MM3 force field as a tool in mechanistic studies of nitrogen inversion processes for alkylamines

We show that the MM3 force field provides a reasonable modelling of the transition state of nitrogen inversion-rotation (NIR) for alkylamines and thus a quantitative estimate of nitrogen inversion barriers for amines of different structures. In general, the NIR barriers for most open-chain, monocyclic and polycyclic amines are now available within 0.7 kcal mol^-1 (standard deviation) of the experimental values. However, for isopropyl-or isobutyl-type amines and pyrrolidine compounds this correlation between the calculated and experimental barriers drops down to 2.7 kcal mol^-1. An effective study of the mechanism of the NIR process, and thus the assignment of NMR-measured barriers, proved possible using MM3-calculated barriers for the isolated rotation (ISR), as well as the concerted NIR. A simple MM3-based test was developed to identify the amines for which an NIR process excludes an isolated rotation process.