Interaction of halide and pseudohalide ions with triplet benzophenone-4-carboxylate: kinetics and radical yields

Abstract

The interaction between triplet benzophenone-4-carboxylate (3BC) and some inorganic anions, X", has been studied by laser flash photolysis over a wide range of anion concentration (up to 6 M), at pH 11 and constant ionic strength. Spectra and extinction coefficients of 3BC and BC1 89" radical were determined by using the benzophenone actinometer. Triplet lifetime measurements gave primary interaction rate constants, kf-(M_1 s'1), close to diffusion controlled for iodide, thiocyanate, and azide (k ~2 X 12109), somewhat less for bromide (2 X 108) and very low for chloride (<106). Radical yields, <pr, as /[X"], pass through a broad minimum around [X-] = 0.01 M, which is close to zero for I", Br", and SCN" and small but finite (-0.015) for N3". At [X"] > 0.1 M, <¿-r increases with [X"] and equivalent amounts of BC'" and X2" radicals are formed, including N6~. At high anion concentrations, plots of versus [X"]"1 are linear. Limiting <pf°values, extrapolated to [X"]"1 = 0, range from 0.60 for SCN" to 0.06 for N3". These results are interpreted, as in our earlier work on anthraquinonesulfonate, in terms of trapping a primary triplet exciplex by a second anion at high concentration to form a termolecular exciplex with low orbital momentum, which may dissociate to the observed radical products (intraradical spin-orbit coupling model). Competitive deactivation processes also occur from these exciplexes and possibly from other high-concentration interactions as well. The overall picture is supported by experiments with mixed anions (SCN" and Cl") that permit separation of the primary and secondary processes and establish that the initial reacting species is the lowest ketone triplet.

Photoinduced oxidation of sea salt halides by aromatic ketones: a source of halogenated radicals

“…The k q values are in reasonably good agreement with the values obtained by Hurley et al (1988) and Shizuka et al (1982) in aqueous and acetonitrile media. The difference (20-50%) is probably due to an ionic strength effect (Douglas et al, 1990;Morrison et al, 1996) or to a solvent effect (Burget and Jacques, 1998;Gutierrez, 2008).…”

“…The errors given as 95% confidence intervals estimated from the data fits. (Murov et al, 1993) G ET (eV) IpH=5.5 water pH=11 water acetonitrile/water (8.2±0.8)×10 9 this work 2.3×10 9 (Hurley et al, 1988) 3.5×10 9 (Shizuka and Obuchi, 1982) (…”

“…After 8 µs when the triplet benzophenone is relaxed, the absorption at 350 nm is still apparent and long-lived (with a lifetime of ca. 15±0.5 µs) and may correspond to bromine radical anion Br •− 2 (Ershov and Janata, 2003;Ershov et al, 2002;Grodkowski and Neta, 2002;Hurley et al, 1988;Zehavi and Rabani, 1972). This radical anion is known to absorb light from 300 to 400 nm with maximum absorption at 355 nm.…”

“…The implications of the presence of such light-absorbing organics and the possible photochemical reactions are not yet well understood in the sea surface microlayer. But clearly, the presence of such photoactive compounds at the air/sea interfaces could give rise to photosensitized processes (Becker et al, 1995;Canonica et al, 2006Canonica et al, , 2000Canonica et al, , 2005Das et al, 1981;Ivanov et al, 2005;Lathioor and Leigh, 2001;Lathioor et al, 1999;Leigh et al, 1996) which may also affect chemistry in the marine boundary layer (MBL) (Reeser et al, 2008). Previously, we showed that chlorophyll may be a photochemical source of halogenated radicals or the driving force for ozone deposition at low wind speed (Clifford et al, 2008).…”

Relaxation of Electronic Angular Momentum in Kramers Systems with Strong Spin-Orbit Coupling. 1. Atomic Radicals in Solution*

“…An important role of unquenched orbital momentum has also been demonstrated in electron transfer ·reactions between triplet excited organic molecules and inorganic anions [14][15][16]. It was pointed out that the quantum yield of free radical formation reflects the degree of "intra radical sac" (lRSaC [15]) that is particularly pronounced in linear radicals (e. g. it is apparent in the linear N 3radical but not in the bent Na i radical) and in atomic radicals (I', Br', Cl').…”