EVIDENCE FOR THE HIGH-ENERGY (COLLINEAR) VAN-DER-WAALS ISOMER OF THE ACETONITRILE DIMER

While the stable configuration of the acetonitrile dimer has antiparal lel dipole moments, recent calculations [Popelier, et al. Faraday Disc uss. 1994, 97, 243] reveal a high-energy isomer in which the dipoles a re collinearly aligned. We report direct evidence for the existence of this isomer using a strategy in which we enforce the less-favorable o rientation of the neutrals by electrostatically binding them to the io dide ion in the I-.(CH3CN)(2) cluster. The collinear dimer is then rel eased by photoexcitation of the dipole-bound excited state of I-.(CH3C N)(2), producing the (CH3CN)(2)(-) anion as the dominant ionic photofr agment. The fragment is mass selected and stripped of the excess elect ron in a modest external electric field (similar to 10 kV/cm), indicat ing that the anion consists of a diffuse electron as expected for a '' dipole-bound'' species. This behavior suggests that the force field of the neutral ''core'' complex supports a minimum in the highly dipolar , collinear configuration.