4848
J. Phys. Chem. 1996, 100, 4848-4852
Mechanism of Photoinduced Isomerization of Alkyl Radicals Trapped in 77 K Solids Hitoshi Koizumi,* Shin Kosugi, and Hiroshi Yoshida Graduate School of Engineering, Hokkaido UniVersity, Kita-ku, Sapporo 060, Japan ReceiVed: September 19, 1995; In Final Form: January 4, 1996X
Photoinduced isomerization of alkyl radicals of n-alkanes trapped in 77 K solids was examined by the ESR method. In crystalline n-alkanes, penultimate radicals (CH3C˙ HCH2-) and interior radicals (-CH2C˙ HCH2-) are converted to primary radicals (C˙ H2CH2-) with irradiation of 254 nm light. This isomerization occurs regardless of the crystal structure. There are isosbestic points in the integrated ESR spectra with different times of the UV irradiation: the ratio of the penultimate to the interior radicals remains constant during the isomerization. This indicates that the interior radicals are directly converted to the primary radicals. The isomerization does not proceed by successive reaction of hydrogen migrations, which mechanism may cause the formation of the penultimate radicals. Photoinduced isomerization from the penultimate radicals to the primary radicals was observed for alkyl radicals in methanol-d4 glass. The photoinduced isomerization may hence proceed by intramolecular reactions and does not require intermolecular abstraction of a hydrogen atom from an adjacent alkane molecule.
R• + hν f R(1)
Introduction Alkyl radicals trapped in low-temperature solids cause isomerization with excitation of ultraviolet light.1-4 Secondary alkyl radicals trapped in 77 K solid alkanes initially isomerize to primary alkyl radicals with irradiation of 254 nm light. This isomerization occurs regardless of the structure of the solids.1 However, the mechanism of the isomerization is still ambiguous, and it has not been explained why the secondary radicals selectively isomerize to the primary radicals. In this paper, we discuss two subjects on the mechanism of the photoinduced isomerization. One is whether the isomerization occurs through successive reactions of hydrogen atom transfers or direct reaction. Several kinds of alkyl radicals are generated in irradiated crystalline n-alkanes. The radicals can be classified into three groups which are distinguishable with ESR: primary (C˙ H2CH2-, R(1)), penultimate (CH3C˙ HCH2-, R(2)), and interior (-CH2C˙ HCH2-, R(3)) radicals. Both R(2) and R(3) isomerize to R(1) with irradiation of 254 nm light. The isomerization from R(3) to R(1) may occur through successive reaction
R(3) + hν f R(2)
(1)
R(2) + hν f R(1)
(2)
R(3) + hν f R(1)
(3)
or direct reaction
We observed the time course of the ESR spectra with the UV irradiation. These observations allowed us to discuss by which mechanism the isomerization occurs. The ratio of the three radicals will show different kinetic behavior in the above two mechanisms: the penultimate radicals are generated by reaction 1 in the mechanism of the successive reaction, whereas the penultimate radicals simply isomerize to the primary radicals in the mechanism of the direct reaction. The other subject is whether the photoinduced isomerization occurs by intramolecular reaction * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +81-11-706-7897. X Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-4848$12.00/0
(4)
or by intermolecular reaction
R• + RH + hν f RH + R(1)
(5)
where R• is R(2) or R(3) and RH is an alkane molecule. We examined the photoinduced isomerization in solids other than neat alkanes, where the alkyl radicals cannot abstract a hydrogen atom from an adjacent alkane molecule. Experimental Section Samples used were decane (stated purity >99%), undecane (>99%), 1-chlorohexane (>95%), 2-chlorohexane (>98%), and 3-chlorohexane (>95%) supplied by Tokyo Kasei Co. Ltd., hexane (>97%) and perdeuterated methanol (99.8%) supplied by Merck, and perdeuterated 2-propanol (>99%) supplied by Janssen Chimica. Alkanes were purified by passage through a column containing activated alumina and silica gel. Perdeuterated methanol containing 5 vol % perdeuterated 2-propanol as a glass-forming agent was used as solvent for hexane and the chlorohexanes. The solvent becomes a glassy solid at 77 K. The solutes are more soluble in the glassy solids than in neat methanol glass. The concentration of hexane was 2 vol %, and the concentrations of the chlorohexanes were 1 vol %. They were degassed by freeze-pump-thaw cycles and sealed in high-purity quartz tubes. All samples were γ-irradiated at 77 K to a dose of 10 kGy. UV irradiations with a low-pressure mercury lamp were also done at 77 K. ESR spectra were measured with a Varian X-band E-line spectrometer. Results Photoinduced Isomerization of Alkyl Radicals in Crystalline n-Alkanes. Figure 1 shows ESR spectra for crystalline decane after γ-irradiation and after successive irradiation of 254 nm light. In the previous paper, we have assigned seven line spectra in Figure 1B or C to R(2). However, this assignment is incorrect. Akimoto et al.5 have recently reported that alkyl radicals in crystalline decane are converted to R(1) with UV irradiation. n-Alkanes (CnH2n+2) with even n (6 < n < 26) crystallize in the triclinic form, while n-alkanes with odd n (11 © 1996 American Chemical Society
Alkyl Radicals Trapped in 77 K Solids
Figure 1. ESR spectra generated from crystalline decane γ-irradiated at 77 K (A) and after successive photolysis for 20 min (B), 60 min (C), and 240 min (D) with 254 nm light, and simulated spectrum for R(1) (E). 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.
< n < 43) do in the orthorhombic form. The observed spectrum of R(1) in the triclinic crystal of decane is different from the spectra of R(1) in the orthorhombic crystals of n-alkanes.1,6 The spectra in Figure 1B and C contain the spectrum of R(1). They are composed of the spectra of R(1), R(2), and R(3). The spectrum becomes the one of R(1) after the UV irradiation for 240 min (Figure 1D). The simulated spectrum for R(1) in Figure 1E shows good agreement with this spectrum. The difference in the spectrum of R(1) in the triclinic and orthorhombic crystals arises from the difference in the coupling constants of the β-protons. It is due to the difference in the conformation of the end CH2 group. The near axial symmetry of the coupling tensor for the two R-protons of both crystals of R(1) arises from rapid exchange of the two protons.7 The concentration of the radicals was nearly constant with UV irradiation of less than 60 min. However, the concentration decreased about 10% with UV irradiation for 120 min and decreased 30% with UV irradiation for 240 min. The isomerization to R(1) was also observed in the crystalline hexane. The spectrum for hexane becomes the one of R(1) after UV irradiation for 30 min (Figure 2C). The integrated spectra of the ESR spectra in Figures 1 and 2 are shown in Figure 3. The intensity of the integrated spectra is normalized to show the same value in area. As indicated with arrows, the spectra for both the alkanes show six isosbestic points. The positions of the isosbestic points depend on the molecules. Although the difference in the positions of four of the points is small, the positions of the innermost points for decane are apparently different from those for hexane. Integrated spectra for R(1), R(2), and R(3) are compared in Figure 3C. The positions of the isosbestic points in Figure 3A and B are indicated. The spectrum for R(1) is the spectrum obtained after the 240 min photolysis of alkyl radicals in crystalline decane, while the spectra for R(2) and R(3) are simulated ones. There are no points where all of the three spectra show the same intensity.
J. Phys. Chem., Vol. 100, No. 12, 1996 4849
Figure 2. ESR spectra generated from crystalline hexane γ-irradiated at 77 K (A) and after successive photolysis for 10 min (B) and 30 min (C) with 254 nm light.
Figure 4 shows ESR spectra for crystalline undecane after γ-irradiation and after successive irradiation with 254 nm light. A significant amount of R(1) is present in triclinic crystals of n-alkanes after radiolysis.6 R(2) and R(3) isomerize to R(1) with UV irradiation.1 The integrated spectra of Figure 4 are shown in Figure 5A. There are isosbestic points. The integrated spectra for R(1), R(2), and R(3) are shown in Figure 5B. The spectrum for the primary radicals is the spectrum obtained after the 30 min photolysis of alkyl radicals in crystalline undecane, while the spectra for the penultimate and interior radicals are simulated ones. There is no point where all of the three spectra show the same intensity. Relationship between Isosbestic Points and Concentration Ratio of Three Alkyl Radicals. ESR spectra for 77 K n-alkanes consist of spectra of the three kinds of alkyl radicals: R(1), R(2), and R(3). The absorption coefficient (H) at magnetic field H is
(H) ) 1(H)χ1 + 2(H)χ2 + 3(H)χ3
(6)
where 1(H), 2(H), and 3(H) are the absorption coefficients for R(1), R(2), and R(3), respectively. χ1, χ2, and χ3 are the mole fractions of R(1), R(2), and R(3). They hence satisfy
χ1 + χ2 + χ3 ) 1
(7)
There are isosbestic points in the integrated ESR spectra during the photoinduced isomerization: six points for hexane and decane, and eight points for undecane. The ESR spectra finally become the spectra for R(1). Hence, at the isosbestic points,
(Hi) ) 1(Hi)
(8)
where Hi is the magnetic field at an isosbestic point. When eqs 7 and 8 are substituted into eq 6, we obtain
2(Hi) - 1(Hi) 3(Hi) - 1(Hi)
)
χ3 χ2
(9)
The ratio of R(3) to R(2) is hence constant during the isomerization. Equation 9 is converted to
4850 J. Phys. Chem., Vol. 100, No. 12, 1996
Koizumi et al.
Figure 4. ESR spectra generated from crystalline undecane γ-irradiated at 77 K (A) and after successive photolysis for 10 min (B) and 30 min (C) with 254 nm light.
Figure 3. Integrated ESR spectra for decane (A) and hexane (B) and for the primary, penultimate, and interior radicals (C): (A) after γ-irradiation (s), after successive photolysis for 20 min (- - -), 60 min (-‚-‚-), and 240 min (‚‚‚) (the positions of the isosbestic points are indicated by arrows); (B) after γ-irradiation (s), after successive photolysis for 10 min (- - -) and 30 min (‚‚‚) (the positions of the isosbestic points are indicated by arrows); (C) primary (s), penultimate (‚‚‚) and interior (-‚-‚-) radicals (the positions of the isosbestic points of decane (b) and hexane (O) are indicated). The spectrum of the primary radicals is that of decane after 240 min of photolysis. The spectra of the penultimate and interior radicals are simulated ones. The following hyperfine coupling constants and line shapes are assumed for the simulation; for the penultimate radicals, aβ ) 2.3 mT for the three β-CH3 protons, aβ ) 3.3 mT for the two β-CH2 protons, A1 ) -1.17 mT, A2 ) -1.96 mT, and A3 ) -3.52 mT for the one R-proton, and the line shape is Lorentzian with ∆Hmsl ) 0.86 mT; for the interior radicals, aβ ) 3.3 mT for the four β-protons, A1) -1.17 mT, A2 ) -1.96 mT, and A3 ) -3.52 mT for the one R-proton, and the line shape is Lorentzian with ∆Hmsl ) 0.86 mT.
1(Hi) )
2(Hi)χ2 + 3(Hi)χ3 χ2 + χ3
(10)
positions of the isosbestic points indicate that the ratio of R(2)/ R(3) for decane is smaller than that for hexane. This agrees with the results reported by Gillbro and Lund.8 In the spectra of undecane, the third isosbestic points from the outside are considerably larger than the values of 2(Hi) and 3(Hi) (Figure 5B). The problem will be in the spectra of R(2) or R(3): the spectra were calculated with the hyperfine coupling constants in the triclinic crystals.6,9 The spectra in the orthorhombic crystal will be a little different from those in the triclinic crystals, as for the spectra of R(1). The difference will arise from the different conformation of the β-protons. Photoinduced Isomerization in 77 K Perdeuterated Methanol Glass. ESR spectra for alkyl radicals of hexane in 77 K perdeuterated methanol glasses are shown in Figure 6A and B. The ESR spectrum after γ-irradiation (Figure 6A) consists of R(2) of hexane and hydroxymethyl radicals (•CD2OD).10 However, the spectrum of the hydroxymethyl radials is restricted within about 5 mT around the center of the spectrum. With irradiation of 254 nm light, the spectrum of R(2) changes to the one in Figure 6B. This is the spectrum for R(1). The ESR spectra of alkyl radicals generated by γ-irradiation of chlorinated hexanes in the methanol-d4 glass are shown in Figure 6C-E: R(1) from 1-chlorohexane (C), R(2) from 2-chlorohexane (D), and R(3) from 3-chlorohexane (E). The ESR spectra in Figure 6B agree well with the ones in Figure 6C. Photoinduced isomerization of R(2) to R(1) hence occurs in deuterated methanol glass. Discussion Successive or Direct Reaction? The concentration ratio of R(2)/R(3) remains constant during the photoinduced isomerization. In order to determine whether the photoinduced isomerization proceeds by the successive or direct reaction, we compare the kinetic behavior of the ratio of R(2)/R(3) for the successive and the direct mechanisms. For the successive mechanism, we here consider the following mechanism k32
R(3) + hν 98 R(2) k21
The difference in the positions of the isosbestic points of decane and hexane arises from the different ratio of R(2)/R(3). The
(11)
R(2) + hν 98 R(1) (12) where k32 is the rate constant for photoinduced isomerization
Alkyl Radicals Trapped in 77 K Solids
J. Phys. Chem., Vol. 100, No. 12, 1996 4851
Figure 6. ESR spectra generated from γ-irradiated hexane dissolved in the glassy matrix of methanol-d4 at 77 K (A) and after 60 min of photolysis with 254 nm light (B). ESR spectra of alkyl radicals generated by γ-irradiation of chlorinated hexanes in the same matrix: spectra of primary radicals from 1-chlorohexane (C), penultimate radicals from 2-chlorohexane (D), and interior radicals from 3-chlorohexane (E). Dotted lines are due to the spectra of radicals from the matrix.
In the direct mechanism, both R(2) and R(3) directly isomerize to R(1): k31
R(3) + hν 98 R(1)
(15)
k21
R(2) + hν 98 R(1) (16) where k31 is the rate constant for photoinduced isomerization from R(3) to R(1) and k21 is the rate constant from R(2) to R(1). The concentration ratio of R(2) and R(3) is [R(2)] [R(2)]0 exp(-(k21 - k31)It) ) [R(3)] [R(3)]0 Figure 5. Integrated ESR spectra for undecane (A) for primary, penultimate, and interior radicals (B): (A) after γ-irradiation (s), after successive photolysis for 5 min (- - -), 10 min (-‚-‚-) and 30 min (‚‚‚); (B) primary (s), penultimate (‚‚‚), and interior (-‚-‚-) radicals (the isosbestic points (b) of undecane are indicated). The spectrum of the primary radicals is that of undecane after the 30 min photolysis. The spectra of the penultimate and interior radicals are simulated ones: they are the same spectra as those in Figure 3C.
from R(3) to R(2) and k21 is the rate constant from R(2) to R(1). When k32 * k21, the concentration ratio of R(2)/R(3) is given by
k32 [R(2)] + ) k [R(3)] 21 - k32 [R(2)]0
{
[R(3)]0
-
}
k32 exp(-(k21 - k32)It) (13) k21 - k32
where [R(2)]0 is the initial concentration of R(2), [R(3)]0 is the initial concentration of R(3), and I is the light intensity. When k32 ) k21, the ratio is
[R(2)]0 [R(2)] ) k32It + [R(3)] [R(3)]0
(14)
The ratio varies with increasing irradiation time for both cases. This behavior cannot explain the experimental results: the ratio of R(2) and R(3) keeps the constant value of [R(2)]0/[R(3)]0 during the isomerization.
(17)
If we assume k21 ≈ k31, eq 17 agrees with the experimental results. The photoinduced isomerization will hence proceed by the direct mechanism. Inter- or Intramolecular Reactions? The photoinduced isomerization of alkyl radicals occurs in methanol-d4 glass: the photoinduced isomerization to R(1) does not require abstraction of a hydrogen from an adjacent alkane molecule. These results indicate that the isomerization reactions occur through intramolecular reactions. The selectivity of the photoinduced isomerization in solid alkanes is independent of the structure of the solids: the secondary radicals are selectively converted to R(1) by photoexcitation.1 This fact also supports that the isomerization occurs through intramolecular reactions. The rate of intermolecular reaction in the solids is expected to depend on the orientation of adjacent molecules. If the isomerization occurs through intermolecular reactions, the selectivity of the isomerization will depend on the structure. In conclusion, the penultimate and the interior radicals directly isomerize to the primary radicals through the intramolecular reaction. The selective isomerization to the primary radicals is hence attained through intramolecular migration of a hydrogen atom in an excited alkyl radical. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan.
4852 J. Phys. Chem., Vol. 100, No. 12, 1996 References and Notes (1) Koizumi, H.; Kosugi, S.; Yoshida, H. J. Phys. Chem. 1994, 98, 11089. (2) Mel'nikov, M. Y.; Fok, N. V. Russ. Chem. ReV. 1980, 49, 131. (3) Sprague, E. D.; Willard, J. E. J. Chem. Phys. 1975, 63, 2603. (4) Iwasaki, M.; Ichikawa, T.; Toriyama, K. J. Polym. Sci. 1967, B5, 423. (5) 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.
Koizumi et al. (6) Iwasaki, M.; Toriyama, K.; Fukaya, M.; Muto, H.; Nunome, K. J. Phys. Chem. 1985, 89, 5278. (7) Fujimoto, M.; Janecka, J. J. Chem. Phys. 1971, 55, 5. (8) Gillbro, T.; Lund, A. Int. J. Radiat. Phys. Chem. 1976, 8, 625. (9) Gillbro, T.; Kinell, P.-O.; Lund, A. J. Phys. Chem. 1969, 73, 4167. (10) Ichikawa, T.; Yoshida, H. J. Phys. Chem. 1992, 96, 7661.
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