Theoretical study of pyrrolidine:: Revised conformational energies and vibrational assignments

The pseudorotational process of pyrrolidine (PYR) and the conformational preference at the N-H position have been thoroughly reinvestigated by means of ab initio methods. To examine electron correlation effects and basis set dependencies, Hartree-Fock (HF), post-Hartree-Fock (MP2, CC, QCI, and Cl) and several density functional (DFT) methods with a large variety of basis sets have been employed. It has been found that both post-Hartree-Fock and DFT methods predict opposed energy differences between the N-H axial and N-H equatorial conformers depending on the size of the basis set. However, according to HF and B3LYP computations with the aug-cc-pVQZ basis set, it could be concluded that the N-H equatorial structure is the most stable conformer of PYR. This prediction is in agreement with the last microwave free jet experiment of Caminati et al. [Caminati, W.; Dell' Erba, A.; Maccaferri, G.; Favero, P. G. J. Mol. Spectrosc. 1998, 191, 45]. On the basis of the results obtained, the previously reported electron diffraction data and microwave experiments have been reinterpreted. Finally, a detailed reassignment of the experimental vibrational frequencies of pyrrolidine and its N-D isotopomer has also been performed by means of the scaled quantum mechanical force fields computed at the HF, MP2, and B3LYP levels with the 6-31G** basis set.