STRUCTURE, VIBRATIONAL FREQUENCIES AND THERMODYNAMIC PROPERTIES OF HYDROGEN PEROXIDE-WATER DIMERS - AN AB-INITIO MOLECULAR-ORBITAL STUDY

High levels of ab initio molecular orbital theory have been used to st udy the structures, binding energies, vibrational frequencies and equi librium constants of hydrogen peroxide-water dimers. The geometries of the different possible conformers were optimized at the HF/6-311 + +G (2d, 2p) level of theory. Five different stationary points have been c haracterized at this level, but only two are minima. The geometries an d vibrational frequencies of these two minima were refined at the MP2/ 6-311 + G(d, p) level. The stretching vibrations of the proton donors show a sizable red-shift. The global minimum corresponds to a five-mem bered ring, where both monomers behave simultaneously as proton donors and proton acceptors. In the second minimum. which lies about 2.2 kca l/mol higher, the hydrogen peroxide monomer behaves as a proton accept or, while water behaves as a proton donor. The binding energies of the se two species were obtained at the QCISD(T)/6-311 + G(2d, 2p) level u sing the MP2 optimized geometries, and the corresponding equilibrium d imerization constants evaluated. A topological analysis of the electro nic charge densities of the dimers is presented.