Orbital interactions and their effects on the conformational stability in six-membered rings containing nitrogen atoms

Conformational preferences and orbital interactions for N-methylpiperidine (1), 1,3-N,N-dimethyl-1,3-diazine (2) and 1,3,5-N,N,N-trimethyl-1,3,5-triazine (3) were analyzed using theoretical calculations and NBO analysis. For compound 1 conformational stability is dictated by 1,3-diaxial interaction, while the conformational preference for compounds 2 and 3 are dictated by orbital interactions. The N-methyl group prefers to adopt the axial orientation for compounds 2 and 3 due to the large orbital interaction LPN→σ*C-N, which only occurs when the methyl groups is in axial orientation.