Ab initio study of the pseudorotation and conformational stability of pyrrolidine

The energy profile for the pseudorotational process of pyrrolidine has been investigated with several ab initio computational methods, and both stable conformers and transition states have been characterized. Theoretical calculations show that pseudorotation is the path preferred for interconversion between the N-H axial and equatorial forms, with a barrier of approximately 0.6 kcal mol(-1). MP2/6-31G** results agree acceptably well with the experimental data from microwave spectroscopy, electron diffraction and vibrational frequencies, which suggest that electron correlation is needed to obtain a reliable description of the conformational preferences. The existence of a free-pseudorotation region predicted by correlated calculations allows the contradictions between the previously reported interpretations of experimental work to be eliminated. The natural bond orbitals (NBO) method was applied on the HF/6-31G** wavefunctions to confirm the existence of delocalization due to the nitrogen lone pair. The relationship between delocalization and the puckering of the axial and equatorial envelope forms is discussed.