Energetics of a low barrier hydrogen bond in nonpolar solvents

A measure of the strength of a low barrier hydrogen bond (LBHB) in apolar organic media was obtained using synthetic molecules derived from Kemp's triacid. The structures feature unusually rigid conformations that enforce intramolecular hydrogen bonds in a dicarboxylic acid, its corresponding acid-amide and their respective conjugate bases. Analysis of proton and deuterium NMR spectra established the formation of a LBHB in the conjugate base of the diacid and a conventional hydrogen bond in the conjugate base of the acid-amide. Through deprotonation equilibria with organic bases, it was determined that the conjugate base of 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 stabilization arising from the LBHB at 25 degrees C. This value is far lower than many estimates but is closer to the recent determinations of Schwartz and Drueckhammer.