New Aspects of Alcohol–Alcohol and Alcohol–Water Interactions: Crystallographic and Quantum Chemical Studies of Antiparallel O–H/O–H Interactions

New modes of interaction, antiparallel O–H/O–H interactions of alcohol–alcohol dimers and alcohol–water dimers, were studied by analyzing data in the Cambridge Structural Database (CSD) and by calculating potential energy surfaces at a very accurate quantum chemical CCSD(T)/CBS level. The data reveal the existence of antiparallel interactions in crystal structures and significant interaction energies. Data from the CSD for alcohol–alcohol dimers show 49.2% of contacts with classical hydrogen bonds and 10.1% of contacts with antiparallel interactions, while for alcohol–water dimers, 59.4% of contacts are classical hydrogen bonds and only 0.6% of contacts are antiparallel interactions. The calculations were performed on methanol, ethanol, and n -propanol dimers. Classical hydrogen-bonded alcohol–alcohol and alcohol–water dimers have interaction energies of up to −6.2 kcal/mol and up to −5.5 kcal/mol, respectively. Antiparallel interactions in alcohol–alcohol and alcohol–water dimers have interaction energies of up to −4.7 kcal/mol and up to −4.4 kcal/mol, respectively. Symmetry-adapted perturbation theory analysis for antiparallel interactions shows their electrostatic nature.