Stereoelectronic Aspects of the Anomeric Effect in Fluoromethylamine

High‐level ab initio calculations have been made for fluoromethylamine to study structural and energetic effects of the relative orientation of the N lone pair to the CF bond. The anti‐conformer (N lone pair anti‐planar to the CF bond) corresponds to the global energy minimum. It has the longest CF distance, the shortest CN distance, and is 7.5 kcal·mol−1 more stable than the related perpendicular conformation (lone pair perpendicular to the CF bond). The syn‐conformation also shows hallmarks of the anomeric effect: long CF bond, short CN bond, and energetic stability when allowance is made for the two pairs of eclipsed hydrogens. The transition state for N inversion is close to the syn‐structure; rotation about the CN bond is strongly coupled with this inversion process. Small bond distance changes of ca. 0.02 Å between parallel and perpendicular conformations are associated with dissociation energy differences of ca. 30 kcal·mol−1. Various criteria for assessing the strength of the anomeric effect are discussed.