A THEORETICAL-STUDY ON THE VIBRATIONALLY STATE-SELECTED HYDROGEN-TRANSFER REACTION - NH3-]NH4++NH2 - AN AB-INITIO MR-SD-CI AND CLASSICAL TRAJECTORY APPROACH((UPSILON)+NH3)

Ab initio MR-SD-CI and classical trajectory calculations have been per formed to elucidate the vibrational mode specificity of the title reac tion, whose reactive cross section is enhanced by vibrational excitati on of the nu2 umbrella-bending mode of NH3+. Potential energy surfaces (PESs) of the reaction have been obtained for vibrationally ground an d excited states (vibrational quantum numbers, upsilon = 0 and 2, resp ectively) by assuming a hydrogen-bonded structure with fixed bending a ngles. The MO calculations show that a hydrogen transfer is composed o f two elementary steps: (1) an electron transfer from NH3 to NH3+ at t he avoided crossing region on the entrance PES, and (2) a proton trans fer in the (NH3 . NH3) + intermediate complex region. The PESs show th at the avoided crossing point shifts to larger intermolecular separati on due to vibrational excitation. Using the ab initio fitted PESs, the classical trajectory calculations elucidate the reaction dynamics. Th e maximum value of the impact parameter (b(max)) for the reaction is i ncreased by the vibrational excitation. Based on these theoretical res ults, a simple reaction model has been proposed, in which the electron capturing volume of NH3+ increases with increasing vibrational quantu m number upsilon.