ENERGETICS OF A LOW BARRIER HYDROGEN-BOND IN NONPOLAR-SOLVENTS

A measure of the strength of a low barrier hydrogen bond (LBHB) in apo lar organic media was obtained using synthetic molecules derived from Kemp's triacid. The structures feature unusually rigid conformations t hat enforce intramolecular hydrogen bonds in a dicarboxylic acid, its corresponding acid-amide and their respective conjugate bases. Analysi s of proton and deuterium NMR spectra established the formation of a L BHB in the conjugate base of the diacid and a conventional hydrogen bo nd in the conjugate base of the acid-amide. Through deprotonation equi libria with organic bases, it was determined that the conjugate base o f the diacid was more stable than the conjugate base of the acid-amide by 2.4 kcal/mol in benzene and 1.4 kcal/mol in dichloromethane. These figures set the upper limits for the free energy of the additional st abilization arising from the LBHB at 25 degrees C. This value is far l ower than many estimates but is closer to the recent determinations of Schwartz and Drueckhammer.