Pseudorotation in pyrrolidine: Rotational coherence spectroscopy and ab initio calculations of a large amplitude intramolecular motion

Pseudorotation in the pyrrolidine molecule was studied by means of femtosecond degenerate four-wave mixing spectroscopy both in the gas cell at room temperature and under supersonic expansion. The experimental observations were reproduced by a fitted simulation based on a one-dimensional model for pseudorotation. Of the two conformers, axial and equatorial, the latter was found to be stabilized by about 29 ± 10 cm^-1 relative to the former one. The barrier for pseudorotation was determined to be 220 ± 20 cm^-1. In addition, quantum chemical calculations of the pseudorotational path of pyrrolidine were performed using the synchronous transit-guided quasi-Newton method at the MP2 and B3LYP levels of theory. Subsequent CCSD(T) calculations yield the energy preference of the equatorial conformer and the barrier for pseudorotation to be 17 and 284 cm^-1, respectively.