STRUCTURE, ENERGETICS AND VIBRATIONAL-SPECTRA OF DIMERS, TRIMERS, ANDTETRAMERS OF HX (X=CL, BR, I)

The title complexes are studied by correlated ab initio methods using a pseudopotential double-zeta basis set, augmented by diffuse sp and t wo sets of polarization functions. The binding energies of the complex es decrease in the order HCl > HBr > HI. In the mixed HX...HX' dimers, the nature of the proton-donor molecule is more important than is the proton-acceptor with respect to the strength of the interaction. Only one minimum is found on the potential energy surface of the trimers a nd tetramers, which corresponds to the C-nh, cyclic structure. Enlarge ment of the complex leads to progressively greater individual H-bond e nergy and HX bond stretch, coupled with reduced intermolecular separat ion and smaller nonlinearity of each H-bond. Electron correlation make s a larger contribution as the atomic number of X increases, The highe st degree of cooperativity is noted for oligomers of HCl and HBr, as c ompared to HI. The nonadditivity is dominated by terms present at the SCF level. The vibrational frequencies exhibit trends that generally p arallel the energetics and geometry patterns, particularly the red shi fts of the HX stretches and the intermolecular modes.