CHARACTERIZATION OF LOW-BARRIER HYDROGEN-BONDS .3. HYDROGEN MALEATE -AN AB-INITIO AND DFT INVESTIGATION

High-level ab initio molecular orbital and density functional theory c alculations predict the existence of a very short-strong hydrogen bond in the monoanion of maleic acid (hydrogen maleate). At all levels of theory (HF, MP2, BLYP, and B3LYP) except B3PW91 the potential energy s urface is predicted to contain two minima, and hence resembles a doubl e well. The barrier to proton transfer via a symmetrical transition st ate is calculated to be very small at all levels of theory. In all cas es the calculated zero point vibrational energy available to the syste m is larger than the calculated barrier for proton transfer, thus the resulting hydrogen bond formed in hydrogen maleate is predicted to be symmetrical. Using the B3PW91 functional and the 6-31+G(d,p) basis set results in a single-well potential and a symmetrically positioned hyd rogen. All correlated methods predict the gas phase hydrogen bond ener gy to be approximately 27 kcal/mol. Effects due to solvents were estim ated using solvent cavity methods. Approximating the solvent as a diel ectric continuum reduces the calculated hydrogen bond energy by roughl y 6 kcal/mol at all levels of theory.