Hydrogen-bond acceptor properties of nitriles: a combined crystallographic andab initio theoretical investigation

Abstract

The hydrogen-bond ability of nitrogen in X—CN···HO—R systems was investigated using crystallographic data retrieved from the Cambridge Structural Database and via ab initio calculations. The classification of hydrogen-bond geometries by the nature of the donor shows that the strength of the interaction increases in the order water ≈ alcohols <phenols. If the hydrogen-bond geometries are classified according to the nature of the acceptor inside each sample of donor, the strongest hydrogen bonds are always observed for ‘push–pull’ nitriles, as observed in solution on the pKHB scale for such molecules. Ab initio molecular orbital calculations are then used to compute descriptors of hydrogen-bond basicity (enthalpy ΔH°, hydrogen-bond length and minimum electrostatic potential of the base) for hydrogen-bond formation between water and 18 nitriles embracing a wide range of structure and basicity. Correlations between the hydrogen-bond basicity scale pKHB and these quantum mechanical descriptors allow the calculation of pKHB values for experimentally inaccessible bases and the treatment of polyfunctional nitriles. They also confirm the super-basicity of the nitrile group of cimetidine, a well known antiulcer drug. This property might be relevant in the molecular recognition of cimetidine by the H2 receptor. Copyright ­© 2000 John Wiley & Sons, Ltd.