3736
J . Phys. Chem. 1987, 91, 3736-3740
harmonic frequencies of 1093 and 1204 cm-' calculated by Takada and Dupuis.22 Finally, the mode identification scheme of Table I11 permits a qualitative description of some of the changes in vibrational structure on excitation from the ground state to the 3s Rydberg state. The frequencies of all three modes A, B, and C diminish slightly from the ground to the excited state. Thus the potential softens somewhat along both the CCC bending and the C H 2 twisting coordinates on excitation of the nominally nonbonding r electron to the 3s Rydberg orbital. The progression in mode A indicates a substantial change in the equilibrium CCC bond angle from ground to excited state. We cannot determine either the sign or magnitude of the change in angle from the available data. The much weaker activity in the torsions B and C indicates that the excited state, like the ground state, is planar or nearly so. The single allyl cation vibrational spacing of 420 f 40 cm-l identified in the photoelectron spectrum of gas-phase allyl is almost surely the CCC bending frequency of the cation. Thus the C C bending frequencies of the 3s Rydberg state and of the cation are the same within current experimental error. More quantitative interpretation of the vibronic band intensity envelopes in REMPI spectra is not possible.
Conclusion We have demonstrated the utility of the new combination of a photolytic source of cold neutral free radicals in a supersonic expansion with the spectroscopic technique of resonance-enhanced multiphoton ionization. The presence of excited vibrational states in the allyl radical beam permitted the determination of both
ground- and excited-state frequencies. The availability of matrix IR and allyl anion photoelectron spectra and especially of highquality ab initio calculations of harmonic frequencies permits a firm identification of observed gas-phase levels with particular low-frequency motions. In the future, we will carry out higher resolution scans of the allyl spectrum and attempt to obtain additional cation vibrational state information from the photoelectron spectroscopy of the cold neutral radical. Use of two probe laser colors will improve the generality of the technique and the signal levels as well. Note Added in Proof. To the extent that the two-photon vibronic transition strength can be approximated by a product of electronic and vibrational (Franck-Condon) factors, bands having Av = f 1 in a nontotally symmetric mode will be symmetry forbidden in an electronically allowed two-photon transition such as 'A, *A2in C,, symmetry. (See Herzberg, G. Electronic Spectra of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1966; pp 173-177 for a discussion of the analogous onephoton case.) If the symmetry labels of Table I11 are correct, then it appears that transitions such as Bh, ,C ,!, etc., are vibronically induced. This seems particularly plausible for a Rydberg state due to its close proximity to other excited states. We thank Dr. J. W. Hudgens for pointing out this possibility.
-
Acknowledgment. We thank Dr. Alec Wodtke for suggesting the quartz photolysis tube, Professor Richard N. Zare for a helpful discussion of possible REMPI line broadening mechanisms, and the National Science Foundation for support of this research under Grant No. CHE-8302856.
Radtcal Catlons of Selectively Deuteriated 3-Methylpentanes and 3-Methylhexanes Produced in y-Irradiated CF,CICFC12 Matrices As Studied by ESR Nobuaki Ohta and Takahisa Ichikawa* Department of Applied Physics and Chemistry, Hiroshima University, Saijyo, Higashi- Hiroshima 724, Japan (Received: December 23, 1986)
The cations of 3-methyl~entane-h,~ (3MP-h) and 3-methylhexane-h16(3MHX-h) are explained with a model where the unpaired electrons are coupled with three trans C-H, protons with respect to the C3-C4 u bonds. The coupling constants of the protons (a") are determined in consideration of the results obtained from the deuteriated compounds: aH(3MP-h) for methylene (C2), branched methyl, and terminal methyl (C,) protons are 52.9, 39.3, and 49.9 G , respectively, and aH(3MHX-h) for methylene (C2), branched methyl, and methylene (C,) protons are 57.1, 39.1, and 61.9 G , respectively. It is suggested that the unpaired electrons in methyl-branched alkane cations higher than 3methylheptane are delocalized over the main-chain C-C c bonds. A mechanism for free radical formation is briefly discussed.
Introduction By the use of electron spin resonance (ESR) spectroscopy Iwasaki and co-workers have found that methyl-substituted butane radical cations in CF2CICFCI2 matrices undergo thermal deprotonation from CH, groups to give primary alkyl radicals.' For methyl-substituted propane radical cations, they have also found that thermal deprotonation from the cations gives alkyl radicals and they have concluded that deprotonation takes place from the bond in which the unpaired electron is highly populated.2 We have observed that y-irradiated glassy 3-methylpentane-h14 (3MP-h) yields mainly the secondary radical which is produced by rupture of the C-H bond on position 23 and that for 3methylhexane-h16(3MHX-h) the secondary radicals on positions (1) Nunome, K.; Toriyama, K.; Iwasaki, M. Chem. Phys. Lett. 1984, 105, 414. (2) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Chem. Phys. 1982, 77, 5891. ( 3 ) Ichikawa, T.; Ohta, N. J . Phys. Chem. 1977, 81, 560.
0022-3654/87/2091-3736$01.50/0
2 and 5 are mainly produced in equal a m o ~ n t s . ~It has been also found that deuterium substitution on positions 2 and 4 in 3MPrh induces selective formation of the tertiary radical CH,CD2C(CH3)CD2CH3.For 3MXH-h, deuterium substitution on positions 2, 4, and 5 resulted in the formation of the tertiary radical at position 3. If these alkyl radicals produced in 3MP and 3MHX glasses are formed through deprotonation from radical cations of 3MP and 3MHX, the unpaired electron sites of the resultant alkyl radicals should reflect a distribution of the unpaired electron densities on the radical cations. At the present, there seems to be no report investigating the ESR spectra of radical cations of selectively deuteriated 3MPs and 3MHXs in halocarbon mat rice^.^ To determine the positions of the coupling protons in the 3MP (4) Ichikawa, T.; Ohta, N. Radial. Phys. Chem., in press. ( 5 ) After submitting the manuscript, we found a recently published paper
about the structure of the radical cation of 3MP-h: Toriyama, K.; Nunome, K.: Iwasaki, M. Chem. Phys. L e u 1986, 132, 456.
0 1987 American Chemical Society
Radical Cations of Deuteriated 3MP and 3MHX
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3737
TABLE I: Abbreviations and Isotopic Purities of Deuteriated 3-Methylpentanes and 3-Methylhexanes
compound 3-methylpentane-3-d 3-methyl-d3-pentane 3-methylpentane-2,2,4,4-d4 3-methylpentane- I , I , I ,5,5,5-d6
abbreviation 3MP-dl 3MP-d3 3MP-d, 3MP-d6
3MHX-dl 3MHX-d3 3-methylhexane-2,2,3,4,4-d5 3MHX-d5 3-methylhexane-2,2,4,4,5,5-d6 3MHX-2d6 3-methylhexane-1 ,1,1,6,6,6-d6 3MHX-1d6 3-methylhexane-2,2,3,4,4,5,5-d73MHX-d,
A
isotopic purity, 7% 80 94 90
85
3-methylhexane-3-d
95
3-methyl-&hexane
99
98 87
P-
99
88
and the 3MHX radical cations and to examine the above idea, we investigate the ESR spectra of radical cations from selectively deuteriated 3MPs and 3MHXs.
Experimental Section 3-Methylpentane-hI4 (3MP-h) and 3-methylhexane-h16 (3MHX-h) were obtained from Chemical Sample Co. with 99% purities. Deuteriated 3MPs and 3MHXs were synthesized; the procedures are described elsewhere? Abbreviations and isotopic purities for the deuteriated compounds are shown in Table I. These 3MPs and 3MHXs were purified by using molecular sieve 13X. CF2ClCFC12from Tokyo Kasei Kogyo was used as received. ESR samples were prepared on a vacuum line and measured amounts of reagents were distilled into quartz ESR tubes. The concentrations of 3MPs and 3MHXs to the freon matrix were 0.3 mol 7%. The ESR samples were y-irradiated with a %o source at 77 K for 15 min at a dose rate of 0.46 kGy min-'. ESR spectra were measured at 77 K by using FE-1X JEOL spectrometer with 100-kHz field modulation. The ESR signals were digitized by a 12-bit A / D converter and fed to an NEC PC-9801 microcomputer. Calculation of the ESR spectra was done with the microcomputer. Gaussian line shapes and isotropic hyperfine coupling constants were used in the calculation. The observed ESR spectra consisted of lines from solute radical cations, solvent radicals, and radicals from radiation-damaged quartz tubes. To eliminate signals from the solvent and the quartz, difference spectra between CF2ClCFC12with and without solutes (Figure 1A) were taken by the use of the microcomputer. Results 3MP Cations. Figure 1B shows the ESR spectrum of 3MP-h cations produced in CF2CICFC12containing 0.3 mol % 3MP-h. The spectrum consists of four lines with an averaged separation of 47.3 G. Free radicals produced by y irradiation of glassy 3MP-h gives a six-line spectrum with an averaged separation of 22.2 G3 and the observed large value of 47.3 G is attributable to proton coupling constants of alkane radical cations in comparison with the values reported.2 For brevity we define here the positions of the protons as follows:
50 G
H
Figure 1. ESR spectra of CF2CICFCI2(A) and 0.3% 3MP-h in CF2C1CFCI2matrices (B) when the samples were y-irradiated at 77 K and measured at 77 K; the calculated spectrum of 3MP-h cations ( C ) .
50 G
w
C(3') C ( 1 )-C(2)-
I
C(3)-CC(4)-CC(5)
Cations of 2-methylbutane produced in CF2ClCFC12have been reported to give a four-line spectrum with a separation of 45 G and the four-line spectrum has been attributed to the interaction of the unpaired electron with the three trans C-H, protons with respect to the C2-C3 bond, i.e. protons on positidns 1, 2' (branched-methyl group), and 4.5*7They have also found that the unpaired electron in methyl-substituted butane cations is mainly localized in the C2-C3 bonds and have concluded that the origin of the trans C-H, proton couplings are due to the ( 6 ) (a) Ohta, N.; Ninomiya, H.; Ichikawa, T. Bull. Faculty Eng. Hiroshima Uniu. 1984, 32, 141. (b) Ohta, N.; Endo, M.;Ichikawa, T. Bull. Faculty Eng. Hiroshima Univ. 1985, 33, 143. (7) Nunome, K.; Toriyama, K.; Iwasaki, M. J . Chem. Phys. 1983, 79, 2499.
Figure 2. ESR spectra of 3MP-d3(A), 3MP-d, (C), and 3MP-d6(E) cations in CF2CICFCI2matrices; calculated spectra of 3MP-d3 (B), 3MP-d4 (D), and 3MP-d6(F) cations.
hyperconjugation effect of the unpaired electron. The cations of 3MP-d, gave a spectrum almost identical with the 3MP-h spectrum, indicating a negligible interaction of the unpaired electron with the deuteron on position 3. For 3 M P - 4 , the spectrum changed into a three-line spectrum with an average separation of 51.3 G (Figure 2A). The result shows interaction of the unpaired electron with a deuteron on position 3'. The spectrum change may be explained as follows: small splittings due to the deuteron (aD = 0 . 1 5 3 ~ are ~ ) smeared out because of the broad line width (25 G) and a three-line spectrum is consequently observed. Both of 3MP-d, and 3MP-d6 yielded similar three-line spectra with average separations of 44.5 and 46.0G , respectively (Figure 2, C and E), indicating the interaction of the
3738 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987
cp
A
A5
/c3\ /H
c2 52.9G
\c4
49.9 G
39.3G
I
I
Cl
H
C< I
c6
6
\
H/C5\C{
39.1G
63\,...yH
61.9 G
I
57.1 G
Ohta and Ichikawa
4 A
- - - - - -I\ ’I I
‘ \
I 1I
: 1I
,
:
;: :!\ ,/-------
;
1
‘
C1
H,
C
393G c3’
I -I
;;
f
:‘!,,
--_
; _ c
$,
; :, ’ !
;\’, I
I
!
/ :,
I
\
I
:,
: I
,;
; ’\
‘,j
> \ I $
\ I
F -41 G
,IC
\\ ‘t.:
1
50G
-41 G Figure 3. Proposed models of the cations for 3MPs (A), for 3MHX-h, 3MHX-d,, 3MHX-d3, 3MHX-d5,and 3MHX-ld6 (B), and for 3MHX2d6 and 3MHX-d7 (C). The C,, C,, C,, and C, carbon atoms are in-plane in models A and B and the C,, C1, C, and C4carbon atoms are in-plane in model C. In model D of the extended type of 3MHX-h cation, all the carbon atoms except C3,are in-plane.
unpaired electrons with a deuteron on positions 2 or 4 for 3MP-d4 and a deuteron on positions 1 or 5 for 3MP-d6. All of the above results indicate that the unpaired electron in the 3MP-h cations interacts with three protons, Le. a proton on positions 1 or 5, a proton on positions 2 or 4, and a proton on position 3’. If we assume that the unpaired electron is mainly localized in the C3-C4 bond as is suggested in methyl-substituted butane cations,’ the observed four-line spectra of the 3MP-h and 3MP-dl cations are attributable to the interaction of the unpaired electron with the three trans C-H, protons (Figure 3A). The three-line spectra observed in other deuteriated 3MPs can be also explained by this assumption. Here, we define the coupling constant of a proton on position 2 as aH(2) and that of a proton on a branched methyl group as aH(3’),etc. The following relations are obtained from the above results:
+ aH(5) + aH(3’) = 47.3 x 3 aH(2) + aH(5) = 51.3 X 2 aH(5) + 4 3 ’ ) = 44.5 x 2 aH(2) + aH(3’) = 46.0 X 2
aH(2)
Solving the relations, we obtained the coupling constants as follows: aH(2) = 52.9 G, aH(5) = 49.9 G, and aH(3’) = 39.3 G.* Figure 1B is the spectra calculated by the use of these coupling constants. Gaussian line shapes with a line width ( A H ) of 25 G are used in the calculation. Figure 2, B, D, and F, shows the calculated spectra for the cations of 3MP-d3, 3MP-d4, and 3MP-d6, respectively. The coupling constants for the deuterons were obtained from relation uD = 0 . 1 5 3 and ~ ~ the value of AH is taken to be 25 G for all cases. 3MHX Cations. Figure 4, A, B, and D, shows the ESR spectra of cations from 3MHX-h, 3MHX-dl, and 3MHX-Id6, respectively. These spectra consists of four lines with an average sep-
+-t Figure 4. ESR spectra of 3MHX-h (A), 3MHX-d, (B), and 3MHX-ld6 (D) cations in CF2CICFC12matrices; difference spectrum between 3MHX-h and 3MHX-1d6 spectra (F); calculated spectra of 3MHX-d, (C) and 3MHX-ld6 cations (E)
aration of 52.7 G, indicating a negligible interaction of the unpaired electron with the protons and deuterons on positions 1, 3, and 6. The results suggest that the unpaired electron in the 3MHX-h cations interact with the three trans C-H, protons (positions 2, 3’, and 5) with respect to the C3-C4 bond as shown in Figure 3B. 3MHX-d3and 3MHX-ds yielded similar three-line spectra with average separations of 59.5 and 50.5 G, respectively (Figure 5, A and C). These three-line spectra can be explained with the model in Figure 3B: interaction with the protons on positions 2 and 5 for 3MHX-d3 cations and with the protons on positions 3’ and 5 for 3MHX-d5 cations. For 3MHX-d7,there exist only one trans C-H, proton respect to the C3-C4 bond and a two-line spectrum will be expected. However, a three-line spectrum with an averaged separation of 46.0 G was observed (Figure 5E), indicating the presence of two coupling protons. The model explaining the three-line spectrum is shown in Figure 3C in which the two trans C-H, protons (positions 1 and 3’) with respect to the C2-C3 bond are coupled with the unpaired electron. 3MHX-2d6 gave a three-line spectrum quite simialr to Figure 5E. The three-line spectrum observed in 3MHX-d5 and 3MHX-d3 also can be ascribed respectively to interaction with the protons on positions 1 and 3’ and the protons on positions 1 and 4. For 3MHX-dS,the average separations of a three-line spectrum expected under these conditions must be equal to that of the 3MHX-d7 spectrum. However, the observed average separation for 3MHX-d7 differs from that for 3MHX-dS. For 3MHX-d3, there is no convincing reason that the unpaired electron does not couple with the protons on position 1 and 3’. At the present, we consider that change in the unpaired electron site from the C3-C4 bond to the C2-C3bond is a special case for the 3MHX-d7cations. Coupling constants may be calculated from the following relations: aH(2) aH(S) + ~ ” ( 3 ’ )= 52.7 X 3
+
aH(2)
+ aH(5) = 59.5 x 2
aH(5) + aH(3’) = 50.5 aH(l)
(8) Errors are estimated to be f0.5 G
- - -- - -
*(
I
X
2
+ aH(3’) = 46.0 X 2
We obtained the values as follows: aH(2)= 57.1 G, aH(5) = 61.9
Radical Cations of Deuteriated 3MP and 3MHX
The Journal of Physical Chemistry, Vol. 91 No. 14, 1987 3739 ~
A
Figure 6. ESR spectrum (solid line) of 2-methylbutane cations in CF2ClCFC12 matrices.
i ;
50 G
I
t--J(
I
I /
1,
’
1
\,,I ,
I ‘.-I
Figure 5. ESR spectra of 3MHX-d3 (A), 3MHX-d5(C), and 3MHX-d7 (E) cations in CF2C1CFCI2matrices; calculated spectra of 3MHX-d3 (B), 3MHX-d5 (D), and 3MHX-d7 (F) cations.
G, aH(3’) = 39.1 G, and a H ( l ) = 52.9 G.8 Figures 4C, 4E, 5B, 5D,and 5 F show the spectra calculated by the use of these coupling constants. The coupling constants for the deuterons were reduced according to relation aD= 0.153aH, but no evident effect for the line shape was observed whether the value of aDis included in the calculation or not. The values of AH were taken as 22.5 G for 3MHX-dl, 30 G for 3MHX-ld6 and 3MHX-d3, and 25 G for 3MHX-d5 and 3MHX-d7. Discussion Structure of the Cations. The hyperfine structures of the radical cations from 3MPs and 3MHXs were assigned by an adaptation of the model proposed by Iwasaki and c o - ~ o r k e r s .The ~ coupling constants for the branched-methyl protons in both 3MPs and 3MHXs show a low value of 39 G in comparison with the values for the methylene protons and the main-chain methyl protons. A value of 39 G, however, seems to be a reasonable order of magnitude in comparison with those reported for the methyl protons of 2,2-dimethylbutane (37.0 G in CF2C1CFClz7)and 2,3-dimethylbutane cations (37.5 G in CF2C1CFC127and 41.0 G in CFC12). The cations of 2-methylbutane in CF2C1CFCl2have been reported to give a four-line spectrum with an averaged separation of 45 G.5 The ESR spectrum is shown in Figure 6 (solid line) and we obtained an averaged separation of 45.5 G. The spectrum can be explained by radicals with three-coupling protons. The spectrum which is simulated in consideration of the uH values obtained from the 3MP cations is shown by dotted line. The aH values used were 52.5,43.9, and 39.1 G for the three protons ( A H = 25 G); averaging these values leads to 45.2 G.Io Iwasaki et al.7 have evaluated B values with the cos2 0 rule. U,
= p~ cos2
e
(1)
Here, a, is the coupling constant for the p proton, p the spin density on the radical carbon atom, B a constant, and 6 the dihedral angle of the trans C-H, bond respect to the C-C bond in which the unpaired electron is mainly localized. They obtained B values (9) Wang, T.; Williams, F. Chem. Phys. Lett. 1981, 82, 177. (10) To assign specifically the a” values we will need to obtain ESR results of selectively deuteriated 2-methylbutanes.
The dotted-line spectrum is calculated.
under the assumption that 6 is 180’ for all the methyl-substituted butane cations. In our result a value of 61.9 G was obtained for a methylene proton in the cations of 3MHXs. Assuming that 6 and p are 180’ and 0.34, respectively (as are taken for the 2methylbutane cation7),we obtain a B value of 182.1 G. Iwasaki et al. obtained a B value of 127 G for the 2-methylbutane cations7 Their B value is small compared to ours, because we obtained the B value by distinguishing aH(methylene) from aH(methyl). It should be noted that our uH value of 61.9 G is close to that of 61 G for in-plane protons in the extended conformer of n-butane cations and that of 63.5 G for in-plane protons in the gauche conformer of n-hexane catiom2 As is shown in Figure 3B, trans C-H, protons H(C2) and H(C5) constitute in-plane protons in the zig-zag C2-C3-C4-C5,plane. Application of the cos2 0 rule to methylene-proton couplings must be examined on a theoretical basis. Deuterium substitution on positions 2, 3, 4, and 5 in 3MHX-h resulted in transfer of the unpaired electron site from the C3-C4 bond to the C2-C3 bond and a three-line spectrum with a separation of 46.0 G was observed. Considering the case where the unpaired electron is mainly localized in the C3-C4 bond, we will expect a two-line spectrum with a separation of 39.1 G due to coupling of one of the branched-methyl protons. The result suggests that the unpaired electron is mainly located on a C-C bond so that hyperfine interactions due to protons are maximized. Toriyama et al. have found that conformers of n-alkane cations are formed in halocarbon matrices:2 n-pentane in CF2C1CFC12 produces the extended and the gauche conformers. Dolivo and Lund” have reported that the proportions of the two types of conformer of the n-pentane cations depend on the deuterium labeling: the proportions of the extended conformers are 50% for n-pentane-hI2and 80% for n-pentane-2,2,4,4-d4. For 3MPs in CF2C1CFC12 matrices, we could not find evidence for the formation of another conformer. The spectrum of 3MHX-ld6 is observed to agree with the calculated spectrum as shown in Figure 4, D and E. The central part of the spectrum of 3MHX-h (Figure 4A), however, disagrees with Figure 4E which indicates the superposition of some other lines on the 3MHX-h cation lines. Figure 4F shows the difference spectrum between the spectra of 3MHX-h and 3MHX-d6.I2 The spectrum consists of three lines with an average separation of 40.6 G and the area under the integrated spectrum is 13% of that of the 3MHX-h spectrum. The three-line spectrum may be attributed to a conformer of 3MHX-h cations. Toriyama et aL2 have reported that n-hexane in CF,C1CFCl2 matrices gives a three-line spectrum with a separation of 41.0 G and the spectrum has been attributed to the u-delocalized cation in which the unpaired electron interacts with the two in-plane end protons in the extended structure. In the present case, this type of interaction is possible as shown in Figure 3D. It is to be noted that the width of the outermost peaks of the 3MHX-dl spectrum is observed to be 82% of that of the 3MHX-h (Figure 4, A and B) which suggests (11) Dolivo, G.; Lund, A. J . Phys. Chem. 1985, 89, 3977. (12) The process is as follows: the intensities of the spectra are adjusted so that the areas under the integrated curves are normalized, the normalized spectrum of 3MHX-d6 is reduced by a factor 0.87 so that the heights of the outermost peaks of the normalized 3MHX-d6 spectrum become equal to those of the outermost peaks of the normalized 3MHX-h spectrum, and then the difference spectrum is taken by using a microcomputer system.
3740 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987
A
c
50 G w
n
Ohta and Ichikawa protonation (eq 2) at the positions on the high unpaired electron
AV
V
Figure 7. ESR spectra of 3MHX-h (A), 3-methylheptane (B), 3methyloctane (C), 4-methylnonane (D), and Smethyldecane (E) cations in CF2CICFCI2matrices.
that interaction with the proton on position 3 in the 3MHX-h cation is not completely negligible. Figure 7 shows the ESR spectra of a series of methyl-substituted alkanes (0.3%) in CF2ClCFC12matrices. The overall separations changed as follows: 158.1 G for 3MHX-h (82 G for the extended conformer), 76.3 G for 3-methylheptane, 58.8 G for 3-methyloctane, 48.0 G for 4-methylnonane, and 37.5 G for 5-methyldecane. The observed decrease in the separations will not be expected from the model where the unpaired electron is mainly localized in one of the C-C u bonds. The result is rather described as delocalized u radicals. The overall separations of n-alkane cations have been reported to decrease as follows: 82 G for hexane, 60 G for heptane, 44 G for octane, 34 G for nonane, and 32 G for decane.I3 The formation of gauche conformers is also possible in our case as discussed by Toriyama et aL2 If extended conformers are exclusively produced, three-line spectra due to the interaction with the two in-plane end protons will be expected. The observed unresolved spectra will possibly consist of mixtures of lines from the extended and gauche conformers. Reactions to Free Radicals. In the section we examine briefly the possibility that the free radicals produced in glassy 3MPs and 3MHXs are directly related to the distributions of unpaired electron densities in the radical cations of 3MPs and 3MHXs.I4 If we assume that the free radicals are produced through de(13) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Phys. Chem. 1981, 85, 2149. (14) We observed that the cations of 3MPs and 3MHXs produced in CF2CICFCI, matrices change into free radicals on warming. Resolution of the free-radical spectra observed was generally poor. Analysis of the ESR spectra is in progress.
densities, we may expect that 3MPs produce secondary radicals on position 2 and primary radicals on positions 5 and 3' and that 3MHXs except 3MHX-2d6 and 3MHX-d, produce secondary radicals on positions 2 and 5 and primary radical on position 3'. Previously, we reported that y-irradiated glassy 3MP-h, 3MP-dl, and 3MP-d3 produce mainly secondary radicals on positions 2 or 4. The results partly correspond with the assumption. Since resolution of the spectra observed in these glassy 3MPs is poor, superposition of the lines from the primary radicals is not eliminated. However, 3MP-d4 produces mainly the tertiary radical on position 3, result which cannot be explained under this assumption. Radiolysis of 3MHX-h, 3MHX-dl, and 3MHX-d, yields mainly secondary radicals on positions 2 and 5.4 The result satisfies the assumption in this case. However, radiolysis of 3MHX-2d6 results in the formation of the tertiary radical on position 3,4 and the unpaired electron in the 3MHX-2d6 cation interacts with the protons on positions 1 and 3' and the deuteron on position 4 in this case. This result disagrees with the assumption. Consequently, the isotope effect that 77 K radiolysis of 3MP-d4 and 3MHX-2d6 mainly yields the tertiary radical cannot be ascribed to deprotonation at the positions of high unpaired electron densities. Ion-molecule reaction 3 is often proposed to explain formation RH+ + R H
-
R'
+ RH+
(3)
of free radicals. If selective transfer of protons and/or H atoms occurs, the reaction will give an explanation for the observed isotope effect. In a previous report,I5 we have found that the yield of R'in y-irradiated glassy 3-methylheptane is reduced by 13.1% when C 0 2 is added as an electron scavenger. If R' is mainly produced through reaction 3, the yield of R' will be explained to increase on addition of C 0 2 , because electron recombination reaction 4 will be hindered. For this reason, we consider that the RH+
+ e-
-
RH*
(4)
ion-molecule reaction is not the main reaction mechanism to form R' in these methyl-branched glassy alkanes. Iwasaki et have considered that reactions 5 and 6 are a possible mechanism to form R': dissociation of an excited cation (RH')* H
+ RH
-
+
+H H2 + R'
R+
(5) (6)
into a carbonium ion and an H atom followed by hydrogen abstraction. We reported previously the isotope effect that photolysis of H I in 3MP-h and 3MP-d4 glasses gives respectively the secondary radical and the tertiary radicaL3 The result seems to indicate the occurrence of selective abstraction by the "hot" H atom. It seems likely that radical formation through reactions 5 and 6 is attributable to the isotope effect observed. (15) Ichikawa, T.; Ohta, N.; Kajioka, H. J . Phys. Chem. 1979, 83, 284. (16) Iwasaki, M.; Toriyama, K.; Nunone, K. Radia?.Phys. Chem. 1983, 21, 141.
