Photoinduced Isomerization of Alkyl Radicals Trapped in Solid Alkanes

Mechanism of Photoinduced Isomerization of Alkyl Radicals Trapped in 77 K Solids. Hitoshi Koizumi, Shin Kosugi, and Hiroshi Yoshida. The Journal of Ph...
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J. Phys. Chem. 1994, 98, 11089-11093

11089

Photoinduced Isomerization of Alkyl Radicals Trapped in Solid Alkanes Hitoshi Koizumi,* Shin Kosugi, and Hiroshi Yoshida Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan Received: April 11, 1994; In Final Form: June 20, 1994@

Photoinduced isomerization of alkyl radicals trapped in y-irradiated 77 K solid alkanes has been studied using electron spin resonance method. Alkyl radicals trapped in the crystalline solids of n-alkanes from n-CgHlg to n-C13Hzg and in the glassy solids of 2- and 3-methylheptanes have been measured. Interior radicals (-CHZCHCHz-) trapped in the orthorhombic crystals of n-alkanes are converted to primary radicals (CH2CH2-) with irradiation of 254 nm light, whereas the interior alkyl radicals in the triclinic crystals of n-alkanes are converted to penultimate alkyl radicals (CH3CHCH*-). This selectivity of photoconversion is explained by the initial formation of the primary radicals in both the crystals and thermal radical conversion to the penultimate alkyl radicals in the triclinic crystals. Selective photoisomerization to primary radicals (CHzC7H15) has also been observed for penultimate radicals ( C H ~ C H C ~ Htrapped I ~ ) in y-irradiated 2- and 3-methylheptanes. Excited alkyl radicals trapped in solid alkanes hence initially isomerize to primary radicals regardless of the structure of the solids.

Introduction Photoinduced conversions of trapped radicals in solid alkanes have been reported by several However, the mechanisms of photoinduced reactions of alkyl radicals are little known and no systematic study of the effect of molecular and crystal structures has been carried out. Willard and co-workers have reported photoinduced isomerization and photoinduced decay of alkyl radicals trapped in n-alkanes2 With exposure to the 254 nm light, interior radicals (-CHzCHCHz-) are converted to penultimate radicals (CH~CHCHZ-). They have only examined n-alkanes of triclinic crystal structure: n-hexane, n-heptane, n-octane, and n-decane. They have also observed a major improvement in the resolution of the ESR spectra of the radicals trapped in y-irradiated 3-methylpentane, 3-ethylpentane, and 3-methylheptane with UV irradiati~n.~ However, they have not concluded whether this spectral change is due to isomerization or conformational change of the radicals. Iwasaki et al. have reported that tertiary alkyl radicals produced in y-irradiated isobutyl bromide and polypropylene are converted to primary radicals with W irradiati~n.~,~ Mel'nikov et al. have studied the photolysis of alkyl radicals trapped in y-irradiated monomethyl- and ethylalkanes. They explained that photoinduced change of the ESR spectra is mainly due to elimination of methyl and ethyl radicak6s7 As pointed out by Willard et aL3 and later as mentioned by Mel'nikov himself,' the elimination of methyl and ethyl radicals appears doubtful. Selectivity of alkyl radical formation by ionizing radiation in solid alkanes much depends on the structure of the solids. Prominent primary alkyl radicals form in the orthorhombic crystal's of n-alkanes, while the yield of the primary alkyl radicals is very low in the triclinic crystals of n-alkanes at 77 K. This has been explained by the difference in the alignment of the two neighboring chains in the orthorhombic and the triclinic crystals.8 The primary radicals trapped in the triclinic crystals can thermally abstract a hydrogen atom bonded to a penultimate carbon of an adjacent molecule and convert to the penultimate radicals, whereas the primary radicals in the orthorhombic crystals cannot abstract it and remain unaltered.

* To whom correspondence should be addressed. @

Abstract published in Advance ACS Abstracts, October 1, 1994.

The selectivity of the alkyl radical formation also much depends on whether the solid is in the crystalline or glassy state. In the glassy solids the penultimate radicals are selectively produced from normal and monomethyl alkanes, while both the penultimate and the interior radicals are produced in the crystalline s ~ l i d s . ~ - It ~ lis then interesting to study how the photoinduced reactions of alkyl radicals depend on the structure of the solids. In this paper we have investigated the photoinduce 1 conversion of alkyl radicals trapped in both the orthorhombic and the triclinic crystals of normal alkanes and in the glassy solids of monomethylalkanes. We have examined the dependence of the conversion on the structure of the solids to get further insight on the photoinduced reactions of excited radicals trapped in the condensed phase.

Experimental Section Samples used were normal alkanes with 8 to 13 carbon numbers and 2- and 3-methylheptanessupplied by Tokyo Kasei Co. Ltd. The n-alkanes were used with no further purification, while 2- and 3-methylheptanes were purified by passage through a column containing activated alumina and silica gel. They were degassed by freeze-pump-thaw cycles and sealed in highpurity quartz tubes. Some samples of n-alkanes were purified with the column, but no significant difference was observed in ESR spectra with the purification. All samples were y-irradiated at 77 K to a dose of 10 kGy. Photolyses with low-pressure mercury lamp were also done at 77 K. ESR spectra were measured with a Varian E-line X-band ESR spectrometer.

Results Photoinduced Isomerization of Alkyl Radicals Trapped in Crystalline n-Alkanes. Figure 1 shows the ESR spectra of y-irradiated 77 K polycrystalline n-alkanes before and after photolysis with 254 nm light. With 30 min UV irradiation, the spectra considerably changed, while the concentrations of the radicals were nearly constant. They were stable for at least 1 week at the temperature of liquid nitrogen. In the spectra of n-C11 and n-C13 the outermost lines much decreased. The spectra became six-line ones with the intensity ratio of 1:3:4: 4:3:1. These spectra are due to primary alkyl radicals (CHz-

0022-365419412098-11089$04.5010 0 1994 American Chemical Society

11090 J. Phys. Chem., Vol. 98, No. 43, 1994

Koizumi et al.

M

A Decane

I F

II

Dodecane

I

5mT

C

4

Nonane

G

n nl 4 * li/lil n

n

Undecane

nn

Tridecane

?,i" 5mT

c---y 1

Figure 1. ESR spectra generated from normal alkanes y-irradiated at 77 K and after successive 30 min photolysis with 254 nm light: n-Cs (A, B), n-C9 (C, D),n-Clo (E,F), ~ - C I(G, I HI,n-Clz (I, J), n-Cl3 (K,L).

CH2-, R(1)). As shown in Figure 2, the simulated spectrum for the primary radicals agrees well with the experimental ones. The spectra of n-C8, n-C9, and n-C10 changed to seven-line spectra. The spectrum of n-C12 also became a similar one to those except for the shape around the center. The spectra of n-CS-Cl0 and C12 are mainly due to penultimate alkyl radicals (CH3CHCH2-, R(2)). As shown in Figure 2, the simulated spectrum for the penultimate radicals agrees with the experimental ones. Although the spectrum for n-C12 and the simulated one for R(2) are a little different around the center, the spectrum for n-C12 will be mainly due to R(2). The change of the shape is due to splitting of the center line of the sevenline spectrum. The shape of the center line gradually changes from n-C8 to n-C12, while the integrated intensity of the line is nearly constant. This would be due to an alteration of hyperfine coupling constants by a small change of molecular conformation or motion. There have been many studies on radicals produced in solid n-alkanes by ionizing r a d i a t i ~ n . * ~ 'Three ~ J ~ kinds of radicals distinguishable by ESR are trapped: primary (CHzCHz-, R(l)), penultimate (CH3CHCH2-, R(2)), and interior (-CHzCHCH2-, R(3)) radicals. According to the results by Iwasaki et al., the yield of R(l) is relatively high in the orthorhombic crystals of n-alkanes (about 40% in y-irradiated n-C13 at 77 K), and the remaining radicals are R(2) and R(3).8 In the triclinic crystals the yield of R(1) is very low and the trapped radicals are mainly R(2) and R(3). The above results hence indicate that R(2) and R(3) are converted to R(1) in the n-C11 and C13 crystals, while R(3) are converted to R(2) in the other crystals with the photolysis of 254 nm light. To confirm the above conclusions, we separated the converted components of the radicals by subtracting the spectrum of the

W-irradiated samples from the one before the irradiation. The integrated spectra of n-C11 are shown in Figure 3. After 60 min, W irradiation, the concentration of R(2) and R(3) is very low and the spectrum is almost the one for R( 1). The spectrum of R( 1) is very sharp, and it is distinguishableeven in a spectrum containing the other alkyl radicals. We then subtract the spectrum of R(l) from the spectrum before the UV irradiation until the sharp peaks of R( 1) vanish. After subtracting 30% of R(1), this is achieved. As shown in Figure 3, a six-line spectrum is obtained. This is the spectrum of R(3). The differentiated spectrum of this is compared with a simulated spectrum for R(3) in Figure 4. They show a good agreement. This indicates that the yield of R(2) in y-irradiated n-C1 1 is low. In y-irradiated n-C 11 about 30% of the trapped radicals are R( 1) and the rest of them are almost R(3). This result agrees qualitatively with the one reported by Iwasaki et al.* They have estimated the ratio of R(2):R(3) in y-irradiated n-C13 is 18532. The integrated spectra of n-C10 are also shown in Figure 3. The spectrum after 60 min UV irradiation is almost that of R(2). Subtracting 30% of this spectrum from the one before the UV-irradiation, a six-line spectrum is also obtained. This spectrum is the one of R(3). As shown in Figure 4, the differentiated spectrum of this agrees well with the simulated one of R(3). In y-irradiated n-C10 about 30% of the trapped radicals are then R(2) and the rest of them are R(3). Within the uncertainty of the estimation, this result agrees with the one for y-irradiated n-C10 single crystal measured by Gillbro and Lund.14 They have reported that the relative concentrations of R(2) and R(3) are 42.5 and 57.5%, respectively. Photoinduced reactions of alkyl radicals trapped in 77 K solid n-alkanes are summarized by the following:

Isomerization of Alkyl Radicals Trapped in Solid Alkanes

n

A

J. Phys. Chem., Vol. 98, No. 43, 1994 11091 U nd ecane

Decane

5mT,

SmT,,

Figure 3. Integrated ESR spectra generated from n-Clo and n-Cl1 at 77 K: the spectra obtained after y-radiolysis (A, D), after successive 60 min photolysis with 254 nm light (B, E), and difference spectra obtained by subtracting 30% of B from A (C) and 30% of E from D

(F).

..........._...___.. -..

-1

Figure 2. Observed and simulated ESR spectra of alkyl radicals photolyzed with 254 nm light: observed (A) and simulated (B) spectra of n-Clo, and observed (C) and simulated (D) spectra of n-Cl1. The following hyperfine coupling constants and line shapes are assumed for the simulation; (B) up = 2.3 mT for three j3-CH3 protons, up = 3.1 and 3.5 mT for two B-CHz protons, a1 = -1.17, a2 = -1.96, a3 = -3.52 mT for one a-proton, and Lorentzian type line shape with A&,I = 0.6 mT; (D) up = 2.0 and 4.0 mT for two B-CHz protons, a1 = -1.76, a2 = -1.96, a3 = -2.9 mT for two a-protons, and Gaussian line shape with A&,,I = 0.5 mT.

-CH2CHCH2-

+ hv - CH2CH2-

5mT

(n-Cl1 and C13) (1) -CH2CHCH2-

+ hv

-C

CH3CHCH2(n-C8-C10, C12) (2)

The crystals of n-C11 and C13 are orthorhombic form, while those of n-C8-C10 and C12 are triclinic form. Hence the above results mean that R(3) are efficiently converted to R(l) in the orthorhombic crystals with exposure to 254 nm light, whereas R(3) are converted to R(2) in the triclinic crystals. Photoinduced Isomerization of Alkyl Radicals Trapped in Glassy Branched Alkanes. Figure 5 shows the ESR spectra of y-irradiated glassy 2-methylheptane before and after W irradiation. With exposure to the UV light the peaks indicated by arrows increased. This spectrum is stable for at least 1 week at the temperature of liquid nitrogen. The difference between the spectra is shown in Figure 5C. This is due to the spectrum of primary alkyl radicals. The simulated spectra for two possible primary radicals (R( la); CH&H(CH3)C&CH2 and R( lb); C S H ~ ~ C H ( C H ~ ) are C Hshown ~) in Figure 7. As shown in Figure 5D, the spectrum simulated from these two spectra with the abundance ratio of R(1a): R(1b) = 1:2 shows good agreement with the spectrum in Figure 5C.

Figure 4. Differentiated spectra of Figure 3C and F (A, B), and simulated spectrum for -CHzCHCHz- (C). The following hyperfine coupling constants and line shapes are assumed: up = 3.3 mT for four B-CHz protons, ul = -1.17, a2 = -1.96, u3 = -3.52 mT for one = 0.8 mT. a-proton, and Lorentzian line shape with MmSl

Figure 6 shows the ESR spectra of y-irradiated glassy 3-methylheptane before and after UV irradiation. With photolysis the intensity of the outermost lines much decreased. This spectrum is also stable for at least 1 week at the temperature of liquid nitrogen. The spectrum after the W irradiation is mainly due to primary radicals (C&I&H(CH3)CH2CH2 or CH2C3H6CH(CH~)C~HS), since the simulated spectrum for these radicals agrees well with the experimental one. Total radical concentrations in both 2- and 3-methylheptanes are constant before and after the W irradiation. Radicals produced in y-irradiated 2- and 3-methylheptane solids are mainly penultimate radicals (CH3CHC6H13),11and hence the above results indicate that the penultimate radicals are converted to the primary radicals with UV irradiation.

Discussion Selectivity of Photoinduced Isomerization in Crystalline n-Alkanes. The selectivity of the photoinduced isomerization

Koizumi et al.

11092 J. Phys. Chem., Vol. 98, No. 43, 1994

0 3-methylheptane

2-methylheptane

A

B

C

D

. I

,

:,: ::,

; j,

~

, 5mT,

j/

Figure 5. ESR spectra generated from 2-methylheptane at 77 K: the spectra obtained after y-radiolysis (A) and after successive 30 min photolysis with 254 nm light (B), the spectrum for a component increased with the photolysis (C), the spectrum simulated from the spectra in Figure 8A (R(1a))and B (R(1b)) with the abundance ratio of R(la):R(lb) = 1:2 (D).

in the crystalline normal alkanes shows similar dependence on the crystal structure to the selectivity of radical formation by ionizing radiation. With the radiolysis, the primary radicals R( 1) prominently form in the triclinic crystals, while in the orthorhombic crystals the yield of R(l) is very low and the penultimate radicals R(2) form. The interior radicals R(3) form in both alkanes. The selectivity of the radical formation in the radiolysis has been explained by the difference in the alignment of the two neighboring chains in the orthorhombic and the triclinic crystals.8 As illustrated in Figure 8, the selectivity in the photoinduced isomerization is explained by a similar mechanism. R(3) initially isomerize to R( 1) both in the triclinic and the orthorhombic crystals. In the triclinic crystal R( 1) can abstract a hydrogen atom bonded to a penultimate carbon atom of an adjacent molecule through a thermal reaction, and R(l) then converts to the more stable R(2). In the orthorhombic crystal R( 1) can abstract none of the hydrogens bonded to the penultimate carbon, and R(l) remains unaltered. This is due to the difference in the alignment of the two neighboring chains in the triclinic and the orthorhombic crystals. The molecular chains of n-alkanes in the orthorhombic crystal are aligned along the c-axis, and their chain ends are on the same plane perpendicular to the c-axis.15 On the other hand, the chain ends of adjacent molecules in the triclinic crystals are shifted along the chain axis by about one methylene unit.16 Hence the nearest hydrogen atom to R( 1) is the one bonded to the terminal carbon atom of the adjacent molecule in the orthorhombic crystals, whereas it is a hydrogen bonded to the second carbon atom in the triclinic crystals. R(1) in the latter crystals can then abstract the hydrogen bonded to the second carbon atom, and they convert to R(2) through the intermolecular reaction. However,

::

:: :;:j $

I

'

:I ::

Figure 6. ESR spectra generated from 3-methylheptane at 77 K: the spectra obtained after y-radiolysis (A) and after successive 30 min photolysis with 254 nm light (B), the spectrum for a component increased wlth the photolysis (C), the spectrum simulated for CnH%+1CH2CH2as in Figure 7A (D).

B

II

I

Figure 7. ESR spectra simulated for CnH%+lCH2CH2(A) and for (B). The following hyperfine coupling constants Cn,H%,+1CH(CH3)CH2 and line shapes are assumed for the simulation: (A) up = 2.0 and 4.0 mT for two B-CH2 protons, ul = -1.76, a2 = -1.96, u3 = -2.90 mT for two a-protons, and Lorentzian type line shape with AHms,= 0.8 mT; (B) ap = 3.3 mT for one B-CH proton, a1 = -1.76, a2 = -1.96, a3 = -2.9 mT for two a-protons, and Lorentzian line shape with A H m l = 0.8 mT.

R(l) in the former crystals cannot abstract this hydrogen at 77 K since the direction of the unpaired electron orbital is not suitable for the abstraction.8 Selective Formation of Primary Radicals in Photoinduced Isomerization. The above results indicate that secondary alkyl radicals (penultimate and interior alkyl radicals) both in crystalline and glassy solid alkanes are initially converted to primary alkyl radicals by photoexcitation. Next we discuss how this selectivity occurs.

Isomerization of Alkyl Radicals Trapped in Solid Alkanes

J. Phys. Chem., Vol. 98, No. 43, 1994 11093

A. Orthorhombic crystal (1 1 s n (odd) s 39 )

a-axisr

B. Triclinic crystal ( n (even) 5 26, n (odd) 5 9 )

a-axis’ Figure 8. Photoinduced reactions of alkyl radicals trapped in the orthorhombic (A) and the triclinic (B) crystals of n-alkanes. Interior alkyl radicals initially convert to primary alkyl radicals in both the crystals. The primary alkyl radicals in the triclinic crystal thermally convert to penultimate alkyl radicals through subsequent hydrogen abstraction from an adjacent molecule.

The photoinduced isomerization can be divided into two kinds of processes: photoexcitation and reaction of excited radicals. They are described as follows: R(pri)

+ hv

R(sec)

+ hv

-

R(pri)*

(3)

R(sec)*

(4)

R(Pri)* \R(Pri)

R(sec)*

(5)

R(sec)

(6)

R(pri)

(7)

R( sec)

(8)

7

where R(pri) and R(sec) are primary and secondary alkyl radicals, respectively. The selectivity in the isomerization is then caused either by difference in the photoabsorption coefficients of the primary and the secondary alkyl radicals or by the selectivity of reactions of the excited alkyl radicals. However, the difference in the photoabsorption coefficients will not explain the high selectivity of the photoisomerization. If we assume that the reactions of the excited radicals show no selectivity on the formation of the primary and the secondary radicals, the maximum concentration ratio of the primary radicals to the secondary radicals is determined by the reciprocal of the ratio of their absorption coefficients. The photoabsorption coefficients of primary and secondary alkyl radicals have been reported by Wendt and Hunziker.17 The absorption coefficient for a primary radical (ethyl radical) at 254 nm is 121 mol dmP3 cm-’, and the coefficients for secondary radicals (2-propyl, 2-butyl, and 2-(3,3-dimethyl)butyl radicals) are 350-400 mol dm-3 cm-’. The concentration ratio of the secondary radicals to the primary radicals is thereby calculated to he between 0.3 and 0.35. However, the measured ratio becon,,: 0.1 after the

60 min photolysis of y-irradiated n-C11, and this high selectivity will not be explained by the difference of the absorption coefficients. Especially in the triclinic crystals the selective formation by the difference of the absorption coefficients is improbable, since the primary radicals are subsequently converted to the penultimate radicals through the thermal reaction. The difference in the absorption coefficients of the penultimate and the intemal radicals will be much smaller than in those of the primary and the secondary radicals. The selective formation of the primary radicals is hence attained through the reactions of the excited alkyl radicals. Studies of the excited states of the alkyl radicals and investigations of the photon energy dependence of the photoinduced isomerization will be helpful to clarify the reason for the high selectivity.

References and Notes (1) Mel’nikov, M. Y.; Fok, N. V. Rus. Chem. Rev. 1980, 49, 131. (2) Wilkey, D. D.; Fenrick, H. W.; Willard, J. E. J . Phys. Chem. 1977, 81, 220. (3) Sprague, E. D.; Willard, J. E. J . Chem. Phys. 1975, 63, 2603. (4) Iwasaki, M.; Ichikawa, T.; Toriyama, K. J. Polym. Sci. 1967, B.5, 423. ( 5 ) Iwasaki, M.; Ichikawa, T.; Toriyama, K. Polym. Lett. 1967,5,423. (6) Mel’nikov. M. Y.: Sklvarenko. V. I.: Fok. N. V. Dokl. Akad. Nauk sssd i974,218,875. (7) Sklvarenko, V. I.; Mel’nikov, M. Y. J . Chem. Soc., Chem. Commun. 1975, 167. (8) Iwasaki, M.; Toriyama, K.; Fukaya, M.; Muto, H.; Nunome, K. 1985, 89, 5278. (9) Ichikawa, T.; Yoshida, H. J. Phys. Chem. 1992, 96, 7656. (10) Ichikawa, T.; Yoshida, H. J. Phys. Chem. 1992, 96, 7661. (11) Koizumi, H.; Hashino, M.; Ichikawa, T.; Yoshida, H. Radiat. Phys. Chem. 1992, 40, 145. (12) Gillbro, T.; Lund, A. In?. J . Radiut. Phys. Chem. 1976, 8, 625. (13) Tilquin, B.; Baudson, T. Radiat. Phys. Chem. 1991, 37, 23. (14) Gillbro, T.; Lund, A. Chem. Phys. 1974, 5, 283. (15) Smith, A. E. J . Chem. Phys. 1953, 21, 2229. (16) Nyburg, S. C.; Liith, H. Acta Crystallogr. 1972, B28, 2992. (17) Wendt, H. R.; Hunziker, H. E. J. Chem. Phys. 1984, 81, 717. .