Ab Initio study of the structure, cooperativity and vibrational properties of the H2O:(HCl)2 hydrogen bonded complex

Ab initio self consistent field (SCF) and second order Moller-Plesset perturbation theory (MP2) calculations have been performed on order to investigate the structure, energetics and spectroscopic properties of H2O:(HCl)(2). Two minima have been located on the energy hypersurface: a cyclic structure without any symmetry element and a bifurcated one with a C-2v symmetry. In this latter complex the two hydrogen bonds are equivalent, whereas in the former there are two relatively strong H-bonds and a weaker one between the terminal sub-units. The calculated energy difference between these two conformers is about 1.0 kcal mol(-1) in favor of the cyclic complex and is very method dependant. The relative stability of the two conformers is mostly governed three-body forces which are attractives in the case of the cyclic structure, and repulsives in the bifurcated complex. The calculated frequency shifts within the cyclic trimer agree with the observed shifts in the argon matrix. (C) 1998 Elsevier Science B.V. All rights reserved.