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A note on the elusive 1E2g state in the two-photon spectrum of benzene
Volume 58, number 4
A NOTE ON THE ELUSNE
CHEMICAL PHYSICS LETTERS
15 October 1978
‘E2p STATE IN THE TWO-PHOTON SPECTRUM OF BENZENE
V. VAIDA*, M.B. ROBIN and N.A. KUEBLER Be11Laboratories. Mwray HdI. New Jersey 07974. US4
Received 28 April 1978 An intense. broad feature appears in both the two-photon thermal lens and multiphoton ionïzation spectra of liquid benzene in the 2000-1800 A region. Such bands in liquids of Iow ionization potentïal are argued to be direct two-photon excïtations into the conduction bands of the lïquids, appearïng three or more eV below the gas-phase tbresholds due to strong solvation of the ionic states. An earlier assignment of the feature in liquid benzene as a transition to the ‘Gg valence state seems uxdikely.
In the region below ‘7.5 eV in benzene vapor, six singlet-singlet excitations are expected, whereas only four have been firmly identified using conventional one-photon and electron energy-loss spectroscopic methods. Within the singlet (x,z*) valence-shell marüfold, the excitations to 1B2u, lBlu and lElu have been convincingly identified, but there remains a one-photon-forbidden transition to lEZg, expected in the vicinity of the transition to IE,, which bas defied detection by conventional means. Beyond the valence shell, excitation from the highest-filed pul0 lelg to the lowest Rydberg orbital, 3s(ak) is ëlectronically forbidden for one-photon excitation, in contrast to that to 3p, whïch is readily observed. The corresponding ek + 3s tnnsition bas been identïfïed in hexamethyl benzene at 42 500 CIII-~ [ 11, and though unobservcd so far in the one-photon spectrum of benzene, its frequency can be estirnated with some confidence. By correlation with the transition frequenties to 3s in a wide range of other hyärocarbons, it is expected that the eQ + 3s transition in benzene will fall24000 cm-1 below the eb ionization potential, i.e. at 50000 cm-l [ 1] , and wïll be two-photon allowed. A dramatic change in the benzene picture bas taken place recently with the advent of two new techniques which are sensitive to weak multiphoton absorptions in nonfluorescing molecules - the multiphoton ion* Present address: Chemistry Department, Harvard Universïty, Cambridge, Massachusetts 02138, USA.
ization and thermal lensing methods. In multiphoton ionization, pulses of dye-laser light are focused into a conductivity cel1 containing an absorbing substance, and the ionization current monitored as the incident wavelength is scanned. The frequenties of the nonionic intermediate states of the absorber are clearly revealed whenever they become resonant with the one-, two-, or three-photon energies, for timder these conditions, the ionization cross section shows a tremendous increase, dïpole selection rules permitting- Using this technique, Johnson [2,3] indeed found a rwo-photon allowed excitation in gaseous benzene, having an origin ar 51085 cm-I_ However, aside from the fact that it is one-photon forbidden and close to the expected (elg,3s) frequency, there is no compelling evidente for a Rydberg assignment. The problem of distinguishing between Rydbe;g and valence shell states bas been encountered repeatedly in the vacuum ultraviolet. and can be resolved using an external perturbation. Thïs usuaUy consists of putting the absorber in a condensed phase of low electronïc mobility, in which valence shell bands are slightly broadened and red-shifted, whereas Rydberg bands are strongly shifted to the blue and so broadened as to appear to be missïng entirely [ 1,4,5]. The condensed-phase test of Johnson’s new transition is reported by Twarowski and Kliger [6], who used the rhermal lensing method_ In this technique the multiphoton laser energy absorbed by the liquid benzene sample is dissipated as he& and the local tempera557
ture changes result in changes of refractive index in the absorptïon volume. These changes of index are then probed with a second, low-energy laser beam, as the wavelength of the Sst laser beam is scanned. Our thermai lens spectrum of hquid benzene, fïg_ 1 (upper), determined as descriied above, is in agreement with the earher work of Twarowski and Kbger. The twophoton therrnal lens spectrum of hquid benzene clearly shows excitations to the vaIence shell lBZu and ‘Bl* states (vibronïcally allowed), but there is no trace of the 51000 cm-l band, thus strongly supportiug a lErs upper state Rydberg assïgnment for this gas-phase t_mnsïtion. The two-photon absorptîon spectrum of Monson and McCIaïn [7] also f&ïIed to reveal an showed excitation in the 50000 cm-l region of hquid benzene, and led them to piace an upper limit of 1 X 10e5: cm4 s/photon molecuIe on the two-photon absorption cross section in tbis region. However, yet another feature appears in the lïquïd
benzene thermal lens spectrum; in the 4000-3600 A one-photorï regïon there is a broad, monotonïcally rïsing signai which is absent in the multiphoton ionization spectrum of the vapor. Becausc the feature appears in a condensed phase, is intense in the two-photon spectrum and is dose to lElu, Twarowsky and Kliger have claimed that thïs is the long-sought valenceshell excitation to ll& ]6]. In thïs note we present an altemate assignment for tbïs feature. Note Brst that 925 eV Îs requïred to ionize benzene in ‘rhe gas phase, but that there is a substantial lowering of the ionïzation potential in liquid benzene due to the polarization stabilization of the ionïc state. There is no direct me-asure of this stabilization for liquid benzene, but Bice and Jortner [9] estimate it as 2.5 eV in crystalhne benzene, and Fuchs and Voltz [IO] have adopted tbis vaIue in their work on the liquid. LeBIanc [i l] bas also considered the question of ionization potentials in organic crystah, and using related spectro-
TWO-F+iOTDN
3 7 0 x Q z az
3
15 October 1978
CHEMICAL PElYSICSLETrERs
Volume 58. number 4
WAVELENGTH,
i
MULÏIPHOIUN IDNIZATRJN LIQUID BENZENE
2-
l0.
, 4500
1
<
1
,
, 4300
,
I
,
, 41;o
I
ONE-PHDTDN
I
t
t
1, 3soD
WAVELENGTH.
t
8
91, 37DO
’
t
1 35 lo
i
Fig. 1. The thennallens (upper) and multiphoton ionïzation spectra (Iower) of liquid benzene. Both spectrawere nxorded uing a W scrvoto maïntaina constantincidentïntensïtyupon the sample, as monitor& wïth an optoacoustic detector chargedwith carbon black [8].
558
VoIume 58. numbes 4
CI-JZMïCAL PHYSICS LETTERS
scopic data deduces the following stahilizationsr naphthalene, 29; anthracene, 3.9; tetracene, 3.8; pyrene, 3.8; and phenanthxene, 3 .Oev. XXere is no obvious trend in tbîs data, but it seems reasonable to assume that the ionizationpotential stabilimtion in liquid benzene will amount to ca. 3 ev. With a liquid-phase ionization potential of 62 eV, the two-photon threshold for the valence band + conduction band transition in benzene is 4000 A. Tbat tbis is quantitatively the threshold reported for the new feature in the thermal lens spectrum is fortuitous; however, it does suggest that the feature in question might be assigned as the direct band-to-band ionization rather than as te,zminating at IE,,. In support of the ionization mechanism, the multiphoton ionization spectrum of liquid benzene, fig. 1, shows the same broad, intense feature, with a thresbold wbich is somewhat to the lowenergy side of tbat in the lens spectrum and visibly overlapped by the excitation to lBlu in the 4400-4100 A region. The relative intensïties of these two features in the ionization spectrum are meaningless as shown, for the ionization below 4 100 A was close to saturation. Moreover, in the thermal lensing experiment, the magnitude of the change in the index of refraction following ionization or tbe subsequent ion recombination is not known quantitatively. Reports of such direct ionization in the one-photon spectra of solutions have been made previously by Nakato et al. ElZ], and Holroyd [13]. The widtb of tbe benzene feature in question is revealing. In liquid benzene, there is stilI no sïgn of a peak at the shortest one-photon wavelength investigated (3590 A), giving an onset-to-peak separation ofwell over 11000 cm-l for the two-photon transition. One sees how unusual this is by comparing it with the intense, broad excitation to lElu in liquid benzene [14], which is only 6000 cm-l from onset to peak. The width of the feature is far more consonant with the width of the conduction band, than wïth any single excïton band. Moreover, if the feature were due to excitation into an exeiton band. then the free-molecule-toliquid shift would have to be at least 11000 cm-1 in order for the transition to be missing in tbe gas pbase down to 3590 A, but present in the liquid beginning at 4000 A (one photon). Sucb a huge vapor/liquid sbift is completely unheard of for a valence-shell excitation which has a null one-photon oscillator strength.
15 October 1978
crystals, i.e., by mutual annihilation of nonconducting excitons. However, if this were the case, then these free-molecule excitations would be seen in tbe gasphase multiphoton ionization spectrum in the 40003600 A region, and tbey are net. In tbis vein, anotber possibility is that tbe feature in question arises from the e& + 3s Rydberg transition which is strongly blueshifted, broadened and overlapped on the high-frequency side by transitions to 4s, Ss, etc. Sucb Wannier/ Franke1 intermediate excitons could be accommodated in the liquïd begïnnïng at 4000 A ïf the ionization potential were sufficiently high, and could themselves gïve rise to ionization either by pairwise annihilation, or by absorption of a third phoron. However, the presence of such Wannier excitons at wavelengtbs below 4000 A (one-photon) implies an ionization potential in the liquid of at least 7.5 eV and probably higher, which is more than 1 eV larger than expectations based on stabïlizations in other organic systems. Consequently, we fee1 that on the basis of the extreme bandwïdrh and its frequency, the 4000-3600 A feature does not have parentage in a state of tbe free molecule, and on tbe basis of energetics, very little of its strength is due to bound Wannier excïtons. Though our arguments for a direct excitation into the conduction band are qualitative only, we do point out that the lAk + lQg
valence-shell assignment is far less likely, and that the eltive %Q_ state of benzene is still missing. If tbe &ignment of tbe broad feature in benzene as being either a direct band-to-bami ionization or what is lcss likely, an excitation into a Warmier-like exciton band is correct, tben sucb broad features should be found in the visible spectra of other liquids having sufficiently low ionization poten&&. Intercstingly, tbe tbermal lees spectrum of liquid butadiene 1151 bas just such a multiphoton feature, stretching from 4200 to beyond 3620 & while in liquid hexatriene 1161 a sünilar feature extends from 5500 to beyond 3800 A. @ith gas-phase adiabatic ionization potentials of 9-06 and 830eV 1171, the onsets reported above yïeld ionïzation-potential stabilizations of 3 .l and 3.8 eV in lic&d butadiene and liquid hexatriene, respectively, if indeed the onsets are these for direct ionization.
Of course, ionization in liquid benzene couId proceed via tbe mecbanism uslually active in molecular 559
Vohune 58. number 4
cKEnacAL
PHYSICS LEiTl.-ERS
Referemes M.B. Rob& Higher excited statesof polyatomk nrokcules, Vols. 1j2 (AcademiePress,New York, 1974 1751. (21 P.M. ióhnson, J. Chem. Phx.. 62 (L975f 4563. (31 P.M. Johnson,J. Chem. Phys. 64 (1936) 4143. [4] S.A. Rite and J- fortner, J_Chem- Phys 44 (1966) 4470. [SI W. Bas&, M.B. Robin and Nh- Kuebler, J. chem. Phys49 (1968) 5007. [6f AJ. Twarowskiand DS. Uigeer,Chem- phys. 20 (í977) 259_ [71 P-R_ bionson and kV&. McClaïn, 3, Chem- Phyr 53 (1970) 29_ IS] W-R Harshbarger and M.B. Robin, Accounts Chem- Res6 C1973j 329. [PI S.A. Rite and J. Jortner,in: Physicsand chem-&ryof the organicsolïd state, Vel. 3, eds D. Fox, M SI. LUbes fll
15 October 11978
and A.We&berger {interscïence.New York, X967) p. 201. [lol C- Fuchs and R. Voltz, chem- Fhyr Letters 18 (1973) 394. fli J o.Ii* Le3lanc Jr., in: physicsand c&mist_v of the OIgauic solid state, Vol. 3, eds. D- Fox, MM- Labes and A. Wekberger (Intencieuce, New York, 1967) p. 133. [121 Y- Nakato, M- Ozaki and H. Tsubomura,J- Phys.Chem. 76 [X972) 2105. 1131 R-k EZolroydand Rx- Rusx& 3. Phys. Chew 78 (1974) 2128. [14] T. Inagakï,J. Chem. Phys. 56 (1972) 2526. fl5] V- Vaida, R.E. Tnrnez,J. Caseyand S.D. Colson, to be published. iI61 A.J. Twarowskiand D.S. -er, Chem- qfiys. Letters50 (1977) 36. [l?f Mi.Beez, G- Biere, N. EO& and E. Heilbronner,Helv. Chïm- Acts 56 (1973) 1028.
A NOTE ON THE ELUSNE
CHEMICAL PHYSICS LETTERS
15 October 1978
‘E2p STATE IN THE TWO-PHOTON SPECTRUM OF BENZENE
V. VAIDA*, M.B. ROBIN and N.A. KUEBLER Be11Laboratories. Mwray HdI. New Jersey 07974. US4
Received 28 April 1978 An intense. broad feature appears in both the two-photon thermal lens and multiphoton ionïzation spectra of liquid benzene in the 2000-1800 A region. Such bands in liquids of Iow ionization potentïal are argued to be direct two-photon excïtations into the conduction bands of the lïquids, appearïng three or more eV below the gas-phase tbresholds due to strong solvation of the ionic states. An earlier assignment of the feature in liquid benzene as a transition to the ‘Gg valence state seems uxdikely.
In the region below ‘7.5 eV in benzene vapor, six singlet-singlet excitations are expected, whereas only four have been firmly identified using conventional one-photon and electron energy-loss spectroscopic methods. Within the singlet (x,z*) valence-shell marüfold, the excitations to 1B2u, lBlu and lElu have been convincingly identified, but there remains a one-photon-forbidden transition to lEZg, expected in the vicinity of the transition to IE,, which bas defied detection by conventional means. Beyond the valence shell, excitation from the highest-filed pul0 lelg to the lowest Rydberg orbital, 3s(ak) is ëlectronically forbidden for one-photon excitation, in contrast to that to 3p, whïch is readily observed. The corresponding ek + 3s tnnsition bas been identïfïed in hexamethyl benzene at 42 500 CIII-~ [ 11, and though unobservcd so far in the one-photon spectrum of benzene, its frequency can be estirnated with some confidence. By correlation with the transition frequenties to 3s in a wide range of other hyärocarbons, it is expected that the eQ + 3s transition in benzene will fall24000 cm-1 below the eb ionization potential, i.e. at 50000 cm-l [ 1] , and wïll be two-photon allowed. A dramatic change in the benzene picture bas taken place recently with the advent of two new techniques which are sensitive to weak multiphoton absorptions in nonfluorescing molecules - the multiphoton ion* Present address: Chemistry Department, Harvard Universïty, Cambridge, Massachusetts 02138, USA.
ization and thermal lensing methods. In multiphoton ionization, pulses of dye-laser light are focused into a conductivity cel1 containing an absorbing substance, and the ionization current monitored as the incident wavelength is scanned. The frequenties of the nonionic intermediate states of the absorber are clearly revealed whenever they become resonant with the one-, two-, or three-photon energies, for timder these conditions, the ionization cross section shows a tremendous increase, dïpole selection rules permitting- Using this technique, Johnson [2,3] indeed found a rwo-photon allowed excitation in gaseous benzene, having an origin ar 51085 cm-I_ However, aside from the fact that it is one-photon forbidden and close to the expected (elg,3s) frequency, there is no compelling evidente for a Rydberg assignment. The problem of distinguishing between Rydbe;g and valence shell states bas been encountered repeatedly in the vacuum ultraviolet. and can be resolved using an external perturbation. Thïs usuaUy consists of putting the absorber in a condensed phase of low electronïc mobility, in which valence shell bands are slightly broadened and red-shifted, whereas Rydberg bands are strongly shifted to the blue and so broadened as to appear to be missïng entirely [ 1,4,5]. The condensed-phase test of Johnson’s new transition is reported by Twarowski and Kliger [6], who used the rhermal lensing method_ In this technique the multiphoton laser energy absorbed by the liquid benzene sample is dissipated as he& and the local tempera557
ture changes result in changes of refractive index in the absorptïon volume. These changes of index are then probed with a second, low-energy laser beam, as the wavelength of the Sst laser beam is scanned. Our thermai lens spectrum of hquid benzene, fïg_ 1 (upper), determined as descriied above, is in agreement with the earher work of Twarowski and Kbger. The twophoton therrnal lens spectrum of hquid benzene clearly shows excitations to the vaIence shell lBZu and ‘Bl* states (vibronïcally allowed), but there is no trace of the 51000 cm-l band, thus strongly supportiug a lErs upper state Rydberg assïgnment for this gas-phase t_mnsïtion. The two-photon absorptîon spectrum of Monson and McCIaïn [7] also f&ïIed to reveal an showed excitation in the 50000 cm-l region of hquid benzene, and led them to piace an upper limit of 1 X 10e5: cm4 s/photon molecuIe on the two-photon absorption cross section in tbis region. However, yet another feature appears in the lïquïd
benzene thermal lens spectrum; in the 4000-3600 A one-photorï regïon there is a broad, monotonïcally rïsing signai which is absent in the multiphoton ionization spectrum of the vapor. Becausc the feature appears in a condensed phase, is intense in the two-photon spectrum and is dose to lElu, Twarowsky and Kliger have claimed that thïs is the long-sought valenceshell excitation to ll& ]6]. In thïs note we present an altemate assignment for tbïs feature. Note Brst that 925 eV Îs requïred to ionize benzene in ‘rhe gas phase, but that there is a substantial lowering of the ionïzation potential in liquid benzene due to the polarization stabilization of the ionïc state. There is no direct me-asure of this stabilization for liquid benzene, but Bice and Jortner [9] estimate it as 2.5 eV in crystalhne benzene, and Fuchs and Voltz [IO] have adopted tbis vaIue in their work on the liquid. LeBIanc [i l] bas also considered the question of ionization potentials in organic crystah, and using related spectro-
TWO-F+iOTDN
3 7 0 x Q z az
3
15 October 1978
CHEMICAL PElYSICSLETrERs
Volume 58. number 4
WAVELENGTH,
i
MULÏIPHOIUN IDNIZATRJN LIQUID BENZENE
2-
l0.
, 4500
1
<
1
,
, 4300
,
I
,
, 41;o
I
ONE-PHDTDN
I
t
t
1, 3soD
WAVELENGTH.
t
8
91, 37DO
’
t
1 35 lo
i
Fig. 1. The thennallens (upper) and multiphoton ionïzation spectra (Iower) of liquid benzene. Both spectrawere nxorded uing a W scrvoto maïntaina constantincidentïntensïtyupon the sample, as monitor& wïth an optoacoustic detector chargedwith carbon black [8].
558
VoIume 58. numbes 4
CI-JZMïCAL PHYSICS LETTERS
scopic data deduces the following stahilizationsr naphthalene, 29; anthracene, 3.9; tetracene, 3.8; pyrene, 3.8; and phenanthxene, 3 .Oev. XXere is no obvious trend in tbîs data, but it seems reasonable to assume that the ionizationpotential stabilimtion in liquid benzene will amount to ca. 3 ev. With a liquid-phase ionization potential of 62 eV, the two-photon threshold for the valence band + conduction band transition in benzene is 4000 A. Tbat tbis is quantitatively the threshold reported for the new feature in the thermal lens spectrum is fortuitous; however, it does suggest that the feature in question might be assigned as the direct band-to-band ionization rather than as te,zminating at IE,,. In support of the ionization mechanism, the multiphoton ionization spectrum of liquid benzene, fig. 1, shows the same broad, intense feature, with a thresbold wbich is somewhat to the lowenergy side of tbat in the lens spectrum and visibly overlapped by the excitation to lBlu in the 4400-4100 A region. The relative intensïties of these two features in the ionization spectrum are meaningless as shown, for the ionization below 4 100 A was close to saturation. Moreover, in the thermal lensing experiment, the magnitude of the change in the index of refraction following ionization or tbe subsequent ion recombination is not known quantitatively. Reports of such direct ionization in the one-photon spectra of solutions have been made previously by Nakato et al. ElZ], and Holroyd [13]. The widtb of tbe benzene feature in question is revealing. In liquid benzene, there is stilI no sïgn of a peak at the shortest one-photon wavelength investigated (3590 A), giving an onset-to-peak separation ofwell over 11000 cm-l for the two-photon transition. One sees how unusual this is by comparing it with the intense, broad excitation to lElu in liquid benzene [14], which is only 6000 cm-l from onset to peak. The width of the feature is far more consonant with the width of the conduction band, than wïth any single excïton band. Moreover, if the feature were due to excitation into an exeiton band. then the free-molecule-toliquid shift would have to be at least 11000 cm-1 in order for the transition to be missing in tbe gas pbase down to 3590 A, but present in the liquid beginning at 4000 A (one photon). Sucb a huge vapor/liquid sbift is completely unheard of for a valence-shell excitation which has a null one-photon oscillator strength.
15 October 1978
crystals, i.e., by mutual annihilation of nonconducting excitons. However, if this were the case, then these free-molecule excitations would be seen in tbe gasphase multiphoton ionization spectrum in the 40003600 A region, and tbey are net. In tbis vein, anotber possibility is that tbe feature in question arises from the e& + 3s Rydberg transition which is strongly blueshifted, broadened and overlapped on the high-frequency side by transitions to 4s, Ss, etc. Sucb Wannier/ Franke1 intermediate excitons could be accommodated in the liquïd begïnnïng at 4000 A ïf the ionization potential were sufficiently high, and could themselves gïve rise to ionization either by pairwise annihilation, or by absorption of a third phoron. However, the presence of such Wannier excitons at wavelengtbs below 4000 A (one-photon) implies an ionization potential in the liquid of at least 7.5 eV and probably higher, which is more than 1 eV larger than expectations based on stabïlizations in other organic systems. Consequently, we fee1 that on the basis of the extreme bandwïdrh and its frequency, the 4000-3600 A feature does not have parentage in a state of tbe free molecule, and on tbe basis of energetics, very little of its strength is due to bound Wannier excïtons. Though our arguments for a direct excitation into the conduction band are qualitative only, we do point out that the lAk + lQg
valence-shell assignment is far less likely, and that the eltive %Q_ state of benzene is still missing. If tbe &ignment of tbe broad feature in benzene as being either a direct band-to-bami ionization or what is lcss likely, an excitation into a Warmier-like exciton band is correct, tben sucb broad features should be found in the visible spectra of other liquids having sufficiently low ionization poten&&. Intercstingly, tbe tbermal lees spectrum of liquid butadiene 1151 bas just such a multiphoton feature, stretching from 4200 to beyond 3620 & while in liquid hexatriene 1161 a sünilar feature extends from 5500 to beyond 3800 A. @ith gas-phase adiabatic ionization potentials of 9-06 and 830eV 1171, the onsets reported above yïeld ionïzation-potential stabilizations of 3 .l and 3.8 eV in lic&d butadiene and liquid hexatriene, respectively, if indeed the onsets are these for direct ionization.
Of course, ionization in liquid benzene couId proceed via tbe mecbanism uslually active in molecular 559
Vohune 58. number 4
cKEnacAL
PHYSICS LEiTl.-ERS
Referemes M.B. Rob& Higher excited statesof polyatomk nrokcules, Vols. 1j2 (AcademiePress,New York, 1974 1751. (21 P.M. ióhnson, J. Chem. Phx.. 62 (L975f 4563. (31 P.M. Johnson,J. Chem. Phys. 64 (1936) 4143. [4] S.A. Rite and J- fortner, J_Chem- Phys 44 (1966) 4470. [SI W. Bas&, M.B. Robin and Nh- Kuebler, J. chem. Phys49 (1968) 5007. [6f AJ. Twarowskiand DS. Uigeer,Chem- phys. 20 (í977) 259_ [71 P-R_ bionson and kV&. McClaïn, 3, Chem- Phyr 53 (1970) 29_ IS] W-R Harshbarger and M.B. Robin, Accounts Chem- Res6 C1973j 329. [PI S.A. Rite and J. Jortner,in: Physicsand chem-&ryof the organicsolïd state, Vel. 3, eds D. Fox, M SI. LUbes fll
15 October 11978
and A.We&berger {interscïence.New York, X967) p. 201. [lol C- Fuchs and R. Voltz, chem- Fhyr Letters 18 (1973) 394. fli J o.Ii* Le3lanc Jr., in: physicsand c&mist_v of the OIgauic solid state, Vol. 3, eds. D- Fox, MM- Labes and A. Wekberger (Intencieuce, New York, 1967) p. 133. [121 Y- Nakato, M- Ozaki and H. Tsubomura,J- Phys.Chem. 76 [X972) 2105. 1131 R-k EZolroydand Rx- Rusx& 3. Phys. Chew 78 (1974) 2128. [14] T. Inagakï,J. Chem. Phys. 56 (1972) 2526. fl5] V- Vaida, R.E. Tnrnez,J. Caseyand S.D. Colson, to be published. iI61 A.J. Twarowskiand D.S. -er, Chem- qfiys. Letters50 (1977) 36. [l?f Mi.Beez, G- Biere, N. EO& and E. Heilbronner,Helv. Chïm- Acts 56 (1973) 1028.