7357-93-9Relevant articles and documents
Roles of Surface Protanation on the Photodynamic, Catalytic, and Other Properties of Polyoxometalates Probed by the Photochemical Functionalization of Alkalies. Implications for Irradiated Semiconductor Metal Oxides
Renneke, Roman F.,Kadkhodayan, Miryam,Pasquali, Marzia,Hill, Craig L.
, p. 8357 - 8367 (2007/10/02)
This paper further addresses the photodynamic and redox properties of polyoxometalates and the legitimacy of these compounds as discrete molecular representations for semiconductor metal oxides. The effect of protonation on redox, photochemical, catalytic, and other basic properties of a representative polyoxometalate, decatungstate (W10O324-), in aprotic media has been examined in detail. Protonation results in minimal perturbation of the electronic absorption spectral features of W10O324- including the HOMO-LUMO gap (band gap in semiconductor formalism) but a shift of ～ 1 V in the ground-state redox potentials (two quasi-reversible one-electron waves at -1.2 and -1.8 V become one quasi-reversible two-electron wave at -0.1 V vs Ag/AgNO3). The quantum yields (Φ) for the photooxidation of several alkanes by W10O324- (homogeneous reactions in CH3CN solution, 25 °C, Ar atmosphere, 322-nm light) all increase substantially upon protonation, with the increases dependent on alkane (ΦH+/ΦnoH+ ≈ 6.9 for cyclooctane (least increase) to ～ 25 for most alkanes). The alkane oxidation products in these processes also change upon protonation from those largely derived from freely diffusing alkyl radical intermediates to those largely derived from carbocation intermediates. The EPR spectra (X band) obtained from photooxidation of alkanes by W10O324- in the absence and presence of acid in frozen CH3CN glasses at 10 K and at 25 °C along with evidence from UV-visible spectra and oxidative titration data establish that the one-electron (EPR-active, S = 1/2) and two-electron (EPR-silent, S = 0) reduced forms of decatungstate, W10O325- and W10O326-, respectively, are produced in a 3:7 mol ratio in the absence of acid while the two-electron reduced form is produced exclusively in the presence of acid. EPR spectra as a function of irradiation time and as a function of acid concentration further establish that the two forms of the one-electron-reduced decatungstate present under acidic conditions are W10O325- and HW10O324-. The quantum yields, organic product distributions, polyoxometalate product distributions, UV-vis spectra, and ground-state redox potentials have all been assessed as a function of the number of equivalents of acid added per equivalent of W10O324-. All change monotonically with added acid, are well correlated, and attain a limiting behavior when ～ 2.5 equiv of acid has been added. The addition of W10O324- to acidified CH3CN solutions of branched alkenes suppresses acid-catalyzed alkene isomerization, providing another independent confirmation of W10O324- protonation. Protonation significantly decreases the rate of reduced decatungstate reoxidation by dioxygen in line with the effect of protonation on the ground-state redox potentials. The rate laws of the protonated and unprotonated forms are very similar: Both are variable order in W10O324-, apparently first order in substrate, and first order in light intensity. A rate law consistent with all the data that covers both protonation phenomena and the important coupled thermal process of electron-transfer oxidation of intermediate radicals has been formulated. The ratios of the rate constant for total deactivation of the excited state to the specific rate constant for alkane photooxidation by the excited state, assessed by double reciprocal plots, are 0.86 and 3.4 for the acidified and nonacidified reactions, respectively. The ratios of the rate constant for radiationless decay to that specifically for alkane photooxidation are 0.095 and 1.63 for the acidified and nonacidified systems, respectively.
Isomerisation des radicaux insatures. III. Radicaux α,α,β-, α,β,γ- et α,α,γ-trimethallyles
Deslauriers, Helene,Collin, Guy J.
, p. 3168 - 3173 (2007/10/02)
α,α,β-, α,β,γ-, and α,α,γ-trimethallyl radicals have been generated in the 147.0-nm gas phase photolysis of 2,3,3-trimethyl-1-butene, 3,4-dimethyl-2-pentene, and 2,4-dimethyl-2-pentene, respectively.Under these conditions, the majority of allyl radicals have an internal energy sufficient for further decomposition: they give rise to the formation of various 1,3-dienes and small amounts of either 1,2- or 2,3-dienes.An internal sigmatropic 1,2-hydrogen atom transfer process is part of the proposed mechanism to explain such products.Moreover, the fragmentation of the trimethyl substituted allyl radicals involves the split of one β(C-C) bond, then one β(C-H), and, to a lesser extent, one central C-CH3 bond.
CYCLIZATION OF C7-ALKANES OVER Pt BLACK CATALYST
Zimmer, H.,Paal, Z.,Tetenyi, P.
, p. 513 - 532 (2007/10/02)
C6-and C5-cyclization of heptane isomers (and also, olefin formation as a related process) over Pt-black have been studied in pulse and circulation systems.Hydrogendeficient conditions favour aromatization, via presumably terminal olefins.C5-Cyclization in the presence of more hydrogen is accompanied by internal olefin formation.Relative reactivities of all heptane isomers have been measured; this shows that cyclization is easier between terminal methyl groups.Optimum hydrogen pressures for both types of cyclization have been determined (and compared with hydrogenolysis, too).Earlier mechanism suggestion for aromatization and cyclopentane formation have been confirmed; the distinction between two types of bond shift mechanisms producing aromatics (from substituted pentanes) and saturated isomers, respectively, has recieved additional support facilitating the identification of these two reactions with mechanisms proposed in the literature.
