Low-barrier hydrogen bonds:Abinitio 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 hydrogen bond (LBHB) system: formic acid–formate anion. In the gas phase, it is found that the hydrogen bond formed is extraordinarily short and strong [ca. 27 kcal/mol at B3LYP/6-31++G(d, p)], with a calculated enthalpy of activation for proton transfer from donor to acceptor that is less than the zero-point vibrational energy available to the system. Several perturbations to this system were studied. Forcing a mismatch of pKas 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 low-barrier hydrogen bond very much. Small variations in the structure of the LBHB results in a decrease in hydrogen-bond strength. Increasing the effective polarity of the cavity surrounding the LBHB was found to have a significant impact on the strength of the hydrogen bond. Implications for enzyme catalysis are discussed.