LOW-BARRIER HYDROGEN-BONDS - AB-INITIO AND DFT INVESTIGATION

High-level ab initio and DFT molecular orbital calculations have been used to investigate the physical properties of a model low-barrier hyd rogen bond (LBHB) system: formic acid-formate anion. Ln the gas phase, it is found that the hydrogen bond formed is extraordinarily short an d strong [ca. 27 kcal/mol at B3LYP/6-31 + + G(d, p)], with a calculate d enthalpy of activation for proton transfer from donor to acceptor th at is less than the zero-point vibrational energy available to the sys tem. Several perturbations to this system were studied, Forcing a mism atch of pK(a)s between donor and acceptor, via the use of substituents , causes the strength of the hydrogen bond to decrease. Microsolvation of the hydrogen-bonded complex does not affect the strength of the lo w-barrier hydrogen bond very much. Small variations in the structure o f the LBHB results in a decrease in hydrogen-bond strength. Increasing the effective polarity of the cavity surrounding the LBHB was found t o have a significant impact on the strength of the hydrogen bond. Impl ications for enzyme catalysis are discussed. (C) 1998 John Wiley & Son s, Inc. J Comput Chem 19: 1345-1352, 1998.