Ab initio and density functional calculations of conformational energies and interconversion pathways in 1,2,3,6-tetrahydropyridine

Hartree–Fock with and without MP2 (frozen core and full) corrections and density functional calculations have been performed on 1,2,3,6-tetrahydropyridine with basis sets 6-31G∗, 6-31+G∗ and 6-311+G∗∗. For all methods which included diffuse functions, the half-chair equatorial N–H conformer was found to be slightly more stable than the half-chair axial conformer, in agreement with experimental results. A detailed comparison for all the methods and basis sets was made with experimental data. These included rotational constants for both the normal and N–D isotopic species, dipole moments and dipole moment components. In addition, several interconversion pathways and barriers between the axial and equatorial conformations were explored by Hartree–Fock and B3LYP with the 6-31+G∗ basis set. The lowest energy pathway between was found to be via the N–H inversion (∼4–5 kcal/mol); via a bent (boat) axial form (∼6–7 kcal/mol) and finally via a bent (boat) equatorial form (∼7–8 kcal/mol). The planar form was found to be ∼10 kcal/mol less stable than the two half-chair forms.