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

Abstract

We present a semiempirical AM1 study of the radical anions of o-, m- and p-halonitrobenzenes and some alkyl substituted derivatives in relation to their σ–π orbital isomerism and the energy of their interconversion (ΔE_σπ). Halobenzene radical anions are also included for comparison. The results obtained with the RHF/CI(5) formalism account for the differences observed in the fragmentation rate of these radical anions under thermal and photochemical conditions. Based on the calculated ΔE_σπ the intramolecular thermal electron transfer from the π system to the σ* C–X bond involved in the fragmentation of the intermediates into an aromatic radical and the anion of the leaving group occurs with considerable energy for the p-, m- and o-chloronitrobenzenes (1a–c) and the p- and m-bromo (2a, b) derivatives. The fragmentation of these radical anions is favoured either from the first or from higher energy excited doublet states. On the other hand, the intramolecular thermal electron transfer is favoured for the 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.