Abstract
Stationary points of the INDO energy hypersurface for various orientations of ethylene in external electric fields of the strength F=0, 2, 4, 6, 8 and 10 × 10^10^ V m^−1^ were found and their characteristics studied by the force constant matrix analysis. Energies, structural parameters, charges, Wiberg indices and dipole moments are presented. The only stable orientation of the ethylene molecule is that for which the CC bond is parallel to the field direction up to F=6 × 10^10^ V m^−1^ (orientation (a) in Fig. 1). Above this value the molecule is structurally unstable and it decomposes to the hydride anion and the C~2~H~3~^+^ cation. Rotational instability was found for two perpendicular orientations of the CC bond with respect to the field vector, in which the field vector was parallel and perpendicular to the molecular plane. Pseudorotations with negative eigenvalues of force constant matrices lead to the stable orientation (a). No stationary points were found when the angle between the CC bond and the field vector was between 0 and 90°. The five longest wavelength vibrational bands are presented for selected orientations and field strengths.