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

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 NH 3 …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 NO 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 corresponding monomeric forms. The charge transfer process associated with the proton migration along the H‐bond is also discussed. It is found that the amount of charge transferred increases with the NO distance and with the RF strength, In order to test the general approach and compare it with previous work, calculations on the real monomeric systems glycine, β‐alanine, and γ‐amino butyric acid was also performed.