Laser flash photolysis studies on hydrogen-atom transfer from the triplet hydroxynaphthylammonium ion to benzophenone via a triplet exciplex. Which group is more reactive for hydrogen-atom transfer, —OH or —NH +3?

Abstract

Laser flash photolysis studies on hydrogen-atom transfer (HT) from the triplet hydroxynaphthylammonium ion (the protonated form of 5-aminonaphth-1-ol, HORNH⁺₃) to benzophenone (BP or CO) in methanol–water (9 : 1 v/v) at 295 K have been carried out in order to elucidate which hydrogen atom of the substituent groups is more reactive for HT, and what is the mechanism. It was found that HT from triplet HORNH⁺₃(³HORNH^+*₃), produced by triplet energy transfer from ³BP*, occurs to BP to yield the hydroxynaphthylamine radical cation (HORNH^˙+₂) and the benzophenone ketyl radical (ĊOH) with efficiencies of 0.92 and 0.48 in the presence of 0.015 and 0.5 mol dm^–3 H₂SO₄, respectively. The decay rate of ³HORNH^+*₃(k_obs) decreases with increasing acid concentration, approaching a constant value at higher acid concentrations. This behaviour of k_obs at higher acid concentrations cannot be explained by the HT mechanism alone for the naphthylammonium ion (RNH⁺₃)–BP system previously reported (S. Kohno, M. Hoshino and H. Shizuka, J. Phys. Chem., 1991, 86, 1297). With an increase of [BP], k_obs increases showing a levelling off at higher [BP], which indicates the formation of a triplet exciplex between ³HORNH^+*₃ and BP. The HT mechanism for the HORNH⁺₃–BP system was interpreted as a composite mechanism. The best-fit kinetic parameters were found to be k_o= 5.0 × 10⁵ s^–1, k_HT=k_e1= 10⁷ s^–1, K₁= 5 × 10² dm³ mol^–1, K′₂= 3 dm³ mol^–1 and k′_d/k_b= 6 × 10² dm³ mol^–1. It is concluded that (i) the hydrogen atom of the NH⁺₃ group (which is more protic than that of OH) is the more reactive for the HT reaction in the present system and (ii) the proposed composite mechanism including two different conformations of the protonated triplet exciplexes, ³(HORNH⁺₃⋯[graphic omitted]OH)* and ³(H⁺₃NROH⋯[graphic omitted]OH)*, is involved in the present HT.