A 2-dimensional ring-puckering potential-energy surface for ethylene carbonate from ab initio and flexible model computations

A two-dimensional ring puckering potential energy surface has been derived for ethylene carbonate using ab initio and flexible model computations. The surface is expressed as a mixed Taylor--Fourier series expansion of the generalized ring puckering coordinates (q, ?). The ab initio molecular energies have been used to determine the degree of truncation of the series which gives an accurate representation of the surface in the region of q, ? space spanned by data from microwave spectroscopy. The ab initio geometries have been used for the kinetic energy part of the flexible model computations. By scaling the coefficients of the ab initio potential it is possible to obtain reasonably good agreement with the observed inversion splitting, fundamental puckering vibrational frequencies and most of the variation in the rotational constants with excitation of the ring puckering vibrations. This potential function has a twisted C2 symmetry equilibrium conformation with a puckering amplitude of 27 pm. The lowest energy path to ring inversion passes through the planar ring conformation where there is a barrier of 2.7 kJ mol- (229 cm-1).