Cooperative effects on the structure and stability of (ethanol)₃-water, (methanol)₃-water heterotetramers and (ethanol)₄, (methanol)₄ tetramers

Density functional theory was used to study (ethanol)₃-water heterotetramers. These clusters form O-H⋯O primary hydrogen bonds and interactions of the C-H⋯O type, called secondary hydrogen bonds. Even though the C-H⋯O interactions are weaker than the primary hydrogen bonds, they seem to influence the stability of some clusters. Analysis of tetramerization energies and enthalpies show that the cyclic geometric pattern formed by four primary hydrogen bonds, where all the molecules are proton acceptor and proton donor at the same time, is the most stable configuration. This configurational preference is a consequence of the existence of cooperative effects, which are described using various indexes, such as O⋯O distances, redshift for stretching frequencies of the donor O-H bonds, and its elongation. Comparison of (ethanol)₃-water heterotetramers data with that of some (ethanol)₄ tetramers, (methanol)₃-water heterotetramers and (methanol)₄ tetramers was carried out and shows a great similarity among the evaluated parameters.