Alcohol Self-Aggregation: the Preferred Configurations of the Ethanol Trimer.

An atomic-level knowledge of the aggregation of archetypal molecular systems is essential to accurately model supramolecular structures and the transition from gas to liquid phase. The structures and forces involved in ethanol aggregation have been investigated using microwave spectroscopy and extensive quantum chemical calculations. Four isomers of the ethanol trimer have been observed and identified based on comparisons between experimental and predicted spectroscopic parameters, and considering collisional relaxation in the supersonic expansion. All observed isomers exhibit O-H ⋯ ${\cdots }$ O hydrogen bonds between the hydroxyl groups forming a six-membered ring. Additionally, secondary C-H ⋯ ${\cdots }$ O hydrogen bonds and H ⋯ ${\cdots }$ H dispersion contacts participate in the stabilization of the complexes with remarkably similar energy contributions. Structures where there is a mixture of gauche and trans conformations of ethanol are favored, with gauche conformations being predominant and no evidence of homochirality synchronization. Our results underscore the critical changes involved in aggregation as the size of the system increases and shed light on the unique properties and behavior of ethanol in chemical and biological systems.