J. Phys. Chem. 1982, 8 6 , 4372-4375
4372
Hindered Rotation of the CF, Group in teff -Butoxy Trifluoromethyl Nitroxide' C. ChatglllalogluZand K. U. Ingold" Division of Chemistty, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR6 (Received: April 13, 1982)
An EPR spectroscopic study of the CF3N(0)OC(CH3),radical reveals that the rotation of the CF3 group is restricted, which provides additional support for the view that alkoxy alkyl nitroxides are less planar at nitrogen than dialkyl nitroxides. The rotation of the CF3 group can be described by the Arrhenius equation with log (Als-') = 13.0 f 0.3 and E, = 14.1 f 0.9 kJ mol-'. At 104 K the individual F hyperfine splittings are +19.97, -2.29, and -0.89 G. It is suggested that negative spin reaches the fluorines by a combination of a through-bond spin polarization and a 1,3 p-p interaction.
Introduction Variable-temperature studies of free radicals in solution by EPR spectroscopy have shown that the hindered rotation of a group which has 3-fold symmetry and is attached directly to a trivalent radical center can only be observed when the configuration at the radical center i s very distinctly n ~ n p l a n a r .That ~ ~ is, for a radical, R,CX(A)B (R3C = H3C, F3C, etc.), hindered rotation of the R3C group has only been observed (by the changes in the EPR spectrum with temperature) when the R3C group does not lie in the AXB plane. Some examples of nonplanar R3CX(A)Bradicals for which hindered rotation of R3C has been observed by EPR spectroscopy and the calculated activation energies for rotation (when available, in kJ mol-,') follow: CH3CTC(C,H3)2;CH$C(CH3)=CCH3; CF3CF2, E , = 11.9;'::"CH3CF2, E , = 9.2; :",' CH3S02, E , = 15;* CF3SO2; CH&(OCH3)2, E, i= 12." Hindered rotation of R3C in a dialkyl nitroxide radical, R3CN(0)R R3CN+(O-)R,has been reported only for the CF3N(0)C(CH3)2COCH3 radical,'l which would appear to be unique in this respect. Other dialkyl nitroxides do not show hindered rotation even at very low temperatures. This is true even of the (CF3)2NO'radical which, despite the presence of two electron-whithdrawing CF3 groups, is planar or quasi-planar,12 the CF3 rotational barrier probably being 1 6 kJ mol-'.12 Alkoxy alkyl nitroxides have nitrogen hyperfine splittings (hfs) in the range 24-29 G,13 these values being about twice as great as the N hfs of dialkyl nitr0~ides.l~ It is generally assumed, albeit without explicit proof, that the enhanced N hfs in alkoxy alkyl
-
(1) Issued as NRCC No. 20635.
(2) NRCC Research Associate 1979-1982. Fischer, H. Helu. Chim. Acta 1976, 59, 880-901. (3) Itzel, H.; (4) Sutcliffe, R.; Lindsay, D.; Griller, D.; Walton, J.; Ingold, K. U. J . Am. Chem. SOC., in press. (5) Meaken, P.; Krusic, P. J. J . Am. Chem. SOC.1973, 95, 8185-8. (6) Chen, K. S.; Krusic, P. J.; Meakin, P.; Kochi, J. K. J . Phys. Chem. 1974, 78, 2014-30. (7) Chen, K. S.;Kochi, J. K. J . Am. Chem. SOC. 1974, 96, 794-801. (8) Chatgilialoglu, C.; Gilbert, B. C.; Norman, R. 0. C. J . Chem. SOC., Perkin Trans. 2 1980, 1429-36. (9) Chatgilialoglu, C.; Gilbert, B. C.; Kirk, C. M.; Norman, R. 0. C. J . Chem. SOC.,Perkin Trans. 2 1979, 1084-8. (10) Gaze, C.; Gilbert, B. C. J . Chem. SOC.,Perkin Trans. 2 1977, 1161-8. (11) Kataumura, Y.;Ishigure, K.; Tabata, Y . J. Phys. Chem. 1979,83, 3152-61. (12) Compton, D. A. C.; Chatgilialoglu, C.; Mantsch, H. H.; Ingold, K. U. J . Phys. Chem. 1981,85, 3093-100. (13) Forrester, A. R. In "Magnetic Properties of Free Radicals"; Fischer, H., Hellwege, K. H. Eds.; Landolt Bornstein, New Series; Springer-Verlag: West Berlin, 1977; Group 11, Part 9c1, pp 192-1006. 0022-365418212086-4372$01.25/0
TABLE I: Rate Constants for Hindered Rotation of t h e C F , G r o u p i n the CF,N(O)OC(CH,), Radical a n d E P R Parameters Used in the Simulations Shown i n Figure l a
T/K 267 213 178 147 125 104
k/s- I 2.34 X 3.33 x 5.97 X 9.19 X 1.06 X 1.10 X
3aF(average)/ Gb a N / G b AH,,/Gb ..
10"
18.18
109
17.67 17.40 17.25 16.86 16.79
10' 10'
lo7 lo6
22.72 22.73 22.76 22.76 22.76 22.86
0.38 0.34 0.36 0.38 0.45 0.60
a Errors: T t 2 K ; k i l o % , aF a n d a N i 0.05 G ; A H , , 0 . 0 2 G. R e a d f r o m experimental spectra.
I
nitroxides are a consequence of (increased) nonplanarity at n i t r ~ g e n , i.e., ' ~ to an increase in the s character of the semioccupied orbital on nitrogen. Strong support for the correctness of this assumption comes from our pSesent observation of CF, hindered rotation in the CF,N(O)OC(CH,), radical.
Experimental Section The CF3N(0)OC(CH3),radical was produced by reaction of CF3N=0 with thermally generated tert-butoxyl or tert-butylperoxyl radicals in the dark as previously described.'' The solvent was CFC1, at temperatures > 160 K and CF3Cl at lower temperatures. Absolutely identical spectra were obtained at 147 K in the two solvents. Spectra were recorded on a Varian E 104 EPR spectrometer. Spectral simulations were performed by using the QCPE program number 209.16 Results EPR Spectra. The EPR spectral parameters for CF3N(0)0C(CH3),at 267 K are g = 2.0059, uN = 22.72 G, uF(3F) = 6.06 G, AHpp= 0.38 G. The spectrum shows evidence of hindered rotation by the CF3group, which becomes even more pronounced at lower temperatures (vide infra). A t (14) For discussions of the reason that F and 0 induce nonplanarity at a neighboring radical center, see, e.g.: Pauling. L. J. Chem. Phys. 1969, 51,2767-9. Dobbs, A.J.; Gilbert, B. C.; Norman, R. 0. C. J . Chem. SOC. A 1971,124-35. Bingham, R. C.; Dewar, M. J. S. J. Am. Chem. SOC.1973, 95,7180-2, 7182-3. Krusic, P. J.; Bingham, R. C. Ibid. 1976,98, 23Ck2. Bernardi, F.; Epiotis, N. D.; Cherry, W.; Schlegel, H. B.; Whangbo, M. W.; Wolfe, S. Ibid. 1976,98, 469-78. Walborsky, H.M.; Collins, P. C. J. Org. Chem. 1976,41, 940-6. Bernardi, F.;Cherry, W.; Shaik, S.; Epiotis, N. D. J . Am. Chem. SOC. 1978 100, 1352-6. (15) Chatgilialoglu, C.; Howard, J. A,; Ingold, K. U. J . Org. Chem., in press. (16) Heinzer, J. Quantum Chemistry Program Exchange, Indiana University, Program 209.
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 22, 1982 4373
CF, Group in tert-Butoxy Trifluoromethyl Nitroxide
SLOW
t$
mI (3F)
--2
--2I
ti
Flgure 2. Correlation diagram showing the relationship between the fast-exchange and slow-exchange limits for the CF, group.
Figure 1. Low-field half of experimental EPR spectrum of CF,N(Ob OC(CH,) is shown on lefl and simulation on right.
139 K satellites due to two 13Catoms in natural abundance could be observed, al% = 4.55 and 8.50 G. We tentatively assign the 4.55-G hfs to the CF, group by analogy with the 5.1-G 13C hfs found for the (CF,),NO’ radical,17J8since calculations suggestlg that the spin density on the vicinal carbon atoms in nitroxides remains practically unchanged as the configuration at N is distorted from planarity. If this is correct, the larger 13Chfs must arise from the central carbon of the tert-butyl group.20 However, a firm assignment could only be made by ’% labeling of the radical. Experiments with CF3N(0)170C(CHJ3gave a”O = 0.93 G at 173 K.15 This particular hfs is quite temperature dependent, decreasing with an increase in temperature,15 da170/dT= -5.2 mG/K from 143 to 243 K. Hindered Rotation of the CF, Group. Line-shape effects are important in the EPR spectrum of CF,N(O)OC(CH,), at 298 K and below. The central lines of the 1:3:3:1 quartets due to the three fluorines (i.e., MIF= k1/2lines), which are slightly broadened at 267 K, broaden further until at 104 K the hindered rotation of the CF, group is “frozen-out” and the spectrum shows three nonequivalent fluorine atoms with hfs of 19.97, 2.29, and 0.89 G (see Figure 1). The two smaller F hfs must be of opposite sign to the larger F hfs both to bring their averaged value (viz., 5.60 G at 104 K) into satisfactory agreement with the 3F ~
~~
(17) Knolle, W. R.; Bolton, J. R. J . Am. Chem. SOC. 1969,91,5411-2. (18) Chatgilialoglu, C.; Malatesta, V.; Ingold, K. U. J. Phys. Chem. 1980.84, 3597-9. (19) Douady, J.;Ellinger, Y.; Rassat, A.; Subra, R. Mol. Phys. 1969, 17,217-37. (20) 0.ur assignment is certainly open to question. Thus, for the (CH,),CNOC(CH,), radical a W l C ) hfs of 9.1 G was assigned to the central carbon of the N-tert-butylgroup.*l The ’% hfs due to the central carbon of the 0-tert-butyl group was not observed.21 (21) Woynar, H.; Ingold, K. U. J. Am. Chem. SOC.1980,102,3813-5.
quartet splitting observed at higher temperatures and to account for the detailed spectral changes that are actually observed (see Figure 1). The broadening of the outer lines at lower temperatures and of the inner lines at higher temperatures can be understood on the basis of the correlation diagram shown in Figure 2. Since the spectral parameters at the slow-exchange limit are known, spectra at intermediate and high temperatures could be accurately simulated16 (see Figure 1) with a three-jump exchange process with interchange of the three sets of CF, group hfs. The F hfs used at 104 K were +19.97, -2.29, and -0.89 G. At higher temperatures where the average F hfs increases, daF(average/F)/dT = +2.8 mG/K (see Table I), the values of laFl were decreased for the two negative F hfs and increased for the positive F hfs by the same percentage and by an amount chosen to given the correct overall value. The N and average F hfs together with the derived rate constants for CF, rotation are tabulated in Table I. The rate constant for hindered rotation can be described by an Arrhenius equation with log (AIS-’) = 13.0 -f 0.3 and E, = 14.1 f 0.9 kJ mol-l, where the errors represent 95% confidence limits.
Discussion Configuration and Conformation of the CF,N(O)OC(CH,), Radical. In R3CC(A)Bradicals the substitution of A,B = H or alkyl by A,B = F or OR produces large deviations from planarity at the radical center. This causes the 13C hfs to increase (cf.!2 the 13C hfs in C H i (38 G), CH3OCH2 (47 G), (CD,0)2CH (98 G), (CH,O),c (153 G)) and the R hfs to decrease ( ~ f . the ~:F ~ hfs due to.the CF, group in CF3CH2(29.6 G), CF,CFH (25.3 G), CF3CF2(11.4 G)). For nitroxides the picture is less simple than for carbon-centeredradicals and the magnitude of aN does not relate directly to the configuration (planarity or otherwise) at nitrogen. This is because the spin density at nitrogen is determined mainly by conjugative delocalization from oxygen, i.e., by the contribution of canonical structure 2
6 R--?-R’
1
0C-
R-b-R’
I+ 2
(22) Brunton, G.;Ingold, K. U.; Roberts, B. P.; Beckwith, A. L. Krusic, P. J. J. Am. Chem. SOC.1977, 99, 3177-9.
J.;
4374
The Journal of Physical Chetnistty, Vol. 86,No. 22, 1982
Chatgilialoglu and Ingold
TABLE 11. N and F Hyperfine Splitting5 (in gauss) for S o m e Selected Radicals radical solvent T/K aN
aF
ref this work this work 24 18 this work
CF")OC(CH,), CH,N(O)OC(CH,), CF,N(O)C(CH,), CF,N(O)CF, CF,N(O)OCF,
CFC1, CFC1, C,H,CH, CFC1, CF,OOCF,
267 24 8 298 295 252
22.72 26.20 12.25 9.4 27.13
6.06 (8.50)a 12.25 8.3b 4.25'
CF,N(O)OR~
!o,OI
298
22.75
6.85
25
a a H ( 3 H ) . There was n o sign of line broadening at this temperature. This radical was generated by photolysis of a nitrom e t h a n e solution of di-tert-butyl ketone, a procedure which did n o t allow lower temperatures t o be reached. N o line broadening even at 1 1 8 K . I z There is line broadening a t this temperature b u t its variation with temperature was n o t explored. See text
to the resonance hybrid. Since structure 2 will be destabilized by electron-withdrawingsubstituents, the N hfs will be decreased by such substituents without there necessarily being any increased flattening at the radical center. We have previously explained the reduced N hfs (relative to those of dialkyl nitroxides) found for (CF3)2NO'1sand for some alkyl trifluoromethyl nitroxidesZ3 in this way. The N and F hfs of CF,N(O)OC(CH,), are compared in Table I1 with those of some related radicals. The electron-withdrawing effect of the CF, group is .manifest in that its N hfs is smaller than for the CH3N(0)OC(CH3)3 radical. The nonplanarity of CF,N(O)OC(CH,), is indicated by the fact that its N hfs is larger and its F hfs smaller than for CF,N(O)C(CH,), and (CF3)2NO'(the latter radical being planar or quasi-planar12). The CF,N(O)OCF, radical (which was generated by photolysis of a 1Ti solution of CF,NO in CF300CF3and was not studied in detail) would appear to be somewhat less planar than CF,N(O)OC(CH,),. A radical produced by photolysis of CF3N02in tetrahydrofuran has been assigned the CF,N(O)OH ~ t r u c t + r e . ~The ~ similarity of its N and F hfs to those of CF,N(O)OC(CH,), and the absence of an H hfs lead us to suggest that it is the CF,N(O)O CH(CH,),O radical. The alkoxy1 170hfs for RN(0)l70R' radicals are all rather small, particularly.when compared with the values of ca. 11.5 G found for RNOC(CH,), radicals.21 Thus, the I7O hfs for (CH3)3CN(0)170C(CH3)3 has been reported to be 1.53 (291 K in benzene):6 and 1.03 (233 K in t ~ l u e n e ) ' ~ G and that for (CH3)3CN(0)OCH(CH3)2 to be 4.6 G (263 K in p r ~ p a n e ) . ' ~ The , ~ ~ larger , ~ ~ 170value for the latter radical is presumably due to the smaller size of the isopropyl group which permits better overlap between the p-type lone pair on the alkoxy oxygen and the semioccupied orbital. For the (CH3)3CN(0)170C(CH3)3 radical steric effects cause twisting about the N-OC(CH3), bond which reduces orbital overlap and hence the I7O hfs. Such twisting, i.e., as in 3, which will be enhanced by raising the
-5.2 mG/K). Furthermore, twisting could produce, via hyperconjugation, the rather large I3C hfs (8.5 G ) tentatively assigned to the central carbon of the tert-butyl group. The fact that the three fluorine atoms in CF3N(0)0C(CH,), become nonequivalent at low temperatures indicates that rotation of the CF, group has been frozen which, in turn, serves to confirm the nonplanar configuration of this radical (vide supra). The preferred CF3conformation is shown in 4. The barrier to rotation (14.1 f 0.9 kJ mol-') (-2.29) (or
F
-0.89)
(CH,),CO
$+
(or-2.29)
@38~-0'8g)
F (+19.97)
4
is comparable to that found for related R,CX(A)B radicals (see Introduction). Of course, the CF, rotational motion might be coupled with an inversion at nitrogen, but this does seem rather unlikely because the N hfs would then be expected to show an appreciable dependence on the t e m p e r a t ~ r ewhich , ~ ~ it does not (see Table I). Signs of the F hfs. The results of a I9FNMR study of (CF3)2NO'and related diamagnetic compounds30 imply that the six equivalent fluorine atoms in this radical must have a positive sign.,' We therefore consider it extremely probable that the average F hfs (or high-temperature uF value) should also be positive. The large F hfs observed a t low temperatures must therefore have a positive sign and the two small F hfs negative signs (see 4). For only one other R3CX(A)Bradical has it been shown experimentally3' that freezing the rotation of the R3C group produces one large, positive R hfs and two small, negative R hfs. The radical is CF3CF2 which, a t low temperatures, has PF hfs of +40.4, -3.14, and -3.14 G.5r6 Stock and Wasielewski,, have recently reviewed possible mechanisms of spin transmission from a radical center to a neighboring CF, group. Four mechanisms were considered, the most important being hyperconjugation which would put positive spin on the fluorine atoms of the
3
temperature, can also be invoked for the CF3N(0)170C(CH,), radical to account for both the small magnitude of the 170hfs and its negative temperature coefficient (viz., (23) (24) (25) (26) (27) (28)
Chatgilialoglu, C.; Ingold, K. U. Can. J. Chem. 1981,59,1745-52. Aurich, H. G.; Czepluch, H. Tetrahedron 1980, 36, 3543-50. Gerlock, J. L.; Janzen, E. G. J.Am. Chem. SOC.1968,90, 1652-4. Pfab, J. Tetrahedron L e t t . 1978, 843-6. Howard, J. A,; Tait, J. C. Can. J. Chem. 1978,56 176-8. For the nitroxyl oxygen of (CH3)&N("0)0CH3, al'O = 18.35 G.24
(29) For a discussion of this point see ref 23. See also: Griller, D.; Ingold, K. U.; Krusic, P. J.; Fischer, H. J. Am. Chem. SOC.1978, 100, 6750-2. (30) Blackley, W. D.; Reinhard, R. R. J. Am. Chem. SOC.1965, 87, 802-5. (31) See, e.g.: Hausser, K. H.; Brunner, H.; Jochims, J. C. Mol. Phys. 1966,10, 253-60. Kreilick, R. W. J.Chem. Phys. 1966,45, 1922-4; 1967, 46, 4260-4. Ingold, K. U.; Brownstein, S. J. Am. Chem. SOC.1975, 97, 1817-8. (32) INDO calculations on CH3SOz give H hfs of f7.04, -2.36, and -2.36 G.' (33) Stock, L. M.; Wasielewski, M. R. Prog. Phys. Org. Chem. 1981. 13, 253-313.
J. Phys. Chem. 1982,86,4375-4379
CF3N(0)OC(CH3),radical. We suggest that negative spin reaches the fluorines by a combination of a through-bond spin polarization from the nitroxide's oxygen atom (which is in the y position with respect to fluorine%)and a 1,3 p p interaction between the semioccupied orbital and the nonbonding p orbitals of the fluorine atoms.35 The (34)For some acyclic alkyl radicals which have negative spin on yhydrogen a t o m in appropriate conformationssee: Ingold, K. U.; Walton, J. C. J. Am. Chem. SOC.1982,104,6167.For a theoretical study of this phenomenon, see: Ellinger, Y.; Rassat, A.; Subra, R.; Berthier, G. J. Am. Chem. SOC.1973,95,2372-3.Ellinger, Y.;Subra, R.; Levy, G.; Millie, P.; Berthier, G. J. Chem. Phys. 1975,62,10-29. (35)A 1,3p-p interaction might produce negative spin density at F either by the mechanism suggested in ref 33 or by the transmission of positive spin to the F 2p orbitals and a subsequent negative spin polarization of the F 1s electrons; see ref 36.
4375
magnitude37of the hfs produced by a 1,3 p-p interaction should be strongly dependent on the spatial separation between the fluorine atoms and the semioccupied orbital. The observed rather large and positive temperature coefficient of the F hfs (+2.8 mG/K) may be partly due to the increased amplitude of the CF3 group's vibrations (e.g., the umbrella motion) as the temperature rises. The conformation of a fluorine atom with respect to the semioccupied orbital will determine the final sign of its hfs.
Acknowledgment. We thank Dr. J. A. Howard for his advice and assistance with some of these experiments. (36)Karplus, M.;Fraenkel, G. K. J. Chem. Phys. 1961,35,1312-23. (37)The actual spin density at F is very small.
Complex between Water and Ammonia Bengt Nelander' and Lelf Nord Thermochemishy Laboratory, Chemical Center, Universit.v of Lund, S-220 07 Lund, Sweden (Received: May 4, 7982; I n Final Form: June 75, 1982)
The 1:l complex between water and ammonia has been studied by means of infrared spectroscopy in argon and nitrogen matrices. Water is H (or D) bonded to ammonia. HDO seems to be exclusively D bonded. The intramolecular vibrations of one of the molecules in the complex are remarkably sensitive to the isotopic composition of the other. The complex is strongly perturbed in nitrogen as compared to argon. In argon, a number of bands were observed in the 650-250-cm-' region. A few of these were also observed in nitrogen. An attempt is made to correlate the complex shifts of v2 of ammonia with the calculated heats of formation for a number of ammonia complexes.
Introduction The ammonia-water complex has been studied theoretically by several authors.1-8 Experimentally the crystalline ammonia hydrates have been investigated in considerable detail,%14but, apart from an observation of a water-induced band in the v2 region of a"onia,l5 nothing has been published on the binary complex. (1)P. A. Kollman and L. C. Allen, J. Am. Chem. SOC.,93,4991(1971). (2)G H.F. Diercksen, W. P. Kraemer, and W. von Niessen, Theor. Chim. Acta, 28,67 (1972). (3)L. Piela, Chem. Phys. Lett., 15, 199 (1972). (4)P. Kollman, J. McKelvey, A. Johansson, and S. Rothenberg,J. Am. Chem Soc., 97,955 (1975). (5)L. C.Allen, J. Am. Chem. Soc., 97,6921 (1975). (6)3. D. Dill, L. C. Allen, W. C. Topp, and J. A. Pople, J . Am. Chem. Soc., 97,7220 (1975). (7)H.Umeyama and K. Morokuma, J. Am. Chem. SOC.,99, 1316 (1977). (8)R. C.Kerns and L. C. Allen, J.Am. Chem. Soc., 100,6587(1978). (9)W. J. Siemons and D. H. Templeton, Acta Crystallogr., 7, 194 (1954). (10)I. Olovsson and D. H. Templeton, Acta Crystallogr., 12, 827 (1959). (11)J. E.Bertie and M. M. Morrison,J. Chem. Phys., 73,4832(1980). (12)G.Sill,U.Fink, and J. R. Ferraro, J. Chem. Phys., 74,997(1981). (13)J. E.Bertie and M. M. Morrison,J. Chem. Phys., 74,4361(1981). (14)C.Thornton, M. S. Khatkale, and J. P. Devlin, J. Chem. Phys., 76,5609 (1981). (15)L. Abouaf-Marguin, M. E. Jacox, and D. E. Milligan, J. Mol. Spectrosc., 67,34 (1977). 0022-3854/82/2088-4375$01.25/0
We therefore felt that a matrix isolation study of the ammonia-water complex would be of considerable interest in itself and also as a convergence point for our (hitherto parallel) investigations of water complexes'6-26and ammonia c ~ m p l e x e s . ~ ~ - ~ l
Experimental Section Two cryostats have been used in the work: an He cryostat which has been described earlier32and a cryostat (16)L. Fredin, Chem. Scr., 4,97 (1973). (17)L. Fredin and B. Nelander, J. Mol. Struct., 16,217 (1973). (18)L. Fredin, B. Nelander, and G. Ribbegird, Chem. Scr., 7, 11 (1975). (19)L. Fredin, B. Nelander, and G. Ribbegird, Chem. Phys. Lett., 36, 375 (19751. (20) L.'Fredin, B. Nelander, and G. Ribbegird, J. Chem. Phys., 66, 4065 (1977). (21)L. Fredin. B. Nelander. and G. Ribbegird. J. Chem. Phvs.. 66. 4073 (1977). (22)B. Nelander, Ber. Bunsenges. Phys. Chem., 82,61 (1978). (23)B. Nelander, J. Chem. Phys., 69,3870 (1978). (24)B. Nelander, J. Chem. Phys., 72,77 (1980). (25)L. Nord, J. Mol. Struct., submitted. (26)L. Nord, J. Mol. Struct., submitted. (27)G. Ribbegird, Chem. Phys. Lett., 25,333 (1974). (28)G.Ribbegird, Chem. Phys., 8, 185 (1975). (29)L.Fredin, B. Nelander, and G. Ribbegird, Chem. Phys., 12,153 (1976). (30)L. Fredin and B. Nelander, Chem. Phys., 15,473 (1976). (31)B. Nelander, J. Mol. Struct., 81,223 (1982).
0 1982 American Chemical Society
