An ab initio study of the electrostatics of protonated amines: application to the molecular mechanics (MM3) force field
The geometries and charge distributions of 20 small ammonium ions have been calculated at the MP2/6-311G** level of theory. ChelpG charges were used to determine the molecular polarities and fundamental bond moments of protonated aliphatic amines. The molecular mechanics (MM3) force field has been updated to reflect the correct polarity of the C–N+ and N+–H bonds, and to more closely reproduce ab initio-derived skeletal bond lengths of ammonium ions. These improvements lead to a force field which accurately reproduces geometric and energetic features of a variety of ammonium ions.