1834
J. Phys. Cham. 1994, 98, 1834-1839
ESR Studies on Jab-Teller Distortion in the Radical Anions and Cations of Hexafluorobenzene Akinori Hasegawa' Department of Chemistry, Kogakkan University, Zse-shi 516, Japan
Masaru Shiotani Faculty of Engineering, Hiroshima University, Higashi- Hiroshima 724, Japan
Yoshimasa Hama Advanced Research Center for Science and Engineering, Waseda University, 3-4- 1 Okubo, Shinjuku, Tokyo 169, Japan Received: August 6, 1993"
ESR spectra were observed for both radical anions and cations of C6Fa in rigid solutions. MO calculations followed by computer simulations were performed for these radical ions which were distorted by Jahn-Teller effects, As for the radical anion, INDO calculations for the 2B1 electronic state resulting from the distortion of the molecular framework to a CZ,symmetry gave simulation spectra satisfactorily fitted to the observed spectrum, while the spectra simulated for one more possible 2A state due to a D2 distortion were clearly different from the observed spectrum. Thus, it has been concluded that the radical anion may be in the 2B1 state. In the case of the radical cation, triplets with a coupling of 13.5 G appeared in either wing region of the spectra observed at low temperatures. Although the MO calculations suggested two possible D2h structures in the *Bzs state with an elongated ring and in the 2B3, state with a compressed ring, the spectra observed for the radical cation were successfully interpreted in terms of the results calculated for only the elongated 2B2g state. Introduction Since ESR spectra of the hexafluorobenzene (C6F6) radical anion generated in an adamantane matrix were observed,l many discussions have been provoked on the structure of the radical anion. Yim and Wood2 have assumed that the D6h symmetry of the parent molecule is retained even after the formation of the radical anion and argued that the large isotropichyperfinecoupling observed for each 19Fnucleus (a(l9F) = 137 G) can be interpreted in terms of a u* structure rather a ?r* structure. On the other hand, a nonplanar chair structure with a puckered ring has been postulated by Symons et al. from the detection of anisotropic spectra of the radical anion in a rigid solution.3 However,Williams et al. observed an isotropic 13C coupling (a(13C) = 12.1 G) in the isotropic ESR spectra of the radical anion and have concluded that the magnitude of this coupling does not support the nonplanar structure but is more consistent with a u* than a ?r* structure in D6h symmetry.4 The optical detection of ESR (ODESR) spectra from recombining radical pairs also gave a large coupling of 133.6 G to 19F in the spectra of the radical anion, C6F6-, in ~olutions.~ In order to explain these isotropic 19Fand 13C hyperfine couplings observed in ESR and ODESR spectra, Shchegoleva et al. have employed the INDO method to several geometrical structures including not only the planar and nonplanar structures already described but also a nonplanar structure arising from the electrostatic repulsion of fluorine atoms as well as the structures distorted by the Jahn-Teller effect.6 As a result of the calculations, it has been concluded that the last model is the most reasonable and that the Jahn-Teller effect gives rise to a C , geometrical structure in a 2B1 electronic state and a D2 structure in a =Astate: which are presented as I and 11, respectively, and have the fluorine atoms shifted from the ring plane. Since these two states are energetically close to each other, degeneracy was considered to be preserved for these B and A types of distortion. Thus, the
* Author to whom correspondence should be addressed. Fax: 0596-271704. * Abstract published in Aduance ACS Absrracts, January 15, 1994. 0022-3654/94/2098- 1834$04.50/0
observed ESR parameters were interpreted in terms of averaging over these two distorted structures.
FL
4* F1
I1 F5
X
On the other hand, Hama et al. observed the ODESR spectra of fluorinated toluene anions and reported that the observed isotropic hyperfine couplings to 'Hand I9F nuclei were perfectly reproduced by INDO calculations only for B1 distortions? In connection with a study of C6F6 using a pulsed electron beam high-pressuremass spectrometer,ab initio MO calculations were carried out for both radical anion and cation of C6F6.9 Regarding the anion, the change of bond lengths and the outof-plane deformation were shown to occur in the anion formation process, and the 2B1 state was concluded to be slightly more energetically stable than the 2A state. Hence, structural distortion by the Jahn-Teller effect may now be acceptable for the radical anion Of CsF6. However, which of the electronic states is responsible for the radical anion, 2B1, *A, or the state averaged over the two, still remains a problem to be solved. Only isotropic spectra have been investigated 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1835
ESR Spectra of C6F6 Ionic Radicals regardingthe radical anion. However, since the observed isotropic 19F couplings are attributable to very small spin densities in the s orbitals of the F atoms, the isotropicspectraare not so susceptible to the structural distortion as compared with anisotropic spectra. Thus, anisotropic spectra for the anions in a rigid solution were investigated over a wide temperature region in this study. Regarding the radical cation, C6F6+, one of the authors of this study and his co-workers initially reported the anisotropic hyperfine couplings to six equivalent F nuclei observed at 170 K after the irradiation of C6F6 in perfluorocyclohexaneat 77 K.l0 On the other hand, ab initio MO calculations for the cation have offered two distorted planar DZh structures in the z B ~(III) , and 2B3, (IV) states with elongated and compressed rings, respect i ~ e l y . ~ JThis l suggests that the six fluorines are no longer equivalent in C6F6+. Therefore, the six equivalent F nuclei observed at 170 K may result from some dynamical averaging. Thus, ESR spectra were observed at low temperatures to detect and analyze spectra attributable to the statically distorted cations in this study.
F4
Fl
F5 I11
F4 IV It may be of particular interest to investigate the distorted structures brought about in both processes of releasing and capturing electrons for such Jahn-Teller active molecules as C6F6.
Experimental Section Solutions containing ca. 1 mol % of hexafluorobenzene in neopentane and in perfluorocyclohexane (cX6FlZ) were prepared in Spectrosil ESR sample tubes on a vacuum line. The samples were irradiated with y-rays from a ~ C source, O the typical total absorption dose being ca. 3 Mrad. ESR measurements were carried out with a Bruker ESP 300E spectrometer operating at 100 KHz modulation and at variable temperatures using an Oxford continuous flow cryostat ESR 900. The field strength was measured using a Bruker EP 035M NMR Gaussmeter.
Results and Discussion Radical Anion of Hexafluorobenzene. Observation of ESR Spectra. ESR spectra for the C6F6- anions produced in a neopentane matrix by y-irradiation at 77 K were recorded at various temperatures from 130to 4 K. In this temperature region, the molecularreorientation of neopentane constitutingthe matrix is considered to be too slow to completely averagethe 19Fhyperfine anisotropy. Typical anisotropic features consisting of parallel and perpendicular componentsappeared in either wing region of the spectra, as shown in Figure 1. The resonance fields giving these components remained constant over this wide temperature region. No new lines appeared, but the line widths, in particular, of the inner lines increased with decreasing temperature. These results may rule out the postulation that the radical anion is in the state averaged over two distorted states of 2B1 and ZA. MO Calculations. The 2B1 and ZA forms optimized by the ab initio UHF STO-3G method have been reported for the C6F6-
i Figure 1. First-derivative ESR spectra of a solid solution of ca. 1 mol % CaFa in neopentane after irradiation at 77 K, observed at various temperatures.
anions.gJ2 According to the calculation, the C-F and C-C bond lengths are changed from the original ones of the neutral form and fluorine atoms are shifted from the ring plane upon the attachment of an electron. The out-of-plane displacement of F atoms has been also suggested from INDO calculations.6 Unfortunately,no theoretical ESR parameters have been reported for the structures optimized by the ab initio method. Thus, isotropiccouplings to '9F nuclei werecalculatedfor these structures using ab initio Gaussian 90 with several bases. For example, a
Hasegawa et al.
1836 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994
,
200
180
.47
t
t
-19.3"
a) I
X
i
F3\
F1
m
X
2-,
Figure 3. Geometries of the c6F.5- anions in the (a) 2 B and ~ (b) 2Astates, used for INDO calculations followed by simulations.
'
51
TABLE 1: Isotropic Hy r f i i Couplings Calculated by the INDO Method for c&- Cthe ZB,state with C-F Bonds Distorted 19.3O from the Ring Plane a (G) nuclei Cald avgd obsda 134.4 139.7 132.3 I9Fi,4 "F2,3,5,6 128.6 I4ci,4
'C2,3.5,6 0 From ref 4.
33.6 3.0
13.2
12.1
TABLE 2 SCF Spin Density Matrices Calculated by the INDO Method for c&- in the zB1 State atom orbital matrix Fi S 0.003 1 Px -0.0001 0.0000 0.0000 PY PI = 0.0000 0.0073 0.0335 PI .O.OOOO 0.0335 0.0422 F2 S 0.0029 Px '0.0215 -0.0123 0.0216 PY P2= -0.0123 0.0068 -0,0120 PI .0.0216 -0.0120 0.0146
I
1
diagonalizations were performed, yielding diagonalized pspin densities (eigenvalues) and the direction of the p orbitals (eignevectors), as shown in Table 3. Neglecting spin densities less than 0.014,the spin densities of 0.0625and 0.0472are in the p orbitals of the FIand Fz atoms, respectively. The direction of the p orbital on F1 makes an angle of 3 1.2"with reference to the z axis, being equal to that of 58.8O with reference to the xy ring plane. Regarding the p orbital of Fz, its direction is almost in the plane including the z axis and the C2-F2 bond, and it makes an angle of 52.8O with reference to the z axis, being equal to -37.2O with reference to the xy ring plane. Spectral Simulations for z B ~State. Computer simulations were performed using the results obtained from INDO calculations. Among the ESR spectra shown in Figure 1, the spectra observed at low temperatures consist of lines with different line widths, reflecting some relaxation or molecular dynamics. However, no important change was observed in the spectra observed at temperatures from 130 to 80 K. Hence, the spectrum recorded at 130 K was selected as the spectrum to be fitted by computer simulation. A program was written for simulation using a second-order treatment for a spin Hamiltonian containing a g tensor and hyperfine tensors for six F nuclei, which have axially symmetric axes in different directions. The isotropic Fcouplingswere slightly corrected to adjust to the observed value, 134.4G. Anisotropy in hyperfine couplings can be obtained from the spin densities in
The Journal of Physical Chemistry, Vol. 98, No. 7, I994
ESR Spectra of C6F6 Ionic Radicals
1837
TABLE 3: Diagonalized Spin Density Matrix DI and Unitary Transformation Matrix VI Used To Diagonalize the Matrix PI in Table 2 by VI*PIVI= Dh for C&- in the 2Bt state. matrix
1 1 1 1
0.0001 0.0000 0.0000 D, = 0.0000 0.0625 0.0000 0.0000 0.0000 -0.0130 1.0000 o.Ooo0 0.0000 VI = 0.0000 0.5187 -0.8550 0.0000 0.8550 0.5 187 -0.0041 0.0000 0.0000 D2 = 0.0000 0.0472 0.0000 0.0000 0.0000 -0.0002 0.0547 0.6945 -0.4615 -0.2589 -0.3908 -0.8833 -0.7962 0.6041 -0.0339
[ [ [
a The V, values boldfaced are the eigenvectors for the eigenvalue boldfaced in D,.
the p orbitals, as previously described, and the value of 280 for a 19F nucleus. Since, in contrast to the isotropic a value, the value of 280has not been authorized for INDO calculations, this value was chosen as an adjustable parameter. The direction of the major hyperfineprincipal axis of F1lies in the yz plane making the angle of 58.8O with reference to the xy ring plane. That of F2 makes the angle of -37.2', as previously shown. At first, the value of 280was assumed to be 800 G, and computer simulation was performed. As a result, anisotropy in the simulated spectra was too exaggerated as compared with that in the observed spectrum, which may suggest the use of a smaller value for 280. When 500 G was assumed for this value, simulation spectra had anisotropic features in the spectral regions which are close to those in the observed spectra. However, the perpendicular component of each outermost line was slightly split, deviating from apparent axial symmetry. Since the results of INDO calculations are merely a guide to interpreting the observed spectra, the direction of the p orbitals may be allowed to be slightly changed so as to give better simulation spectra. Small changes by ca. 4O,giving angles of 5 5 O for FI and -41O for F2 with reference to the ring plane, brought about the simulation spectrum shown in Figure 4a. The ESR parameters used for the simulation are given in the caption of the figure. As for the outermost components,the features and regions of the spectral lines are in complete agreement with those observed. Regarding the inner components,however, agreement is not very good. Hereupon, if the large number of the parameters used in this simulation is taken into consideration, one may recognize that the perfect reproduction of the observed spectrum by computer simulationis impossible and that agreement between this simulatedspectrumand the observed one is rather satisfactory, as seen from comparison of the spectra in Figure 4. Further change in the value of 280was of no use to improve the simulation spectra. It is noteworthy that the INDO calculations for the 2 B ~ geometry optimized by the ab initio UHF STO-3G method9led us to the simulation spectra consistingof nine major components, which was in marked contrast to the seven observed lines. Simulation spectra could not be improved substantially for this geometry. Accordingly, the ESR spectra observed for the C6F6 anions in rigid solutions may be interpreted in terms of the anions in such a ZBI state as shown in Figure 3a, at the present stage. Nevertheless, there is still one more model to be examined, i.e., a 2A state in a 0 2 structure. Hence, INDO calculations followed by spectral simulation were also performed for the 2A state in a manner similar to that done for the ZB1 state.
I
9109.4MHz
Figure 4. (a) First-derivative ESR spectrum for C6F6- in the ZB1state simulated with the following parameters: g, = g, = 2.0015,g, = 1.9994; All = 173.2G,A* = 126.3 G for Fl,.; A! = 154.2G,A 1 = 118.8 G for F&3,5,6;a linewidth of 3.5 G. See the text regarding the direction of the axes of hyperfine tensors. (b) First-derivative ESR spectrum for the C6F.5- anion observed at 130K.
MO Calculations and Simulations for zA State. INDO calculations for the anion in the ZA state were carried out by changing the angle made between the ring plane and the C-F bonds to the F atoms which are out of the ring plane. An energy minimum was obtained for an angle of 12O. However, the calculated isotropic F couplings, 1 1 8 G for F I ,in~ the ring plane and 78 G for F2,3,5,6out of the ring plane, were too small to agree with the 134.4 G observed for the six equivalent F nuclei. Neglecting the increase in energy, we can choose 25O for this angle (Figure 3b), because it gives isotropic couplings of ~(F1,4) = 116 G, a(F2,3,5,6) = 124 G, a(14C1,4)= -7 G, and U ( ~ ~ C Z , ~ , S , ~ ) = 27 G, which give average values close to the observed a(F) = 134.4 G and a(14C) = 12.1 G. Thus, it should be emphasized that as far as isotropic spectra are concerned, it is difficult to propose the most preferable state among ZB1,2A,and the average of these two states. Spin density matrices for FI in the ring plane and F2 out of the ring plane were diagonalized. Using these results and the value of 280 as an adjustable parameter, spectral simulations were performed. For 280 = 500 G,the outermost components have three unambiguous extreme peaks which are characteristic of orthorhombic symmetry, as shown in Figure 5 . They are in clear contrast to the two peaks appearing in the observed spectrum (Figure 4b). These three peaks appeared regardless of the magnitude of 280 and the small changes in the direction of the p orbitals on F2,3,5,6. They result from the fact that the p orbitals having spin densities of 0.0588 on these four F atoms make angles of f59.8O with reference to the xy ring plane, while the p orbitals
1838 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994
Hasegawa et al.
170 K
Figure 5. First-derivative ESR spectrum for C6Fs- in the 2A state simulatedwith the followingparameters: gxx= gYy= 2.0015 , g, = 1.9994; Ail = 148.9 G, A l = 117.3 G for F1.4; ,411 = 167.1 G, A 1 = 123.0 G for Fz.3,5,6; a linewidth of 3.5 G. See the text regarding the direction of the axes of hyperfine tensors.
withspindensitiesof0.0422on F1,41ieonthey axis. Thissituation is quite different from that obtained from the 2B1 state. As for the inner components, fitting is inferior to that in the case of the 2B1state. Accordingly, it may be concluded that the CbFb-anion inarigidsolution hasthe2BI electronicstateasaresultofdistortion by the Jahn-Teller effect. Radical Cation of Hexsfluorobenzene. Observation of ESR Spectra. As mentioned in the Introduction, one of the present authorsand his co-workers have reported the anisotropic hyperfine couplings due to six equivalent fluorines for the C6F6 radical cation based on their preliminary ESR study.1° Here we fully present our experimental results on C6F6+. The radical cation was radiolytically generated in the perfluorocyclohexane (e C6F12) matrix, and the ESR spectra were observed over a wide temperature range from 170 to 4 K. The spectral line shapes were found to be drastically changed with temperature as shown in Figure 6. In the figure are shown three typical temperaturedependent ESR spectra of C6F6+recorded at 170,77, and 10 K. The 170 K spectrum clearly shows seven equally spaced hyperfine lines due to six magnetically equivalent fluorines. The spectral line shape is characteristic of axially symmetric g and hyperfine (A) tensors and is well simulated by using the following ESR parameters: All = 67.7 G and A L = ca. 0 G for six equivalent fluorines, gll = 2.0020 and gL = 2.0060. With decreasing temperature, the inner hyperfine lines become broader and almost disappear at 77 K, but the outermost lines remain unchanged. With further decreasing temperature, the outermost bands become sharper and then they are split into three lines with an equally spaced 13.5 G as observed for the 10 K spectrum. We can reasonably attribute the observed triplet to the parallel hyperfine component due to two equivalent fluorines of C6F6+. Then, the six fluorines of C6F6+ can be divided into two groups: one is the two equivalent fluorines giving the 13.5 G splitting and the other is four equivalent fluorines. Only the outermost components due to the four fluorines have been clearly observed with a 394-G splitting in each wing region of the spectra. Thus we attribute a splitting of 98.5 G ((394G)/4) to one of the four equivalent fluorines at lower temperature. The line width of the triplet significantly depends on both the line and temperature: that of the outermost lines is the smallest and the spectrum observed a t 10 K is the most unambiguous. Note that the 4 K spectrum was essentially the same as the 10 K spectrum except for the line width. Consistent with this argument, the inner bands due to the four equivalent fluorines, i.e. M I= f l components, were not clearly observed at 10 K, probably because of a selective line width broadening: the M I = 0 component should be overlapped with the matrix signals a t the center band. The observed g and 19Fhyperfine tensors are characteristic of a planar *-type radical cation similar to the radical cations of
10K
.
100 0
”’
Figure 6. First-derivativeESR spectra of a solid solution of ca. 1 mol 9%of C6F6 in perfluorocyclohexaneafter irradiation at 77 K, observed at
three different temperatures. pentafluoropyridine and other fluorinated pyridines and benzenes.lh Similar dynamics has been observed for the radical cations of Jahn-Teller active molecules such as C6H6+9’3C2H6+, C-C&+, c - C ~ H ~ +and , C-C6H12+.14 We now concentrate on the distorted structure of C6F6+ which gives the two sets of the hyperfines due to two and four equivalent fluorines of C6F6+. The structural distortion may be responsible for a static Jahn-Teller effect of C6F6+ which has a degenarate HOMO in the neutral form. MO Calculations. Two papers have reported MO calculations for the radical cations using the ab initio method.9Jl The parent molecule, C6F6, has a doubly degenerated HOMO. Through the loss of an electron, the bonding part of the C-C bonds becomes weaker and, as a consequence, elongated. On the other hand, the antibonding part becomes stronger and compressed. Thus, two distorted DZh structures are possible: a BO state with an elongated ring (111) and a 2B3, state with a compressed ring (IV).9 It has been also revealed that the elongated form is only 1.8 kcal/mol (STO-3G) or 0.7 kcal/mol (3-21-G) more stable than the compressed form.g Therefore, it may be of interest to investigate which of the states is responsible for the radical cation, the elongated states, the compressed states, or the average of the two. From the character of the HOMO, large splittings due to two equivalent F nuclei can be expected from the compressed state, while small splittings can be done from the elongated state. The observed wing features of the triplet with a coupling of 13.5 G may strongly support the 2B2, state in the elongated D2h symmetry (III). Unfortunately, no ESR parameters have been given from the ab initio MO calculations. Thus, INDO calculations were performed to obtain ESR parameters. The elongated form (111)optimized by the ab initio UHFSTO3G method has been reported.g For comparison, geometrical optimization was computed by the MNDO method,I5 giving
ESR Spectra of C6F6 Ionic Radicals
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1839
TABLE 4 Hyperfine Couplings Calculated by the INDO Method for C&6+ in 'BQ mtb Du Structure and the Geometries Optimized by (A) ab initiog and (B) MNDO Methods isotropic 19Fcoupling, a (GI nuclei Fl.4
F2.3,5,6 avg
A
B
-13.6 63.4 37.7
-16.2 68.9 40.5
obsd"
ca. 23 ~~~
~
anisotropic I9Fcoupling, 2B = 2 ( 4 - AL)/3 (G) 2Bo nuclei F1,4
F2,3,5,6
800 G
700 G
600 G
A
B
A
B
A
B
obsdb
-8.8 60.2
-11.8 64.7
-7.7 52.6
-10.4 56.6
-6.6 45.1
-8.8 40.5
ca.-9 ca.66
From ,411 = 67.7, Al = ca. 0 G at 170 K. From All = -13.5, A L = ca. 0 G for F1.4; All = 98.5, A L = ca. 0 G for F2,3,5,6 at 10 K.
geometric parameters similar to those for III: r(C1-Q = 1.441 A, r(C2-C3) = 1.491 A, r(C1-FI) = 1.311 A, r(CrF2) = 1.294 A, LC6CIC2 = 120.2', and LClCzF2 = 119.0'. ESR parameters were calculated for these two geometries by the INDO method, and the results are given in Table 4. From the spectrum observed at 170 K, hyperfine couplings of All = 67.7 and A L = ca. 0 G were obtained for the six equivalent F nuclei. These values correspond to an isotropic value a = ca. 23 G. The isotropic couplings averaged over six F nuclei are 37.7 and 40.5 G for the geometries optimized by the (A) ab initio and (B) MNDO methods, respectively, and they are not so far from the observed value. Anisotropy in the hyperfine coupling can be obtained from the major spin density on the pporbital and the value of 280 chosen as an adjustable parameter. The anisotropies obtained for the two geometries (A and B) are also given in Table 4. On the other hand, the spectrum observed at 10K gave anisotropic hyperfine couplings: All = (-)13.5 and A L = ca. 0 G for F1,4and All = 98.5 and Al = ca. 0 G for F2,3,.5,6,where the negative value was deduced from the calculated negative spin densities. These values give anisotropies in coupling, ca. (-)9 and ca. 66 G, respectively. As seen from Table 4, the anisotropies calculated with 280 = 800 G for both geometries of A and B are in good agreement with the values obtained from the observed spectrum. The observed parallel couplings of (-)13.5 G for the two F and 98.5 G for the four F nuclei give an average value of 61.2 G. This value is in approximate but reasonable agreement with 67.7 G observed at 170 K. Thus, it may be concluded that the C6F6+cation has only the elongated D2h structure with the 2B2, state regardless of the calculation results, implying that the two forms are energetically close to each other? It is noteworthy that the radical cation of benzene, C&+, has been revealed by ESR observation to undergo the other type of distortion to give the compressed D2h structure,13quite in contrast to the present result for C6F6+. Ab initio calculations gave two contradictoryresults.11 At the UHF level, the elongated structure is 2.0 kcal/mol more stable than the compressed structure for both C6H6' and C6F6+. On the other hand, at the level including electron correlation for the u electrons by means of second-order Mdler-Plesset perturbation, the compressed structure for C6H6+ and t h e elongated structure for C6F6+ are more stable, although the two distorted structures are within 0.1 kcal/mol of each other for both cases." It may be of much interest to consider the reasons why C6F6+ takes an elongated D2h structure with the 2B2, state in the static form, whereas C6H6' takes a compressed D2h structure with the 2B3, state. The F2p*electrons in C6F6+ participate in not only a conjugation so as to stabilize the u electron systems but also a "plus inductive effect" so as to destabilize the electron systems.16 The destabilization is more effective for the 2B2, state since the associated structure has four considerably shortened C-F bonds
and four C atoms with large spin densities, whereas the 2B3, state has two shortened C-F bonds and two C atoms with large spin den~ities.~ Thus, the C6F6+cationmay have 2B2,state. In accord with this discussion, ESR evidence shows that the mono- and 1,4-di-fluorobenzeneradical cations result in b,,-like SOMOs, whereas the benzene radical cations possessing more than four fluorines take bz,-like SOMOs. The details will be reported elsewhere. The present study on a semiempiricalINDO level has suggested that the C6F6- anion takes the cb structure with out-of-plane fluorinesin the 2B1state, while the C6F6+cation has the elongated planar D2h structure in the 2B2, state. In other words, the radical anion undergoes a nonplanardeformationfrom the original planar structure of the neutral molecule, whereas the cation remains planar. Similar results have been obtained in our previous studies for the radical ions formed from planar tetrafluoroethylene molecules: the F2C=CF2- anion takes a nonplanar chair form,I7 while the F2C=CF2+ cation keeps a planar structure.'* Further systematicstudieswill be required to investigateif such difference in structural deformation between the radical anion and cation of the same molecule is common among molecules containing fluorines.
Acknowledgment. We are grateful to Mr. H. Toshiro of Hiroshima University for calculations using the semiempirical MNDO method. We also thank Dr. H. Muto of Government Industrial Research Institute for helpful discussions and Dr. S. Yamabe of Nara Universityof Education for informingus of the geometrical parameters that did not appear in their paper. Prof. F. Williams is also acknowledged for first introducing the authors (A.H. and M.S.) to ESR study of C6F6-when they stayed at the University of Tennessee in the middle 1970s. References and Notes (1) (a) Williams, L. F.;Yim, M. B.; Wood, D. E. J . Am. Chem. Soc. 1973, 95,6475. (b) Shiotani, M.; Williams, F.J. Am. Chem. Soc. 1976,98, 4006. (2) Yim, M. B.; Wood, D. E. J. Am. Chem. Soc. 1976, 98, 2053. (3) Symons, M. C. R.; Selby, R. C.; Smith, I. G.; Bratt, S.W. Chem. Phys. Lett. 1977, 48, 100. (4) Wang, J. T.; Williams, F. Chem. Phys. Lett. 1980, 71. 471. (5) Anisimov, 0. A.; Grigoryants, V. M:; Molin, Yu. N. Chem. Phys. Lett. 1980, 74, 15. (6) Shchegoleva, L. N.; Bilkis, I. I.; Schastnev, P . V. Chem. Phys. 1983, 82, 343. (7) Reference 6 has erroneously represented these two states in terms with bl and a2 orbitals, respectively, in the C, group. (8) (a) Yabe, K.; Douguchi, K.;Sohma, T.; Yamaguchi, H.; Hama, Y. Prep. Symp. Radiat. Chem. Jpn. 1990, 33, 19. (b) Yabe, K.; Sohma, T.; Hirama, R.; Hama, Y. Prep. Symp.Radiar. Chem. Jpn. 1991, 34, 1. (c) Sohma, T.; Kokubu, K.; Yabe, K.; Hama, Y. Prep. ESR Symp. Jpn. 1992, 31, 42. (9) Hiraoka, K.; Mizuse, S.;Yamabe, S.J. Phys. Chem. 1990,94,3689. (10) (a) Shiotani, M.; Kawazoe, H.; Sohma, J. J . Phys. Chem. 1984,88, 2220. (b) Shiotani, M. Mag. Res. Reuiew 1987, 12, 333. (1 1) Raghavachari, K.; Haddon, R. C.; Miller, T. A.; Bondybey, V. E. J. Chem. Phys. 1983, 79, 1387. (12) The authors of ref 9 have informed us that two kinds of angles are lacking for C-F bonds in the optimized geometries shown in Figure 6 of their paper and that they are -2.9O for four C-F bonds in C, structure and 19.0° for four C-F bonds in 0 2 structure, with reference to the ring plane. (13) Iwasaki, M.; Toriyama, K.; Nunome, K. J. Chem. Soc., Chem. Commun. 1983, 320. (14) .For example: (a) Toriyama, K.In Radical IonicSystems; Lund, A,, Shiotani, M., Eds.; Kluwer Academic: Dordrecht, 1991; Chapter I 4 (b) Lindgren, M.; Shiotani, M. In RadicallonicSystems;Lund, A., Shiotani, M., Eds.; Kluwer Academic: Dordrecht, 1991; Chapter 1-5. (15) The MNDO MO calculations were performed using the program MOPAC/386, whichisbasedontheMOPAC 5.0(QCPENo.455),ofTORAY system center. (16) (a) Sheppard, W. A.; Sharts, C. M. Organic Fluorine Chemistry; Benjamin: New York, 1969;Chapter 3, p 18. (b) Chambers, R. D. Fluorine in Organic Chemistry; John Wiley & Sons: New York, 1973; Chapter 4, p 64. (17) Hasegawa, A.; Symons, M. C. R. J. Chem. SOC.,Faraday Tram. 1 1983, 79, 1565. (18) Hasegawa, A.; Symons, M. C. R. J. Chem. Soc., Faraday Trans. 1 1983, 79, 93.
