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 b een used to investigate the dependence of the strength of a typical lo w-barrier hydrogen bond on geometrical distortions. In gas phase simul ations, HF/6-31+G(d,p), MP2/6-31+G(d,p), and B3LYP/6-31+G(d,p) level c alculations reveal that the short-strong hydrogen bond formed between a formic acid molecule and a formate anion is very sensitive to both t he hydrogen bond length and the hydrogen bond angle. A 0.5 Angstrom le ngthening of the low-barrier hydrogen bond results in a weakening of t hat bond by over 6 kcal/mol. A 1.0 Angstrom lengthening of the hydroge n bond results in an approximately 12 kcal/mol decrease in the calcula ted strength of the corresponding hydrogen bond. Similarly, an angle b ending 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 kc al/mol. Implications for enzyme catalysis are discussed.