Structure and spectra of HOCl(H2O)(n) clusters, n=1-4: A theoretical calculation

The geometrical structure, binding energy, and vibrational spectra of small clusters of HOCl and water molecules, HOCl(H2O)(n), n = 1-4, have been inv estigated at the MP2 revel of theory, using triple-Z, 6-311++G(d,p), basis sets. The ab initio results predict for the clusters an almost planar ring skeleton, made up of the O atoms and one H atom from each molecule, linked by hydrogen bonding to the next oxygen. The species with one water unit pre sents two almost equally stable conformers, syn and anti, in agreement with previous calculations. Two stable structures have also been found for clus ters with three and four molecules of water, one where the H atom of the HO Cl molecule makes part of the ring, and another one in which the Cl atom is in the ring. The more stable in either case is found to be the H-in molecu le, with a difference of 35.6 kJ/mol in optimized energy. The predicted spe ctra for these two species are very different. As the number of water units increases, the corresponding clusters become more stable, and a number of physical effects is predicted. The more important one is the increase in th e intramolecular O-H bond length, which is responsible for a large reductio n in the wavenumber of the corresponding O-H vibration. The results indicat e the formation of hydrogen bonds and a partial proton transfer from HOCl t o the water ring structure. In a parallel calculation, water clusters, (H2O )(n), n = 1-5, have also been investigated. Our results agree well with tho se of previous authors.