Stability, metastability, and unstability of three-electron-bonded radical anions.: A model ab initio theoretical study

The stability of OthereforeO, NthereforeN, SthereforeS, PthereforeP, and SithereforeSi three-electron bonds in anionic radicals isoelectronic to dihalogen radical anions is studied by means of ab initio calculations on model systems. The difficulty of generating the dissociation energy profiles of such anions and their rearrangement to neutral species is solved by a practical method which consists of calculating the neutral and anionic energy profiles separately and shifting the curves with respect to each other to match the experimental energy gap between the asymptotes. Here the neutral and anionic reaction profiles are calculated at the CASPT2 and MP2 levels, respectively. The calculations predict that the OthereforeO bond is likely to be observed in anions of the type [ROthereforeOR](.-), where R is any alkyl substituent or carbon chain. The anion Si2H6.- is found to be a metastable species, with a fair barrier to electron detachment. The barrier is much smaller for N2H4.- and P2H4.-, thus precluding experimental observation. However, these species can be stabilized by electron-attractor substituents, the effect of which can be quantitatively estimated by means of the parent anion's diagrams and some fast complementary calculations. An example is given with the [CF(3)HNthereforeNHCF(3)](.-) anionic complex.