INFLUENCE OF ISOTOPIC SUBSTITUTION ON STRENGTH OF HYDROGEN BONDS OF COMMON ORGANIC GROUPS

Abstract

Although the electronic contribution to the strength of a H–bond is unaffected by isotopic substitution, the heavier mass of deuterium compared with protium lowers some of the vibrational frequencies in the complex. The binding energy of the complex, which includes zero–point and thermal vibrational energies, can thus be altered by several tenths of a kcal mol⁻¹ by H/D substitution. Ab initio calculations are used to analyze this phenomenon in a number of common organic functional groups that are prone to form H–bonds: hydroxyl, carboxyl and amide, both self–complexing as homodimers and with water molecules as partners. It is found that any site of D–substitution increases the complexation energy; however, the bridging sites show a stronger preference for D over H than do the non–bridging, or terminal, sites. Hence D–bonding can be considered to be stronger than H–bonding in these functional groups. Of the groups considered, the energetic preference for D over H is greater in the hydroxyl group, so deuterium would be expected to gravitate toward solvent water molecules in isotopic scrambling experiments. The increments in H–bonding energy resulting from each site of substitution are addititve in cases of multiple substitution. © 1997 John Wiley & Sons, Ltd.