Cubane Radical Cation in Liquid Hydrocarbons: Time-Resolved Fluorescence DetectedMagnetic Resonance Study

摘要

The structure and rearrangement of strained hydrocarbons and radical cations ingeneral, and those of the (CH)8 group in particular, have attracted considerable experimental and theoretical attention. The electronic structure of the cubane radical cation (cubane(.+)) is particularly interesting, since the cubane molecule has the highest strain energy of the set and belongs to the O(h) point group. The photoelectron spectroscopic study and ab initio STO-3G and MINDO/3 calculations by Heilbronner et al. indicate that cubane has triply degenerate highest occupied molecular orbitals. Upon ionization it undergoes Jahn-Teller distortion. The symmetry is lowered from Q(h) to C(2v). An EPR study of cubane photolyzed in a neon matrix at 4 K was reported by Knight et al. in 1987. However, Knight repeated these experiments recently and concluded that cubane(.+) was not observed in his experiments. In view of this finding, we conclude that the cubane radical cation was not observed previously and that in our studies of cubane(.+) rearrangements in Freon matrices we could not directly observe the EPR spectrum of cubane(.+) in several Freon matrices at 77 K. Cubane(.+) undergoes rearrangement at 80 K and higher temperatures, and one cannot utilize a Freon matrix to stabilize and observe its ERA spectrum in the temperature range (110-120 K) where resolved EPR spectra of radical cations in Cf2ClCFCl2 are usually obtained. We report here a time-domain study of cubane(.+) utilizing the time-resolved fluorescence detected magnetic resonance (FDMR) technique. We have observed a radical cation, which we assign to cubane(.+) in liquid hydrocarbon solution.