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 C--C 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 C--C 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 C--X 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.