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The structure of the CFCl3 radical anion
Volume 149, number 3
CHEMICAL PHYSICS LETTERS
19 August I988
THE STRUCTURE OF THE CFC13RADICAL ANION
L. BONAZZOLA, J.-P. MICHAUT and J. RONCIN Laboratoire
de Physico-Chimie
des Rayonnements,
Bdtiment 350, Universitc! ParisAd,
91405 Orsay Cedex, France
Received 2 April 1988; in final form 28 June I988
Exposure of trichlorofluoromethane in the presence of tetrahydropyran or some other impurities at very low concentration to @‘Coyrays at 77 K gives the CFCI, radical anion. From the analysis of its EPR spectrum and from ab initio geometry optimization, it is concluded that the anion has CJ, symmetry.
..
1. Introduction
:’ :
Organic radical cations have been extensively studied by ESR spectroscopy in the past ten years [ 11. They are produced in irradiated frozen freon matrices. The advantage of the use of fluorine- and chlorine-containing compounds as matrices is to give matrix radicals which do not interfere with the studied cations because their spectra are broadened by the large anisotropy of fluorine and chlorine couplings. On warming the sample containing the cations, some neutral matrix radicals may appear [ 21. Their formation is due to secondary reactions during the decay of the cations. The present work concerns a primary radical anion formed in the most commonly used matrix for radiolytic preparation of cations: trichlorofluoromethane (CFC&).
III, III
Ill
,111 Iti
II,,I,,
I,
,
i
2, Experimental results Two spectra (recorded at 157 K) of y-irradiated CFC13containing respectively 10s3 mol% CHjOH (a) and 3 x lo-” mol% tetrahydropyran (b) are given in fig. 1. Co y rays were used for irradiation at 77 K (dose of 1 Mrad). No ESR signals due to the anions CFCl, were detected after irradiation of pure CFCL and further annealing to 157 K. This is probably due to recombination of CFCl? with the electron loss centers, migrating by hole transfer as soon as molecular motion becomes important. In the presence of impurities (CH30H or tetrahydropyran) these 316
II
CFCICI -n I: CFClCl
Fig. 1. (a) ESR spectrum of CFClj containing IO-’ mol% CH,OH after exposure to Co y rays at 77 K. (b) ESR spectrum of CFCl, containing 3 x I Om6mol% tetrahydropyran after exposure to Co yraysat77K.
0 009-26 14/88/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
19 August 1988
CHEMICAL PHYSICS LETTERS
Volume 149, number 3
Table 1 Experimental g tensors and A hypertine coupling tensors for CFCl, and CF,ClRadical
g1
gll
Nucleus
A, (G)
CFCI,
2.0037
2.0136
j5Cl F
(-)
CF>Cl- a1
2.0070
2.002 1
=Cl F
AI, (G)
18.3 107.4
0 0
17.4 87.4
43.2 197.3
‘) Ref. [6].
holes are trapped provided the ionization potential of the impurity is less than that of CFC&. If the initial concentration of tetrahydropyran (THP) is > 10e5 mol%, the spectrum assigned by Symons et al. [ 31 to THP+ appears. For concentrations below 1Oe5 mol%, spectrum (b) is observed. It can be seen that the two spectra (a) and (b) arise from the same radical even if the centre of spectrum (b) is obscured by one or several interfering paramagnetic species. The shape of the two spectra is due to axially symmetric g and hyperfine T tensors of randomly oriented radicals in a powder. The central line corresponds to T,,= 0 and the other lines correspond to a coupling between the free electron and three nuclear spins, 1=3/2 (Cl), and one nuclear spin I= l/2 (F). Consequently, the observed radical comes from the matrix CFC&. Formation of the cation CFCl: can be eliminated because it has been shown for Ccl: that only two of the chlorine atoms are equivalent and that the Jahn-Teller effect implies a trigonal bipyramidal structure [ 41. Can the formed radical be the anion CC&F-? Reference has been made to this anion in the literature [ 51: by using a tetramethylsilane matrix (TMS), the observed spectrum is very broad (400 G) and unresolved. In the present work, the measured couplings (table 1) indicate that the three chlorine atoms are equivalent, which means that the radical has C3, symmetry and that a rapid reorientation occurs about the C, axis at 157 K. 3. Discussion CF,Cl- radical anions have been observed in TMS matrices [ 61. It has been shown that they possess Cj, symmetry and that the CF3 group rotates rapidly about the symmetric C-Cl axis. On the other hand,
substitution of the central carbon atom with a silicon atom changes the structure of the anion from C3” symmetry (CF,Cl-) to trigonal bipyramidal (SiF&l- [ 71). In view of these contrasting results it seemed desirable to perform full geometry optimization on CFCl; and CClF?. The equilibrium geometries were determined from ab initio unrestricted SCF calculations by means of a minimization procedure using the gradient of the potential energy as described by McIver and Komornicki [ 81. Calculations were done with the program GAMESS using a minimal STO-3G basis set. The calculated spin densities and equilibrium geometry for CFCl, and CClF,- anions are reported in fig. 2 and table 2. It is well known [ 91 that the theoretical study of negative ions involves complications not encountered for neutral and cationic species. For reliable calculations within the LCAO MO finite atomic basis approach, diffuse atomic orbitals have to be added to the core and valence atomic basis sets . Because of computational expense, our calculations were limited to minimal basis sets; nevertheless, good agreement between theory and experiment is achieved.
4. Spin densities and structure Experimental spin densities have been calculated from the measured couplings using the atomic parameters listed by Morton and Preston [ lo], 4.1. CCI,F Calculation shows that this anion has a CsVstructure (see fig. 2). Most of the spin density on the chlorine atom lying in the XZ plane is in the 3p, orbital (perpendicular to the Z direction of the C-F bond). The three chlorine atoms become equivalent 317
Volume 149, number 3
CHEMICAL PHYSICS LETTERS
E : - 1499.076482
19 August 1988
hartrce
Fig. 2. Calculated equilibrium geometry for CF,CI- and CC&F- anions [5]
by rapid reorientation about Cz. The measured total Cl tensor (18.3; 18.3; 0) decomposes into an anisotropic tensor (6.1; 6.1; - 12.2) and an isotropic coupling of 12.2 G. This corresponds to a tensor of 24.2; - 12.2; - 12.2 G relative to the motionless radical and to a spin density of 19.5Ohcompared to the calculated value of 27.3%. The calculated 2s spin 318
density at fluorine is negative. Ifwe consider that the measured couplings are negative,, the total tensor (- 107.4; - 107.4; 0) decomposes into an anisotropic tensor (-35.8; - 35.8; 71.6) and an isotropic coupling of - 71.6 G. The calculated spin density p in the 2p orbitals of fluorine is also negative. The density is nearly the same in 2p,, 2p,, and 2p, or-
19 August 1988
CHEMICAL PHYSICS LETTERS
Volume 149, number 3
Table 2 Experimental and calculated spin densities for CFClr and CF,ClOrbital
Nucleus
CFCl,expt.
2s 2P.y 2P,. 2P:
C
3s 3P, 3P, 3P,
Cl a’
2s 2P, 2P, 2P,
F
” Cl in the xz plane (fig. 2).
CF,Clexpt. [ 61
talc. 0.213 -0.113 -0.113 0.206
0.006 0.195 a’
- 0.004 -0.057 b’ -0.057 c, b) b, 2p,- f 2p,.
‘) 24-f
0.008 0.273 -0.002 0.001
0.013
- 0.002 -0.014 -0.014 -0.013
0.007
0.136
0.058 *z)
Zp,. d, Fin thexz plane (fig. 2).
bitals. Experimental results require that p( 2p,) Jp( 2p,) =p( 2pY)- $(2p,) = - 5.7%. Calculation shows - in agreement with experiment - that no direct spin density appears at the fluorine atoms; its couplings come from spin polarization induced through the bonds. No measurement of 13C couplings have been performed. Strong polarization of orbitals inducing an apparent 2p, spin density (p) of 32% is predicted (p(2pZ)apparent=p(2p,) f[P(2PX)+p(2P,)l). A large 2s spin density at C is also expected. Thus the calculated CJ, electronic structure of CC&F- fits the experimental couplings quite well. It is interesting to compare the changes in structure from the parent molecule to the anion CC13F-: the C-F bond lengthens very slightly; a flattening of the pyramidal group Ccl3 is expected associated with a lengthening of the C-Cl bonds. The unpaired electron resides in an a, antibonding orbital mainly composed of the 2p orbitals of the chlorine atoms, perpendicular to the C-F bond, of the 2s and the 2p, orbital of the carbon (experimental spin density of z 50%). The very slight lengthening of the C-Cl bond in CC13F- with respect to the molecule does not suggest the well-known dissociative electron attachment behaviour of CC&F resulting in the loss of chloride ions [ 5,111. In the CFCl, matrix, the spectra of fig. 1 disappear slowly at 160 K without any transformation leading to the neutral carbon-centered radical CFCl;: this may arise
talc.
talc. [ 51
0.194 0.017 0.017 0.500
0.277 -0.013 -0.013 0.196
0.006 0 0 0.170
0.003 0 0 0.388
0.001 - 0.022 -0.022 0.053
0.004 0.015 0.001 0.003
e’2p,- f (2p,t_2p,).
from the larger mobility of positive charges neutralizing the anion before dissociation is made possible by the softening of the matrix, 4.2. CF,ClCNDO/Z calculations [ 51 suggest that the unpaired electron is in an antibonding o* orbital which is composed largely of the p orbitals from carbon and the halogen lying along the CJVsymmetry axis (CCl) of the anion. We have also performed a geometry optimization of CF,Cl- (see fig. 2): 17O/o of the spin density is in the 2p, chlorine orbital (in agreement with experimental results). Most of the spin density for the fluorine atoms lies in 2p orbitals. The spin density at the carbon atom lies in 2s and 2p, orbitals. A lengthening of the C-Cl bond (with respect to the neutral species) is expected from ab initio calculations (from 1.80 to 2.54 A). CND0/2 calculations give C-Cl = 1.94 A. Such a large variation in bond length is also predicted for disulfur anions: the S-S antibonding o* orbital changes the bond length from 2.06 8, in the molecule to 2.62 A in the anion [ 121. This change is expected to be even greater (S-S = 2.80 A) at the MPn/6-31G* level [ 131. The main structural modification caused by the capture of an electron by a CClF3 molecule is elongation of the C-Cl bond length. This result agrees with the dissociation of the anion into a neutral radical and a Cl- ion, known experimentally [ 61. 319
Volume 149, number 3
CHEMICAL PHYSICS LETTERS
5. Conclusion The EPR spectrum of the CC&F- anion has been observed in a CCISF matrix in the presence of impurities at very low concentration. Analysis of the experimental data and ab initio calculations lead to the conclusion that this anion has C3”symmetry as does CF$l-, reported some years ago. The experimental study of negative ion formation in CF$l and CFC13under low-energy electron impact [ 141 in the gas phase shows that Cl- coming from CFC& is by far the most abundant ion (relative intensity Cl-/ CFC&= 1000 compared to Cl-/CF,C1=2.5). These results are not readily interpretable in the light of the above structure calculations: during the formation of the respective anions a very slight elongation of the C-Cl bond length is expected in CC&F, and a large elongation of the C-Cl bond length is expected in CFSCl. Determining the potential curves for Cl- removal, i.e. calculation of the energies of CFC& and CF,Cl versus C-Cl distance, is probably the best way to understand the dissociative electron attachment mechanism, provided the geometry of the starting molecules is reliable, which was probably not the case in previous work [ 111.
320
19 August 1988
References [l] M.C.R. Symons, Chem. Sot. Rev. (1984) 393. T. Shida, Y. Nosaka and T. Kato, J. Phys. Chem. 82 (1978) 695. [2] B.W. Walther and F. Williams, J. Chem. Phys. 79 (1983) 3167; X.Z. Qin, Q.X. Guo, J.T. Wang and F. Williams, J. Chem. Sot. Chem. Commun. (1987) 1553. [ 3 ] M.C.R. Symons and B.W. Wren, J. Chem. Sot. Perkin II (1984) 511. [4] J.R. Bews and C. Glidewell, J. Mol. Struct. 71 (1981) 287. [ 51 A. Hasegawa, M. Shiotani and F. Williams, Faraday Discussions Chem. Sot. 63 (1978) 157. [6] A. Hasegawa and F. Williams, Chem. Phys. Letters 46 (1977) 66. [ 71 A. Hasegawa, S. Uchimura and M. Hayashi, J. Magn. Reson. 38 (1980) 391. [ 81 J.M. McIver Jr. and A.J. Komomicki, Chem. Phys. Letters 10 (1971) 303. [9] J. Simons and K.D. Jordan, Chem. Rev. 87 (1987) 535. [lo] J.R. Morton and K.F. Preston, J. Magn. Resort. 30 (1978) 577. [ 111 SD. Peyerimhoff and M.J. Buenker, Chem. Phys. Letters 65 (1979) 434. [ 121 L. Bonazzola, J.P. Michaut and J. Roncin, J. Chem. Phys. 83 (1985) 2727. [ 13] R. Franzi, H. Geoffroy, M.V.V. Sheddy and J. Weber, J. Phys. Chem. 91 (1987) 3187. [ 141 E. Illenberger, M.U. Scheunemann and H. Baum@rtel, Chem. Phys. 37 (1979) 21.
CHEMICAL PHYSICS LETTERS
19 August I988
THE STRUCTURE OF THE CFC13RADICAL ANION
L. BONAZZOLA, J.-P. MICHAUT and J. RONCIN Laboratoire
de Physico-Chimie
des Rayonnements,
Bdtiment 350, Universitc! ParisAd,
91405 Orsay Cedex, France
Received 2 April 1988; in final form 28 June I988
Exposure of trichlorofluoromethane in the presence of tetrahydropyran or some other impurities at very low concentration to @‘Coyrays at 77 K gives the CFCI, radical anion. From the analysis of its EPR spectrum and from ab initio geometry optimization, it is concluded that the anion has CJ, symmetry.
..
1. Introduction
:’ :
Organic radical cations have been extensively studied by ESR spectroscopy in the past ten years [ 11. They are produced in irradiated frozen freon matrices. The advantage of the use of fluorine- and chlorine-containing compounds as matrices is to give matrix radicals which do not interfere with the studied cations because their spectra are broadened by the large anisotropy of fluorine and chlorine couplings. On warming the sample containing the cations, some neutral matrix radicals may appear [ 21. Their formation is due to secondary reactions during the decay of the cations. The present work concerns a primary radical anion formed in the most commonly used matrix for radiolytic preparation of cations: trichlorofluoromethane (CFC&).
III, III
Ill
,111 Iti
II,,I,,
I,
,
i
2, Experimental results Two spectra (recorded at 157 K) of y-irradiated CFC13containing respectively 10s3 mol% CHjOH (a) and 3 x lo-” mol% tetrahydropyran (b) are given in fig. 1. Co y rays were used for irradiation at 77 K (dose of 1 Mrad). No ESR signals due to the anions CFCl, were detected after irradiation of pure CFCL and further annealing to 157 K. This is probably due to recombination of CFCl? with the electron loss centers, migrating by hole transfer as soon as molecular motion becomes important. In the presence of impurities (CH30H or tetrahydropyran) these 316
II
CFCICI -n I: CFClCl
Fig. 1. (a) ESR spectrum of CFClj containing IO-’ mol% CH,OH after exposure to Co y rays at 77 K. (b) ESR spectrum of CFCl, containing 3 x I Om6mol% tetrahydropyran after exposure to Co yraysat77K.
0 009-26 14/88/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
19 August 1988
CHEMICAL PHYSICS LETTERS
Volume 149, number 3
Table 1 Experimental g tensors and A hypertine coupling tensors for CFCl, and CF,ClRadical
g1
gll
Nucleus
A, (G)
CFCI,
2.0037
2.0136
j5Cl F
(-)
CF>Cl- a1
2.0070
2.002 1
=Cl F
AI, (G)
18.3 107.4
0 0
17.4 87.4
43.2 197.3
‘) Ref. [6].
holes are trapped provided the ionization potential of the impurity is less than that of CFC&. If the initial concentration of tetrahydropyran (THP) is > 10e5 mol%, the spectrum assigned by Symons et al. [ 31 to THP+ appears. For concentrations below 1Oe5 mol%, spectrum (b) is observed. It can be seen that the two spectra (a) and (b) arise from the same radical even if the centre of spectrum (b) is obscured by one or several interfering paramagnetic species. The shape of the two spectra is due to axially symmetric g and hyperfine T tensors of randomly oriented radicals in a powder. The central line corresponds to T,,= 0 and the other lines correspond to a coupling between the free electron and three nuclear spins, 1=3/2 (Cl), and one nuclear spin I= l/2 (F). Consequently, the observed radical comes from the matrix CFC&. Formation of the cation CFCl: can be eliminated because it has been shown for Ccl: that only two of the chlorine atoms are equivalent and that the Jahn-Teller effect implies a trigonal bipyramidal structure [ 41. Can the formed radical be the anion CC&F-? Reference has been made to this anion in the literature [ 51: by using a tetramethylsilane matrix (TMS), the observed spectrum is very broad (400 G) and unresolved. In the present work, the measured couplings (table 1) indicate that the three chlorine atoms are equivalent, which means that the radical has C3, symmetry and that a rapid reorientation occurs about the C, axis at 157 K. 3. Discussion CF,Cl- radical anions have been observed in TMS matrices [ 61. It has been shown that they possess Cj, symmetry and that the CF3 group rotates rapidly about the symmetric C-Cl axis. On the other hand,
substitution of the central carbon atom with a silicon atom changes the structure of the anion from C3” symmetry (CF,Cl-) to trigonal bipyramidal (SiF&l- [ 71). In view of these contrasting results it seemed desirable to perform full geometry optimization on CFCl; and CClF?. The equilibrium geometries were determined from ab initio unrestricted SCF calculations by means of a minimization procedure using the gradient of the potential energy as described by McIver and Komornicki [ 81. Calculations were done with the program GAMESS using a minimal STO-3G basis set. The calculated spin densities and equilibrium geometry for CFCl, and CClF,- anions are reported in fig. 2 and table 2. It is well known [ 91 that the theoretical study of negative ions involves complications not encountered for neutral and cationic species. For reliable calculations within the LCAO MO finite atomic basis approach, diffuse atomic orbitals have to be added to the core and valence atomic basis sets . Because of computational expense, our calculations were limited to minimal basis sets; nevertheless, good agreement between theory and experiment is achieved.
4. Spin densities and structure Experimental spin densities have been calculated from the measured couplings using the atomic parameters listed by Morton and Preston [ lo], 4.1. CCI,F Calculation shows that this anion has a CsVstructure (see fig. 2). Most of the spin density on the chlorine atom lying in the XZ plane is in the 3p, orbital (perpendicular to the Z direction of the C-F bond). The three chlorine atoms become equivalent 317
Volume 149, number 3
CHEMICAL PHYSICS LETTERS
E : - 1499.076482
19 August 1988
hartrce
Fig. 2. Calculated equilibrium geometry for CF,CI- and CC&F- anions [5]
by rapid reorientation about Cz. The measured total Cl tensor (18.3; 18.3; 0) decomposes into an anisotropic tensor (6.1; 6.1; - 12.2) and an isotropic coupling of 12.2 G. This corresponds to a tensor of 24.2; - 12.2; - 12.2 G relative to the motionless radical and to a spin density of 19.5Ohcompared to the calculated value of 27.3%. The calculated 2s spin 318
density at fluorine is negative. Ifwe consider that the measured couplings are negative,, the total tensor (- 107.4; - 107.4; 0) decomposes into an anisotropic tensor (-35.8; - 35.8; 71.6) and an isotropic coupling of - 71.6 G. The calculated spin density p in the 2p orbitals of fluorine is also negative. The density is nearly the same in 2p,, 2p,, and 2p, or-
19 August 1988
CHEMICAL PHYSICS LETTERS
Volume 149, number 3
Table 2 Experimental and calculated spin densities for CFClr and CF,ClOrbital
Nucleus
CFCl,expt.
2s 2P.y 2P,. 2P:
C
3s 3P, 3P, 3P,
Cl a’
2s 2P, 2P, 2P,
F
” Cl in the xz plane (fig. 2).
CF,Clexpt. [ 61
talc. 0.213 -0.113 -0.113 0.206
0.006 0.195 a’
- 0.004 -0.057 b’ -0.057 c, b) b, 2p,- f 2p,.
‘) 24-f
0.008 0.273 -0.002 0.001
0.013
- 0.002 -0.014 -0.014 -0.013
0.007
0.136
0.058 *z)
Zp,. d, Fin thexz plane (fig. 2).
bitals. Experimental results require that p( 2p,) Jp( 2p,) =p( 2pY)- $(2p,) = - 5.7%. Calculation shows - in agreement with experiment - that no direct spin density appears at the fluorine atoms; its couplings come from spin polarization induced through the bonds. No measurement of 13C couplings have been performed. Strong polarization of orbitals inducing an apparent 2p, spin density (p) of 32% is predicted (p(2pZ)apparent=p(2p,) f[P(2PX)+p(2P,)l). A large 2s spin density at C is also expected. Thus the calculated CJ, electronic structure of CC&F- fits the experimental couplings quite well. It is interesting to compare the changes in structure from the parent molecule to the anion CC13F-: the C-F bond lengthens very slightly; a flattening of the pyramidal group Ccl3 is expected associated with a lengthening of the C-Cl bonds. The unpaired electron resides in an a, antibonding orbital mainly composed of the 2p orbitals of the chlorine atoms, perpendicular to the C-F bond, of the 2s and the 2p, orbital of the carbon (experimental spin density of z 50%). The very slight lengthening of the C-Cl bond in CC13F- with respect to the molecule does not suggest the well-known dissociative electron attachment behaviour of CC&F resulting in the loss of chloride ions [ 5,111. In the CFCl, matrix, the spectra of fig. 1 disappear slowly at 160 K without any transformation leading to the neutral carbon-centered radical CFCl;: this may arise
talc.
talc. [ 51
0.194 0.017 0.017 0.500
0.277 -0.013 -0.013 0.196
0.006 0 0 0.170
0.003 0 0 0.388
0.001 - 0.022 -0.022 0.053
0.004 0.015 0.001 0.003
e’2p,- f (2p,t_2p,).
from the larger mobility of positive charges neutralizing the anion before dissociation is made possible by the softening of the matrix, 4.2. CF,ClCNDO/Z calculations [ 51 suggest that the unpaired electron is in an antibonding o* orbital which is composed largely of the p orbitals from carbon and the halogen lying along the CJVsymmetry axis (CCl) of the anion. We have also performed a geometry optimization of CF,Cl- (see fig. 2): 17O/o of the spin density is in the 2p, chlorine orbital (in agreement with experimental results). Most of the spin density for the fluorine atoms lies in 2p orbitals. The spin density at the carbon atom lies in 2s and 2p, orbitals. A lengthening of the C-Cl bond (with respect to the neutral species) is expected from ab initio calculations (from 1.80 to 2.54 A). CND0/2 calculations give C-Cl = 1.94 A. Such a large variation in bond length is also predicted for disulfur anions: the S-S antibonding o* orbital changes the bond length from 2.06 8, in the molecule to 2.62 A in the anion [ 121. This change is expected to be even greater (S-S = 2.80 A) at the MPn/6-31G* level [ 131. The main structural modification caused by the capture of an electron by a CClF3 molecule is elongation of the C-Cl bond length. This result agrees with the dissociation of the anion into a neutral radical and a Cl- ion, known experimentally [ 61. 319
Volume 149, number 3
CHEMICAL PHYSICS LETTERS
5. Conclusion The EPR spectrum of the CC&F- anion has been observed in a CCISF matrix in the presence of impurities at very low concentration. Analysis of the experimental data and ab initio calculations lead to the conclusion that this anion has C3”symmetry as does CF$l-, reported some years ago. The experimental study of negative ion formation in CF$l and CFC13under low-energy electron impact [ 141 in the gas phase shows that Cl- coming from CFC& is by far the most abundant ion (relative intensity Cl-/ CFC&= 1000 compared to Cl-/CF,C1=2.5). These results are not readily interpretable in the light of the above structure calculations: during the formation of the respective anions a very slight elongation of the C-Cl bond length is expected in CC&F, and a large elongation of the C-Cl bond length is expected in CFSCl. Determining the potential curves for Cl- removal, i.e. calculation of the energies of CFC& and CF,Cl versus C-Cl distance, is probably the best way to understand the dissociative electron attachment mechanism, provided the geometry of the starting molecules is reliable, which was probably not the case in previous work [ 111.
320
19 August 1988
References [l] M.C.R. Symons, Chem. Sot. Rev. (1984) 393. T. Shida, Y. Nosaka and T. Kato, J. Phys. Chem. 82 (1978) 695. [2] B.W. Walther and F. Williams, J. Chem. Phys. 79 (1983) 3167; X.Z. Qin, Q.X. Guo, J.T. Wang and F. Williams, J. Chem. Sot. Chem. Commun. (1987) 1553. [ 3 ] M.C.R. Symons and B.W. Wren, J. Chem. Sot. Perkin II (1984) 511. [4] J.R. Bews and C. Glidewell, J. Mol. Struct. 71 (1981) 287. [ 51 A. Hasegawa, M. Shiotani and F. Williams, Faraday Discussions Chem. Sot. 63 (1978) 157. [6] A. Hasegawa and F. Williams, Chem. Phys. Letters 46 (1977) 66. [ 71 A. Hasegawa, S. Uchimura and M. Hayashi, J. Magn. Reson. 38 (1980) 391. [ 81 J.M. McIver Jr. and A.J. Komomicki, Chem. Phys. Letters 10 (1971) 303. [9] J. Simons and K.D. Jordan, Chem. Rev. 87 (1987) 535. [lo] J.R. Morton and K.F. Preston, J. Magn. Resort. 30 (1978) 577. [ 111 SD. Peyerimhoff and M.J. Buenker, Chem. Phys. Letters 65 (1979) 434. [ 121 L. Bonazzola, J.P. Michaut and J. Roncin, J. Chem. Phys. 83 (1985) 2727. [ 13] R. Franzi, H. Geoffroy, M.V.V. Sheddy and J. Weber, J. Phys. Chem. 91 (1987) 3187. [ 141 E. Illenberger, M.U. Scheunemann and H. Baum@rtel, Chem. Phys. 37 (1979) 21.