Reactivity of some condensed heterocyclic cations towards ortho-positronium atoms in acetonitrile solution

The effect of changes in chemical structure and charge distribution of mole cules on their reactivity towards an orthopositronium (o-Ps) atom was inves tigated by positron annihilation lifetime spectroscopy. Molecules with very similar structural frameworks were selected as model systems. Bridgehead n itrogen-containing condensed heterocyclic pyridoazinium salts with 1-3 nitr ogen atoms and their methyl-substituted analogues were studied in acetonitr ile solutions. In both groups the reactivity towards o-Ps atoms increased d rastically with the increasing number of N atoms, while only a slight decre ase of reactivity was observed due to methyl substitution. The strong elect ron-withdrawing effect of the N atoms and the weak electron-donating charac ter of the methyl group accounted for these observations. In contrast to th e reaction with o-Ps atom, methyl substitution affected drastically the che mical reactivity of these molecules in ring opening reactions with secondar y amines, where the methylated compounds did not react even under forcing c onditions. In the light of the o-Ps lifetime measurements a possible explan ation for this fact is that the methyl group sterically hinders the attack of the secondary amines. (C) 2001 Elsevier Science B.V. All rights reserved .