Cation Binding Effect on Hydrogen Bonding

Cation binding effects on nearby H bonds are studied for the imidazole dimer. The binding of the cations. H⁺, Na⁺, Zn(OH)⁺, and Zn²⁺, is found to have a significant effect on both the H bond energy and the equilibrium internuclear separation of the H bond. The net stabilization energy ranges from 8.0 kcal for Na⁺ to 28.0 kcal for Zn²⁺. The polarization of the dimer is also appropriate to increase the binding of another imidazole. Proton transfer is also studied for the dimer itself and the cation perturbed dimer. Double wells are found in all cases in the energy curve for proton transfer with a fixed N-N' distance between the imidazole monomers. With the exception of Zn²⁺ a substantial barrier is calculated for the proton transfer reaction. Compact effective potentials (CEP) were used in the calculations. Comparisons of the orbital energies and gradient optimized geometry of the CEP calculation for imidazole with a GTO 4-31G all-electron calculation are quite satisfactory.