Classical and distonic radical cations: A valence bond approach
Hiberty, Philippe C.
- 化学化工－已发表论文 
The conventional radical cations arising from the ionization of CH3CH2X (X=F, OH, NH2, Cl, SH, PH2), and their distonic isomers, CH2CH2XH center dot+, were studied by means of standard Moller-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 impaired electron is in an orbital that lies in the plane of the heavy atoms, while the C states have their impaired 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 kJmol(-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 kJmol(-1)), except for the less electronegative X groups of the series, namely X=SH and PH2. In these two cases, together with X=NH2, 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.