Microwave spectrum and ab initio computations for ethylene carbonate. Part 1.--Conformation and ring inversion

The conformation and ring inversion in ethylene carbonate have been investigated using microwave spectroscopy and ab initio computations. The vibrational satellite spectra of the normal isotopic species of ethylene carbonate and the rotational constants of Bäckvall et al.(Tetrahedron Lett. 1980, 21, 4985) for cis- and trans-[1,2-2H2]ethylene carbonate show the molecule to have a twisted C2 equilibrium conformation. The vibrational satellite spectra have been analysed in terms of independent ring-bending and twisting vibration. A potential function for the twisting vibration derived from the variation of the rotational constants with vibrational state, and vibrational energy separations obtained from relative intensities and Coriolis perturbations give an equilibrium twist angle of 19° and a barrier to ring inversion of 2.8 kJ mol-1. The twist angle has also been obtained from inertial data, and a value of 15° is obtained using two methods of calculation. Ab initio computations of the ring-puckering potential-energy surface have been made using STO-3G and STO-3-21 G orbitals and complete geometry optimization. The STO-3G computations predict a planar C2? equilibrium geometry, but with the twisting vibration being lower in frequency and more anharmonic than the bending vibration. The STO-3-21G computations predict a C2 conformation with an equilibrium twist angle of 14° and barrier to inversion through the planar ring conformation of 1.1 kJ mol-1. The computed barrier to ring inversion by pseudorotation is 14.3 kJ mol-1.