Theoretical study of CH4-CH4, CHF3-CH4, CH4-H2O, and CHF3-H2O dimers.

We have studied systems with typical hydrogen bonding and others with interaction involving hydrogen. CH(4)-CH(4), CH(4)-H(2)O, CHF(3)-CH(4), and CHF(3)-H(2)O dimers were studied using MPWB1K, PBE1PBE, MP2, and QCISD levels of theory with a large number of basis functions. The Pople 6-31+G(2d), 6-311++G(2d,2p), and 6-311++G(3df,3pd) as well as Dunning augmented aug-cc-pVDZ and aug-cc-pVTZ basis sets were used. The dimer geometries were fully optimized. An optimal basis set was determined for these systems to achieve a suitable compromise between accuracy and computational feasibility. A proper strategy was found for the electronic property calculations of dimers studied: the use of aug-cc-pVDZ as the optimal basis set at MP2 level. Dipole moments, polarizabilities, BSSE effects, and DeltaZPE were also analyzed for these dimers.

The hydrogen peroxide-rare gas systems: quantum chemical calculations and hyperspherical harmonic representation of the potential energy surface for atom-floppy molecule interactions.

A quantum chemical exploration is reported on the interaction potentials of H2O2 with the rare gases, He, Ne, Ar, Kr, and Xe. Hydrogen peroxide (the simplest example of chiral molecule in its equilibrium geometry) is modeled as rigid except for the torsional mode around the O-O bond. However, on the basis of previous work (Maciel, G. S.; et al. Chem. Phys. Lett. 2006 432, 383), the internal mode description is based, rather than on the vectors of the usual valence picture, on the orthogonal local representation, which was demonstrated useful for molecular dynamics simulations, because the torsion around the vector joining the center-of-mass of the two OH radicals mimics accurately the adiabatic reaction path for chirality changing isomerization, following the torsional potential energy profile from equilibrium through the barriers for the trans and cis geometries. The basic motivation of this work is the determination of potential energy surfaces for the interactions to be used in classical and quantum simulations of molecular collisions, specifically those leading to chirality changes of possible relevance in the modeling of prebiotic phenomena. Particular attention is devoted to the definition of coordinates and expansion formulas for the potentials, allowing for a faithful representation of geometrical and symmetry properties of these systems, prototypical of the interaction of an atom with a floppy molecule.