Molecular recognition in 1 : 1 hydrogen-bonded complexes of oxirane and trans-2,3-dimethyloxirane with ethanol: a rotational spectroscopic and ab initio study

Abstract

High resolution rotational spectroscopy complemented by ab initio calculations has been used to elucidate the diastereomeric interactions in 1 : 1 complexes of ethanol, a transient chiral alcohol, hydrogen-bonded to oxirane (achiral) or trans-2,3-dimethyloxirane (DMO, 2 stereocenters). Two conformers of oxirane⋯ethanol and three conformers of DMO⋯ethanol have been identified, and their structures as well as their stability ordering have been determined. This completes, together with previous results on the propylene oxide⋯ethanol complex (N. Borho and Y. Xu, Angew. Chem., 2007, 119, 2326–2329; Angew. Chem., Int. Ed., 2007, 46, 2276–2279.), the study of a set of model systems with zero, one, and two methyl functional groups at the hydrogen bond acceptor oxirane. The dependence of the observed rotational line intensities on pressure, nozzle temperature, and different carrier gases has been investigated for the case of DMO⋯ethanol. This provides insight into the kinetical and thermodynamical influence on the formation of different conformers. Comparison of the subtle energy differences among the complexes and within each set of conformers allows for a detailed analysis of molecular recognition in this benchmark system.