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 - ·(CH 3 CN) 2 cluster. The collinear dimer is then released by photoexcitation of the dipole-bound excited state of I - ·(CH 3 CN) 2, producing the (CH 3 CN) 2 - anion as the dominant ionic photofragment. The fragment is mass selected and stripped of the excess electron in a modest external electric field (∼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.