Electronic and structural effects determining rotational barriers about the C–N bond in enamines of pyran-4-one and thiopyran-4-one—A theoretical MO ab initio approach to the interpretation of experimental results

Abstract

Ab initio MO calculations were carried out in 2-chloro-6-NR 2-pyran-4-ones, 2-chloro-6-NR 2-thipyran-4-ones and in their protonated forms with the aim of discussing the electronic and structural effects modulating the rotational barriers about the C–N partial double bond measured in previous research. The barriers in these molecules depend on NR 2 substituents (pyrrolidino, dimethylamino, piperidino and morpholino groups were examined), on the O/S atoms in the pyranone/thiopyranone rings and differ in neutral and protonated forms. The results of calculations, carried out at different levels of theory (HF/6-31G ∗//HF/6-31G ∗, HF/6-31G ∗∗//HF/6-31G ∗∗, B3LYP/6-31G ∗∗, MP2/6-31G ∗∗/MP2/6-31G ∗∗) and simulating the effect of a solvent (self-consistent isodensity polarized continuum model) were employed to analyse the effect of the different molecular fragments on the barrier to internal rotation about the partial C–N double bond. Parallel trends are found for calculated and experimental results as a function of changes in molecular structure even though the calculated barriers for the ions are overestimated.