CHARACTERIZATION OF LOW-BARRIER HYDROGEN-BONDS 4 - BASIS-SET AND CORRELATION-EFFECTS - AN AB-INITIO AND DFT INVESTIGATION

High-level ab initio and density functional calculations conclude that formic acid and formate anion form a very short, strong hydrogen bond in the gas phase. This system is proposed as a model for the interact ions present during many enzyme-catalysed reactions. The results of an extensive basis set study suggests that 6-31+G(d,p) is an excellent a nd sufficient basis set for the further study of these types of system . Hartree-Fock calculations consistently underestimated the energy of interaction between a formic acid and a formate anion (22 kcal mol(-1) ) while overestimating the potential energy barrier for transfer of a proton from formic acid to formate anion (2 kcal mol(-1)). Calculation s at correlated levels of theory, including DFT, show a stronger energ y of interaction (hydrogen bond energy) between formic acid and format e anion (27 kcal mol(-1)), and predict little or no barrier (0.3 kcal mol(-1)) for proton transfer. The density functional methods (BLYP and B3LYP) perform almost identically to other correlated ab initio metho ds such as MP2, MP3, and MP4. (C) 1998 Elsevier Science B.V.