O. Mo et al., STRUCTURE, VIBRATIONAL FREQUENCIES, AND THERMODYNAMIC PROPERTIES OF HYDROGEN-PEROXIDE DIMERS - AN AB-INITIO MOLECULAR-ORBITAL STUDY, The Journal of chemical physics, 100(4), 1994, pp. 2871-2877
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
.