Raman and Infrared Wavenumbers, Normal Coordinate Analyses, Barrier to Internal Rotation and Ring Inversion in 4-Cyclopentenecarboxaldehyde Based on Ab Initio Calculations

The conformational stability of 4-cyclopentenecarboxaldehyde was investigated by ab initio calculations with the 6-311G* basis set. The calculations were carried out at restricted Hartree-Fock (HF) and second-order Möller-Plesset (MP2) levels. From the calculation, the molecule was predicted to exist as a complex mixture of both axial and equatorial conformations at ambient temperature. The potential function in the equatorial molecules was determined for the CHO asymmetric torsion. The vibrational wavenumbers were computed at the HF level and the zero-energy corrections were included in the calculated barrier. Normal coordinate calculations were carried out and potential energy distributions were calculated for the trans-equatorial and gauche-equatorial conformers of the molecule. The calculated vibrational wavenumbers for the two conformers were scaled and compared with those observed experimentally for similar molecules.