J . Phys. Chem. 1992, 96. 1525-1527
0 -/ -L.I
0
L&_
1 2 -
100
L
I
-
L
200 300 Timelfs
-
._-L
400
500
Figure 3. Variance of the distribution of Ar-Ar interatomic distances (in units of d)versus time for three densities at 300 K. As evident from Figures 1 and 2, the barrier crossing time is shorter than 100 fs.
a mass effect.Ig The relative distance of H to the OH center of mass becomes essentially the "new" OH bond distance. It therefore fails to impart too much momentum to the "old" OH bond distance and hence fails to displace it. To probe the state of the rare gas environment, we have examined both individual Ar atom trajectoric id ensemble averages. Caging of the (small sized) reactants vciurs only at such densities that solvent atoms are also caged so that diffusion is considerably slowed down. But diffusion is a longer time scale (19) Schechter, I.; Levine, R. D.; Gordon, R. G.J . Phys. Chem. 1991,95, 8201.
1525
phenomenon, and on the time scale of barrier crossing there is no discernible differences in the dynamic response of the solvent. We illustrate this point in Figure 3 which shows the variance of inter Ar atom distances versus time. Most previous simulations in liquid Ar at 300 K were carried out at p* = 0.83 when no caging is evident. If the time axis of Figure 3 is scaled by an order of magnitude, so that it runs to 5 ps, then the behavior at p = 0.83 will be seen to be diffusive ((9) a t ) . On the time scales of interest to us, the behavior of the solvent at p = 1.68 is not qualitatively different from that at p = 0.83. In conclusion, we found that even at very high densities where caging does take place, the response of the rare gas environment is still slow on the time scale of a direct bimolecular reaction. This can possibly be used to simplify the experimental study of the dynamics of such reactions which are typically over in a vibrational period. The motion from the reactants valley over the barrier and to the products valley exhibits the same dynamics in either a fluid or a glass environment. Differences due to the changes in the solvent dynamics as the density is increased are manifested only at longer times. A detailed discussion of this separation of time scales, with the short time dynamics being governed essentially by the potential energy of the isolated reactants, is being prepared for publication.
Acknowledgment. We thank Professors W. M. Gelbart, D. Kivelson, R.Lynden-Bell, M. F. Nicol, M. Ross,and K. R.Wilson for useful discussions and comments. This work was supported by the US.-Israel Binational Science Foundation, BSF, Jerusalem, Israel.
Near-Infrared Absorption Spectra of C,, Radical Cations and Anions Prepared Simultaneously in Solid Argon Zbigniew Gasyna, Lester Andrews,* and Paul N. Schatz* Department of Chemistry, University of Virginia, Charlottesuille, Virginia 22901 (Received: November 13, 1991)
The codeposition of Ca vapor with excess argon and argon resonance radiation has produced strong new absorptions at 973 and 1068 nm in solid argon at 11 f 1 K. A similar experiment with CC14 added to serve as an electron trap reduced the yield of the 1068-nm band with little effect on the 973-nm absorption. The 973-nm band is assigned to Cm*+produced by photoionization and the 1068-nm band to C60*- formed by electron capture. These identifications are in excellent agreement with glassy matrix, solution, and photoelectron spectra.
The laboratory synthesis of Cmand other carbon cages known as fullerenes has spawned a host of studies on this highly symmetrical soccerball-shaped molecule. The reader is referred to a comprehensive recent review by Kroto, Allaf, and Balm' and to sources for rapid and continuing literature updatesa2 Since the discovery3 in 1985 of the stability of c60, attributed to a truncated icosahedral cage structure, a host of studies have confirmed the I,, symmetry of the molecule and established its precise geometry. The low ionization energy (7.6 eV) and high electron affinity (2.6 eV)49Smake electron-transfer properties unusually interesting (1) Kroto, H. W.; Allaf, A. W.; Balm, S . P. Chem. Reu. 1991, 91, 1213. (2) Two good methods of keeping abreast of this rapidly moving field are ( I ) the Bucky News Service, send e-mail message to
[email protected]. u p e n n d u ; and (2) a complete and continuing bibliography from R. E. Smalley, e-mail to
[email protected]. (3) Kroto, H. W.; Heath, J. R.; OBrien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (4) Lichtenberger, D. L.; Nebesny, K. W.; Ray, C. D.; Huffman, D. R.; Lamb, L. D. Chem. Phys. Lett. 1991, 176, 203.
for Cs0 and facilitate the preparation and study of the molecular cation and anion. Accordingly, Shida and co-workers have obtained electronic absorption spectra of Cm radical anions and cations following radiolysis in different glassy matrices a t 77 K using proven technology developed at Kyotoa6 EPR spectra of these species in different solutions have also been reported.'-I0 Very recently, the near-infrared absorption spectrum of Cm*-has also been observed following controlled-potential coulometry.I0 ( 5 ) Yang, S. H.; Pettiette, C. L.; Conceicao, J.; Cheshnovsky, 0.;Smalley, R. E. Chem. Phys. Lezr. 1987, 139, 233. (6) Kato, T.; Kodama, T.; Shida, T.;Nakagawa, T.; Matsui, Y.; Suzuki, S.; Shiromaru, H.; Yamauchi, K.; Achiba, Y. Chem. Phys. Lett. 1991, 180, 446. (7) Kukolich, S. G.;Huffman, D. R. Chem. Phys. Lett. 1991, 182, 263. (8) Krusic, P. J.; Morton, J. R.; Preston, K. F.J . Am. Chem. Soc. 1991, 11., 3. 6274. .~ (9) Keizer, P. N.; Morton, J. R.; Preston, K. F.; Sugden, A. K. J . Phys. Chem. 1991, 95, 7117. (10) Greaney, M. A.; Gorun, S. M. J . Phys. Chem. 1991, 95,7142. Kato. T.; Kodama, T.; Oyama, M.; Okazaki, S.; Shida, T.Chem. Phys. Lerr. 1991, 186, 35.
0022-365419212096-1525%03.00/0 0 1992 American Chemical Society
1526 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992
Letters
Io 1 0 5
0.5 '60 1
O C
I
0.5
t I
200
I
400
I
I
600
I
I
BOO
I
I
I
I
I
9500
10000
]
1000
h /nm
Figure 1. Absorption spectra of samples prepared with argon resonance irradiation during deposition of C , in argon matrices at 11 f 1 K. New species trapped (a) in a neat argon matrix and (b) in an argon matrix containing -0.1% CCI,,.
Since solutions and glassy matrices are known to be more perturbing environments than inert gas matrices,I1-l3 we have prepared Ca radical anions and cations simultaneously in solid argon using matrix photoioinization methods developed in Charlottes~ille.'I-'~We report here electronic spectra of C60*- and (&'+ in solid argon and provide some evidence for Jahn-Teller and spin-orbit couplings of both c6{+and C60*- within the matrix cage. The basic matrix photoionization technique employed here has been described earlier.l'-I5 A 6-mm-0.d. quartz tube, ring-sealed to a piece of 12-mm-0.d. quartz tube, served as a windowless argon resonance lamp (1 1.6 and 11.8 eV). The microwave-powered argon discharge radiation and flowing argon gas were directed at the 11 f 1 K sapphire window along with c 6 0 vapor effusing from a Knudsen cell at about 400 OC as described previously.I6 The preliminary magnetic circular dichroism (MCD) spectrum for Ca*+was obtained in the same manner except that the sample was deposited on a sapphire window at a temperature of 5-6 K in the center of the bore of a superconducting solenoid. Nearinfrared-visible and ultraviolet absorption spectra were recorded on a Cary 17 spectrophotometer using expanded scales. The MCD spectrum was recorded using a previously described in~trument.~' Several matrix experiments were done codepositing C60with concurrent argon resonance radiation. Representative absorption spectra are shown in Figure la; the 253- and 326-nm ultraviolet C,, bands were observed as in the earlier study,I6 and new product bands were observed in the near-infrared region at 973 and 1068 nm. Moving the discharge lamp orifice further away from the cold window (from 3 to 4 in. distance) in separate experiments favored the 973-nm band over the 1068-nm band by about 20%. Clearly resolved triplet structure was reproducibly observed on the sharp 1068-nm band, and there were indications of a similar though less well-resolved triplet on the 973-nm band. The near-infrared region is illustrated in Figure 2a using an expanded wavenumber scale. A preliminary MCD measurement of the 973-nm band is illustrated in Figure 2b. These samples were photolyzed with filtered and full radiation from a medium-pressure (11) Andrews, L. Annu. Rev. Phys. Chem. 1979, 30, 79. (1 2) Andrews, L. In Radical Ionic Systems; Lund, A., Shiotani, M., Eds.; Kluwer Academic Publishers: Dordrecht, 1991; p 5 5 . (13) Andrews, L.; Kelsall, B. J.; Blankenship,T. A. J . Phys. Chem. 1982, 86, 2916. (14) Prochaska, F. T.; Andrews, L. J. Chem. Phys. 1977,67, 1091. (1 5 ) Andrews, L.; Tevault, D. E.; Smardzewski, R. R. Appl. Spectrosc. 1978, 32, 157.
(16) Gasyna, Z . ; Schatz, P. N.; Hare, J. P.; Dennis, T. J.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Chem. Phys. Lerr. 1991, 183, 283. (17) Lund, P. A.; Smith, D.; Jacobs, S. M.; Schatz, P. N. J . Phys. Chem. 1984, 88, 31.
I
I
11000
10500
11500
WC.'~
Figure 2. Near-infrared spectra of C60'-and Cm*+in solid argon on an expanded wavenumber scale: (a) absorption spectrum from Figure l a ( A = absorbance) and (b) magnetic circular dichroism spectrum ( M = AL - AR per tesla). The vertical arrows locate the vibronic side bands listed in Table I.
mercury arc (175 W Philips H39KB) without significant change in the spectra. Furthermore, C, samples deposited without concurrent argon resonance radiation also showed no change on mercury arc photolysis. Two additional experiments were done adding an Ar/CC14 = 500/1 gas stream at approximately equal flow rate with the argon from the discharge lamp during condensation with C,, vapor. The best spectrum thus obtained is illustrated in Figure lb. In comparing the spectra with and without CCl.,, three points are noteworthy: (1) the spectrum of C,, was unchanged, (2) a new weak broad band was observed at 420 nm, which has been assigned by many authors to a reactive CCl, species produced by radiation,I8J9and (3) the relative intensities of the two near-infrared product bands have been reversed. In fact, the 973-nm band absorbance is comparable in the two experiments, but the 1068-nm band absorbance was reduced by a factor of about 4 upon the addition of CC14 to the matrix sample. Accordingly, vibronic structure can be associated with the appropriate species by comparing Figure l a and l b spectra. Finally, photolysis of similar CC14-dopedsamples deposited without argon resonance radiation exhibited no change. We assign the sharp peaks at 973 and 1068 nm to C6O'+ and C,'-, respectively, for the reasons given below and suggest the following basic processes during matrix formation: c60 hv (1 1.6, 11.8 eV) C60'+ + e(1)
+
-
Process 1 is based on the obvious expectation that the resonance radiation can ionize neutral parent molecules, and process 2 reflects the appreciable electron affinity of C,, which permits other parent molecules to act as electron traps in the solid Ar matrix. The role of electron traps in preserving matrix isolated cations has been discussed previously.'1-13 The reasons for the above assignments follow. (1) The radiolysis method of Shida produces parent anions in MTHF glasses and parent cations in Freon matrices. The location and general appearance of the 973-nm solid argon band are in excellent agreement with the 980-nm band in a Freon matrix assigned to C60*+,and the same applies to the 1068-nm band in solid argon compared with the 1076-nm band in glassy MTHF assigned to Since the extinction coefficients of the 328-nm C, and 1076-nm C60*- band maxima are comparable in MTHF glass: we estimate that the argon matrix samples contain roughly 10-20% as much C,*- as c60. Furthermore, the absorptions of transient molecular ions are typically slightly red-shifted in going (18) Andrews, L.; Prochaska, F. T. J . Phys. Chem. 1979, 83, 368. (19) Maier, G.; Reisenauer, H.P.; Hu, J.; Hess, A., Jr.; Schaad, L. J. Tetrahedron Lett. 1989, 30, 4105.
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1527
Letters TABLE I: Frequencies of Major Ca'- and CW*+Bands and Fine Structure in Solid Argon at 11 1 K"
*
Cas+
c60'-
vlcm-'
Alcm-'
9360 9390 9440
30 75
9680 9800 10140 10550 10800
320 440 780 1190 1440
vlcm-'
A1cm-l
10280 10310 sh 10 360 sh
30
10 54OC
260
80
"The first three rows show the splitting of the main bands, and lower rows give frequencies of vibronic sidebands. The band origins are accurate to A5 cm-l. b A are measured from the origin band, accuracy & l o cm-I. 'This band is probably overlapped by a Ca*-band, but the CCI4 experiment clearly associates the main intensity with C,o'+.
from an Ar to a glassy m a t r i ~ . ' ~ J * In ~ ~addition, the present result is also in agreement with the much broader 1064-nm band produced by coulometry in CH2C12/toluene solution and assigned to Ca'-.Io The observation of virtually the same main bands in solid argon as in glassy matrix and solution spectra removes the possibility of any chemical reaction or interaction with the substrate that might complicate the identification of Ca'+ and Ca'-. (2) The photoelectron spectrum of Cmin a thin film shows two strong broad bands at 7.6 and 8.9 eV.4 In the gas phase these bands reveal sharper vertical ionization energies separated by 1.27 f 0.02 eV.23 The sharp matrix band at 973 nm (1.274 f 0.001 eV) is in excellent agreement with the separation between the first two photoelectron bands, which is expected for the first electronic absorption of the molecular cation. In the case of naphthalene cation the agreement between sharp gas-phase photoelectron band difference and the absorption band energy in solid argon is within the measurement error.I3 (3) The addition of CC14 provides competition for c 6 0 in the electron capture process. Assuming the argon resonance photon flux is only slightly attenuated by the added CC14, the yield of Ca*+ sh6uld be comparable but the yield of c60'- should be reduced owing to the competitive electron capture process (3). Thus, the CCl., doping experiment clearly identifies the 1068-nm band with the anion. CC14 + e- CC13 + C1(3)
-
In Table I the observed frequencies are summarized for both species. The first three rows describe the triplet structure of the main band for both the anion and cation. The vibronic intervals for (260.- include bands near those reported for c60'- in MTHF glass and CH2C12/toluenesolution.6*I0It appears that the medium affects the vibronic intensity; the two lower frequency vibronic bands displaced at 320 and 440 cm-l are stronger in solid argon whereas vibronic bands spaced 1210 cm-l in the glass and 1507 cm-l in solution are stronger. In all three vibronic spectra of Ca'in different media, the reported vibronic intervals satisfactorily match aqand h, Raman bands for Ca in a thin The single vibronic interval for Ca'+ at 260 cm-l also matches the h, Raman fundamental of Ca at 273 cm-l. These a, and h, (Jahn-Teller) vibrational modes can reasonably be expected to have vibronic activity in the subject electronic transitions. Typically, cation transitions are slightly red-shifted by an argon matrix and red-shifted still more by a Freon matrix from the gas-phase value. The naphthalene cation is the best case for comparison. The photoelectron spectrum predicted an electronic (20) Andrews, L.; Friedman, R. S.; Kelsall, B. J. J . Phys. Chem. 1985,89, 4016. (21) Andrews, L.; Friedman, R. S.; Kelsall, 8. J. J . Phys. Chem. 1985,89,
4550. ... (22) Kelsall, B. J.; Andrews, L. J . Phys. Chem. 1984, 88, 5893. (23) Lichtenberger, D. L. To be published. (24) Bethune, D. S.; Meijer, G.; Tang, W. C.; Rosen, J. J.; Golden, W. G.; Seki, H.; Brown, C. A,; de Vries, M. S. Chem. Phys. Lett. 1991, 179, 181.
transition at 14 920 & 160 cm-l in the gas phase, and the argon matrix origin was observed at 14810 f 3 cm-l, in agreement within experimental error, but a small (1 10 cm-I) red shift by solid argon ifthe experimental uncertainties are neglected. The same transition was observed at 14 490 f 20 cm-I in a Freon matrix25some 320 cm-I below the solid argon value. The separation between sharp vertical gas-phase photoelectron band origins is 10240 f 160 cm-1 for Cas2) This is intermediate between the Freon matrix (10 200 cm-l) and argon matrix (10 280 values. In the case of the very large molecular radical cation Ca*+, solid argon could in fact provide repulsive interactions and cause a blue shift relative to the gas-phase transition. Finally, we comment briefly on the triplet structure observed on the main peaks of both Can- and Ca*+. The triplet structure is most clearly apparent in Ca*-, and the splittings (Table I) are far too small to be associated with intramolecular vibrations of the anion. The triplet structure is less apparent in the cation but emerges much more clearly in the corresponding MCD spectrum where a sharp positive spike corresponds to the lowest energy absorption component and a broader MCD contour of opposite sign is observed with two minima corresponding to the absorption shoulders. (The change of sign of the MCD spectrum immediately excludes the possibility that the triplet structure could be due to cations a t different matrix sites.) Note that the magnitudes of the triplet splittings are very similar in both c60*- and c60*+. The triplet structure, and especially the MCD pattern for Ca*+, is very reminiscent of the 2S -,2P transition of Li and N a atoms isolated in Xe matrices,26the main differences being the better resolution of the latter and an oppositely signed MCD pattern. In Li/Xe and Na/Xe, the triplet structure can be convincingly interpreted as due to the combined effect of spin-orbit and Jahn-Teller coupling in the 2Pexcited state, where both couplings are (almost) entirely mediated by the Xe lattice.26 We suggest that the triplet structure observed in Cm'+ and Ca'is also a consequence of the combined effects of Jahn-Teller and spin-orbit coupling. Following Shida et a1.,6 the electronic transitions observed for Ca'+ and Ca*- are respectively assigned as 2H, -,2H, and *TI" 2T1,. In each case the excited state splits into two spin-orbit components (2H = G3/2 @ ISl2and 'TI = EIl2@ G3/2).27 We note that the splitting between the first peak and the center of gravity of the second and third is 4 5 cm-', which is of the order of magnitude expected since t(2pc) = 3 1 cm-I.l6 The Jahn-Teller effect would involve coupling of the excited-state electron (or hole) with the host lattice, as is the case in Li/Xe and Na/Xe26 and in other alkali metals in noble gas hosts.28 Clearly, the transitions involved in the present cases are different from 2S 2P, but in analogy to that c a ~ e , ' we ~.~~ speculate that the ratio of first MCD to zeroth absorption moment will be proportional to S;fflkT. In that event, MCD amplitudes will vary inversely with T,and tefffor the excited states can be deduced. It should also be possible to distinguish this suggested mechanism from a purely static (crystal field) explanation of the triplet s t r ~ c t u r e . 'In ~ the case of Li/Xe and Na/Xe, teff= -200 cm-1.26 We anticipate a positive tefffor C60*+and C60*- in solid argon.28 It is the sign of this quantity which determines the sense (sign) of the bisignated MCD pattern.17*26,28A detailed MCD study of both species will be reported in the near future.
-
-
Acknowledgment. We gratefully acknowledge financial support from N S F Grants C H E 88-20764 and C H E 89-02456, the gift of a purified Ca sample from H. W. Kroto and co-workers, and permission to quote unpublished gas-phase photoelectron spectra of c 6 0 from D. L. Lichtenberger. (25) Shida, T.; Iwata, S. J . Am. Chem. Soc. 1973, 95, 3473. Shida, T. Annu. Rev. Phys. Chem. 1991, 42, 55. (26) Rose, J.; Smith, D.; Williamson, B. E.;Schatz, P. N.; OBrien, M. C. M. J . Phys. Chem. 1986, 90, 2608. (27) Herzberg, G. Molecular Spectra and Molecular Structure. III. Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand: Princeton, NJ, 1966; Appendix 111. (28) Samet, C.; Rose,J. L.;Schatz, P. N.; OBrien, M. C. M. Chem. Phys. Lett. 1989, 159, 567.
