Interaction of halide and pseudohalide lons with the triplet state of 1-nitronaphthalene. Effect of acidity: a flash photolysis study

Abstract

The interaction of halide, pseudohalide and hydroxide ions (X–) with the lowest excited triplet state of 1-nitronaphthalene (3NN) has been investigated by laser and conventional flash photolysis. One-electron transfer was demonstrated for CN–, SCN– and OH– ions, and the results for the quenching effect of the inorganic anions examined have been interpreted in terms of one-electron transfer from X– to 3NN. The effect of acidity on the triplet lifetime and on the reactivity of 3NN towards these anions was studied. In particular, it was found that CI– ion quenches the triplet state of NN with a second-order rate constant, k2, which at H0≈–2.0 is higher by a factor of 5 × 103 than that measured at neutral pH. The dependence of k2 on the acidity of the medium enabled the pKa of the protonated 3NN to be determined as –0.66 ± 0.02 in 25%(v/v) ethanol–water. A satisfactory correlation of log k2 with the free-energy change, estimated from the electrode potentials of the couples involved, was obtained using the Rehm–Weller empirical free-energy relationship. A ground-state concentration dependence of the lifetime of 3NN was found in acetonitrile, and in aqueous acetonitrile, and the self-quenching suggested to involve one-electron transfer. In 50%(v/v) aqueous ethanol the 1-hydroxyethyl radical, generated in the H-atom abstraction reaction of 3NN with ethanol, was shown to reduce NN by both an outer-sphere and an addition/elimination mechanism.