Initial quantum yields φ for the formation of DMPO-OH adducts in clear, aerated, unbuffered TiO2 sols irradiated at 295 nm were determined by kinetic electron paramagnetic resonance spectrometry as function of the 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap concentration, photon irradiance I o, and added dichlorobenzene DCB, at 298 K. φ steadily increases with [DMPO] in the range 0.3 mM to 0.3 M, extrapolating to φ [DMPO]→∞ = 0.54 ± 0.10, in contrast with the constant yields obtained for the homogeneous generation of OH radicals in the photodissociation of H2O2 at 254 nm under similar conditions. In TiO2 sols at [DMPO] = 0.6 mM, φ (≈0.002) remains constant over a 20-fold variation of Io , but decreases upon addition of comparable DCB concentrations. These observations prove that (1) DMPO reacts with photogenerated holes and/or OH radicals on the surface of TiO2 particles rather than in the solution bulk, (2) it is possible to approach quantitative carrier trapping, (3) DCB is as reactive as DMPO toward some of the photogenerated carriers, and (4) the recombination of a single carrier pairthe usual event in small particles at moderate photon fluxesis a pseudo-first-order process. Photocatalysis in semiconductor sols is a stochastic process not amenable to conventional kinetic analysis based on rate expressions involving continuous concentration variables. Quantum yields are largely controlled by the anodic (h + D → D+), rather than by the slower (e + O2 → O2 •-) cathodic charge transfer reaction.
This has been ascribed to very favorable stereoelectronic factors associated to the chroman ring in stabilizing the derived phenoxyl radical (aT-) [4]. In this connection a quantitative measure of enhanced radical stabilization would be provided by the difference of 0-H bond energies in phenol itself and aTH [5,61:
Values of AH, = (-1 2.4 k 1.6) kJ mol-', AS, = (-18.5 k 5.6) J K-l mo1-l for reaction (3) (see text), corresponding to an 0-H bond energy of 322.2 kJ m0I-l in 1, and to a 14.5 J K-' mol-' entropy loss for the CH,O-Ar (Ar = aromatic) libration in 2 relative to I , are derived from the temperature dependence of the equilibrium constant K,, determined by EPR spectrometry in benzene-toluene media, between 251 and 304 K. These results allow, for the first time, discrimination between enthalpic and entropic effects on the rates of (0) H-atom abstraction by peroxyl radicals from 4-methoxyphenols and related species.It is generally acknowledged that phenolic antioxidants operate by scavenging peroxyl radicals RO, to form stabilized phenoxyls XO, viu relatively fast H-atom transfer reactions. The effect of various aromatic ring substituents on the rate of reaction (1 ), presumably correlating with the thermochemicalstability of the resulting XO radicals, has been explored systematically. Thus, it has been found that alkyl groups, which provide hyperconjugative stability, and a 4-methoxy group, which mixes the SOMO (singly-occupied molecular orbital) with a p-type 0-atom lone pair orbital, increase the value of k , . However, these effects are not strictly additive. As previously noted, the naive application of these rules of thumb leads, for example, to the prediction that a-tocopherol (a-T), the most efficient natural antioxidant, in vivo as well as in vitro, should be as reactive as 4-methoxy-2,3,5,6-tetramethylphenol (MTAP), and only 50% more reactive than 4-methoxy-2,3,6-trimcthylphenol (MTIP) at 300 K, at wide variance with experimental d a t a 3It was argued that stereoelectronic effects and the freezing of the p-alkoxy internal rotation, with the concomitant loss of internal entropy along the reaction coordinate during H-atom abstraction, impeded both in M-T and MTAP, but not in MTIP, could account for the observed discrepancies.2bWe have recently carried out EPR equilibrium measurements in radical buffers by a method that provided a direct value of E,(O-H) = 318 kJ mol-' in a-T (Ed = bond dissociation enthalpy),' only 8.4 kJ mol-' smaller than the similarly substituted MTAP.6 Based on these data, the well established linear free energy correlation (2), which assumes entropies of [E,(XO--H)/kJ mol-'1 = 420.1 -12.85 log (k,/mol-' dm3 spl) (2) reaction and activation independent of the nature of X substituents in reaction ( l),2b,4 yields k,(a-T)/k,(MTAP) = 4.5, instead of the experimental value of 8.2 at 300 K. Thus, this correlation performs poorly even for a-T and MTAP, although thep-substituents remain locked in both cases (albeit in parallel and perpendicular configurations, respectively). It appears that different factors, perhaps more subtle than those considered so far, control the thermochemistry and reactivity of phenolic antioxidants.Since one of the goals of physical organic chemistry is to investigate the effect of substituents on stability and reactivity, and considering the availability of well establ...
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