Evaluating the formation of salt-bridges: a molecular orbital study

Quantum chemical calculations were carried out to explore salt-bridge formation in the complexes of hydrogen chloride with ammonia, methylamine, dimethylamine, trimethylamine and pyridine to provide `bench-marksʹ for calculations on larger systems, such as neutral and protonated peptides. Among the selected model systems there are molecular complexes and zwitterionic structures as well. Geometry optimization was carried out at the MP2, B3LYP and B3PW91 levels of theory in conjunction with average and large basis sets. The results demonstrate that the above density functionals perform reasonably well for the investigated systems. The calculated geometrical data are in agreement with the corresponding experimental values of ammonia–hydrogen chloride, trimethylamine–hydrogen chloride and pyridine–hydrogen chloride.