photoinduced Isomerization of Protiated and Partially Deuterated Hexyl Radicals. Figure 1 shows ESR spectra of a 77 K single crystal of n-hexane-d14 containing 2 mol % n-hexane-h14 after γ-irradiation and after successive irradiation of 254 nm light. The spectrum after γ-irradiation in Figure 1A is mainly due to 2-hexyl-h13 radicals. A simulated spectrum for 2-hexyl-h13 radical in Figure 1B agrees well with the spectrum in Figure 1A. After the photolysis of 60 min, the spectrum changed to that in Figure 1C. This is due to the isomerization from secondary to primary radicals.4,5 A simulated spectrum of 1-hexyl-h13 radical is shown in Figure 1D. Intensity of 1-hexyl radical increases, while that of 2-hexyl radicals decrease with the irradiation of 254 nm light. More than 60% of secondary hexyl in Figure 1A is converted to primary hexyl radical in Figure 1C. Figure 2 shows ESR spectra for a single crystal of hexaned14 containing 2 mol % n-hexane-1,6-d2. The spectrum after γ-irradiation in Figure 2A is mainly due to 2-hexyl-1,6-d2 radicals. Simulated spectrum for 2-hexyl-1,6-d2 is shown in Figure 2B. The spectrum consists of triplet splitting of a β-deuteron and splitting of an R- and four β-protons. The hfcc for the β-deuteron is 0.34 mT. It is significantly smaller than the value of 0.40 mT that is calculated from the hfcc for the methyl protons of 2-hexyl-h13 and the magnetic moment ratio of H/D. After the photolysis of 120 min, the spectrum changed to Figure 2C. Simulated spectrum for 1-hexyl-1,6-d2 is shown in Figure 2D. The intensity of 1-hexyl-1,6-d2 increases, while that of 2-hexyl-1,6-d2 radicals decreases with the irradiation of 254 nm light: the photoinduced isomerization from secondary to primary alkyls also occurs. The quantum yield of the isomerization is, however, much lower than that for hexyl-h13. Figure 3 shows ESR spectra of a single crystal of hexaned14 containing 2 mol % n-hexane-1,1,6,6-d4, and Figure 4 shows
Takada et al.
Figure 1. ESR spectra of a 77 K single crystal of n-hexane-d14 containing 2 mol % of n-hexane-h14: after γ-irradiation (A); a simulated spectrum for 2-hexyl-h13 (B); after successive irradiation of 254 nm light (C); a simulated spectrum for 1-hexyl-h13 (D). The following hyperfine coupling constants and line shapes are assumed for the simulations: aβ ) 2.59 mT for three β-CH3 protons, aR ) 3.32 mT for the R-proton, and aβ ) 3.12 and 3.60 mT for β-CH2 protons, and Lorentzian line shape with ∆Hmsl ) 0.24 mT for 2-hexyl-h13; aR ) 1.94 mT for two R-protons, aβ ) 3.70 and 2.90 mT for β-CH2 protons, and Lorentzian line shape with ∆Hmsl ) 0.20 mT for 1-hexyl-h13.
those containing 2 mol % n-hexane-1,1,1,6,6,6-d6, respectively. Figures 3A and 4A show the ESR spectra after γ-irradiation, while Figures 3C and 4C show the ESR spectra after the successive photolysis of 254 nm light. Simulated spectra for 2-hexyl-d4 and for 2-hexyl-d6 are shown in Figures 3B and 4B, and those for 1-hexyl-d4 and for 1-hexyl-d6 are shown in Figures 3D and 4D. These spectra indicate that the photoinduced isomerization also occurs in the solids. 1-Hexyl radicals increase as 2-hexyl radicals decrease with irradiation of 254 nm light. The quantum yield is, however, much lower than that for 2-hexyl-h13. The hfcc of β-methyl deuterons for 2-hexyl-d4 is 0.36 mT. It is also less than the value of 0.40 mT which is calculated from the hfcc of β-methyl protons for 2-hexyl-h13, but a little larger than that for 2-hexyl-d2. The hfcc of β-methyl deuteron for 2-hexyl-d6 is 0.40 mT. It is the same as that calculated from the hfcc of β-methyl protons for 2-hexyl-h13. Quantum Yield for Protiated and Partially Deuterated Hexyl Radicals. Figure 5 shows the time course of the concentration of the protiated and partially deuterated 2-hexyl radicals with irradiation of 254 nm light. The relative concentration is obtained from the peak height of the ESR spectra. The concentration of 2-hexyl-2,5,5-d3 radicals in hexane-2,2,5,5-d46 is also plotted. Relative values of the quantum yields of the decay are obtained from their slopes. They are tabulated in Table 1. The concentration of the 2-hexyl radicals decreases also by conversion to nonradical products as well as by the photoinduced
Isotope Effect on Isomerization of Alkyl Radicals
J. Phys. Chem. B, Vol. 104, No. 4, 2000 705
Figure 2. ESR spectra of a 77 K single crystal of n-hexane-d14 containing 2 mol % of n-hexane-1,6-d2: after γ-irradiation (A); a simulated spectrum for 2-hexyl-1,6-d2 (B); after successive irradiation of 254 nm light (C); a simulated spectrum for 1-hexyl-1,6-d2 (D). The peaks indicated by asterisks in Figure 2A are caused by imperfection of the single crystal; the crystal contains parts in which the orientation of n-hexane molecules is different from that in the main part of the crystal. It was identified by comparison with the powder spectrum of polycrystalline solid of the sample. The following hyperfine coupling constants and line shapes are assumed for the simulations: aβ ) 2.90 mT for two β-CH2D protons, aβ ) 0.34 mT for the β-CH2D deuteron, aR ) 1.68 mT for the R-proton, and aβ ) 3.12 and 3.60 mT for β-CH2 protons, and Lorentzian line shape with ∆Hmsl ) 0.2 mT for 2-hexyl1,6-d2; aR ) 2.9 mT for the R-proton, aR ) 0.48 mT for the R-deuteron, and aβ ) 3.70 and 3.1 mT for β-CH2 protons, and Lorentzian line shape with ∆Hmsl ) 0.2 mT for 1-hexyl-1,6-d2.
isomerization to 1-hexyl radicals. Time course of the total concentration of radicals in neat 77 K n-hexane-h14 is shown in Figure 5. They are obtained by double integration of the ESR spectra. The decay of radicals in neat 77 K n-hexane-d14 was the same as that in neat 77 K n-hexane-h14 within experimental errors. The decay of total concentration of the partially deuterated hexyl radicals in 77 K n-hexane-d14 is hence nearly the same as that in neat 77 K n-hexane-h14. Hexyl radicals will hence decay by the following mechanism: Φi2
2-hexyl + hν 98 1-hexyl Φi3
3-hexyl + hν 98 1-hexyl ΦD1
1-hexyl + hν 98 non-radical products ΦD2
2-hexyl + hν 98 non-radical products ΦD3
3-hexyl + hν 98 non-radical products
(1) (2)
Figure 3. ESR spectra of a 77 K single crystal of n-hexane-d14 containing 2 mol % of n-hexane-1,1,6,6-d4: after γ-irradiation (A); a simulated spectrum for 2-hexyl-1,1,6,6-d4 (B); after successive irradiation of 254 nm light (C); a simulated spectrum for 1-hexyl-1,1,6,6-d4 (D). The following hyperfine coupling constants and line shapes are assumed for the simulations: aβ ) 2.85mT for the β-CHD2 protons, aβ ) 0.36 mT for two β-CHD2 deuteron, aR ) 3.40 mT for the R-proton, and aβ ) 3.12 and 3.60 mT for β-CH2 protons, and Lorentzian line shape with ∆Hmsl ) 0.2 mT for 2-hexyl-1,1,6,6-d4; aR ) 0.48 mT for two R-deuteron, and aβ ) 3.70 and 2.9 mT for β-CH2 protons, and Lorentzian line shape with ∆Hmsl ) 0.2 mT for 1-hexyl-1,1,6,6-d4.
non-radical products are the same for all the hexyl radials,
ΦD ) ΦD1 ) ΦD2 ) ΦD3
(6)
Total concentration of hexyl radicals [hexyl] is then given by
[hexyl] ) [1-hexyl] + [2-hexyl] +[3-hexyl] ) [hexyl]0 exp(-ΦDIt) (7) where [hexyl]0 is the total concentration before the irradiation of 254 nm light, I the intensity of light, and t the irradiation time. Concentration of 2-hexyl radicals [2-hexyl] is then given by
[2-hexyl] ) [2-hexyl]0 exp(-(Φi2 + ΦD)It) ) [2-hexyl]0 exp(-ΦIt)
(8)
where Φ is the quantum yield for the decay of 2-hexyl radicals. The value of Φi2/Φi2(2-hexyl-h13) is then calculated from Φ/Φ(2-hexyl-h13) and ΦD/Φ(2-hexyl-h13) by
(3)
Φi2/Φi2(2-hexyl-h13) ) {Φ/Φ(2-hexyl-h13) ΦD/Φ(2-hexyl-h13)}/{1 - ΦD/Φ(2-hexyl-h13)} (9)
(4)
The values for Φi2/Φi2(2-hexyl-h13) are tabulated in Table 1.
(5)
Discussion
where Φij is the quantum yield of photoinduced isomerization, ΦDj the quantum yield of conversion to nonradical products for j-hexyl radical. We here assume that the absorption coefficients at 254 nm and the quantum yields of the conversion to the
Secondary Isotope Effect on the Photoinduced Isomerization. As shown in Table 1, the quantum yield of the photoinduced isomerization Φi is much decreased by the deuteration of one or two of hydrogens in each methyl group. The values of Φi/Φi(2-hexyl-h13) for 2-hexyl-1,6-d2 and for
706 J. Phys. Chem. B, Vol. 104, No. 4, 2000
Takada et al. TABLE 1: Quantum Yields for the Photoinduced Decay of 2-Hexyl Radicals in 77 K Hexane-d14 Single Crystals radical
Φ/Φ(hexyl-h13)a
Φi2/Φi2(hexyl-h13)b
2-hexyl-h13 2-hexyl-1,6-d2 2-hexyl-1,1,6,6-d4 2-hexyl-1,1,1,6,6,6-d6 2-hexyl-2,5,5-d3c hexyl total
1 0.25 0.21 0.21 0.80 0.17d
1 0.09 0.05 0.04 0.76
a Φ, quantum yield of the decay of 2-hexyl radicals; Φ(hexyl-h ), 13 quantum yield for 2-hexyl-h13. b Φi2, quantum yield of the photoinduced isomerization of 2-hexyl radicals; Φi2(hexyl-h13), quantum yield for 2-hexyl-h13. c Φ in 77 K polycrystalline solids of hexane-2,2,5,5-d4 (ref 6). d Quantum yield of the decay of total concentration of radicals in 77 K hexane-h14; ΦD/Φ(hexyl-h13).
consider only an isotope effect on the atom of the transfer, the proportions of H and of D in the methyl groups solely determine whether the atom is H or D. Only the mass of the atom then influences on the efficiency for the transfer. Within this approximation, the quantum yield Φi is obtained by Figure 4. ESR spectra of a 77 K single crystal of n-hexane-d14 containing 2 mol % of n-hexane-1,1,1,6,6,6-d6: after γ-irradiation (A); a simulated spectrum for 2-hexyl-1,1,1,6,6,6-d6 (B); after successive irradiation of 254 nm light (C); a simulated spectrum for 1-hexyl1,1,1,6,6,6-d6 (D). The following hyperfine coupling constants and line shapes are assumed for the simulations: aβ ) 0.40 mT for two β-CD3 deuteron, aR ) 2.8 mT for the R-proton, and aβ ) 3.12 and 3.60 mT for β-CH2 protons, and Lorentzian line shape with ∆Hmsl ) 0.27 mT for 2-hexyl-1,1,1,6,6,6-d6; aR ) 0.48 mT for two R-deuteron, and aβ ) 3.70 mT for the β-CHD proton, aβ ) 0.45 mT for the β-CHD deuteron, and Lorentzian line shape with ∆Hmsl ) 0.2 mT for 1-hexyl1,1,2,6,6,6-d6.
Figure 5. Concentration of 2-hexyl-h13 (O), 2-hexyl-1,6-d2 (0), 2-hexyl-1,1,6,6-d4 (9), and 2-hexyl-1,1,1,6,6,6-d6 (4) in 77 K single crystal of n-hexane-d14 and of 2-hexyl-2,5,5-d3 in 77 K polycrystalline solid of n-hexane-2,2,5,5-d4 (b), total concentration of radicals in 77 K single crystal of n-hexane-h14 (×) as a function of irradiation time of 254 nm light.
2-hexyl-1,1,6,6-d4 indicate that the photoinduced isomerization is affected by a large secondary isotope effect, which is an isotope effect on atoms not participating in bond breaking or forming in the reaction. The photoinduced isomerization from secondary to primary alkyl radicals is an intramolecular hydrogen transfer; a hydrogen atom in the methyl groups transfers to the radical site. If we
1 1 Φi ) NH Φi(2-hexyl-h13) + ND Φi(2-hexyl-1,1,1,6,6,6-d6) 6 6 (7) where NH and ND are the number of H and of D atoms in the methyl groups, respectively. The value of NH is 4 for 2-hexyl1,6-d2 and 2 for 2-hexyl-1,1,6,6-d4. Φi/Φi(2-hexyl-h13) is estimated to be more than 2/3 for 2-hexyl-1,6-d2 and more than 1/3 for 2-hexyl-1,1,6,6-d4. These values are significantly larger than the experimental values; the large isotope effect for 2-hexyl1,6-d2 and for 2-hexyl-1,1,6,6-d4 is not explained only by a primary isotope effect, which is a effect on atoms participating in bond breaking or forming in the reaction. The degree of the secondary isotope effect depends on the position of the deuteration. The deuteration of the 2,5-hydrogens causes a much smaller secondary isotope effect than the deuteration of the end methyl groups; Φi/Φi(2-hexyl-h13) for 2-hexyl-2,5,5-d3 is 0.76. Hindered Rotation of Partially Deuterated Methyl Groups. The hfcc of β-methyl deuterons for 2-hexyl-1,6-d2 and for 2-hexyl-1,1,6,6-d4 is significantly less than the value which is calculated from the hfcc of β-methyl protons of 2-hexyl-h13 and the magnetic moment ratio of H/D. However, the hfcc of β-methyl deuterons for 2-hexyl-1,1,1,6,6,6-d6 is nearly the same as the calculated one. Similar results have been reported about CHD2CD27, CH2DCD2,7 and DCH2C(CH3)210 radicals. The difference in the hfcc for the radicals arises from the difference in hindered rotation of symmetric and asymmetric methyl rotators.8 The hfcc of β-protons aβ depends on the azimuthal angle θ between the axis of the p orbital containing the unpaired electron and the β-proton; the value of aβ is approximately proportional to cos2 θ11. The values of aβ for three β-CH3 are not the same at a fixed position, but β-CH3 groups of alkyl is exchanged fast enough by hindered rotation of the methyl groups. The values of aβ are thereby averaged to be the same value for alkyl radicals in 77 K solid alkanes. In the case of symmetric methyl rotator, CH3- or CD3-, there are three equivalent equilibrium positions of the methyl groups, as shown in Figure 6A. In the case of an asymmetric rotator, CH2D- and CHD2- in parts B and C of Figure 6, however, three positions are not energetically equivalent. This is because the center of mass of the radicals is different among the three positions.12,13 The smaller value of the hfcc of the deuterons
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J. Phys. Chem. B, Vol. 104, No. 4, 2000 707
Figure 6. Possible equilibrium conformations for CH3CHCH2-, CD3CHCH2- (A), CH2DCHCH2- (B), and CHD2CHCH2- (C).
Figure 8. Schematic illustration of SOMO (A), SOMO-1 (B), and valence excited state (C) of 2-hexyl radical.
Figure 7. Schematic illustration of the mechanism of the photoinduced isomerization of alkyl radicals. A secondary radical is excited to a Rydberg state. It is converted to the valence excited state. Two hydrogen atoms of the methyl groups are positively charged in the valence excited state. One of the hydrogen atoms then transfers to the radical site.
indicates that the deuterons preferentially occupy the in-plane positions as shown in parts B and C of Figure 6. Mechanism of the Photoinduced Isomerization. We have proposed a mechanism through a valence excited state for the photoinduced isomerization of alkyl radicals (Figure 7).6 The mechanism and the preferential conformation of C-D bond at the position in the plane of the C-C-C bonds can explain the secondary isotope effect on the isomerization. Our mechanism is as follows. An alkyl radical is at first excited to a 3s Rydberg state;14,15 it is converted to a valence excited state. The valence excited state is a state in which an electron of the next lower molecular orbital to the singly occupied molecular orbital (SOMO-1) is excited to the singly occupied molecular orbital (SOMO) (Figure 8). The SOMO-1
of alkyl radicals will be a similar orbital to the SOMO of cation radicals of alkanaes,16,17 which is delocalized over the molecular chain and two hydrogens of the end methyl groups at the position in the plane of the C-C-C bonds. The excitation of an electron from the SOMO-1 to the SOMO results in the two hydrogens being positively charged, while the carbon of the radical site is negatively charged. A proton from a methyl group can then transfer to the radical site. A secondary alkyl radical thereby isomerizes to a primary alkyl radical. The deuterons of β-CH2D in 2-hexyl-1,6-d2 and of β-CHD2 in 2-hexyl-1,1,6,6-d4 preferentially occupy the in-plane position. Hydrogen atoms in the position are positively charged in the valence excited state; an atom at this position transfers to the radical site in the mechanism. Deuteration of the atom of the transfer decreases the yield of the isomerization.5 The partial dueteration of the methyl groups hence cause the large isotope effect. The deuteration at the 2,5-positions of 2-hexyl radicals causes the small isotope effect. This is because the deuterons at 2,5-positions are only spectators in the mechanism. In conclusion, the isotope effect on the quantum yields for the photoinduced isomerization of 2-hexyl radicals with partially deuterated methyl groups is well explained by our mechanism. The photoinduced isomerization of alkyl radicals will proceed through the lowest valence excited state in the solid phase. Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan. References and Notes (1) Mel’nikov, M. Ya; Smirnov, V. A. Handbook of Photochemistry of Organic Radicals: Absorption and Emission Properties, Mechanisms, Aging; Begell House: New York, 1996. (2) Pshezhetskii, S. Ya.; Kotov, A. G.; Milinchuk, V. K.; Roginskii, V. A.; Tupikov, V. I. EPR of Free Radicals in Radiation Chemistry; John Wiley & Sons: New York, 1974; Chapter VIII. (3) Koizumi, H.; Kosugi, S.; Yoshida, H.; J. Phys. Chem. 1994, 98, 11089.
708 J. Phys. Chem. B, Vol. 104, No. 4, 2000 (4) Koizumi, H.; Kosugi, S.; Yoshida, H.; J. Phys. Chem. 1996, 100, 4848. (5) Takada, T.; Koizumi, H.; Kagei, K.; Ichikawa, T.; Yoshida, H.; J. Phys. Chem. B 1997, 101, 4379. (6) Takada, T.; Koizumi, H.; Ichikawa, T.; Chem. Phys. Lett. 1999, 300, 253. (7) Fessenden, R. W.; Schuler, R. H. J. Chem. Phys. 1963, 39, 2147. (8) Fessenden, R. W. J. Chim. Phys. 1964, 61, 1570. (9) Gillbro, T.; Lund, A.; Radiat. Phys. Chem. 1976, 8, 625. (10) Lloyd, R. V.; Wood, D. E.; J. Am. Chem. Soc. 1975, 97, 5986.
Takada et al. (11) Heller, C.; McConnell, H. M. J. Chem. Phys. 1960, 32, 1535. (12) Quade, C. R.; Lin, C. C. J. Chem. Phys. 1963, 38, 540. (13) Liu, M.; Quade, C. R.; J. Mol. Spectrosc. 1991, 146, 238. (14) Wendt, H. R.; Hunziker, H. E. J. Chem. Phys. 1984, 81, 717. (15) Lengsfield, B. H., III.; Siegbahn, P. E. M.; Liu, B. J. Chem. Phys. 1984, 81, 710. (16) Toriyama, K.; Nunome, K.; Iwasaki, M. J. Phys. Chem. 1981, 85, 2149. (17) Toriyama, K.; Nunome, K.; Iwasaki, M. J. Phys. Chem. 1982, 77, 5891.
