Self‐consistent reaction field calculations on the proton transfer in ammonia‐formic acid systems as a model for hydrogen bonding in amino acids in solution

Abstract

The reaction field (RF) model of solvent effects, implemented within the SCF‐CNDO/2 scheme of calculation, has been applied to analyze the proton transfer in the NH3…HCOOH system in the presence of several polarizable media. The aim of such a study was to characterize the tatutomeric equilibrium between the neutral and zwiterionic forms of H‐bonded amino acids in aprotic solvents. Qualitative results concerning the energetics of this equilbrium show the stabilization of two different H‐bonded complexes, corresponding to two separate minima in the free energy surface. These well known double minima potentials are found to be dependent on both the intermolecular NO distance and the strength of the reaction field. The behavior of this model is qualitatively consistent with experimental observations of nitrogen‐substituted amino acids in solution: both show, for low values of the dielectric constant, tautomeric equilibria where the H‐bonded complexes appear to be more stable than the corr