Pseudorotation in heterocyclic five-membered rings: Tetrahydrofuran and pyrrolidine

Using the 6-31G** basis set, the HF-SCF and Moller-Plesset second-order (MP2) perturbation calculations have been carried out for tetrahydrofuran (THF) and pyrrolidine (PY) with the symmetries given hy pseudorotation. On the whole, the MP2 calculations give better results on the puckered structures and energetics of both molecules, which are consistent with diffraction and spectroscopic results. From the MP2/6-31G** calculations, the twist conformation T-4(3) is found to be the most stable one for THF, and the twist T-1(2)(ax) and the envelope (1)E(ax) forms appear to be energetically identical and most feasible for PY. We investigate the correlation between the puckering amplitudes and the sum of deviations of endocyclic bond angles from the standard value. The better correlation may support that the revised pseudorotation model proposed here is more appropriate to describe the puckering of non-equilateral five-membered rings than earlier models.