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J . Phys. Chem. 1984, 88, 4078-4082
carbonyl stretching mode spectrum displays a prominent feature at 1741 cm-' and a shoulder at approximately 1726 cm-'. What are the origins of these differences in the lipid conformation in DPPC-EAN and DPPC-water dispersions? The tighter packing seen in the lamellar state DPPC-EAN dispersions may relate to the strongly ionic (1 1.7 M in ions, ref 6) nature of the solvent. The highly ionic medium will tend to attenuate the repulsive dipole-dipole interaction between DPPC head groups, thus reducing the lateral spreading pressure exerted on the membrane. A decrease in the head-group contribution to the spreading pressure tends, in turn, to inrease the temperature of the acyl chain order-disorder transition.2s The free energy of transfer of a methylene group from water to a micelle at 25 OC has been estimated at -680 cal/mol, in comparison to -370 cal/mol for transfer of a methylene group from EAN to a micelle." The smaller free energy of transfer of hydrocarbon to solvent in DPPC-EAN dispersions, as compared to that in DPPC-water dispersions, makes it less unfavorable to expose hydrocarbon chains to the EAN solvent. Since more hydrocarbon is exposed to solvent in the micellar form than in the liquidcrystalline state, micelle formation is accomplished relatively more easily in EAN than in DPPC. The result is to increase the chain length at which the lamellar-micellar transition becomes a gel (28) Nagle, J. F. J . Membr. Biol. 1976, 27, 233.
to liquid-crystalline bilayer transition from 12 carbons in water17 to 18 carbons in EANa6 Similar effects of the hydrocarbon to solvent free energy of transfer would be expected in other solvents; further work is under way to determine whether the stability of liquid-crystalline bilayers with respect to micelle formation can be predicted from a consideration of this factor. In summary, we have demonstrated that dispersions of DPPC in the fused salt EAN are tightly packed in an orthorhombic subcell in the lamellar state; tight packing results from attenuation by the solent of dipole-dipole interactions between neighboring head groups. This subcell packing arrangement of the acyl chains is characterized by distinctive lipid spectral features in the methylene deformation region at 1420 cm-I, in the 1720-1800cm-' C=O stretching mode region, and in the low-frequency rotary lattice mode region at 124 cm-'. At approximately 59.5 "C a phase transition occurs in which the lamellar dispersion assumes a micellar state. Micelle formation is more favored in EAN than in water because of the smaller free energy of transfer of the acyl chain groups from the hydrocarbon region of the bilayer to the EAN solvent than to the water solvent. Acknowledgment. We thank Dr. D. F. Evans for generously providing us with EAN for this experiments and Drs. V. A. Parsegian and W. C. Harris for numerous stimulating discussions. Registry No. DPPC, 2644-64-6; EAN, 221 13-86-6;water, 7732-18-5.
Electron Spin Resonance Studies on Propylene Radical Cation' M. Shiotani,* Y. Nagata, and J. Sohma Faculty of Engineering, Hokkaido University, Sapporo 060, Japan (Received: June 24, 1983: In Final Form: February 13, 1984)
An ESR spectrum of the propylene radical cation has been observed in y-irradiated solid solutions of propylene in CC1,F or SF6. The assignment of the spectrum was confirmed by using propylene-& CH3CH=CD,. In contrast with previous studies, the CH3 group was found to rotate freely even at 77 K in both matrices and a a-type structure was deduced from the experimental values of the proton hf splittings and the INDO results. It was also found that the proton hf splittings of the radical cation depended on both matrix and temperature. In connection with these results the twisted form from planarity is discussed. The allyl radical was formed by decay of the radical cation in both via ion-molecular reaction. Moreover, a CH3CHCH2Fradical formed by a fluorine atom addition was observed in SF6.
Introduction The radical cations of olefins are believed to be important intermediates in reactions such as cationic polymerizations2 and catalytic r e a ~ t i o n . ~They are also interesting from the standpoint of electfonic structure. Although the ground-state structure of neutral olefins is planar, nonplanar (twisted) structures for olefin radical ions have been s ~ g g e s t e d . ~ -The ~ formation of solute radical cations in y-irradiated organic glasses containing small amounts of olefins has been studied by Shida and Hamilllo using optical spectroscopy. They have shown that the positive charge (1) A part of this study has been presented at the 25th Japanese Radiation Chemistry Symposium (Sendai), Oct 1982. (2) For example Tsuji, K.; Yoshida, H.; Hayashi, K. Polym. Lett. 1967, 5, 313. (3) For example: Shik, S . J. Caral. 1983, 79, 390. (4) Marry, S.; Thomson, C. Chem. Phys. Lett. 1981,82, 373. (5) Raddon-Row, M. N.; Randau, N. G.; Houk, K. N. J. Am. Chem. SOC. 1982, 104,
1143.
(6) Bellville, D. J.; Bauld, N. L. J . Am. Chem. SOC.1982, 104, 294. (7) Hasegawa, A.; Symons, M. C. R. J. Chem. Soc., Faraday Trans. 1 1983, 79, 1565. They have discussed the nonplanar structure of CzF4--. (8) Kira, M.; Nakagawa, H.; Sakurai, H. J. Am. Chem. SOC.1983,105,
6983.
(9) Nakatsuji, H. J . Am. Chem. Soc. 1973, 95, 2084. (10) Shida, T.; Hamill, W. H. J . Am. Chem. SOC.1966, 88, 5376.
0022-3654/84/2088-4078$01.50/0
resulting from ionization of the matrices can be trapped by added olefins with lower ionization potentials than the matrices. Ichikawa et al." reported the first ESR spectrum of olefin radical cation, i.e., tetramethylethylene radical cation formed in a y-irradiated glassy solution of 3-methylpentane. Then, Shida et a l l z reported ESR studies on some simple olefin and diene radical cations produced in irradiated trichlorofluoromethane (CC13F). Recently, Toriyama et al.I3 reported the radical cation of propylene, CH3CH=CH2+-, produced in CC13F. On the basis of the assumption that rotation of the CH3 group was hindered, they analyzed the spectrum observed at 77 K by using the isotropic hf splittings of a(CH2) = 11 and 6 G, a(CH3) = 23.5, 23.5, and 47.0 G,and a(CH) N I) G. However, in the present studies using partially deuterated propylene, CH3CH=CD2,we have found that the CH3 group is rotating freely even at 77 K. In the present paper, we report our experimental spectra of propylene radical cations, CH3CH=CH2+. and CH3CH=CDz+-, generated in either CC13F or SF6 matrix. The spectra were (11) Ichikawa, T.; Ludwig, R. K. J . Am. Chem. SOC.1969, 91, 1023. (12) Shida, T.; Egawa, Y.; Kubodera, H.; Kat0 T. J. Chem. Phys. 1980, 73, 5963. (13) Toriyama, K.; Nunome, K, Iwasaki, M. J . Chem. Phys. 1982, 77,
5981.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4019
ESR Studies on Propylene Radical Cation
TABLE I: ESR Parameters of Some Olefin Radical Cations Related to the Present Studies
cation
matrix CCl,FCClF, CC1;F
temp, K
g
100 77
2.0030 f 0.0003 2.0033 f 0.0003
isotropic proton hf splittings, G dCH,) = 23.3 a(CH;) = 24.0; a(CH) = 7 . 0 ; a(CH2) =
77
2.0033 f 0.0003
a(CH3) = 24.0; a(CH) = 7 . 0 ; a(CD2) =
ref our studyZS present Htudy
23.0. 12.0
CCIpF CClpF
130
2.0033 f 0.0003
CC13F
130
2.0033 f 0.0003
SF6
77
2.0033 f 0.0003
SF6
17
2.0033 f 0.0003
77 160
2.0033 f 0.0003 2.0033 f 0.0003
77 77 77 77 71
2.0033 f 0.0003 2.0033 f 0.0003
CC13F CC13F CClF2CClF2 SF6 3MP CClpF CCIpF CClpF CClSF CC13F
77
7.5. 1.P a ( C H 3 i = 24.0; a(CH) = 9.0; a(CH2) = 16.0, 9.0 a(CH3) = 24.0; a(CH) = 9.0; a(CD2) = 2.5, 1.4“ a(CH3) = 27.0; a(CH) = 5.0; a(CH2) = 16.0, 14.0 a(CH3) = 27.0; a(CH) = 5.0; a(CD2) = 2.5, 2.2‘ a(CH3) = 16.2; a(CH2) = 14.0 a(CH3) = a(CH2) = 16.2; u(”F of CC1,F) = 5.2 a(CH3) = a(CH2) 16.1 a(CH3) E a(CH2) N 17 a(CH1) = 16.5 a(CH;) = 17.2 a(CH3) = 23.9; n(CH) = 9.8 (for both cis and trans) a(CH2) = 44.2 a(CH) = 64.2 All(4F)= 146.3; A, Ob
Deuterium hf splittings of CD2 belonging to CH3CH=CD2+.. Anisotropic splitting. successfully analyzed in terms of isotropic proton hf splittings and the experimental hf splittings were compared with the theoretical ones obtained by the INDO M O method.14 The hf splittings were both temperature and matrix dependent. Decay of the radical cation was accompanied by allyl radical formation in both matrices, probably through an ion-molecular reaction. Furthermore, addition of fluorine atom to propylene was observed in SF6.
c H~c H-c A.Exp. 77K
\
H:
/CC13F
present study present study
present study present study present study 12
present study present study present study 11 12 12
8 8 36
r”’ \ AAA
Experimental Section Propylene (CH3CH=CH2, >99.9%) was obtained from Takachiho Kagaku Kogyo K. K., propylene-l,l-d2 ( C H 2 C H 4 D 2 , >99 D-atom %) from Merck Sharp and Dohme, Freon-1 1 (CC13F, >99%) from Tokyo Kasei Co., and sulfur hexafluoride (SF,) from Allied Chemicals. Samples of solid solutions containing generally ca. 1 mol % propylene in either SF6or CC13F were prepared by standard vacuum techniques in Spectrosil tubes. The radicals of interest were generated by y-irradiation of the solid solution at 77 K, by using the same procedure employed in our previous studies on radical cations.15 The y-irradiation dose was less than 1 Mrd and the ESR spectra were recorded a t temperatures up to the disappearance of the radicals (Le,, from 77 to ca. 150 K). A JEOL X-band spectrometer (JES-PX-1X) was used to record the ESR spectra. The magnetic field strengths in the free-electron region were calibrated by using the hf splitting of a standard Mn2+/Mg0 sample. Results and Discussion E S R Spectra of the Propylene Radical Cation. Propylene Cation in CC13F. The spectra shown in Figure 1 were recorded at 77 (A) and 130 (B) K after y-irradiation of a solid solution of ca. 1 mol % propylene in CC13F. Although the spectra A and B are apparently different from each other, their reversible interconversion with change of temperature suggests that they originate from the same paramagnetic species. When partially deuterated propylene (CH3CH=CD2) was used instead of normal (14) INDO calculations were carried out by using the CNDO-INDO MO program presented in: Pople, J. A.; Beveridge, D. L. “Approximate Molecular Orbital Theory”; McGraw-Hill: New York, 1970, modified by H. Itoh and deposited in the Program Library at the Hokkaido University Computer Center. (15) (a) Shiotani, M.; Nagata, Y.; Tasaki, M.; Sohma, J.; Shida, T. J . Phys. Chem. 1983,87, 1170. (b) Shiotani, M.; Kawazoe, H.; Sohma, J. Ibid. 1984, 88, 2220. See the references therein for the previous ESR studies on various radical cations in solid matrices.
Figure 1. ESR spectra of a y-irradiated solid solution of ca. 1 mol 75 CH3CH=CH2 in CC13F: (A and B) experimental spectra at 77 and 130 K; (A’ and B’) simulated spectra of propylene radical cations using ESR parameters listed in Table I and Lorentzian line width of 5.0 G.
propylene, the spectra were simplified and a quartet with additional small splittings was observed, as shown in Figure 2. The isotropic quartet of 24.0 G with a binominal relative intensity is directly attributed to a freely rotating CH3 group. This splitting was independent of temperatures between 77 and 145 K. The spectra observed for the CH3CH=CH2+. were successfully analyzed by taking the splitting of 24.0 G for the CH3 group into account. The best-fit simulated spectra are compared with the experimental ones in Figure 1. The hf splittings used for the simulation are listed in Table I. The agreement between the
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The Journal of Physical Chemistry, Vol. 88, No. 18, 1984
H
Figure 3. ESR spectra of a y-irradiated SF6 containing ca. 1 mol % CH3CH=CD2 (A) and CH3CH=CH2 (B) recorded at 77 K. A' and B' are the simulated spectra of propylene radical cation using hf splittings listed in Table I and Lorentzian line width of 7 G.
Figure 2. .ESR spectra of a y-irradiated CClpFcontaining ca. 1 mol % CH3CH=CD2: (A and B) experimental spectra at 77 and 130 K; (A' and B') simulated spectra of propylene radical cation using ESR parameters listed in Table I and Lorentzian line width of 7.0 G.
experimental and simulated spectra is excellent for the spectrum observed a t the higher temperature, 130 K. However, for the spectrum recorded at the lower temperature, 77 K, the relative intensities or the line shape of the calculated spectrum deviates considerably from the observed ones, although the resonance line positions coincide well with each other. The agreement was greatly improved when a broad singlet was superimposed on the spectrum.16 The obtained isotropic hf splittings are a(CH) = 9.0 G, a(CH2) = 16.0, 9.0 G, and a(CH3) = 24.0 G for the higher temperature type of spectrum in which the splitting of a CH proton accidentally becomes equal to that of one of the inequivalent CH2 protons. However, for the lower temperature type of spectrum, the splittings were changed to a(CH) = 7.0 G and a(CH2) = 23.0, 12.0 G while the a(CH3) remained unchanged. In order to confirm this, the spectral simulation was carried out for CH3CH=CD2+. by using CD2 deuterium hf splittings reduced by a factor of 6.5 (magnetic moment ratio of H to D) from those of the corresponding CH2. The agreement between the experimental and calculated spectra (Figure 2) further supports the above analysis. Thus, we conclude that the hf splittings of CH and CH2 are temperature dependent. The observation of six proton's hf splittings conclusively shows that the spectrum is due to the propylene radical ion. Judging from the high ionization potential (IP1: 11.8 eV)" and the high electron affinity of CC13Fused as a matrix, the spectrum is solely attributed to the radical cation of propylene (IP1: 9.7 eV).'* The hf splittings in the literature relevant to the propylene radical cation are listed in Table I. The observed proton hf splittings are of a reasonable order of magnitude for the 7-type radical cations of olefins. l 2 9 l 3 The isobutene radical cation, (CH3)2C=CH2+., generated in CC13F has been reported to give hf splittings of 16.5 and 14.0 G at 77 K for the methyl and methylene protons, respectively.'2 The (16) Toriyama, K.; Nunome, K.; Iwasaki, M., private communication. They suggested it to us originally and we confirmed it. (17) Shida, T.; Nosaka, Y . ;Kato, Y . J . Phys. Chem. 1978, 82, 695. (18) (a) Dewer, M. J. S.; Worley, S. D. J. Chem. Phys. 1969,50,654. (b) Forst, D. C.; Sandhv, J. S. Indian J . Chem. 1971, 9, 1105.
methyl proton splitting observed was considerably less than that for the propylene radical cation. We repeated the same experiment in order to see how the spectrum of the hf splittings changed upon warming the sample. It was found that the spectral resolution was greatly improved upon warming and a nonet of doublets with 16.2 and 5.2 G was clearly observed at 160 K.18 The nonet was attributed to two methyl protons and methylene protons which gave accidentally equal splitting at the higher temperature of 160 K. The additional doublet of 5.2 G is attributed to one fluorine belonging to the matrix CC13F.19 Such a superhyperfine splitting due to matrix atom has been often observed in CC13F.20 Thus, the methylene protons of (CH3)2C=CH2+- in CC13F were found to give a slightly temperature-dependent hf splitting, while the methyl proton splitting was independent of temperature. This observation is consistent with the CH3CH=CH2+. in CC13F. The possibility of superhyperfine splitting due to matrix fluorine atom should be ruled out with respect to the doublet of 7 G (or 9 G ) attributed already to the C H proton of CH3CH=CH2+.in CC13F. The reason is that the same doublet has been observed for the CH3CH=CH2+. in SF6, where no such superhyperfine splitting SF6as is expected, judging from the previous ~ t u d i e s ' ~ using J a matrix. Propylene Cation in SF,. SF6 has been used as a matrix in ESR studies on radical cations13 because of its high ionization potential (IPl: 15.7 eV).22 Therefore, we used SF6as a matrix instead of the CC13F in order to confirm generation of the propylene radical cation and the ESR parameters obtained in the CC13F. A y-irradiated CH3CH=CD2/SF6 gave the quartet, similar to that in CC13F, below the solid-solid phase transition of SF6,94.3 K,23as shown in Figure 3A. However, splitting of the quartet was 27.0 G, which is larger than that in the CC13F (24.0 G). The spectrum observed a t 77 K for irradiated (19) (a) The same experimental results were reported by: Yoshida, H.; Ogasawara, M. "Proceedings of the 25th Japanese Radiation Chemistry Symposium"; Japanese Society of Radiation Chemistry: Sendai, 1982; p 107. (b) When the isobutene radical cation was generated in CClzFCCIFz, the nonet of 16.1 G without additional doublet was observed. (20) (a) A superhyperfine splitting of 3.4 and 5.0 G has been observed for both of the radical cations of 1,2-difluorobenzene and toluene in CC1,F (Shiotani, M.; Kawazoe, H.; Sohma, J., unpublished). (b) A superhyperfine splitting of 13.9 G has been also reported for C2F4+.in CC13F (ref 36). (21) Shiotani, M.; Nagata, Y.; Sohma, J. J. Phys. Chem. 1983, 86,4131. (22) Potts, A. W.; Lempka, H. J.; Streets, D. G.; Price, W. G. Phifos. Trans. R. SOC.London, Ser. A 1970, 268, 59. (23) (a) Michel, J.; Deffored, M.; Rigny, P. J . Chim. Phys. Phys.-Chim. Biof. 1970, 69, 31. (b) Garg, S. K. J . Chem. Phys. 1977, 66, 2517.
ESR Studies on Propylene Radical Cation
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4081
r ~ ( ~ 2 a\ ~), ( 0 5 6 )
5 OG (-63)
r5\\
\
"f lkqH?
H3C6
270G (283)
H3 (14:;
,60G
20
(126)
Figure 4. Experimental proton hf splittings compared with the theoretical ones (values in parentheses) calculated for the optimized planar structure by the INDO MO method. The geometrical parameters used are rC4 = 1.40 8,rCd = 1.45 A, rC+ = 1.09 8,LC4CIH3= 122.5",LC4CiHz = 125.0°, L C ~ C ~ =H 119.0°, ~ and LCIC4H3= 124.0". Comparison of the geometrical parameters of the radical cation and the parent molecule's indicates that the C=C bond length is increased, as expected for loss of a bonding *-electron, the other paremeters including the bond
667.5
-
10
Y)
3
0,
-
66 8.0
Y
c)
0
> Y
angles remaining almost unchanged.
CH3CH=CH2/SF6 is shown in Figure 3B. The spectrum differs in the number of hf lines relative to that in CC13F at the same temperature. The spectrum of Figure 3B was successfully simulated by using the splittings listed in Table I. All proton hf splittings appeared slightly different in the SF6 system from those in the CC1,F. The quartet observed in the CH3CH=CD2+-/SF6 was also reasonably simulated by using the proton and deuterium splittings listed in Table I (Figure 3A'). Thus, the hf splittings of the propylene radical cation reveal a rather strong matrix dependence, as well as a temperature dependence. INDO Calculation and Structure. For comparison with the experimental results, INDO M O calculation^'^ were performed for an isolated CH3CH=CH2+radical cation. The conformations considered were either planar or twisted at the C=C bond. First, INDO optimization was carried out for the planar structure. An overall agreement between the calculated hf splittings and the experimental ones for the SF6 system is rather good (Figure 4), although the larger difference of 11 or 7 G between inequivalent methylene proton hf splittings observed for the lower or higher temperature type spectrum in CC13Fis not successfully reproduced by the calculation. Note that the calculated hf splittings of the methyl protons are an average of the values obtained for various configurations of the methyl group. Although the ground-state structure of neutral olefins is planar, the possibility of a twisted structure has been suggested for the olefin radical ~ a t i o n . ~ - ~For , * , ~example, the twist angles of ca. 25", ca. 15", and ca. 40' have been suggested for the CH2= CH2+., (CH3)2C==CH2+.,and (CH3)2C=C(CH3)2+-, respectively, on the basis of a careful analysis of the vibrational structure of the photoelectron spectrum of ethylene and MINDO calculations.6 I N D O calculations were also performed for the twisted configuration. The energy minimum was found at a configuration with twist angle of 45", which is 0.91 eV more stable than the optimized configuration for the planar structure. However, the calculated proton hf splittings change sharply with the twist angle and the agreement between the experimental and calculated splittings becomes worse, especially for the C H proton.24 Optimization at any other twisted configuration different from a twist angle of 45O gave higher energy than that at the 45O twisted structure according to our calculation. Moreover, the calculated hf splittings were found far from the experimental ones for these twisted structures as shown in Figure 5. Thus, we conclude that the propylene radical cation observed has a planar structure judging from the hf splittings. It is interesting to compare proton hf splittings of methylsubstituted ethylene radical cations with each other as a function of the degree of twisting. A hf splitting of CH2=CH2+. is 23.3 (24) The isotropic hf splittings calculated for the optimized geometry of twisted structure are as follows: a(CH) = 50.1 G, a(CHZ) = 18.7, 23.4 G, and a(CH3) = 16.2 G. The eometrical parameters used are r c e = 1 . 3 5 A, rCPH= 1.09A, rC4 = 1.501,LC,C,H, = 122', LC4ClH2= 123O, LHSC4C1 = 117O, LC,C,C, = 13!', and a twisted angle of 45' at C4=C1 (numbering of the atoms IS shown in Figure 4).
w -10
0
30
+
60
90
Figure 5. Dependence upon the twist angle (6)of the isotropic hf splittings and total energies given by INDO calculations.
G for four equivalent protons (ref 25 and Table I), which is about 2 times larger than that expected for the planar structure judging from the a-proton hf splitting of hydrocarbon T radical.26 On the other hand, the hf splittings reported for cis- (and trans-) CH3CH=CHCH3+. are a(-CH,) = 23.9 G and a(=CH) = 9.8 GelzThe latter splitting of 9.8 G corresponds to that of the planar 7r radical. Thus, the CH2=CH2+. seems to be twisted, but the CH3CH=CHCH3+. must have a planar structure. The CH3CH=CH2+- and (CH3)2C=CHz+. are interesting cases to compare with the CH2=CH2+- and CH3CH=CHCH3+.. According to Bellville and Bauld; the (CH,),C=CH,+. has a smaller angle of twist than the CH2=CH2+., as mentioned above. Their results are consistent with the ESR data showing that the a-proton hf splitting of (CH3)2C=CH2C. (16.2 G) is smaller than that of CH2=CH2+. (23.3 G).27 The methyl proton hf splitting of CH3CH=CH2+. is larger than that of (CH,),C=CH,+-, but the opposite tendency is obtained for the C H proton, as shown in Table I. This result indicates that a twist angle of CH3CH=CH2+. is smaller than that of (CH3)2C=CHz+. (15°)6 and that the CH3CH=CH2+. might have a lower barrier to becoming planar than the (CH3)zC=CH2+. (4.3 X eV).6,28 This argument might provide the solution to the problem of large matrix and (25) Recent1 we succeeded in observing ESR spectra of the ethylene radical cations, k 2 H 4 + . ,13C2H4+.,and 12C2D4+., generated in CCI2FCC1FZ or CC1F2CC1Fz. The quintet of 23.3 G has been observed for four equivalent rotons of 12C2H4+*at 100 K. Note added in proof The ESR spectrum of P3C2H4f.was successfully analyzed in terms of an isotropic proton hfs of 23.3 G and axial symmetric "C hfs of A,,= 32.5 G and A , = 8.0 G. A twist angle of 45 h 5" was concluded on the basis of ESR line shape simulation of 13C2H4+..Shiotani, M.; Nagata, Y.;Sohma, J. J . Am. Chem. SOC.,in press. (26) The isotropic a-proton hf splitting of CH3, a typical ?r-type planar radical, is 23.0 G (for example: Fessenden, R. W.; Schuler, R. J . Chem. Phys. 1965,43, 2704). Therefore, ca. 10-G hf splitting is expected for the a-proton of CH2=CHz+. with planar structure. The INDO calculation results in a splitting of -10.9 G for the planar CH2=CHz+.. (27) A strong dependence of the a-proton hf splitting on the twist angle is predicted by the INDO calculation on CH3CH=CH2+., in which the splitting changes its sign from negative to positive at ca. 5 O for the C H and ca. 10' for the CH2 with increasing of the angle (Figure 5 ) . It has been confirmed that this tendency holds true for the CHz=CH2+. and the (CH3)2C=CH2+. by using the INDO method. We believe that the a-proton hf splittings of both cations, CH2=CHz+. and (CH3)zC=CH2+., have a positive sign based on the INDO and absolute values of the splittings (23.3 and 16.2G). On the other hand, the methyl proton hf splitting (with a positive sign) decreases by increasing the twist angle. (28) This argument has been derived on the basis of the MINDO calculations, because the MINDO calculations seem to be much more reliable than the other calculations including the INDO in regard to the twist angle and potential barrier.
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The Journal of Physical Chemistry, Vol. 88, No. 18, 1984
TABLE II: Isotropic Hf Splittings of Some Neutral Radicals Related to the Present Studies radical
matrix
CH,i-iCH=CH, CHii-iCHi-iCH; CHzi-iCHi-iCH, CH2yCHi-iCH2 CHSCH-CHzF CHjCH-CD2F
CCIaF SF6 c-C~H~ xenon SF6
CH3CH-CH3 (CH,)ZC-CH,F
CH$HzCH:,
(CH,),C. CHZ-CHZF CH2-CH2F CH27CH2F CH3C=CH2
SF, SF, C(CH314 SF6 SF6
C-C& allene-ethane
temp,
K
g
91 132 150 184 95 95
2.0025 zt 0.0003 2.0025 f 0.0003 2.00254 2.0032 2.0037 0.0003 2.0037 f 0.0003
188 110
2.0035 zt 0.0003
128 112 132 164 100
2.0035 zt 0.0003 2.0035 f 0.0003 2.00222
isotropic hf splittings and their assignment, G 15.1, 14.3; u(CH) = 4.1 14.0; a(CH) '= 3:7 14.83, 13.93; a(CH) = 4.06 14.95, 14.00; a(CH) = 4.15 12.8; a(CH3) = a(CH) = 25.2 z(F) = 87.3; a(CH2) z(F) 89.9; a(CD2) < AH (2.5 G), a(CH3) = n(CH) = 25.2 z(CH3) = 24.68; a(CH) = 22.1 1 z(CH3) = 23.4; a(H of CH2F) = 5.7; a(F of CH2F) = 122.6 z(CH3) = 22.7 z(F) = a(P-CH2) = 32.0; ~(cx-CHZ)= 22.6 z(F) = 35.9; a(P-CH) = 30.7; ~ ( ( Y - C H= ~ )22.6 z(F) = 46.17; a(P-CH2) = 27.83; ~ ( ( Y - C H=~ )22.16 19.48; a(CH2) = 57.89, 32.92 z(CH,)
u(CH,) n(CH,) z(CH2) z(CH2)
temperature dependence of hf splittings which had been observed only for the CH3CH=CH2+., but not the CH2=CH2+-, (CH3)2C=CH2+., and CH3CH=CHCH3+.. That is, the CH3CH=CH2+- in SF6may interchange between the two configurations corresponding to the energy minimum rapid enough to give the apparent planarity on the ESR time scale because of the low barrier and the soft SF6 matrix. The same argument is possible a t the higher temperature for the CH3CH=CH2+- in CC1)F. In fact, it has been often observed that the radicals trapped in CC1,F g.ive rotational-state spectra above ca. 100 K because of the matrix ~oftening.'~,'~ On the other hand, the large difference between the CH2 hf splittings observed for the lower temperature type spectrum in CC13Fsuggests a possible rehybridization of the CH2 group probably due to increasing of either the matrix rigidity or the potential barrier upon cooling or both. This explains why the INDO calculations do not agree with the observation in regard to twisting. Allyl Radical Formation. When the sample was warmed above 145 K, the CH3CH=CH2+. in CC13F decayed monotonically without giving any new radical for the lower concentration of ca. 1 mol % propylene. However, with increasing propylene concentration, the spectrum due to CH3CH=CH2+. was gradually replaced by a new spectrum. This new spectrum consists of a well-resolved isotropic triplet of triplets of doublets with a1(2H) = 15.1 G, a2(2H) = 14.3 G, and a3(lH) = 4.1 G. This spectrum, without doubt, corresponds to the allyl radical CH2~CH,CH2, which is formed by a removal of one proton from the propylene radical cation. The hf splittings of the allyl radical available in the l i t e r a t ~ r eare ~ ~listed , ~ ~ in Table I1 in order to compare with the present ones. A formation of the allyl radical only in the concentrated solution of propylene in CC13F strongly indicates an ion-molecular reaction,)' since the solute dimer or aggregates are likely to be in the concentrated solution. The allyl radical formation was also clearly observed for the propylene/SF, system. When an irradiated solid solution of propylene in SF6was warmed just above the phase transition of 94.3 K,23 the propylene radical cation was irreversibly converted into the allyl radical (the radical conversion being almost 100%). As with the CC1,F system, this observation suggests that the solute radical cation can diffuse in the matrix consisting of bulky molecules like SF6 to react with the solute molecule to form the allyl radical by an ion-molecular reaction above the phase transition. Fluorine Atom Addition to Propylene in SF,. When a y-irradiated solid solution of propylene in SF6was recorded just above 94.3 K, the originally anisotropic SF5. and SF,-- s p e ~ t r a ~ ~ (29) Fessenden, R. W.; Schuler, R. H. J . Chem. Phys. 1963, 39, 2147. (30) Cook, M. D.; Roberts, B. P. J . Chem. SOC.,Chem. Commun. 1983, 264. (31) Such an ion-molecular reaction has been reported for the alkene system as well as the alkane.'2~'3~1s~39 (32) Fessenden, R. W.; Schuler, R. H. J . Chern. Phys. 1966, 45, 1845. (33) Morton, J. R.; Preston, K. F. Chem. Phys. Letr. 1973, 18, 98. (34) Morton, J. R.; Preston, K. F. ACS Symp. Ser. 1978, No. 66 and papers cited therein.
= = = =
ref present present 29 30 present present
study study study study
29 35 (our study) 29 35 (our study) 35 (our study) 37 29
were sharply changed into isotropic ones and concomitantly a well-resolved isotropic spectrum assigned to CH3CH-CH2F became clearly visible together with the allyl radical. It was experimentally confirmed that the radical had been already formed at 77 K and the concentration was not changed before and after warming the sample to just above the phase transition. An assignment of the spectrum was confirmed by help of partially deuterated propylene, CH3CH=CD2. The hf splittings evaluated are listed in Table I1 together with those of related radicals. Fluorine atom addition to an unsaturated bond of solute hydrocarbon in irradiated SF6has been discussed in our recent paper on the fluorovinyl radical.21 It is interesting to note the following two points with respect to the CH3CH-CH2F radical. One is that the fluorine atom adds to the carbon at a site opposite to the C H 3 group. This sort of site-selective reaction has been observed for some alkynes and alkenes such as CH3C=CH, C2H5C=CH, and (CHJ2C=CH2, which!eact with fluorine atoms to form the radicals CH3C=CHF, C2H5C=CHF, and (CH3)2C-CH2F, respectively, in irradiated SF6? The other is related to the hf splittings. The hf splittings for the.alky1 radical with one p-fluorine such as CH2-CH2F, (CH3)CH-CH2F, and (CH3)2C-CH2F, are listed in Table 11. While an a-proton is replaced by a methyl gFoup, the /3-fluorine splitting increases drastically from 32 (CH2CH2F) to 123 ((CH3)2CCH2F)G; at the same time, that of a proton decreases from 27 to 5.7 G. The splittings of (CH3)CH-CH2F fall between those of the former two radicals. This strong substitution effect on the p-fluorine and proton hf splittings is reasonably explained in terms of hindered internal rotation in fluoroalkyl radical^.)^ The details will be published in a separate paper.35 Note Added in ProoJ Recently Toriyama et al. have reported an alternative assignment of hyperfine splittings of CH3CH= CH2+. which is consistent with the present studies. They have also discussed the twisted structure of methyl-substituted ethylene radical cations (trimethylethylene and propylene) on the basis of proton hyperfine splittings. Toriyama, K.; Nunome, K.; Iwasaki, M.; Chem. Phys. Lett. 1984, 107, 8 6 . Acknowledgment. The present research was partially supported by the Subsidy for Scientific Research of the Ministry of Education in Japan (Grant No. 58550510). We thank the referees for several helpful suggestions which have served to clarify our interpretations. Registry No. CH3CH=CH2+., 34504- 10-4; CH3CH=CD2+*, 90885-94-2; CH~CHTCH., 6067-68-1; CH3CHCH2F*, 40499-20-5.
~ ~(35)~ Shiotani, - ~ ~ M.; Nagata, Y.; Sohma, J. "Proceedings of the 25th Japanese Radiation Chemistry Symposium"; Japanese Society of Radiation Chemistry: Sendai, 1982; p 51. The details will be published. (36) Hasegawa, A,; Symons, M. C. R. J . Chem. SOC., Faraday Trans. 1 1983, 79, 93. They have demonstrated that the C2F4+-is a r-type radical with planar structure. (37) Chen, K. S . ; Krusic, P. J.; Meakin, P.; Kochi, J. K. J . Phys. Chem. 1974, 78, 2014. (38) Linde, 0. R.; Christensen, D. J . Chem. Phys. 1969, 35, 1374. (39) Kubodera, H.; Shida, T.; Shimokoshi, K. J . Phys. Chem. 1981, 85, 2583.
