Ab initio NQR study of piperidine umbrella inversions

Two conformers of piperidine interconvert through nitrogen inversion. In this equilibrium, conformers with equatorial N-H appear dominant over those with axial N-H. In this study, substituent (X) effects are investigated on nitrogen inversions, for axially substituted 3-X-piperidines, using ab initio calculated NQR parameters, where X=H, F, Cl, Br, CH3, CF3, OH, CHO, COOH, CN, NH2 and NO2. These 12 species are in three forms. They may have an axial sp3 N-H (1X), a planar sp2 N-H (2X), and/or an equatorial sp3 N-H (3X). The axial minima, 1X, appear to go through transition states (TS), 2X, and convert to equatorial minima, 3X. Ab initio and DFT methods are employed to study these equilibria (1X (min)⇌2X(TS)⇌3X(min)). Energies, structural parameters as well as 2H (2H-N) and 14N quadrupole coupling constants (χ2H and χ14N, respectively) are calculated for 1X, 2X and 3X. The order of χ2H magnitude appears to be: 2X>3X>1X. The same order is obtained for χ14N when X=F, Cl, Br, CH3, CF3, COOH, CN, NH2 and NO2. For X=H, OH and CHO, χ14N trend changes to: 2X>1X>3X. According to our B3LYP/6-31 g (d,p) and MP2/6-31g(d,p) data, the equilibrium shifts in favor of 1X over 3X form is demonstrated, when X is F, Cl, Br, COOH, NO2, CN or CF3. On the other hand, equatorial 3X dominate over axial 1X when X=H, CH3, OH and NH2.