Ab initio study of the dipole-bound anion (H2O…HCl)−

Abstract

The (H2O…HCl)− anion has been studied at the coupled cluster level of theory with single, double, and noniterative, triple excitations whereas lowest energy structures have been determined at the second-order Mo/ller–Plesset level. The vertical electron detachment energy and the adiabatic electron affinity were found to be 475 and 447 cm−1, respectively, and they agree very well with the maximum of the dominant peak in the photoelectron spectrum of (H2O…HCl)− at 436 cm−1, recorded by Bowen and collaborators. Our results indicate that electron correlation contributions to the electron binding energy are important and represent ∼70% of its total value. Both for the neutral and the anion, the two equivalent Cs symmetry minima are separated by a C2v transition state and the energy barrier amounts to only 77 cm−1. Thus the equilibrium structures averaged over vibrations are effectively planar (C2v). The Franck–Condon (FC) factors, calculated in harmonic approximation, indicate that the neutral complex formed in photoelectron spectroscopy (PES) experiments may be vibrationally excited in both soft intermolecular and stiff intramolecular modes. The theoretical photoelectron spectrum based on the calculated FC factors is compared to the experimental PES spectrum of Bowen et al.