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 (1_X), a planar sp² N–H (2_X), and/or an equatorial sp³ N–H (3_X). The axial minima, 1_X, appear to go through transition states (TS), 2_X, and convert to equatorial minima, 3_X. Ab initio and DFT methods are employed to study these equilibria (1_X (min)2_X(TS)3_X(min)). Energies, structural parameters as well as ²H (²H–N) and ¹⁴N quadrupole coupling constants (χ_2H and χ_14N, respectively) are calculated for 1_X, 2_X and 3_X. The order of χ_2H magnitude appears to be: 2_X>3_X>1_X. The same order is obtained for χ_14N when X=F, Cl, Br, CH₃, CF₃, COOH, CN, NH₂ and NO₂. For X=H, OH and CHO, χ_14N trend changes to: 2_X>1_X>3_X. According to our B3LYP/6-31 g (d,p) and MP2/6-31g(d,p) data, the equilibrium shifts in favor of 1_X over 3_X form is demonstrated, when X is F, Cl, Br, COOH, NO₂, CN or CF₃. On the other hand, equatorial 3_X dominate over axial 1_X when X=H, CH₃, OH and NH₂.