HYDROGEN-BONDING OF CARBONYL, ETHER, AND ESTER OXYGEN-ATOMS WITH ALKANOL HYDROXYL-GROUPS

An attractive way to study intermolecular hydrogen bonding is to combi ne analysis of experimental crystallographic data with ab initio-based energy calculations. Using the Cambridge Structural Database (CSD), a distributed multipole analysis (DMA)-based description of the electro static energy, and intermolecular perturbation theory (IMPT) calculati ons, hydrogen bonding between donor alkanol hydroxyl groups and oxygen acceptor atoms in ketone, ether, and ester functional groups is chara cterized. The presence and absence of lone pair directionality to carb onyl and ether or ester oxygens, respectively, can be explained in ter ms of favored electrostatic energies, the major attractive contributio n in hydrogen bonding. A hydrogen bond in its optimum geometry is only slightly stronger when formed to a ketone group than to an ether grou p. Hydrogen bonds formed to carbonyl groups have similar properties in a ketone or ester, but the ester O-2 differs from an ether oxygen due to various environmental effects rather than a change in its intrinsi c properties. For (E)-ester oxygens, there are few hydrogen bonds foun d in the CSD because of the competition with the adjacent carbonyl gro up, but the interaction energies are similar to an ether. Hydrogen bon ds to O-2 of (Z)-esters are destabilized by the repulsive electrostati c interaction with the carbonyl group. The relative abundance of nonli near hydrogen bonds found in the CSD can be explained by geometrical f actors, and is also due to environmental effects producing slightly st ronger intermolecular interaction energies for an off-linear geometry. (C) 1997 by John Wiley & Sons, Inc.