Molecules in an electric field: Some structural and spectroscopic consequences

It is theoretically shown that under the influence of an appropriately directed external electric field (ε) in H₂CO or a similar molecule (of the XYC = O type), the barrier to planar inversion in the ³n π* state decreases as the field strength increases. The molecule assumes a forced planar geometry in the ³n π* state rather abruptly when ε_(applied) > ε_(critical). This phenomenon of field-induced change of structure of an isolated molecule has the attributes of a first order phase transition. The effects should be spectroscopically detectable in the inversion spectrum. The existence of another critical field strength at which the molecule makes a more or less continuous transition from the normal convalent to a purely ionic structure is demonstrated and rationalized through a simple model.