Laser Flash Photolysis Study on the Hydrogen Atom Transfer Reaction from Triplet 2-Naphthylammonium Ion to Benzophenone. Remarkable Proton Effect

Article abstract of DOI:10.1021/j100167a025

The laser flash photolysis study at 355 nm in the 2-naphthylammonium ion (RNH3⁺) and benzophenone (BP) system has been carried out in methanol-water (9:1 v/v) at 290 K.At first, triplet benzophenone (³BP^*) produced by fast intersystem crossing of ¹BP^* excited at 355 nm is quenched by RNH3⁺ with almost diffusion-controlled rates (1.1x10¹⁰ M^-1 s^-1 at = 0.015 M; 3.9x10⁹ M^-1s^-1 at = 0.5 M).The triplet 2-naphthylammonium ion (³RNH3^+*) is produced in the nanosecond region by triplet energy transfer from ³BP^* to RNH3⁺ with efficiencies of 0.41 ( = 0.015 M) and 0.33 ( = 0.5 M) at +> = 3.0x10^-3 M and at = 1.12x10^-2 M, competing with hydrogen abstraction of ³BP^* from solvent (methanol) or RNH3⁺ molecules.Subsequently, in the microsecond region the hydrogen atom transfer (HT) reaction from ³RNH3^+* to ground BP occurs effectively to give the naphthylamine cation radical (RNH2^.+) and the ketyl radical (>C^.OH).The decayl rate (τ_obsd)^-1 of ^3RNH3^+* increases with an increase of the BP concentration, and at higher BP concentrations a leveling off is observed.The (τ_obsd)^-1 value decreases drastically with an increase in acid concentration.The mechanism of the HT reaction can be accounted for by the intracomplex HT reaction of the triplet exciplex ³(RNH3⁺ *** >CO)^* with a rate constant of ca. 5x10⁶ s^-1.At higher acid concentrations, the triplet complex is decomposed by protonation to give free ³RNH3^+* and BP, resulting in suppression of the rate for the HT reaction.