Classical and Distonic Radical Cations: A Valence Bond Approach

The conventional radical cations arising from the ionization of CH 3 CH 2 X (X=F, OH, NH 2 , Cl, SH, PH 2 ), and their distonic isomers, CH 2 CH 2 XH .+ , were studied by means of standard Møller–Plesset and G2 methods, and by an ab initio valence bond method. Among the conventional structures, two distinct states are considered. In the so‐called c ′ states, the unpaired electron is in an orbital that lies in the plane of the heavy atoms, while the c ′′ states have their unpaired electron in an orbital lying out of the plane. It is shown that c ′ states are, as a rule, more stable than the c ′′ states, by up to approximately 20 kJ mol −1 depending on the nature of X, owing to a stabilizing interplay of resonating structures. While the geometries of the c ′′ states are rather similar to those of the neutral molecules, some of the c ′ states display very different geometries, characterized by elongated CC bonds, particularly when X=F or, to a lesser extent, when X=OH or Cl. These peculiar geometric features are rationalized by the valence bond analysis, which reveals that the CC bond in these species is better viewed as a two‐center, one‐electron bond. The distonic radical cations are generally more stable than the conventional ones (by 20–100 kJ mol −1 ), except for the less electronegative X groups of the series, namely X=SH and PH 2 . In these two cases, together with X=NH 2 , the radical cation displays a classical distonic structure, as regards the geometry and electronic state. On the other hand, considerable CX elongation is found for X=F or Cl. In these last cases, the valence bond analysis shows that the radical cation is better viewed as an ion–molecule complex between an ionized ethylene and a neutral HX molecule. The electronic structure of the distonic radical cation with X=OH lies between the two previous limiting descriptions.