Isotope Effect on Photoinduced Isomerization of Alkyl Radicals

Publication Date (Web): May 29, 1997 ... The large isotope effect may arise in the course of either the isomerization in the excited state or the rela...
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J. Phys. Chem. B 1997, 101, 4379-4382

4379

Isotope Effect on Photoinduced Isomerization of Alkyl Radicals Trapped in 77 K Solid Alkanes Tomoya Takada, Hitoshi Koizumi,* Koichi Kagei, Tsuneki Ichikawa, and Hiroshi Yoshida Graduate School of Engineering, Hokkaido UniVersity, Kita-ku, Sapporo 060, Japan ReceiVed: December 16, 1996; In Final Form: February 26, 1997X

Photoinduced isomerization of protiated and deuterated hexyl radicals in 77 K solid n-hexanes was examined. 2- and 3-hexyl-h13 radicals in n-hexane-h14 isomerize to 1-hexyl-h13 radicals with the irradiation of 254 nm light. This isomerization is suppressed by replacing the six hydrogen atoms of the methyl groups with deuterium atoms. The quantum yield for the isomerization of 2- and 3-hexyl-1,1,1,6,6,6-d6 in n-hexane-1,1,1,6,6,6-d6 and of 2- and 3-hexyl-d13 in n-hexane-d14 is less than 1/16 of that of 2- and 3-hexyl-h13 radicals in n-hexaneh14. Photoinduced isomerization of hexyl-h13 radicals in 77 K n-hexane-d14 was also examined. 2- and 3-hexylh13 radicals isomerize to 1-hexyl-h13 radical even in the solid. The isomerization may hence occur through an intramolecular reaction. The large isotope effect may arise in the course of either the isomerization in the excited state or the relaxation process of the excited primary radical.

Introduction Excited radicals may cause reactions different from ground state radicals or excited molecules: excited radicals have both unpaired electrons and internal energy. Comparison of reactivity between the excited and ground radicals or between excited radicals and excited parent molecules is of fundamental interest. Studies on the reactions of excited radicals are also of practical interest. Radicals generally absorb light of longer wavelength than that of their parent molecules.1,2 Radicals generated in liquids or in solids of the parent molecules can then be excited by light without disturbance of the absorption of the parent molecules. Radicals generated in materials may hence act as a photosensitizer. Knowledge on the reactions of the excited radials will be necessary to understand deterioration of materials on exposure to light. However, little is known about their reactions compared to ground state radicals or excited molecules.2,3 Excited secondary or tertiary alkyl radicals trapped in lowtemperature solids isomerize to primary alkyl radicals. This isomerization is induced by irradiation of light of shorter wavelength than ca. 300 nm.2,4-7 It occurs regardless of the structure of the solids.4,5 However, it has not been explained why the secondary radicals isomerize to the primary radicals. In the present work, we have studied the photoinduced isomerization of protiated and deuterated alkyl radicals in 77 K solid alkanes. We discuss two subjects concerning the mechanism of the isomerization and try to gain insight into the origin of the selectivity of the isomerization. One subject is the isotope effect on the isomerization reaction. We examined photoinduced isomerization of hexyl-1,1,1,6,6,6-d6 radicals. The isomerization of secondary or tertiary alkyl radicals to primary alkyl radical is a hydrogen transfer reaction from an end methyl group. The effect of the deuteration of the methyl groups will then give some insight into the mechanism of the isomerization. The other subject is whether the photoinduced isomerization occurs through intramolecular or through intermolecular reactions. In the previous paper, we have examined the isomerization of 2- and 3-hexyl radicals in 77 K perdeuterated methanol * To whom correspondence should be addressed. E-mail: koizumih@ pc.cfa.hokudai.ac.jp. Fax: +81-11-706-7897. X Abstract published in AdVance ACS Abstracts, May 1, 1997.

S1089-5647(96)04074-6 CCC: $14.00

(methanol-d4) glass and concluded that the isomerization may occur through intramolecular reactions.5 Methanol-d4 containing 5 vol % perdeuterated 2-propanol as a glass-forming agent was used as the solvent for protiated n-hexane (n-hexane-h14) in the experiment. We assumed that hexyl-h13 radicals generated by γ-irradiation of the glass containing 2 vol % n-hexane-h14 were surrounded by methanol-d4 and cannot abstract hydrogen from an adjacent n-hexane-h14 molecule. However, there is a small possibility that small crystals of n-hexane-h14 are separated out, and hexyl-h13 radicals are generated in the crystals. In the present work we then performed further experiments with 77 K perdeuterated crystalline n-hexane (n-hexane-d14) containing n-hexane-h14. n-Hexane-h14 dissolves well in hexane-d14. Hexyl-h13 radicals have no adjacent n-hexane-h14 molecules at low concentrations of n-hexane-h14. Experimental Section Samples used were n-hexane (n-hexane-h14, stated purity >97%) supplied by Kanto chemicals and n-hexane-d14 supplied by Aldrich (99 atom % D). n-Hexane-1,1,1,6,6,6-d6 was synthesized from adipic acid dimethyl ester. The starting material was converted to hexane-1,6-diol-1,6-d2 using lithium aluminum deuteride; the hexanediol was brominated using phosphorous tribromide; the brominated compound was finally converted to n-hexane-1,1,1,6,6,6-d6 by the reaction with lithium triethylborodeuteride.8 The samples were degassed by freeze-pump-thaw cycles and sealed in high-purity quartz tubes. All samples were γ-irradiated at 77 K with a dose of ca. 10 kGy. UV-irradiation with a low-pressure mercury lamp was also done at 77 K. ESR spectra were measured with an X-band spectrometer (Varian E-line series). Results Photoinduced Isomerization of Protiated and Deuterated Hexyl Radicals. Figure 1 shows ESR spectra of 77 K crystalline n-hexane-h14 after γ-irradiation and after successive irradiation of 254 nm light.5 The spectrum after γ-irradiation is composed of 2-hexyl-h13 and 3-hexyl-h13 radicals.9,10 After 30 min photolysis, the spectrum considerably changed, while the concentration of the radicals was nearly constant. The spectrum after the photolysis is assigned as 1-hexyl radical© 1997 American Chemical Society

4380 J. Phys. Chem. B, Vol. 101, No. 22, 1997

Figure 1. ESR spectra generated from crystalline n-hexane γ-irradiated at 77 K (A) and after successive photolysis for 30 min (B) and simulated spectrum for 1-hexyl-h14 (C). The following hyperfine coupling constants and line shape are assumed for the simulation: aβ)3.7 and 2.9 mT for the two β-protons, A1 ) -1.76 mT, A2 ) -1.96 mT, and A3 ) -2.9 mT for the two R-protons, and the line shape is Lorentzian with ∆Hmsl ) 0.5 mT.

Takada et al.

Figure 3. Experimental ESR spectra (A) generated from crystalline n-hexane-1,1,1,6,6,6-d6 γ-irradiated at 77 K (s) and after successive photolysis for 60 min (‚‚‚); simulated ESR spectra (B) for 1-hexyl1,1,6,6,6-d5 and 2- and 3-hexyl-1,1,1,6,6,6-d6 with the abundance ratio of 20:15:65 as in Figure 2C (s) and for 20% conversion of 2- and 3-hexyl-d6 radicals to 1-hexyl-d6 (‚‚‚). It was assumed that 2- and 3-hexyl-1,1,1,6,6,6-d6 radicals isomerize to 1-hexyl-1,1,2,6,6,6-d6 and 1-hexyl-1,1,3,6,6,6-d6, respectively. The following coupling constants and line shape are assumed for the simulation: 1-hexyl-1,1,2,6,6,6-d6 is simulated with aβ ) 3.3 mT for the β-proton, 0.51 mT for β-deuteron, A1 ) -0.27 mT, A2 ) -0.30 mT, A3 ) -0.45 mT for the two R-deuterons, and the line shape is Lorentzian with ∆Hmsl ) 0.7 mT; 1-hexyl-1,1,3,6,6,6-d6 is simulated with aβ ) 3.3 for the two β-protons, A1 ) -0.27 mT, A2 ) -0.30 mT, A3 ) -0.45 mT for the two R-deuterons, and the line shape is Lorentzian with ∆Hmsl ) 0.65 mT.

Figure 2. ESR spectra generated from crystalline n-hexane-1,1,1,6,6,6d6 γ-irradiated at 77 K (A) and after successive photolysis for 60 min (B) and simulated spectrum for 1-hexyl-1,1,6,6,6-d5, 2-hexyl-1,1,1,6,6,6d6 and 3-hexyl-1,1,1,6,6,6-d6 with the abundance ratio of 20:15:65 (C). The following hyperfine coupling constants and line shape are assumed for the simulation: 1-hexyl-1,1,6,6,6-d6 is simulated with aβ ) 3.3 for the two β-protons, A1 ) -0.27 mT, A2 ) -0.30 mT, A3 ) -0.45 mT for the two R-deuterons, and the line shape is Lorentzian with ∆Hmsl ) 0.65 mT; 2-hexyl-1,1,1,6,6,6-d6 is simulated with aβ ) 3.1 and 3.5 mT for the two β-protons, A1 ) -1.17 mT, A2 ) -1.96 mT, A3 ) -3.52 mT for the a-proton, aβ ) 0.35 mT for the three deuterons, and the line shape is Lorentzian with ∆Hmsl ) 0.65 mT; 3-hexyl-1,1,1,6,6,6d6 is simulated with aβ ) 3.3 for the four β-protons, A1 ) -1.17 mT, A2 ) -1.96 mT, A3 ) -3.52 mT for the R-proton, and the line shape is Lorentzian with ∆Hmsl ) 0.65 mT. Figure 4. Integrated ESR spectra of Figure 3A (A) and 3B (B).

h13.5,11 A simulated spectrum for 1-hexyl-h13 in Figure 1C agrees well with the experimental one in Figure 1B. With the irradiation of UV-light, 2- and 3-hexyl radicals hence isomerize to 1-hexyl-h13 radicals. Figure 2 shows ESR spectra of 77 K crystalline n-hexane1,1,1,6,6,6-d6 after γ-irradiation and after successive irradiation of 254 nm light. Contrary to the result on 77 K n-hexane-h14, the spectrum changes little after 60 min photolysis of 254 nm light. The spectra in Figure 2A,B are assigned as the spectrum of the overlap of 1-hexyl-1,1,6,6,6-d5, 2-hexyl-1,1,1,6,6,6-d6 and 3-hexyl-1,1,1,6,6,6-d6. The ratio of the radicals is estimated to be 20:15:65. The simulated spectrum with this abundance

ratio in Figure 2C agrees well with the experimental spectra in Figure 2A,B. ESR spectra before and after the UV-irradiation are compared again in Figure 3A, and their integrated spectra are shown in Figure 4A. The amount of conversion from 2-hexyl-1,1,1,6,6,6d6 and 3-hexyl-1,1,1,6,6,6-d6 to 1-hexyl-d6 is estimated by comparison with simulated spectra. Simulated spectra for 2and 3-hexyl-1,1,1,6,6,6-d6 with the abundance ratio of 3:1 and for 20% conversion from the mixture of 2- and 3-hexyl1,1,1,6,6,6-d6 to 1-hexyl-d6 are shown in Figure 3B. Their integrated spectra are shown in Figure 4B. 2-Hexyl-1,1,1,6,6,6-

Isotope Effect on Photoinduced Isomerization

Figure 5. ESR spectra generated from crystalline n-hexane-d14 g-irradiated at 77 K (A) and after successive photolysis for 60 min (B). ESR spectra for 1-hexyl radical generated from n-hexane-h14 by γ- and successive UV-irradiation (C) is shown for comparison.

J. Phys. Chem. B, Vol. 101, No. 22, 1997 4381 n-hexane-d14. However, the spectra of hexyl-d13 radicals are restricted within about 5 mT around the center because hyperfine coupling constants for deuteron are about 1/6.5 of the hyperfine coupling constants of protons.12,13 Figure 6A is assigned as the spectrum of the mixture of 2and 3-hexyl-h13 radicals, while Figure 6B is assigned as the spectrum of 1-hexyl-h13 radical. Photoinduced isomerization from 2- or 3-hexyl-h13 to 1-hexyl-h13 radical hence occurs even in n-hexane-d14. The spectrum of 1-hexyl-h13 radicals in 77 K n-hexane-h14, the same spectrum in Figure 1B, is compared in Figure 6C. This spectrum agrees well with the spectrum in Figure 6B. The higher resolution of the spectrum in n-hexaned14 than the resolution of the spectrum in n-hexane-h14 arises from smaller superhyperfine coupling of deuterons than that of protons. Discussion Isomerization between 2- and 3-Hexyls. The spectrum of 77 K n-hexane-1,1,1,6,6,6-d6 changes little with the UVirradiation. This also means that the isomerization from 2-hexyl1,1,1,6,6,6-d6 to 3-hexyl-1,1,1,6,6,6-d6 occurs to a small degree, and this indicates that 3-hexyl radical directly isomerizes to 1-hexyl radical:

3-hexyl + hν f 1-hexyl

(1)

The isomerization does not occur through a successive mechanism:

Figure 6. ESR spectra generated from crystalline n-hexane-d14 containing 2 mol % n-hexane-h14 γ-irradiated at 77 K (A) and after successive photolysis for 60 min (B).

d6 is assumed to isomerize to 1-hexyl-1,1,2,6,6,6-d6 and 3-hexyl1,1,1,6,6,6-d6 to 1-hexyl-1,1,3,6,6,6-d6. The difference in the experimental spectra before and after the 60 min UV-irradiation is much less than difference in the simulated spectra before and after the 20% conversion. The amount of the conversion will be much less than 10%. Simulations with different degrees of conversion for 2- and 3-hexyl-1,1,1,6,6,6-d6 also result in the same conclusion. More than 80% of 2- and 3-hexyl-h13 radicals in hexane-h14 isomerize to 1-hexyl-h14 radicals after the 30 min UV-irradiation. It is estimated from the intensity of the outermost lines in Figure 1B. The quantum yield of the photoinduced isomerization for hexyl-d6 radicals will hence be less than 1/16 of the quantum yield for hexyl-h13 radicals. Figure 5 shows ESR spectra of 77 K crystalline n-hexaned14 after γ-irradiation and after successive irradiation of 254 nm light. The spectra are nearly the same. The quantum yield of photoinduced isomerization of hexyl radicals-d13 in 77 K n-hexane-d14 is also very low. Photoinduced Isomerization in n-Hexane-d14. Figure 6 shows ESR spectra of 77 K crystalline n-hexane-d14 containing 2 mol % n-hexane-h14 after γ-irradiation and after successive irradiation of 254 nm light. The spectrum changed with the UV-irradiation. In the figure full lines show spectra due to only hexyl-h13 radicals. Hexyl-d13 radicals are generated from

3-hexyl + hν f 2-hexyl

(2)

2-hexyl + hν f 1-hexyl

(3)

In the successive mechanism, reaction 2 will be little affected by the deuteration. This reaction should occur even for 3-hexyld6. 2-Hexyl-d6 should be accumulated with UV-irradiation when reaction 3 is suppressed. However, this accumulation has not been observed. This result confirms the conclusion in the previous paper:5 we have observed the time course of the ESR spectra of protiated alkyl radicals with UV-irradiation and have concluded that internal alkyl radicals (-CH2CHCH2-) directly isomerize to 1-alkyl radicals (CH2CH2-). Inter- or Intramolecular Reactions? The photoinduced isomerization of hexyl-h13 radicals occurs even in n-hexaned14. This indicates that the isomerization may occur through intramolecular reactions. Hexyl-h13 radicals are surrounded by n-hexane-d14. Hexyl-h13 radicals cannot abstract a hydrogen atom from an adjacent n-hexane-h14 molecule. Isomerization from 2- and 3-hexyl-h13 radicals to 1-hexyl-h13 radicals in n-hexane-d14 is possible only through intramolecular reactions. Mechanism of the Photoinduced Isomerization. The selective isomerization to primary alkyl radical is attained through intramolecular reaction of an excited alkyl radical. The large isotope effect on the isomerization was observed. On the basis of the results, we here try to discuss a more detailed mechanism of the isomerization. A schematic illustration of potential curves for the isomerization is shown in Figure 7. The first excited state of the alkyl radicals was assigned as a Rydberg state.14,15 However, information on the potential curves for the excited state is scarce. The potential curve for the Rydberg state is then estimated from information on cations of alkyl radicals, which will show similar structure to those of the Rydberg states. Primary alkyl cations are less stable than secondary alkyl cations.16 The geometries of the primary alkyl cations are different from those of the primary alkyl radicals in the gas phase.16,17 The structures of proton-added cycloalkane were suggested.17 In the solid phase,

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Takada et al. The tunneling mechanism in the excited state can be examined with photolysis of perdeuterated primary radical. If the isomerization between the primary and secondary radicals proceeds through a tunneling mechanism, isomerization from the primary to the secondary radical will not be observed with the photolysis. However, if there is no isotope effect on the isomerization in the excited state, the photoinduced isomerization to the secondary radical will be observed. In the latter case the observed isotope effect should arise from the isotope effect on the relaxation process. The perdeuterated primary radical will hence isomerize to the secondary radical in the excited state before it relaxes to the ground primary radical.

Figure 7. Schematic illustration of the mechanism of the photoinduced isomerization of alkyl radicals. A photoexcited secondary radical isomerizes through a tunneling mechanism to the primary radical in the excited state. The excited primary radical rapidly relaxes to the ground primary radical.

the position of the carbon atoms in the excited primary radical can deviate only a little from the position of those in the ground primary radical, while hydrogen atoms may change their position to stabilize the excited primary radical. The structure of the excited primary radical would hence be a hydrogen-bridged structure. A similar structure is suggested for excited ethyl radicals in the gas phase.18,19 A probable mechanism of the photoinduced isomerization is indicated in Figure 7: a photoexcited secondary radical isomerizes to a primary radical in the excited state, and it relaxes to the ground primary radical. The primary alkyl is less stable than the secondary radical even in the excited state. The excited primary radical may return to the excited secondary radical. The relaxation of the excited to the ground primary radical should hence be much more efficient than the relaxation of the secondary radical. The large isotope effect of the photoinduced isomerization indicates that the relaxation process of the excited radical is influenced by the deuteration and/or the isomerization in the excited state proceeds through a tunneling mechanism. Comparison of lifetimes of protiated and deuterated secondary radicals in the excited state will hence verify the mechanism. The mechanism expects that the lifetime of the protiated secondary radical is much shorter than that of the deuterated secondary radical.

Acknowledgment. This work is partly supported by a Grantin-Aid for Scientific Research (No. 08240202) from the Ministry of Education, Science and Culture, Japan. References and Notes (1) Yoshida, H. In Handbook of Radiation Chemistry; Tabata, Y., Ito, Y., Tagawa, S., Eds.; CRC Press: Boca Raton, FL, 1991; p 558. (2) Mel’nikov, M. Y.; Fok, N. V. Russ. Chem. ReV. 1980, 49, 131. (3) Mel’nikov, M. Y. Photochemistry of organic radicals; Begell House Inc. Publishers: New York, 1996. (4) Koizumi, H.; Kosugi, S.; Yoshida, H. J. Phys. Chem. 1994, 98, 11089. (5) Koizumi, H.; Kosugi, S.; Yoshida, H. J. Phys. Chem. 1996, 100, 4848. (6) Sprague, E. D.; Willard, J. E. J. Chem. Phys. 1975, 63, 2603. (7) Iwasaki, M.; Ichikawa, T.; Toriyama, K. J. Polym. Sci. 1967, B5, 423. (8) Brown, H. C.; Krishnamurthy, S. J. Am. Chem. Soc. 1973, 95, 1669. (9) Lund, A. J. Phys. Chem. 1972, 76, 1411. (10) Gillbro, T.; Lund, A. Int. J. Radiat. Phys. Chem. 1976, 8, 625. (11) Akimoto, H.; Ohta, N.; Ichikawa, T. Abstracts of Papers, The 37th Symposium on Radiation Chemistry, Japan, Sapporo; Japanese Society of Radiation Chemistry: Takasaki, 1994; Abstract 1P09. (12) Ichikawa, T.; Yoshida, H. J. Phys. Chem. 1992, 96, 7661. (13) Pshezhetskii, S. Y.; 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. (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) Schultz, J. C.; Houle, F. A.; Beauchamp, J. L. J. Am. Chem. Soc. 1984, 106, 3917. (17) Raghavachari, K.; Whiteside, R. A.; Pople, J. A.; Schleyer, P. v. R. J. Am. Chem. Soc. 1981, 103, 5649. (18) Ruscic, B.; Berkowitz, J.; Curtiss, L. A.; Pople, J. A. J. Chem. Phys. 1989, 91, 114. (19) Blomberg, M. R. A.; Liu, B. J. Chem. Phys. 1985, 83, 3995.