A quantitative investigation of the transannular amine-ketone (N...C=O) interaction in medium-sized heterocycles.

The transannular N...C=O interaction in several medium-sized heterocycles has been investigated by force-field methods. Conformational searching has been performed at the molecular mechanics level using different methods and the conformers so generated have been reoptimized at RHF/6-31G(d). For selected conformers, N...C distances are reported, together with transannular bond orders and atomic charges. Good agreement with available X-ray crystallographic data is obtained for the transannular bond distance in cryptopine. Changes in the partial atomic charges derived from the electrostatic potential provide good support for the donor-acceptor model of transannular interactions. Partial charges derived with other methods do not give satisfactory results. Some force fields do not reproduce the transannular interaction very well. This is demonstrated and rationalized, and modifications are suggested and tested for these force fields with good results when comparing diagnostic geometric features with X-ray data.