PHOTOELECTRON-SPECTROSCOPY OF THE GAS-PHASE S(N)2 REACTION INTERMEDIATES I-.CH3I AND I-.CD3I - DISTORTION OF THE CH3I AT THE ION-DIPOLE COMPLEX

The I- . CH3I and I- . CD3I intermediates in the gas-phase SN2 identity reaction, I- + CH3I --> CH3I + I-, are isolated, cooled in a free jet, and characterized using negative ion photoelectron spectroscopy. The spectra appear similar to bare I- but shifted to lower electron kinetic energy by the known binding energy of the complex. Vibrational fine structure in each spin-orbit subband is dominated by a simple vibrational progression in the nu3 mode of CH3I (nominally the C-I stretch). Franck-Condon analysis indicates that this bond is elongated in the complex by about 3% relative to bare CH3I with observable distortions in the C-H stretch and H-C-X bond angle (the umbrella mode). We discuss these observations in the context of the charge transfer (e.g., valence bond) model for the S(N)2 potential energy surface.