A theoretical approach to understanding the fragmentation reaction of halonitrobenzene radical anions

We present a semiempirical AM1 study of the radical anions of o-, m- and p- halonitrobenzenes and some alkyl substituted derivatives in relation to the ir sigma-pi orbital isomerism and the energy of their interconversion (Delt a E-sigma pi). Halobenzene radical anions are also included for comparison. The results obtained with the RHF/CI(5) formalism account for the differen ces observed in the fragmentation rate df these radical anions under therma l and photochemical conditions. Based on the calculated Delta E-sigma pi th e intramolecular thermal electron transfer from the pi system to the sigma* C-X bond involved in the fragmentation of the intermediates into an aromat ic radical and the anion of the leaving group occurs with considerable ener gy for the p-, m- and o-chloronitrobenzenes (la-c) and the p- and m-bromo ( 2a, b) derivatives. The fragmentation of these radical anions is favoured e ither from the first or fi;om higher energy excited doublet states. On the other hand, the intramolecular thermal electron transfer is favoured for th e p-, m-, o-iodo (3a-c) and o-bromo (2c) derivatives. The results obtained for some alkyl substituted halonitrobenzene radical anions are in agreement with their known experimental fragmentation rates.