Ab Initio Study of Chiral Discrimination in the Glycidol Dimer

Chiral discrimination, the ability
of a chiral molecule to exhibit different weak intermolecular interactions
than its mirror image, is investigated for dimers of oxiranemethanol
(glycidol). In this regard, high-level ab initio calculations were
performed to study the chiral recognition effects in the homochiral
and heterochiral dimers of glycidol. Fourteen dimer structures, seven
homochiral and seven heterochiral, were studied: they all feature
two intermolecular O–H···O hydrogen bonds. These
structures have been determined with the second-order Møller–Plesset
perturbation theory (MP2) using the aug-cc-pVTZ basis set and verified
to pertain to actual local minima. The benchmark interaction energy
values were computed using MP2 extrapolated from the aug-cc-pVQZ and
aug-cc-pV5Z bases with a higher-level correction from a coupled-cluster
calculation in the aug-cc-pVTZ basis. The global minimum structure
is a homochiral one, with the two hydrogen bonds forming a part of
a ring with eight heavy atoms. A similar heterochiral structure has
a binding energy smaller by about 0.6 kcal/mol. The largest diastereomeric
energy difference is about 1.0 kcal/mol. Further insight into the
origins of chiral discrimination was provided by symmetry-adapted
perturbation theory (SAPT) and a functional-group SAPT (F-SAPT) difference
analysis to investigate the direct and indirect effects of two −H/–CH2OH substitutions leading from an achiral ethylene oxide dimer
to the chiral glycidol dimer. Last but not least, harmonic frequency
shifts relative to a noninteracting glycidol molecule were calculated
and analyzed for all conformations to get insight into the origins
of chiral discrimination. It is found that the largest frequency shifts
are related to the effect of hydrogen bonding on the O–H stretch
mode, the stability of the ring involving both hydrogen bonds, and
the transition between two nonequivalent minima of the glycidol molecule.