The mechanism of aromatic nitration in solution: Marcus theory and semiempirical molecular orbital calculations on NO2+ and NO+ as one-electron oxidants

Abstract

Marcus theory calculations of rates of electron transfer (ET) have been carried out for attack by NO⁺ and NO₂⁺ on various reactive aromatic substrates including alkylbenzenes and naphthalene. While reasonable agreement between calculation and experiment is obtained for nitrous acid catalysed nitration, the calculated rate of electron transfer in the nitration of naphthalene by nitronium ion is too low by between nine and fifteen orders of magnitude. Possible reasons for this discrepancy are discussed and it is suggested that H₂NO₃⁺ may be the ET agent responsible for the previously observed CIDNP effects in this system. Semi-empirical molecular orbital calculations have been carried out on relevant species, including reaction coordinates for the nitration of naphthalene in the 1- and 2-positions. Transition state energies suggest that attack in the 1-position is very much favoured by the ET process.