The recently introduced nucleophilicity index (N) of unsaturated compounds, which are typical pi-nucleophiles, correlates (R = 0.9229) with their singlet-triplet energy splitting (Delta(S-T)) between the ground singlet and the excited (3)(pi pi*) state. The latter quantity is proposed for the activation barrier (E-a) determination of the nucleophile-electrophile addition reactions. A new approach for calculation of the E-a value is developed for reactions between alkenes and the p-methoxybenzhydryl cation. This scheme involves some simple DFT parameters, namely, the Delta(S-T) of the reactants and products. The experimentally available E-a values for the above reaction correlate well (R = 0.9944) with the calculated data. (C) 2014 Elsevier B.V. All rights reserved.