"Electromers" of the tetramethyleneethane radical cation and their nonexistence in the octamethyl derivative: interplay of experiment and theory.

Bicyclopropylidene 1a and its octamethyl derivative 1b are subjected to ionization by X-irradiation in solid argon. In accord with previous experiments, this treatment leads to the spontaneous opening of both cyclopropylidene rings, as does ionization of 1b by gamma-irradiation in CFCl(3) at 77 K. The resulting tetramethyleneethane (or bisallyl) radical cations 2a+* and 2b+* are distinguished by a broad band in the NIR. In the case of 2a+*, wavelength-selective photolyses reveal the presence of two interconvertible species with very similar yet distinct spectra. Based on DFT and CASSCF/CASPT2 calculations, these spectra are assigned to two "electromeric" forms of 2a+* which differ in the nature of the singly occupied MO. The NIR bands correspond to charge-resonance transitions between states with fully delocalized spin and charge. Calculations predict that similar electromers should also exist in 2b+* which shows a much weaker NIR band, but no corresponding experimental evidence could be found. On the other hand, the ESR spectrum of 2b+* indicates that, in contrast to 2a+*, the spin is largely localized in one of the two allylic moieties in 2b+*. Although no theoretical method is presently available that would permit an accurate modeling of the opposing factors favoring localized or delocalized structures in molecules such as 2a+* or 2b+*, the observed trends can be satisfactorily rationalized on the basis of semiquantitative considerations. In particular, the important role of vibronic coupling in shaping the potential surfaces for such systems is emphasized.