The quantum yields (fobs) relating to the colloidal TiO2-sensitized photo-oxidation of ring methoxysubstituted benzylic alcohols (D) were determined. The true quantum yields (f0) were obtained from a Langmuir– Hinshelwood isotherm treatment of fobs at different [D]. In the light of the suggested photo-oxidation mechanism, an equation was deduced where f0 is expressed as a function of the electron transfer (ket), back electron transfer (kÿet) and benzylic deprotonation (kp) rate constants. In particular, the lower f0 value of 3-CH3O-vs 4-CH3O-benzyl alcohol (1, with lower Ep) should principally depend on the difference in ket, while the decrease in f0 on going from alcohol 1 to the a-methyl derivative (4) should be due to the difference in kp (stereoelectronic effect). The adsorption equilibrium constants under irradiation (KD) were also obtained from the above equation and the values are similar for the considered substrates, except for 4. In fact, this substrate shows a lower KD value, probably because of the steric hindrance of the methyl group to the OH adsorption (preferential site) on TiO2. Finally, both the inter- and intramolecular deuterium isotope effect (kH/kD = 1.8 and 1.6, respectively) are consistent with a kinetically significant Ca—H bond breaking following the electron-transfer step.
Photo-oxidation of some benzylic alcohols sensitized by colloidal TiO2 in CH3CN. A Kinetic mechanistic study through quantum yield determinations
ROL, Cesare;SEBASTIANI, Giovanni Vittorio;DEL GIACCO, Tiziana;
2000
Abstract
The quantum yields (fobs) relating to the colloidal TiO2-sensitized photo-oxidation of ring methoxysubstituted benzylic alcohols (D) were determined. The true quantum yields (f0) were obtained from a Langmuir– Hinshelwood isotherm treatment of fobs at different [D]. In the light of the suggested photo-oxidation mechanism, an equation was deduced where f0 is expressed as a function of the electron transfer (ket), back electron transfer (kÿet) and benzylic deprotonation (kp) rate constants. In particular, the lower f0 value of 3-CH3O-vs 4-CH3O-benzyl alcohol (1, with lower Ep) should principally depend on the difference in ket, while the decrease in f0 on going from alcohol 1 to the a-methyl derivative (4) should be due to the difference in kp (stereoelectronic effect). The adsorption equilibrium constants under irradiation (KD) were also obtained from the above equation and the values are similar for the considered substrates, except for 4. In fact, this substrate shows a lower KD value, probably because of the steric hindrance of the methyl group to the OH adsorption (preferential site) on TiO2. Finally, both the inter- and intramolecular deuterium isotope effect (kH/kD = 1.8 and 1.6, respectively) are consistent with a kinetically significant Ca—H bond breaking following the electron-transfer step.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
