Theoretical study of the influence of electric fields on hydrogen-bonded acid-base complexes

Matrix effects on the optimized geometries and the electronic properties of acid-base complexes XHB, with HX = HF, HCl, HBr, HCN and B = NH3, have been modeled using ab initio methods (6-31G** and 6-311++G** basis sets) in two different ways. Model A corresponds to the Onsager SCRF model, and model B corresponds to a homogeneous electric field F = 2qe(o)/r(e)(2) = 2.88 x 10(5)q V/cm of varying strength generated by two distant charges +qe(o) and -qe(o) of opposite sign placed at distances of r(e) = 100 Angstrom. In both models, the minima and reaction coordinate of proton transfer has been calculated. As the electric interactions are increased, both models predict an increase of the dipole moments associated with a proton shift from X to B, i.e., a conversion of the molecular to the zwitterionic complexes. Both models predicts double minima for some electric fields; in model B electric fields are found where the neutral complex is not stable, evolving to the ion pair complex. These fields can be used to characterize the acidity of the donor toward the base without the necessity of assuming a proton-transfer equilibrium. In both models a similar field-induced correlation between the two hydrogen bond distances r(1) drop X ... H and r(2) drop H ... B is observed for all configurations. This correlation indicates in the molecular complexes a hydrogen bond compression when the proton is shifted toward the base and in the zwitterionic complexes a widening. The minimum of the X B distance r(1) + r(2) occurs when the proton-transfer coordinate r(1) - r(2) = r(01) - r(02), where r(01) and r(02)represent the distances X ... H and H ... B+ in the free donors.