Molecular mechanics (MM4) and ab initio study of amide-amide and amide-water dimers

As an extension of earlier amide work, hydrogen bonding parameters in amides have been developed for the molecular mechanics force field MM4. These parameters were obtained from studies on several configurations for each of the following complexes: formamide dimer, formamide-water, N-methylacetamide dimer, and N-methylacetamide-water. MM4 hydrogen bonding distances and binding energies were compared and fit to ab initio calculations on these systems reported by previous authors, preferably those done at the MP2 level of theory using double- or triple-xi basis sets, in addition to calculations carried out in the present study at the MP2/6-311++G(2d,2p) level. The MM4 optimized structures for the lowest energy configurations of the formamide-water complex were also compared to the existing results from microwave studies on these systems.