Chiral Self Recognition: Interactions in Propylene Oxide Complexes

Abstract

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.

Recent developments in symmetry‐adapted perturbation theory

“…In my group, we have recently applied F-SAPT differences to elucidate the subtle noncovalent effects leading to chiral discrimination in the propylene oxide dimer. 304…”

“…Beyond near‐equilibrium intermolecular complexes, F‐SAPT has also been applied to elucidate interactions in the transition states of some organocatalyzed reactions, attempting to explain their stereoselectivity and enantioselectivity. In my group, we have recently applied F‐SAPT differences to elucidate the subtle noncovalent effects leading to chiral discrimination in the propylene oxide dimer …”

Ab Initio Study of Chiral Discrimination in the Glycidol Dimer

“…In the last few years, the chiral discrimination effects have been investigated, − theoretically and experimentally, for several representative gas-phase intermolecular complexes. The systems studied include glycidol dimer, butan-2-ol dimer, propylene oxide dimer, , and the propylene oxide–glycidol complex.…”

Description of Chiral Complexes within Functional-Group Symmetry-Adapted Perturbation Theory—The Case of (S/R)-Carvone with Derivatives of (−)-Menthol

“…About the same time, Hemmati and Patkowski applied the same methods for the interaction of propylene oxide complexes. 9 Out of the tested methods, the F-SAPT proved to be the most useful in determining which functional groups are responsible for modulation of the strength of the interaction between chiral molecules. Here, in order to further advance our experience, we decided to study the applicability of F-SAPT for a description of similar noncovalent complexes.…”

“…We also examined the dimers of chlorophyllide and its artificial diastereoisomer with perturbational techniques in ref . About the same time, Hemmati and Patkowski applied the same methods for the interaction of propylene oxide complexes . Out of the tested methods, the F-SAPT proved to be the most useful in determining which functional groups are responsible for modulation of the strength of the interaction between chiral molecules.…”