4074
J . Phys. Chem. 1984, 88, 4074-4078
Effects of Solvent on Blomembrane Structure: Raman Spectroscopic Investigation of Dipalmitoylphosphatidylcholine Dlspersed in N-Ethylammonium Nitrate Timothy J. O'Leary* and Ira W. Levin* Laboratory of Chemical Physics, National Institute of Arthritis, Diabetes, and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20205 (Received: January 27, 1984)
The lamellar to micellar phase transition characteristics of dipalmitoylphosphatidylcholine (DPPC) dispersed in N-ethylammonium nitrate (EAN) were investigated by Raman spectroscopy. Below the transition temperature, attenuation of dipolar repulsive forces between neighboring head groups by the solvent results in an ordered lamellar structure in which the acyl chains pack in an orthorhombic subcell. This bilayer state is characterized by distinctive vibrational Raman spectral patterns in the 1440-cm-' methylene deformation and the 1740-cm-' C 4 stretching mode regions. The relatively sharp (1 O C breadth) lamellar to micellar phase transition is observed at 59.5 O C , a dramatic increase in temperature above that observed for the DPPC-water bilayer gel to liquid-crystallinephase transition. On melting of the low-temperaturelamellar state, the decreased free energy of transfer of the acyl chains from the lipid hydrocarbon chain region to the EAN solvent, compared to that of the water system, promotes micellization, rather than liquid-crystalline bilayer formation.
Introduction The mechanisms by which solvents such as water influence the structures of biological membranes have not been established. Although phospholipids form bilayers in a variety of solvents such as water,'-3 glycerol," chl~roform,~ and N-ethylammonium nitrate (EAN),6 the resulting lamellar systems exhibit widely different structural and thermodynamic properties. Thus, the precise structure of a liposome depends critically not only on lipid chain length, degree of chain saturation, and class of polar head group but also on the presence and the nature of the solvent. Hence, lipids that form interdigitated bilayers in glycerol at 35 OC4 are noninterdigitated in water' and micellar in c h l ~ r o f o r m .Despite ~ the polymorphism displayed by the liposomal preparations, the various solvents evince basic similarities. Since these solvents are, at least, moderately polar, they are expected to exhibit considerable order in the head-group region and to penetrate, in general, only slightly into the acyl chain portion of the liposome. These solvents both induce major changes in the conformations of the lipid head group and interface regions and decrease the temperature of the acyl chain order-disorder transition in comparison to anhydrous s y ~ t e m s . ~To * ~some extent, these physical changes mimic the structural alterations observed after an initial melting of anhydrous polycrystalline lipid in studies where bilayer properties are observed in sequential heating and cooling cycle^.^-^ Recently, N-ethylammonium nitrate, a fused salt, was shown to disperse distearoylphosphatidylcholine(DSPC) as gel and liquid-crystalline bilayer assemblies. Although EAN is a completely ionic medium, the solvent is structurally similar to water in that the highly polar fused salt is capable of forming an extensive network of hydrogen bonds.lOJ1 Unlike water, however, it is not characterized by extensive fluctuations between structural forms of different densities and geometrical arrangements; therefore, the heat capacities of solution for many compounds in EAN are small compared to those of the water systems and are comparable (1) Chapman, D. Q. Rev. Biophys. 1975, 8 , 185. (2) Israelachvili, J. N.; Marrelja, S.; Horn, R. G. Q. Rev. Biophys. 1980, 13, 121. (3) Schnitzky, M.; Barenholz, Y. 1978, Biochim. Biophys. Acta 1978, 515, lhl _ " I .
(4) McDaniel, R. V.; McIntosh, T. J.; Simon, S. A. Biochim. Biophys. Acta 1983, 731, 97. (5) OLeary, T. J.; Levin, I. W., unpublished data. (6) Evans, D. F.; Kaler, E. W.; Benton, W. J. J . Phys. Chem. 1983, 87, 533. (7) Bush, S. F.; Adams, R. G.; Levin, I. W. Biochemistry 1980, 29,4429. (8) OLeary, T. J.; Levin, I. W. J . Phys. Chem. 1984, 88, 1790. (9) Chapman, D.; Williams, R. M.; Ladbrooke, B. D. Chem. Phys. Lipids 1967, I , 445. (10) Evans, D. F.; Chen, S.-h.; Schriver, G.W.; Arnett, E. M. J . Am. Chem. SOC.1981, 103, 481-482. (11) Evans, D. F.; Yamouchi, A,; Roman, R.; Casassa, E. 2. J . Colloid Interface Sci. 1982, 88, 89-96.
to those for other polar organic solvents such as dimethyl sulfoxide.I2 For both this reason and because of the ability of highly ionic solvents such as EAN to attenuate the interaction forces between neighboring head group dipoles in the bilayer plane, as well as the solvation forces between bilayer^,^,'^ conformational differences are expected between phospholipids dispersed in EAN and liposomes dispersed in water. Both microscopy and X-ray diffraction studies reveal both similarities and differences between the melting characteristics of DSPC-EAN and DSPC-water dispersions. Although bilayers are formed in both solvents, the pretransition and main transition occur at higher temperatures in the DSPC-EAN dispersions.6 Furthermore, the area per lipid molecule is reported to be larger in EAN than in water.6 Since vibrational spectroscopic markers enable one to monitor conformational changes in the head group, glycerol backbone, and acyl chain regions of the phospholipid bilayer,14 Raman spectroscopy represents an ideal experimental technique by which to explore the influence of EAN on lipid structure. Distinct spectral features characteristic of the acyl chain regions may be used not only to identify unambiguously morphological states of the lipid, such as hexagonally or orthorhombically packed gel phases, liquid-crystalline, and micellar states, but to monitor the temperature dependence of interconversions between these various f ~ r r n s . ' ~ - ' ~ Information on the enthalpy of lipid melting is also obtained from the temperature-dependent spectral changes.17
Methods and Materials Dipalmitoylphosphatidylcholine (DPPC) was obtained from Sigma Chemical Co., recrystallized from ethanol, and lyophilized from chloroform for 48 h at torr. N-Ethylammonium nitrate (EAN) was a gift of Dr. D. F. Evans. (The preparation of EAN was described by Dr. Evans in ref 6.) Lipid dispersions were prepared by suspending DPPC in EAN at 70-80 "C for several hours, while mechanically shaking the suspension. After sealing the samples (29% DPPC by weight) in Kimex glass capillary tubes (1.25-mm i.d.), the dispersions were cycled through the phase transition temperature several times by repeated heating and cooling of the tubes. Raman spectra were recorded with a Spex Ramalog 6 spectrometer equipped with holographic gratings. Excitation with 514.5-nm radiation from a Coherent CR-12 argon (12) Mirejovsky, D.;Arnett, E. M. J . Am. Chem. SOC.1983, 105, 1112. (13) Lee, L. J.; McAlister, M.; Fuller, N.; Rand, R. P.; Parsegian, V. A. Biophys. J. 1982, 37, 657. (14) Levin, I. W. In "Advances in Infrared and Raman Spectroscopy"; Clark, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1984; Vol. 11, pp 1-48. (15) Lippert, J. L.; Peticolas, W. L. Proc. Natl. Acad. Sci. U.S.A. 1971, 68, 1572. (16) Yellin, N.; Levin, I. W. Biochemistry 1977, 16, 642. (17) Huang, C.; Lapides, J. R.; Levin, I. W. J . Am. Chem. SOC.1982, 104, 5926.
This article not subject to US. Copyright. Published 1984 by the American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4075
DPPC Dispersed in N-Ethylammonium Nitrate N-ETHYLAMMONIUM NITRATE 25OC
TABLE I: Raman Spectra of Liquid N-Ethylammonium Nitrate at 16 'C rei int assigntn freq, cm-'
f P
I
1
I
J
675
d
U
L
U 1525
WAVENUMBER DISPLACEMENT icm-ll
Figure 1. Survey Raman spectrum of liquid N-Ethylammonium nitrate (EAN) at 25 "C in the 675-1525-cm-' interval.
ion laser delivered 200-800 m W at the sample. Temperature profiles for determining phase transition characteristics were constructed, however, from spectra determined with 200-mW incident power. Spectral resolution of the spectrometer was - 5 cm-'. Spectral frequencies, calibrated with argon ion laser lines,I8 are reported to f 2 cm-'. Depending upon the spectral interval of interest, spectra, acquired with a Nicolet NIC-1180 data system interfaced to the spectrometer, were signal averaged for 1-40 scans with a uniform scan rate of 1 cm-'/s. Sample temperatures were maintained by placing the sample capillary within a thermostatically controlled brass mount, whose temperature was maintained by a flowing solution of ethylene glycol and water. A copper-constantan thermocouple, placed adjacent to the sample capillary within the brass housing, recorded the temperature. Spectra of the dispersed lipid were recorded, both below and above the liposomal melting transition, in the 675-1 525-, 1700-1800-, and 2800-31OO-cm-' spectral regions in order to obtain information on acyl chain, head group, and glycerol backbone conformations. Order-disorder temperature profiles were constructed from the peak height intensity ratios Z109o/Zi130, for the gauche and trans acyl chain C-C stretching modes, respectively, and 12850/12880, for the methylene CH2 symmetric and asymmetric stretching modes, respectively. These ratios were recorded a t 1 OC intervals from 48 to 67 "C. The contribution of EAN to the Raman scattered signal in the 1080-1 130-cm-' region was negligible. Although EAN does contribute intensity to the DPPC C-H stretching region in the 2880-3300-cm-' interval, spectral subtractions of EAN are not required in using the 12850/12880 peak height intensity ratios either in obtaining a reliable value for T , or in estimating the melting interval. In performing E A N spectral subtractions for DSPC-EAN mixtures, however, we used the intense symmetric NO3- stretching mode a t 1044 cm-I of neat EAN, scanned a t the appropriate temperature, to determine normalization factors. Survey infrared spectra of EAN were recorded with a Perkin-Elmer Model 580B spectrophotometer as a thin film between Irtran I1 plates.
Results and Discussion Although we do not wish to emphasize the spectra of EAN in the present paper, we briefly summarize the salient vibrational features of this fused salt. The NO3- moiety reflects D,,, point group symmetry in that single frequencies for the fundamentals of the liquid fused salt are observed for the Raman-active modes at 1044 cm-I, the vl(a'') symmetric N - 0 stretching motion, 1402 cm-', the v3(e') asymmetric N - O stretching motion, and 720 cm-', the v4(e') 0-N-0 deformation mode.19 The out-of-plane nitrogen (18) Craig, N. C.; Levin, I. W. Appl. Spectrosc. 1979, 33, 475. (19) Janz, G. J.; Wait, S. C. In "Raman Spectroscopy"; Szymanski, H.,
Ed.; Plenum Press: New York, 1967; pp 139-167.
7 14
3240 3115 2996 2981 2948 2935 2915 2888 1652 1617 1472 1459 1402 1374 1332 1196 1044 988 98OC 873 825' 720
iI
61 79 102 90 43 58 83 59 24 52 16 16 18 14 1000 14
brd 153 34
N H 3 + antisym str NH1+ - sym . str C H 3 asym str C H 2 asym str CH, sym str (FR) CH, sym str C H 3 sym str (FR) 2uzb NH3'def C H 3 asym def C H 2 scissors u3, NO3- asym str CH, sym def C-N str C H 3 rock CH, twist u , , NO3- sym str C-C str NH3' rock C H 3 rock u2, out-of-plane nitrogen distortion v4, N O 2 def
+
"NO3 modes assigned on the basis of D3* local symmetry. Overtone of infrared-active out-of-plane nitrogen distortion. Cobservedin the infrared spectrum of a thin-film sample. FR and br represent Fermi resonance component and broad, respectively.
N-ETHYL AMMONIUM NITRATE 25OC
2800
2900
3000
3100
3200
3300
WAVENUMBER DISPLACEMENT (cm-ll
Figure 2. Raman spectrum of liquid EAN at 25 " C in the C-H and N-H stretching mode regions.
distortion, ~ ~ ( a " is~ inactive ), in the Raman spectrum but is observed in the infrared spectrum at 825 ~ m - l . ' 2v2 ~ is observed as a sharp feature at 1652 cm-' superimposed upon the broad NH3+ deformation mode at 1617 cm-'. The breadth of this feature is attributed to hydrogen bonding characteristics of the fused salts. As shown in Figure 1, a relatively strong Raman feature, assigned to a mixture of the CH3 rocking and C-C stretching motions of the ethyl group, appears at 873 cm-1.20 A summary of the spectrum appears in Table I. Vibrational assignments for the C-H and N-H stretching mode regions are also summarized in Table I. Figure 2 displays the NH3+symmetric and asymmetric stretching modes at -3 115 and 3240 cm-', respectively. The Fermi resonance components of the methyl C-H symmetric stretching mode appear at 2888 and 2935 cm-1.21 The methyl asymmetric C-H stretching mode, degenerate under local symmetry, shows components at 2981 and 2996 cm-'. The methylene symmetric and asymmetric stretching
-
(20) Schachtschneider, J. H.; Snyder, R. G. Specrrochim. Acta 1963, 19, 117. (21) Hill, I. R.; Levin, I. W. J . Chem. Phys. 1979, 70, 842.
4076
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984
O'Leary and Levin A. DPPC: EAN 47%
1
P P
2'01
I
1 .o
2800
3100
B. D!PC: 0.0 1
I
I
I
I
I
I
1
I
I
52
54
56
58
60
62
64
66
68
T
EAN 63'C
loci
Figure 3. Temperature profile for DPPC-EAN dispersions derived from the I,ow/Il130 peak height intensity ratios as indices. A phase transition at -59.5 OC is apparent. The chain disorder at high temperatures, as evidenced by the relatively high values for the intensity ratios, is greater than that for liquid-crystallinesystems and is typical of micellar forms.
modes are assigned to the inflection at 291 5 cm-' and the feature at 2948 cm-', respectively. The temperature profile for the DPPC-EAN dispersion derived from the C-C stretching mode region peak height intensity ratios 11,?90/{1130 (Zgauche/Ztrans) is shown in Figure 3. The transition midpoint T, occurs at approximately 59.5 OC; the relatively sharp transition is largely complete within 1 "C. The temperature profile based on the 12935/12880 peak height intensity ratio also reflects a relatively narrow melting range, centered at approximately 59.5 OC. Visual examination of the capillary tube above the melting temperature reveals the nonturbid liquid characteristic of micellar dispersions. The conclusion that the phase transition is lamellar to micellar rather than bilayer gel to bilayer liquid crystalline is corroborated both by the characteristic 28003 lOO-cm-' C-H stretching region spectrum (Figure 4B), which shows the degree of melting typical for known micellar systems,22 and by the sharpness of the transition. The relative intensity differences between the lipid 2853-, -2897-, and 2935-cm-' features (Figure 4B) are particularly distinctive between the micellar and liquid-crystalline states. Assignments for these spectral features have been previously d i ~ c u s s e d . ' Figure ~ ~ ~ ~ 4 also presents the comparisons of DPPC-EAN liposomes with DPPC liposomes in water. In particular, parts A and C of Figure 4 compare the gel-state order for DPPC in the EAN and water solvents, respectively. The characteristic liquid-crystalline spectrum for DPPC-water liposomes is seen in Figure 4D. Representative spectra of DPPC-EAN dispersions below and above T , and DPPC-water dispersion below the acyl chain order-disorder phase transition are seen in Figure 5 and 6. Figure 5A shows the characteristic spectral feature at 1420 cm-', which is clearly indicative of acyl chain packing in an orthorhombic s u b s ~ e 1 1for ~ ~ DPPC-EAN *~~ dispersions in the lamellar state. While a weak feature at this frequency24is seen in DPPC-water dispersions which have been incubated at temperatures below that of the subtransition ( 18 "C, ref 25) for several days, the relative intensity of this orthorhombic marker, -54% of the 1437-cm-l methylene deformation mode, exceeds that which we have seen previously in phospholipid dispersions. In DSPC-water dispersions at -180 OC, the peak height intensity of the 1420-cm-' feature is -24% of the 1437-cm-I feature.24 The 1420-cm-' feature disappears on melting, as the subcell is altered (Figure 5B). Further confirmation of an orthorhombic-packed subcell appears in a low-frequency rotary lattice mode for the DPPCEAN dispersion (spectrum not shown) at 124 cm-I. As shown
-
-
(22) Wu, W.; Huang, C.; Conley, T. G.; Martin, K. B.; Levin, I. W. Biochemistry 1982, 21, 5957. (23) Boerio, F. S.; Koenig, J. L. J . Chem. Phys. 1970, 52, 3425. (24) Yellin, N.; Levin, I. W. Eiochim. Eiophys. Acta 1977, 489, 177. (25) Chen, S . C.; Sturtevant, J. M.; Gaffney, B. J. Proc. Nurl. Acud. Sci. U.S.A. 1980, 77, 5060.
kE l , , , , , z 5
28W
31W
C. DPPC: WATER 0°C
::
5
i
d
" 1
28W
3100
D. DP-PC: WATER 45'C
28W
2860
,
I
2920
2980
.
3040
,
3100
WAVENUMBER DISPLACEMENT Icm-ll
Figure 4. Raman spectrum of the 2800-3 100-cm-I C-H stretching region for DPPC liposomes in various solvents: (A) the ordered gel state at 47 "C in EAN; ( B ) the micellar state at 63 OC in EAN (Spectral contributions from the EAN solvent have been subtracted in (A) and (B).); (C) the gel state of DPPC at 0 OC in water; (D) the liquid-crystalline state of DPPC at 45 "C in water.
by Olf and Fanconi,26 the odd n-paraffins which exhibit orthorhombic subcell packing display a band series, corresponding to rotation about the chain axis, whose frequencies are independent of chain length and appear between 120 and 125 cm-I. A second striking difference between DPPC-EAN and DPPC-water dispersions is observed in the 1740-cm-I C=O stretching mode region (Figure 6A,C), a spectral interval sensitive to the conformation of the glycerol backbone. DPPC-EAN dispersions exhibit spectra with two clearly resolved features at 1726 and 1740 cm-', characteristic of the two inequivalent lipid acyl chains.' These features merge on melting (Figure 6B), giving rise to a spectrum similar to that of DPPC-water dispersions, in (26) Olf, H. G.; Fanconi, B. J . Chem. Phys. 1973, 59, 534.
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4077
DPPC Dispersed in N-Ethylammonium Nitrate
A. DPPC: EAN 4 7 ° C
8 2
2
A. DPPC: EAN 48°C
f
h
1700
1800
6. DPPC: EAN 6 3 ° C v)
f I
A
6. DPPC: EAN 63OC
II
c I
1700
1800
C. DPPC: WATER O°C m
c
C. DPPC: WATER 0°C
wf
1700
1720
1740
1760
1780
1800
WAVENUMBER DISPLACEMENT icm-') 1526
575
WAVENUMBER DISPLACEMENT 1crn-l)
Figure 5. (A) Raman spectrum of DPPC-EAN liposomes at -46 "C with the EAN solvent contribution subtracted. (The intense EAN feature at 1044 cm-l is incompletely subtracted due to small differences in the EAN spectrum in the presence of DPPC.) The characteristic spectral pattern for an orthorhombic subcell packing is observed in the 1440-cm-' methylene deformation region. (B) Raman spectrum of DPPC-EAN liposomes at 63 O C . Chain disorder is evidenced by the intense 1080-cm-l feature, decreased intensity of the 1130-cm-I feature, disappearance of the 1420-cm-' feature, and loss of resolution in the 1437- and 1461-cm-l region. (C) Raman spectrum of DPPC-water liposomes at 0 "C. No feature is seen at 1420 cm-l (this sample was not incubated at low temperatures). Other features of chain ordering are similar to the DPPC-EAN bilayers.
which only one clearly defined spectral feature is observed. Infrared spectra recorded by Cameron and M a n t ~ c of h ~DPPC~ water dispersions incubated below the subtransition temperature also show features at 1727 and 1741 cm-' of nearly equal intensity, although they are considerably less well resolved than those shown in Figure 6 . The appearance of two distinct carbonyl features in the observed (and deconvoluted) spectra were att; ibuted by Cameron and Mantsch to partial dehydration of the pl.cwholipid in this crystalline form,27since anhydrous polycrystallie-c DL-DPPC exhibits two carbonyl stretching modes at 1720 and 1738 (27) Cameron, D. G.; Mantsch, H. H. Biophys. J . 1982, 38, 175.
Figure 6. (A) Raman spectrum of DPPC-EAN liposomes at 47 OC in the carbonyl stretching mode region. Clearly resolved spectral features at 1727 and 1741 cm-', characteristic of inequivalent acyl chain packing in an orthorhombic subcell, are apparent. (B) Raman spectrum of DPPC-EAN liposomes at 63 "C. The 1727- and 1741-cm-' features are no longer resolved, although a weak shoulder at 1726 cm-l is still present, indicating persistent inequivalence of the sn-1 and sn-2 chains. ( C ) Spectrum of DPPC-water bilayers at 0 O C .
~ m - ' .We ~ interpret the Cameron and Mantsch frequency data27 to reflect specifically an acyl chain orthorhombic subcell (vida supra). Bilayer dehydration may occur, but the infrared data are not necessarily confirmatory. In addition, experiment^^,^ have shown that the disappearance of well-resolved spectral features at 1721 and 1740 cm-' is not only a function of hydration but also of glycerol backbone conformational changes that are induced by either solvation or melting of the anhydrous, polycrystalline lipid. For these reasons, we interpret the appearance of the spectral features in Figure 6 as being due to an accentuation of the inequivalent environments of the one- and two-chain carbonyl moieties when the acyl chains are ordered in an orthorhombic subcell. We also note that the resolution of the two carbonyl stretching mode components is directly related to the relative intensity of the orthorhombic subcell marker at 1420 cm-'. The acyl chain inequivalence, as reflected by the frequencies of the two primary carbonyl components, largely disappears, together with the 1420-cm-' feature, on melting. In the micellar state the
4078
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
