Characterization of low-barrier hydrogen bonds .7. Relationship between strength and geometry of short-strong hydrogen bonds. The formic acid formate anion model system. An ab initio and DFT investigation

High-level ab initio and density functional theory calculations have been used to investigate the dependence of the strength of a typical low-barrier hydrogen bond on geometrical distortions. In gas phase simulations, HF/6-31+G(d,p), MP2/6-31+G(d,p), and B3LYP/6-31+G(d,p) level calculations reveal that the short-strong hydrogen bond formed between a formic acid molecule and a formate anion is very sensitive to both the hydrogen bond length and the hydrogen bond angle. A 0.5 Angstrom lengthening of the low-barrier hydrogen bond results in a weakening of that bond by over 6 kcal/mol. A 1.0 Angstrom lengthening of the hydrogen bond results in an approximately 12 kcal/mol decrease in the calculated strength of the corresponding hydrogen bond. Similarly, an angle bending distortion of the hydrogen bond by as little as 30 degrees can lead to a weakening of the hydrogen bond interaction by more than 5 kcal/mol. Implications for enzyme catalysis are discussed.