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 pK_a 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.