Intramolecular inversions, structure and conformational behavior of gaseous and liquid N-cyanopiperidine. Comparison with other 1-cyanoheterocyclohexanes

Abstract

The molecular structure and intramolecular inversions of N-cyanopiperidine (CNP) were studied by various quantum chemical methods and using synchronous gas electron diffraction/mass spectrometry (GED/MS) and IR spectroscopy. The contribution of the equatorial conformer was found GED 52(6), IR 60(6), B3LYP 77, B3LYP-D3 67, M062X 59, MP2 54%. Interpolation using CCSD(T)-F12/CBS predicts Eax–Eeq = 0.26 kcal/mol. The NBO analysis shows a strong conjugation of the nitrogen atom lone pair with the triple bond making the r(N–CCN) = 1.355(3) Å distance as short as a typical sesquilateral bond.A low energy barrier for the Eq→Ax transition of 1.0–2.3 kcal/mol in CNP leads to difficulties in NMR conformational ratio studies. This leaves the GED method almost as the only experimental method to study both structural and conformational properties.Conformational properties of the related compounds are found to depend on the X–CCN bond distance and conjugation between the CN group and the lone pair on the heteroatom. The contribution of the axial form of 1-cyano-heterocyclohexanes increases in the series of heteroatoms X in the cyclohexane ring: N→C→Si→P.