3 years ago

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

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 Cα atoms along with their Heq 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 CO bond and the two lone pairs leads to coexisting intermediate and axial, in 1-piperidinecarboxamide, and axial and equatorial forms in N, N-dimethyl-1-piperidinecarboxamide and 1,1′-carbonyldipiperidine. The conformational ratio for the latter ones is Eq:Ax≈50:50%. The steric effect and conjugation stabilize the axial conformer.

Publisher URL: www.sciencedirect.com/science

DOI: S0040402017307664

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