On the activation of σ-bonds by electric fields: A Valence Bond perspective

The activation of non-polar σ-bonds induced by an electric field is studied from the perspective of the Valence Bond theory. As representative examples we study the dissociation of the H–H and C–H bonds of molecular hydrogen and methane, respectively, under the experience of an homogeneous as well as an heterogeneous field oriented along the bond axis. For all cases, the increase in the electric field have similar effects: (i) the stabilization of the potential energy, (ii) an increment of the equilibrium bond length and (iii) the transition from an homolytic dissociation mechanism to an heterolytic one when the bond is subjected under a strong enough field. These general observations are thoroughly explained using a simple Valence Bond model that involve the increment of the resonance energy between the covalent and the ionic structures, and the curve crossing between the two structures after some field strength.