The role of OH…O and CH…O hydrogen bonds and H…H interactions in ethanol/methanol–water heterohexamers

Bioethanol is one of the world’s most extensively produced biofuels. However, it is difficult to purify due to the formation of the ethanol–water azeotrope. Knowledge of the azeotrope structure at the molecular level can help to improve existing purification methods. In order to achieve a better understanding of this azeotrope structure, the characterization of (ethanol)₅–water heterohexamers was carried out by analyzing the results of electronic structure calculations performed at the B3LYP/6-31+G(d) level. Hexamerization energies were found to range between −36.8 and −25.8 kcal/mol. Topological analysis of the electron density confirmed the existence of primary (OH…O) hydrogen bonds (HBs), secondary (CH…O) HBs, and H…H interactions in these clusters. Comparison with three different solvated alcohol systems featuring the same types of atom–atom interactions permitted the following order of stability to be determined: (methanol)₅–water > (methanol)₆ > (ethanol)₅–water > (ethanol)₆. These findings, together with accompanying geometric and spectroscopic analyses, show that similar cooperative effects exist among the primary HBs for structures with the same arrangement of primary HBs, regardless of the nature of the molecules involved. This result provides an indication that the molecular ratio can be considered to determine the unusual behavior of the ethanol–water system. The investigation also highlights the presence of several types of weak interaction in addition to primary HBs. [Figure not available: see fulltext.]