Cooperative C-H...O hydrogen bonding in CO(2)-Lewis base complexes: implications for solvation in supercritical CO(2).

Understanding the fundamental principles for the design of CO(2)-philic materials is of growing importance due to the potential for enabling "green" chemistry and technologies in liquid and supercritical CO(2) as alternative solvent systems. Recently, there have been numerous efforts to develop hydrocarbon-based CO(2)-philes containing carbonyl groups, which are known to interact through a Lewis acid-Lewis base (LA-LB) interaction with CO(2) molecules, thereby providing the necessary solvation energy for dissolution. In this work, we investigate the role of a weaker, but cooperative, C-H...O hydrogen bond as an additional stabilizing interaction in the solvation of polycarbonyl moieties with hydrogen atoms attached directly to the carbonyl carbon or to the alpha-carbon atom. Ab initio calculations are performed on simple intermolecular complexes of CO(2) with compounds capable of acting as Lewis bases. Systems studied in different interaction configurations include formaldehyde, acetaldehyde, acetic acid, and methyl acetate, as model carbonyl compounds, and dimethyl sulfoxide as a model system for the sulfonyl group. Interaction energies, vibrational frequencies, charge transfer, and other molecular properties are calculated. Results indicate that C-H...O hydrogen bonds may be an important stabilizing interaction that merits consideration in the design of future CO(2)-philes.