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 Å lengthening of the low-barrier hydrogen bond results in a weakening of that bond by over 6 kcal/mol. A 1.0 Å 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° can lead to a weakening of the hydrogen bond interaction by more than 5 kcal/mol. Implications for enzyme catalysis are discussed.