High electric field effects on the acrylonitrile molecule: An ab initio study

The behaviour of a vinylic molecule (acrylonitrile) submitted to the high electric field occurring during cathodic polymerization has been investigated by ab initio calculations. In this approach the electrochemical double layer has been modelized by homogeneous fields (up to 2×10¹⁰ V/m). It is found that the field induces minor geometrical deformations in the molecule (<4° and <0.02 Å). But, owing to its orientation along the CN bond, it enhances the electron attraction effect of the nitrile group, an intensity of 5 × 10⁹ V/m being needed to induce on the vinylic bond an effect of the same order of magnitude as that of CN itself. An opposite σ/π electron transfer is also pointed out, suggesting how important is the σ-π separation in conjugated system when subjected to an external field. Very high field intensity (10¹⁰ V/m) is needed to induce significant changes in the electronic density and electrostatic potentials of the molecule, the particular behaviour of the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) suggesting an increased ability for the CH₂ group to attract electron rich functional groups when such a high field intensity is reached. Finally the position trans relative to the CN group is the one most affected by the electric field; this behaviour provides a clue for monitoring the electronic properties of the molecule in view of designing selective polymerization reactions.