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

High levels of ab initio molecular orbital theory were used to study t he structures, binding energies, vibrational frequencies, and equilibr ium constants of hydrogen peroxide dimers. The geometries of the diffe rent initial structures considered 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 of them were minima. The geom etries of these two minima were refined at the MP2/6-311 + G(d,p) leve l. Their vibrational frequencies, calculated at the same level of theo ry, show a sizeable redshift of the stretching vibrations of the proto n donors. The global minimum corresponds to a six-membered ring of C-i symmetry, while the second minimum is a five-membered ring, which lie s about 1.1 kcal mol(-1) above the global one. The formation of the la tter implies a considerable enhancement of the dipole moment. The bind ing energies of these two species were obtained at the QCISD(T)/6-311 + G(2d,p) level using the MP2 optimized geometries. The equilibrium di merization constants for hydrogen peroxide are considerably smaller th an those for water, due to significant entropic effects. A topological analysis of the electronic charge densities of the dimers shows that both cyclic minima present weaker hydrogen bonds than noncyclic dimers .