J. Phys. Chem. 1994,98, 11331-11336
11331
Li+CO2- and Na+C02- Generated in Argon Matrices: An ESR Study Ralf Kdppe and Paul H. Kasai" IBM Research Division, Almaden Research Center, 650 Harry Road, Sun Jose, Califomia 95120-6099 Received: June I I , I994@
Li+CO2- and Na+C02- charge-transfer complexes were generated by cocondensation of alkali-metal atoms and CO2 molecules in argon matrices at -4 K. The ESR spectra of Li'COZ- of both the C, form and the cyclic CzVform and Na+C02- of the CzVform were observed. The g-tensors, the 13C hyperfine coupling tensors, and the alkali-metal atom hyperfine coupling tensors of the CzVcomplexes were determined. The unpaired electron distribution (-55% at C and -35% at two oxygen atoms of the COz moiety and -10% at the alkali-metal atom) determined from these hfc tensors is in good agreement with that given by the MNDO molecular orbital method. In the case of the Li/COZ/Ar system, a strong orientation effect was observed for the resulting Li+CO2- complexes. The orientation mechanism consistent with the observed effect is presented and demonstrated.
Introduction
It has been shown experimentally that carbon dioxide, CO2, has a negative molecular electron affinity of -0.60 f 0.2 eV.' It follows that an "isolated CO2- anion" would not exist; the CO2- anion, however, could apparently be stabilized by complexing with a countercation. Jacox and Milligan2 detected, by IR spectroscopy, formation of NaC02, KCOz, and CsCOz upon cocondensation of the alkali-metal atoms and COZin argon matrices. Based on the normal coordinate analyses they concluded the formation of the charge-transferspecies M+C02with Czv symmetry and an OCO angle of -130". Using ab initio SCF methods Yoshioka and Jordan3predicted two stable isomeric forms, I and 11, for M+C02- (M = Li, Na).
I
I1 'M
Structure I has Czv symmetry and a theoretical stabilization energy (with respect to neutral C02 and metal atom) of 0.85 eV for Li+COz- and 0.34 eV for Na+C02-. Structure I1 of C, symmetry is less stable; the stabilization energy is 0.83 eV for LiT0.2- and 0.14 eV for Na+C02-. Both isomeric forms of LifC02- were found by Margave et alS4in their matrix isolation IR study of the Li/COz/Argon system. Only Na+COz- of the CzV form was observed from the Na/COgAr system, and photoirradiation was required for its f o r m a t i ~ n . ~ , ~ ESR spectra of Li+CO2- and Na+COz- generated in alkalimetal formate by X-ray or y-ray irradiation had been studied exten~ively.~-~ ESR spectra of Li+COz- and Na+C02- generated upon condensation of alkali-metal atoms in COZmatrices had also been reported.l0," In each of these cases, M+C02of CzVsymmetry was formed. The unpaired electron was found mainly on the carbon, and small but characteristic hfc (hyperfine coupling) structures due to the alkali-metal nucleus were observed. Exact disposition of the metal cation in the M+C02complex is influenced sensitively by the polarity of the host matrix. Thus Aiso (the isotropic component of the hfc tensor) of the 'Li nucleus of Li+CO2- generated in lithium formate is -3 G,8 whereas that formed in a COz matrix is -1 1 G." The @
Abstract published in Advance ACS Abstracts, October 1, 1994.
0022-3654/94/2098-11331$04.50/0
Aiso of 23Naof Na+C02- generated in sodium formate is -9 G,6 whereas that formed in a C02 matrix is -23 G.l0 No ESR study of the alkali-metal/C02/Ar system has been reported. We report here the ESR spectra of Li+CO2- and Na+C02generated by the cocondensation method in argon matrices. As expected, much larger Aiso of the metal atom nuclei, 15 G for Li+CO2- and 41 G for Na+C02-, were observed. The unpaired electron distribution (-55% at C and -35% at two oxygen atoms of the COz moiety and -10% at the alkali-metal atom) determined from the 13C and alkali-metal atom hfc tensors is in good agreement with that given by the MNDO molecular orbital method. Most interestingly, in the case of the Li/COz/ Ar system, a strong orientation effect was observed for the resulting Li+COz- complexes. The orientation mechanism consistent with the observed effect is discussed and demonstrated. Experimental Section
A liquid helium cryostat that would enable trapping of vaporized metal atoms in an inert gas matrix and examination of the resulting matrix by ESR has been described earlier.lZ Spectra can be recorded while the flat deposition target (sapphire) is oriented parallel or perpendicular to the field lines of the magnet. In the present series of experiments, alkali-metal atoms (Li or Na) were evaporated from a resistively heated stainless steel tube (250 "C for Na and 470 "C for Li) and were trapped in argon matrices containing a controlled amount of COz (-0.4%). The ESR spectrometer used was an IBM Model ER200D system, and a low-frequency (375 Hz) field modulation was used for the signal detection. All the spectra reported here were obtained while the matrix was maintained at -4 K. The spectrometer frequency locked to the sample cavity was 9.433 GHz and a typical microwave power level was -2 pW. For photoirradiation of the matrix, a light beam from a high-pressure xenon-mercury lamp (Oriel, 1 Kw unit) was passed through a water filter and a broad band interference filter of choice and was focused on the cold finger 40 cm away. Research grade argon and CP grade carbon dioxide were obtained from Matheson, and 13Cenriched (enrichement '99%) carbon dioxide was obtained from Aldrich. 0 1994 American Chemical Society
11332 J. Phys. Chem., Vol. 98,No. 44, 1994 0
A
1 1
A
I
I
5 s
Figure 1. ESR spectra of the Li/C0~(0.4%)/Arsystem: (a) observed as prepared: (b) observed after irradiation with blue light ( I = 400 f 50 nm). The inner pair of the quartet due to ’Li atoms (A) and the outer components of the triplet due to 6Li atoms are indicated. The signals B and C are ascribed to Li+CO2- complexes (see text).
Spectra and Analyses Li+C02-. The ESR spectrum of Li atoms isolated in an argon matrix was reported some time ago.I3 There are two naturally abundant Li isotopes, 7Li (natural abundance = 92.6%, I = 3/2) and 6Li (natural abundance = 7.4%, I = 1). The spectrum is thus dominated by a sharp quartet (Aiso = 143 G) due to 7Li atoms; a triplet of weaker intensity and smaller spacings (Aiso = 55 G) due to 6Li atoms is also readily recognized. An argon matrix in which COz and Li atoms had been cocondensed appeared dark green. Figure l a shows the ESR spectrum of the matrix thus prepared; the central section encompassing the inner two components of the quartet of 7Li atoms is shown. The outer components of the triplet due to isolated 6Li atoms are recognized as indicated. The remaining signals, the broad quartet B and singlet C, were observed only when COz and Li atoms were cocondensed. When the matrix (of Figure la) was irradiated with blue light (A = 400 f 50 nm) for eight minutes, the signals due to isolated Li atoms decreased and the singlet C disappeared almost completely, while the quartet B increased substantially (Fig. lb). Margrave et al. observed, in their matrix-isolation IR study, the conversion of Li+CO2- of the C, form to that of the C2, form on exposure to visible light.4 The quartet B in Figure 1 is hence assigned to Li’COz- of the CzVform and the singlet C to Li+COz- of the C, form. The quartet pattern of B is undoubtedly due to the hfc interaction with the 7Li nucleus. In either form, the unpaired electron is expected to reside mainly on the carbon atom, and a much smaller Li hfc interaction is expected for C,Li+COz-. The ESR spectrum of M+C02- is expected to be given by the spin-Hamilotnian of the following form: %pin
= P(g.P.$x
Koppe and Kasai
+ gpySy + gP&) + W2.x + Ay’ySy + A J 2 2 (1)
where the first term represents the Zeeman interaction and the second part the hfc interaction with the metal nucleus. The principal axes of the g-tensor and that of the hfc tensor would be coincident for C2” Li+COz- but not for C, Li+COz-.
Figure 2. ESR spectra of CzVLi’C02- observed from the L E O 2 (0.4%)/Arsystem after irradiation with blue light: (a) the magnetic field parallel to the matrix plane and (b) the magnetic field perpendicular to the matrix plane. The absorption shapes of each hyperfine components are drawn and the field positions associated with the principal axes of the g-tensor are indicated. The signals seen at extreme left are due to isolated 6Li atoms.
Most interestingly the quartet signals B of C2, Li+COZrevealed a dramatic effect of preferential orientation of the species within the matrix. As stated earlier, the cold finger for the matrix deposition is a flat spatula-shaped sapphire rod. Figure 2 shows the quartet B of the Li/C0~(0.4%)/argonsystem observed after irradiation with blue light (A = 400 f 50 nm) with the magnetic field parallel (Figure 2a) and perpendicular (Figure 2b) to the plane of the cold finger. When the magnetic field is parallel to the plane of the rod (Figure 2a), for each hyperfiie component,three “singular” signals can be recognized at the positions associated with the three principal axes (of the g and hfc tensors) as indicated. The relative strengths of the signals are not exactly those expected from an ensemble of randomly oriented radicals (vide infra). We then note that, when the magnetic field is perpendicular to the plane of the cold finger (Figure 2b), two of the “singular” signals gain prominence conspicuously at the expense of the third. The orientation mechanism that would account for the observed effect is presented in the Discussion section. Not unexpectedly, when the matrix was prepared with simultaneous photoirradiation during deposition, the preferential orientation was essentially absent. The quartet B observed from the Li/coz(o.4%)/Ar system, prepared with simultaneous irradiation with blue light (A = 400 f 50 nm), is shown in Figure 3a. The three singular signals of each hyperfine component are recognized as shown. The g-tensor and the ’Li-hfc tensor determined accordingly are presented in Table 1. Fig. 3b shows the computer-simulated spectrum based on these tensors.I4 In order to ascertain the number of COz molecules in each complex and to determine the I3C hfc tensors, we then repeated the experiment using 13C-enriched (>99%) carbon dioxide. Figure 4a shows the ESR spectrum observed from the Li/13C02(0.4%)/Ar system (prepared in the dark). A broad doublet with the spacing of -90G and a doublet-of-quartet with the doublet spacing of -180 G are recognized as indicated. The former is assigned to C,Li+C02- and the latter to CZ, Li+COzof I3C isotope. In accord with the earlier result, when the matrix (of Figure 4a) was irradiated with blue light, the broad doublet disappeared (Figure 4b). Though some sections are masked by strong signals due to Li atoms, sufficient details of the doublet-of-quartetsare seen to permit accurate determination of the 13Chfc tensor. The
J. Phys. Chem., Vol. 98, No. 44, 1994 11333
Li+CO2- and Na+C02- Generated in Argon Matrices
1
*
2.I
"
Z
"
y z n
y r
I
y
I
7s
Figure 3. (a) ESR spectrum of C2, LifC02- observed from the Lil c02(0.4%)/Arsystem prepared with simultaneous irradiation with blue light. The sharp triplet (indicated by arrows) is due to isolated 6Li atoms. (b) ESR spectrum simulated on the basis of the g-tensor and the 7Li hfc tensor given in Table 1. The signals due to 6Li atoms (0.05%) were superposed.
e Figure 4. ESR spectra of the LilL3CO2(0.4%)/Arsystem: (a) observed as prepared: (b) observed after irradiation with blue light (1= 400 f 50 nm). (c) ESR spectrum of Cz, Li+I3CO2- simulated on the basis of the g-tensor and the 7Li and I3C hfc tensors given in Table 1.
TABLE 1: g-Tensors, the I3C hfc Tensors, and the Alkali-Metal Atom hfc Tensors of Li+COZ- and Na+C02Generated in Formate Crystals and in Argon Matrices Y
X
Z
Aiso
~
g A(7Li) A(I3C)
2.0008 -3.3 207
LiHCO2 (ref 8) 2.0026 1.9971 -3.3 -3.3 G 156 156
-3.3 G 174 G
g A(Li) A(I3C)
LilCOJAr (current work) 2.0008 1.9967 2.0033 16.5 15.3 14.4 214.5 168.0 168.0
15.4 G 183.5 G
A(W)
2.0019 10.3 202.0
NaHC02 (refs 6 and 9) 2.0034 1.9980 8.1 8.0 153.7 154.6
8.7 G 170.1 G
g A(Na) A(I3C)
NdCOJAr (current work) 2.0029 2.0023 1.9979 40.0 39.5 42.5 196.0 150.0 153.0
40.7 G 166.3 G
g
13Chfc tensor thus determined is given in Table 1. Figure 4c shows the simulated spectrum of C2" Li+l3C02- based on the g-tensor and the 'Li hfc tensor (determined from the normal species) and the 13C hfc tensor determined here. Na+ Cot-. The ESR spectrum of Na atoms isolated in an argon matrix was also examined some time ago.13 The spectrum comprises a sharp quartet due to the hfc interaction with the 23Na nucleus (A,,, = 330 G). In contrast to the Li/C02/Ar system, the ESR spectrum of the NdC02/Ar system as prepared showed only the ESR signals due to isolated Na atoms. Irradiation of the matrix with yellow light (A = 600 f 50 nm covering the Na 3s 3p transition) resulted in decrease of the Na atom signals and concurrent appearance of a prominent quartet with successive spacings of -40 G centered about the position of g = 2.00 (Figure 5a). Jacox and MilliganZand Margrave et aL5 observed spontaneous formation of M+C02- in an argon matrix for M = Li, K, and Cs but formation of Na+C02- only after irradiation of the matrix with visible light. Only Na+C02- of the C2, form was observed. The quartet pattern of Figure 5a is hence assigned
-
Figure 5. (a) ESR spectrum of Na+C02- observed from the NdC02(0.4%)/Ar system after irradiation with yellow light (1 = 600 f 50 nm). (b) ESR spectrum simulated on the basis of the g tensor and the 23Nahfc tensor given in Table 1.
to C2" Na+C02-. In contrast to Li+CO2- spontaneously formed in argon matrices, Na+C02- photoinduced in argon matrices showed only a slight indication of preferential orientation. The singular points in each hyperfine component (of Figure 5a) are recognized as shown. The g-tensor and the 23Nahfc tensor of C2, Na+C02- thus determined are given in Table 1. Figure 5b shows the simulated spectrum based on these tensors. The ESR spectrum of C2" Na+13C02- observed from the Na/ l3CO2(0.4%)/Ar system after irradiation with yellow light is shown in Figure 6a. The expected doublet-of-quartet pattern was recognized as indicated, and the 13C hfc tensor was determined (Table 1). Figure 6b shows the simulated spectrum of C2" Na+l3C02- based on the g-tensor and the 23Nahfc tensor (determined from the normal species) and the 13C hfc tensor determined here.
Koppe and Kasai
11334 J. Phys. Chem., Vol. 98,No. 44, 1994
0 80MO
Y O G
Figure 6. (a) ESR spectrum of CzVNa+I3C02- observed from the Na/ *3C02(0.4%)/Ar system after irradiation with yellow light (A = 600 f 50 nm). (b) ESR spectrum simulated on the basis of the g tensor and the 23Naand I3C hfc tensors given in Table 1.
Discussion Earlier experimental works (IR and ESR) clearly showed the existence of charge-transfer species M+C02- (M = Li, Na, K, Cs) of Czv symmetry (structure I). For Li+CO2- the existence of the isomer of C, symmetry (structure 11) was predicted theoretically3and was later confirmed experimentally? A large difference between the force constants, e.g., ALiO) = 0.45 mdyn/A for CzVLi+CO2- and ALiO) = 1.46 mdyn/A for C,Li+CO2-, suggests a more covalent bonding for the latter. In the present ESR study of the Li/COz/Ar system, an ESR spectrum most likely due to C, Li+COZ- was indeed observed. The spectrum revealed only the average g value (2.00) and average 13Chfc constant (-90 G), however. In the absence of additional information, further discussion of the species is not possible. The following discussions pertain only to Li+COz- and Na+C02- of the CzVform. The g-tensors, the metal atom hfc tensors, and the 13C hfc tensors of Li+CO2- and Na+C02determined presently are compiled in Table 1. The corresponding values determined earlier in formate crystals are also shown for comparison. The g-tensors and the 13C hfc tensors of M+C02- complexes generated in argon matrices and those generated in formate crystals are conspicuously similar, while the metal hfc tensors are quite different. Much can be learned from the g-tensors and the hfc tensors of the complexes determined presently. g-Tensor. The n-orbital system of the linear C02 molecule comprises three doubly degenerate levels, the bonding orbitals of the form a4(2p), b[4(2p)o1 4(2p)02], the nonbonding orbitals of the form 4(2p)ol - 4(2p)02, and the antibonding orbitals of the form a4(2p)c - b[4(2p)o1 4(2p)021. The eight n electrons would fill the bonding and nonbonding orbitals. In the case of the anion, COz-, the unpaired electron is hence expected to reside in one of the antibonding n orbitals, or, if the anion is bent, in the orbital correlated to it. The situation is depicted in Figure 7. Here the 0-C-0 direction (of linear COz) is identified with the y-axis, and the x-axis is identified with the CzVaxis of bent COz. A perusal of the situation by the EHT method15 revealed that, on bending, very little happens
+
+
+
0,
= a+lp,)c
0
- bl+l~.101 + 4(Px)ozl
0 = m0 + c +(sic + d + ~ S ) L < Figure 7. The n-orbital system of linear and bent Cot- anions (without Lif ion) given by EHT calculations.
to any of these n orbitals except the antibonding n orbital coplanar with the plane of the bend.I6 The energy values given at the right end of the figure are those of the EHT calculation. Relatively small hfc interactions of the metal nuclei observed for Li+COZ- and Na+COz- indicate that the disposition of the valence electrons in these complexes is closely approximated by that of the bent C02- anion. The SOMO (semifilled molecular orbital) of the complex is thus surmised to be as follows:
= U4(2Px)c - H m P X ) o l + 4(2Px)021 + c4(2s)c + d4(2S),i (2) It has been shown that, for a radical with a nondegenerate ground state IO),deviation of the g-tensor from the spin-only value, g, = 2.0023, is given as follow^:'^ gi - g, =
-2nC n
(0ILiIn>(. lLjlO> En-
Eo
(3)
Here i (=x, y, z ) represents a principal axis of the g-tensor, L, the orbital angular momentum operator, and 1 the one-electron spin-orbit coupling constant. The summation is performed for all the excited states. In evaluating eq 3 in terms of LCAOMO's, only one-centered integrals need be retained, and for each atomic integral the spin-orbit coupling constant of the particular atom is used. Within the manifold given in Figure 7, only excited levels that can give nonvanishing terms in eq 3 are n : and a,. It follows immediately that g, I 2.0023, g, > g, (reflecting the effect of filled a, orbital), and g, -= gd (reflecting the effect of vacant n : orbital). The deviation of g, is expected to be uniquely large because of the small separation (6E) between the n,"and n,*levels. The identification of the principal axes of the observed g-tensor to the molecular axes (as shown in Figure 7) is based on these considerations. The spin-orbit coupling constants of oxygen and carbon atoms (estimated from atomic spectroscopy data)'* are 0.0093 and 0.0018 eV, respectively. Substituting these values and 6 E =
Li+CO2- and Na+C02- Generated in Argon Matrices
J. Phys. Chem., Vol. 98, No. 44, 1994 11335
1.3 eV in eq 3 and setting a2 = 0.40 and b2 = 0.18 in eq 2 (the values estimated below from the hfc tensors), one predicts g, = 1.996 in reasonable agreement with the observed value. HFC Tensors and SOMO. The hfc interaction of a particular magnetic nucleus is determined essentially by the distribution of the unpaired electron in the immediate vicinity of the nucleus. The 13Chfc tensor of M+C02- is thus expected to be axially symmetric about the x-axis and may be analyzed as follows:19 All
+ udip
=
=
-
(4)
Here Aiso and Adip are the isotropic and dipolar components of the hfc tensor resulting from the spin densities in the 4(2s)c and 4(2px)c orbitals, respectively. The analysis, thereby, of the observed 13C hfc tensor of LifC02- yields Aiso(13C)= 183.5 G, and Ad,,(13C) = 15.5 G. Comparison of these values with the atomic values,20ALo(13C)= 1349 G and A&,(’3C) = 38.4 G, would yield the spin densities at the respective carbon orbitals. The spin density in the Li 2s orbital may be assessed from the observed Li coupling constant in comparison with that of isolated Li atoms (Aiso= 143 G). The balance of spin density may be assumed to be in the oxygen 2p, orbitals. The unpaired electron distribution (hence the coefficients in eq 2) are thus determined as follows: a’ = g(2pJC = 0.40 (0.36)
b2 = g(2px), = 0.18 (0.17)
c2 = ~ ( 2 s=) 0.14 ~ (0.12)
d2 = ~ ( 2 s =) 0.11 ~ ~ (0.13) We also examined the spin density distribution by the MNDO method in the MOPAC package.21 The structural parameters were taken from Jordan’s ab initio calc~lation.~ The results are given above in parentheses. The calculated values are in excellent agreement with those deduced from the observed hfc tensors. The MNDO calculation predicted, in addition, g(2px)~i = 0.04. The observed anisotropy of the 7Li hfc tensor, however, is not believed to be due primarily to Li 2p orbitals. The anisotropic component of a hfc tensor between an unpaired electron and a magnetic nucleus is given by eq 5 where r is the distance between the unpaired electron and the nucleus.
We estimated the dipolar interaction between the 7Li nucleus and the unpaired electron (distributed over the COz moiety as delineated above) by performing numerical evaluation of eq 5 using the Slater orbitals of carbon and oxygen. We obtained Adipr = t-2.1, Adip,y = -0.6, and = -1.5 G, readily accounting for the magnitude and signs of the anisotropy of the observed 7Li hfc tensor (Adipr = +1.1, Adip,, = -0.1, and Adip,z =
Y
-1.0 G).
Orientation. Preferential orientation of species trapped in rare gas matrices has been observed from time to time. It appears to occur when the molecular species impinging on the target matrix has a correct kinetic energy so that, during deposition, the advancing shallow surface layer of the matrix remains fluid. The orientation effect is most often encountered with linear molecules (e.g., CuF;?,BO, et^.)^*,^^ and molecules with a well-defined molecular plane (e.g., NO;?,NF;?, Cu(NO3);?,
Figure 8. Preferential orientation of LifC02- in argon matrices: neutral COZmolecules are trapped with their molecular axes oriented parallel to the matrix plane (the plane of the figure), and the Li’C02complex is formed when a lithium atom assumes an equatorial position around the carbon atom as indicated. Computed spectra based on “perfect orientation’’and the g-tensor and ’Li hfc tensor of the complex are shown for the two canonical orientations of the cold finger.
etc.).22,24,25The preferred orientation is usually that in which the molecular axis or the plane lies “flat”, parallel to the surface of the cold finger. Noted exceptions are diatomic molecules with a large dipole moment such as VO and ZnF.26x27These molecules were found to orient with the molecular axis perpendicular to the matrix surface. It is pertinent to note that, in either mode of orientation, when the magnetic field is perpendicular to the plane of the rod, only signals associated with one particular principal axis are observed accentuated. In contrast, the ESR spectra of the Li/CO;?/Arsystem shown in Figure 2 are such that, when the magnetic field is perpendicular to the matrix surface, the signals along two principal axes, x and z , are accentuated. The signals along the y-axis are accentuated when the field is applied parallel to the matrix plane. Clearly the orientation is not dictated by the molecular plane of the complex. The observed orientation effect can be accounted for if we assume (1) that neutral C02 molecules are trapped with their molecular axes orientated parallel to the matrix plane and ( 2 ) that the charge-transfer complex Li+CO*- is formed when a lithium atom migrates through the shallow fluid layer and assumes an equatorial position around the carbon atom of a C02 molecule. The situation is depicted in Figure 8 identifying the plane of the figure with that of the matrix surface. Hereon, when the magnetic field is parallel to the matrix plane (say, in the horizontal direction of the figure), the resonance positions of the Li+CO2- complexes are determined by the angle 8 between the molecular axis (of the original C02) and the magnetic field and the azimuthal position of the Li+ ion on the equatorial circle. Singular signals are expected at the positions associated with all three principal axes. The pattern differs from that due to an ensemble of randomly oriented radicals in that the solid angle factor, sin (e),is absent for the oriented matrix. When the magnetic field is perpendicular to the matrix plane, the resonance position depends only on the azimuthal position
Koppe and Kasai
11336 J. Phys. Chem., Vol. 98, No. 44, I994 of the Li+ ion. Singular signals are hence seen at the positions associated with the x and z axes only. The simulated spectra for the two canonical orientations of the cold finger based on “perfect orientation” and the g-tensor and the 7Li hfc tensor of the complex (Table 1) are also shown in Figure 8. The variances betweeen the simulated spectra and those observed (Figure 2) are ascribed to incompleteness of the orientation. Acknowledgment. Ralf Koppe is grateful for a financial support of Deutsche Forschungsgemeinschaft for his stay at IBM Almaden Research Center. References and Notes (1) Compton, R. N.; Reinhardt, P. W.; Cooper, C. D. J . Chem. Phys. 1975, 63, 3821. (2) Jacox, M. E.; Milligan, D. E. Chem. Phys. Lett. 1974, 28, 163. (3) Yoshioka, Y.; Jordan, K. D. Chem. Phys. Lett. 1981, 84, 370. (4) Kafati, Z. H.; Hauge, R. H.; Billups, W. E.; Margrave, J. L. J . Am. Chem. SOC. 1983, 105, 3886. (5) Kafafi Z. H.; Hauge, R. H.; Billups, W. E.; Margrave, J. L. lnorg. Chem. 1984, 23, 171. (6) Cook, R. J.; Whiffen, D. H. J. Phys. Chem. 1967, 71, 93. (7) Atkins, P. W.; Keen, N.; Symons, M. C. R. J . Chem. SOC. 1962, 2873. ( 8 ) Sharp, J. H.; Symons, M. C. R. J . Chem. SOC.A 1970, 3075.
(9) Dalal, N. S.; McDowell, C. A.; Park, J. M. J. Chem. Phys. 1975, 63, 1856.
(10) Bennett, J. E.; Mile, B.; Thomas, A. Trans. Faraday SOC. 1965, 61, 2357.
(11) Borel, J. P.; Faes, F.; Pittet, A. J. Chem. Phys. 1981, 74, 2120. (12) Kasai, P. H. Acc. Chem. Res. 1971, 4, 329. (13) Jen, C. K.; Bowers, V. A.; Cochran, E. L.; Foner, S. N. Phys. Rev. 1962, 126, 1749. (14) Kasai,P. H. J . Am. Chem. SOC. 1972, 94, 5950. (15) Hoffmann, R. J . Chem. Phys. 1963, 39, 1307. (16) Walsh, A. D. J . Chem. SOC. 1953, 2226. (17) F‘ryce, M. H. L. Proc. Phys. SOC. (London) Ser. A 1950, 63, 25. (18) Moore, C. E. Natl. Stand. Re5 Data Ser. (US.Natl. Bur. Stand.) 1971, 35. (19) Smith, W. V.; Sorokin, P. P.; Gelles, I. L.; Lasher, G. J. Phys. Rev. 1959, 115, 1546. (20) Morton, J. R.; Preston, K. F. J . Magn. Reson. 1978, 30, 577. (21) MOPAC (V.5) BY J. P. Stewart of Frank J. Seiler Research Laboratory, U. S . Air Force Academy, Colorado Springs, CO. (22) Kasai, P. H.; Whipple, E. B.; Weltner, W., Jr. J . Chem. Phys. 1966, 44, 2581. (23) Knight, L. B.; Wise, M. B.; Davidson, E. R.; McMurchie, L. E. J . Chem. Phys. 1982, 76, 126. (24) Kasai, P. H.; Whipple, E. B.; Weltner, W., Jr. J . Chem. Phys. 1965, 42, 1120. (25) Kasai, P. H.; Whipple, E. B. Mol. Phys. 1965, 9, 497. (26) Kasai, P. H. J . Chem. Phys. 1968, 49, 4979. (27) Knight, L. B.; Mouchet, A.; Beaudry, W. T.; Duncan, M. J . Mag. Reson. 1978, 32, 383.
