Quantum scattering calculations of energy transfer and isomerization of HCN/HNC in collisions with Ar

Quantum scattering calculations are reported for zero impact parameter collisions of Ar with HCN/HNC for three fixed angles of attack of Ar with respect to the CN axis, in the total energy range 16 000-20 000 cm(-1), and using a new Ar-HCN interaction potential based on ab initio data. We find that this interaction potential only weakly couples localized HCN and localized HNC states. As a result, although isomerization is energetically possible in much of the energy range considered, the probability of collision-induced isomerization is found to be small. Detailed analysis of our scattering results shows that "head on" collisions in which the Ar atom impinges on the H end of the molecule are more effective in promoting T --> V energy transfer than are "nearly perpendicular" and "tail on" (opposite to the H atom) collisions. Significant energy transfer processes between translation and vibration involve the bending mode nu(2) either through pure bend excitation/deexcitation or through smaller Delta E processes in which a larger number of nu(2) quanta are exchanged for a smaller number of nu(1) or nu(3) quanta. Examination of our distributions of inelastic transition probabilities for highly excited states, including a delocalized state, suggests that they mimic a biexponential gap distribution. (C) 2000 American Institute of Physics. [S0021-9606(00)01309-X].