Chiral Self Recognition: Interactions in Propylene Oxide Complexes

We elucidate the subtle energetic effects that give rise to chiral recognition in the propylene oxide dimer. Specifically, we investigate six homochiral (RRx) and six heterochiral (RSx) structures of this complex, with the RRn-RSn pair sharing the same pattern of weak O···H-C hydrogen bonds but subtly differing in energy due to chiral effects. The interaction energies for the 12 structures are computed at various levels of electronic structure theory and basis set up to the complete basis set limit of the coupled-cluster approach with single, double, and perturbative triple excitations (CCSD(T)). These benchmark interaction energies are compared to the results of various approximate approaches, both density functional theory-based and wave function-based. We find that while the RRn-RSn diastereomeric energy differences exhibit a great deal of error cancellation between the individual interaction energies, most approximate methods have a hard time even reproducing the correct signs of these differences consistently. The origins of the RRn-RSn differences are elucidated by several symmetry-adapted perturbation theory (SAPT) analyses ranging from ordinary intermolecular SAPT to a functional-group SAPT (F-SAPT) decomposition of direct and indirect H → CH₃ substitution effects leading from achiral ethylene oxide complexes to chiral propylene oxide ones. It is shown that the largest diastereomeric energy differences are correlated to the variations in the electrostatic and dispersion SAPT contributions. Finally, the effect of chiral interactions on the vibrational frequencies of a propylene oxide molecule is investigated, showing that the interaction results in largest frequency shifts, splittings, and chiral discrimination effects in the lowest, torsional vibrational mode of the noninteracting monomer.