Structure, energetics and vibrational spectra of dimers, trimers, and tetramers 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 two sets of polarization functions. The binding energies of the complexes 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 and tetramers, which corresponds to the C-nh, cyclic structure. Enlargement of the complex leads to progressively greater individual H-bond energy and HX bond stretch, coupled with reduced intermolecular separation and smaller nonlinearity of each H-bond. Electron correlation makes a larger contribution as the atomic number of X increases, The highest degree of cooperativity is noted for oligomers of HCl and HBr, as compared to HI. The nonadditivity is dominated by terms present at the SCF level. The vibrational frequencies exhibit trends that generally parallel the energetics and geometry patterns, particularly the red shifts of the HX stretches and the intermolecular modes.