Orbital interaction between electron lone pair and carbonyl group in N-trifluoroacetylpiperidine and N-piperidine amides: Planar and non-planar nitrogen bond configurations

Molecular structure of N-trifluoroacetylpiperidine (NFAPi) was studied by synchronous gas-phase electron diffraction/mass spectrometry (GED/MS), IR spectroscopy and quantum chemical (QC) calculations. Due to the influence of strong conjugation between the electron lone pair on the nitrogen atom and the double bond of the carbonyl group, NFAPi may exist as only one conformer, in which the trifluoroacetyl group is located in an intermediate, between axial and equatorial, position. Nine atoms (two C-alpha atoms along with their H-eq atoms, the nitrogen atom, the carbonyl group and one CF fragment of the trifluoromethyl group) form a single plane. In contrast, partition of the conjugation between the double C=O bond and the two lone pairs leads to coexisting intermediate and axial, in 1-piperidinecarboxamide, and axial and equatorial forms in N, N-dimethy1-1-piperidinecarboxamide and 1,1'-carbonyldipiperidine. The conformational ratio for the latter ones is Eq:AX approximate to 50:50%. The steric effect and conjugation stabilize the axial conformer. (C) 2017 Elsevier Ltd. All rights reserved.