The singlet–triplet energy splitting of π-nucleophiles as a measure of their reaction rate with electrophilic partners

The recently introduced nucleophilicity index (N) of unsaturated compounds, which are typical π-nucleophiles, correlates (R = 0.9229) with their singlet–triplet energy splitting (ΔS–T) between the ground singlet and the excited 3(ππ∗) state. The latter quantity is proposed for the activation barrier (Ea) determination of the nucleophile–electrophile addition reactions. A new approach for calculation of the Ea value is developed for reactions between alkenes and the p-methoxybenzhydryl cation. This scheme involves some simple DFT parameters, namely, the ΔS–T of the reactants and products. The experimentally available Ea values for the above reaction correlate well (R = 0.9944) with the calculated data.