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.