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

The (H2O...HCl)(-) anion has been studied at the coupled cluster level of t heory with single, double, and noniterative, triple excitations whereas low est energy structures have been determined at the second-order Moller-Pless et level. The vertical electron detachment energy and the adiabatic electro n affinity were found to be 475 and 447 cm(-1), respectively, and they agre e very well with the maximum of the dominant peak in the photoelectron spec trum of (H2O...HCl)(-) at 436 cm(-1), recorded by Bowen and collaborators. Our results indicate that electron correlation contributions to the electro n binding energy are important and represent similar to 70% of its total va lue. Both for the neutral and the anion, the two equivalent C-s symmetry mi nima are separated by a C-2v transition state and the energy barrier amount s to only 77 cm(-1). Thus the equilibrium structures averaged over vibratio ns are effectively planar (C-2v). The Franck-Condon (FC) factors, calculate d in harmonic approximation, indicate that the neutral complex formed in ph otoelectron spectroscopy (PES) experiments may be vibrationally excited in both soft intermolecular and stiff intramolecular modes. The theoretical ph otoelectron spectrum based on the calculated FC factors is compared to the experimental PES spectrum of Bowen (C) 1999 American Institute of Physics. [S0021-9606(99)30331-7].