Roth H, Clark T, Kisch H (1999)
Publication Status: Published
Publication Type: Journal article
Publication year: 1999
Publisher: Wiley-VCH Verlag
Book Volume: 5
Pages Range: 208-217
Journal Issue: 1
DOI: 10.1002/(SICI)1521-3765(19990104)5:1<208::AID-CHEM208>3.0.CO;2-0
The mechanism of the photodehydrodimerization of 2,5-dihydrofuran (2,5-DHF) by suspended zinc sulfide powders was investigated through emission, inhibition, adsorption, and quantum yield studies. Zinc and cadmium ions influenced the emission intensity only marginally hut strongly inhibited the reaction, so the photoreactive surface sites were not identical with the emitting states. which had lifetimes in the 0.1-24 ns range. Adsorption isotherms for these metal ions and 2,5-DHF in aqueous solution indicated the presence of mono- and multilayer adsorption. Zn2+ and Cd2+ were both adsorbed physically and by metal sulfide precipitatation, hut in the case of Cd2+ a lattice exchange mechanism with ZnS afforded CdS and dissolved zinc ions also, C-13 NMR spectra and the good agreement between the calculated number of zinc sites and the measured amount of 2,5-DHF in the saturated solvent-solute surface monolayer indicated that 2,5-DHF was adsorbed perpendicular to the: surface at all the available zinc sites,The true adsorption constant (170 +/- 30 L mol(-1)) was consistent with the pseudo-constant (260+/-50 L mol(-1)) obtained from the concentration dependence of the reaction rate, C-13 NMR signals of the alpha-carbon atoms of 2,5-DHF and THP adsorbed onto ZnS from the gas phase were downfield shifted by 1.5 and 8.7 ppm as compared to those of the free substrates. respectively. The downfield shift of the olefinic signals was about 0.4 ppm. it is postulated that a dissociative electron transfer from adsorbed 2,5-DHF to the reactive hole afforded a proton and the dihydrofuryrl radical. The corresponding C-H bond dissociation energies were calculated by ab initio methods for various substrates. As expected, the apparent quantum yield of various substrates increased linearly with decreasing C-H bond dissociation energy. Tnc: intermediate dihydrofuryl radical dimerized to the products in the adsorbed state, as indicated bit the linear increase of the square root of the reaction rate with increasing 2,5-DHF surface concentration and by competition experiments with THF/?5-DHF mixtures. The reaction inhibition by Zn2+ and Cd2+ could be analyzed in terms of the Stern-Volmer model only when the surface concentration was considered. Inhibition by Cd2+ is about three times faster than inhibition by Zn2+, in which case no zinc metal was observed.
APA:
Roth, H., Clark, T., & Kisch, H. (1999). Heterogeneous photocatalysis, part XIX - Semiconductor type A photocatalysis: Role of substrate adsorption and the nature of photoreactive surface sites in zinc sulfide catalyzed C-C coupling reactions. Chemistry - A European Journal, 5(1), 208-217. https://doi.org/10.1002/(SICI)1521-3765(19990104)5:1<208::AID-CHEM208>3.0.CO;2-0
MLA:
Roth, Henryette, Timothy Clark, and Horst Kisch. "Heterogeneous photocatalysis, part XIX - Semiconductor type A photocatalysis: Role of substrate adsorption and the nature of photoreactive surface sites in zinc sulfide catalyzed C-C coupling reactions." Chemistry - A European Journal 5.1 (1999): 208-217.
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