Evidence for the high-energy (collinear) van der Waals isomer of the acetonitrile dimer

While the stable configuration of the acetonitrile dimer has antiparallel dipole moments, recent calculations [Popelier, et al. Faraday Discuss. 1994, 97, 243] reveal a high-energy isomer in which the dipoles are collinearly aligned. We report direct evidence for the existence of this isomer using a strategy in which we enforce the less-favorable orientation of the neutrals by electrostatically binding them to the iodide ion in the I-.(CH3CN)(2) cluster. The collinear dimer is then released by photoexcitation of the dipole-bound excited state of I-.(CH3CN)(2), producing the (CH3CN)(2)(-) anion as the dominant ionic photofragment. The fragment is mass selected and stripped of the excess electron in a modest external electric field (similar to 10 kV/cm), indicating 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.