Ab initio potential energy surface, infrared spectra, and dynamics of the ion-molecule complexes between Br− and H2, D2, and HD

Abstract

A three-dimensional potential energy surface (PES) for the Br(-)-H(2) complex is computed using the ab initio CCSD(T) method and an extended basis set. The PES has two equivalent minima at the linear geometries (equilibrium interfragment distance R(e)=3.34 A and interaction energy D(e)=670 cm(-1)) separated by the barrier at the T-shaped configuration (interfragment distance R(e)=4.03 A and barrier height of 610 cm(-1)). Ab initio points are fitted to a flexible analytical form and used in the variational rovibrational energy level calculations. Simulated infrared spectra of the Br(-)-H(2) and Br(-)-D(2) complexes in the monomer stretching excitation region are in good agreement with the measured ones. Nonstatistical intensity ratios of the complexes of para- and ortho-monomers are qualitatively explained by monomer ligand exchange reactions. Predissociation of the complexes containing vibrationally excited monomers is analyzed and shown to proceed through the near-resonant vibration-to-rotation energy transfer. For complexes involving Br(-) and the HD monomer, two energetically low-lying states are predicted, corresponding predominately to the Br(-)-DH and Br(-)-HD isomeric forms. The results demonstrate the close similarity of the bromide containing complexes to their analogs containing the chloride anion.