Coupled cluster investigation of the axial and equatorial isomers of pyrrolidine

Abstract

The CCSD(T) methodology coupled with Dunning's correlation-consistent basis sets was employed to study the relative energies of the axial and equatorial pyrrolidine. The enthalpies of formation of both isomers were computed including corrections for relativistic and core-valence effects as well as anharmonic effects. The accurate determination of the lowest energy isomer is a very complex task because of the very small energy gap. At the CCSD(T)/CBS level, equatorial pyrrolidine is 0.10 kcal/mol lower in energy than axial pyrrolidine. When core-valence, scalar relativistic and spin-orbit effects are included, the relative energy is similar, 0.11 kcal/mol. However, when zero-point energies are taken into account, some difficulties are observed. Anharmonic effects on ZPEs were not possible to be accurately estimated as problems were detected in the lowest vibrational modes. Thus, the question of which is the lowest energy isomer of pyrrolidine remains open until accurate ZPEs can be determined for both isomers. The enthalpy of formation of pyrrolidine is predicted to be −2.7 ± 1 kcal/mol at 298 K.