Glycidol dimer: anatomy of a molecular handshake

Abstract

Chiral self-recognition, the ability of a molecule to distinguish between a copy and a mirror copy of itself via intermolecular interactions, is demonstrated for dimers of glycidol (oxiranemethanol) in a supersonic jet expansion. The infrared OH-stretching spectra of homochiral and heterochiral dimers differ from each other and exhibit an unexpected spectral complexity. A systematic quantum chemical study of the conformational degrees of freedom reveals two important types of hydrogen bond topology in glycidol dimers and up to 10 important conformations in the adiabatic expansion. These dimer conformations derive from two out of eight monomer conformations which are preformed and stabilized by intramolecular hydrogen bond contacts. All important conformations have two intermolecular OH-O hydrogen bonds. In the most stable conformations, identical copies of glycidol appear to interact more strongly with each other than with optical antipodes. Secondary interactions such as CH-O contacts are predicted to contribute importantly to chiral discrimination. The spectral complexity in the OH-stretching region can be rationalized qualitatively by harmonic predictions at HF, B3LYP and MP2 levels using small basis sets. Higher level calculations based on this conformational landscape exploration are initiated. They should become increasingly feasible for such a small prototype and will be desirable in order to achieve a quantitative understanding of chiral recognition. Experimentally, the addition of small amounts of Ar to the He expansion is shown to enhance conformational relaxation in the jet.