Photochemical homogeneous

Hill, C. L. Renneke, R. F. Faraj, M. K. Sustained Thermal and Photochemical Homogeneous Catalytic Functionalization of Hydrocarbons by Polyoxometalates. In The Role of Oxygen in Chemistry and Biochemistry, Ando, W., Moro-Oka, Y. Eds. Elsevier New York, 1988 pp 185-191. 

Another claim of a thermal and photochemical homogeneous (or topotactic) polymerization of the diacetylene 30 a (Scheme 2.1.9) is questionable. The claim had to be modified several times, it was admitted later that the polymorph required crystal solvent (e.g. 0.5 dioxane) [57], and 30 a was no longer discussed in the review of the same research group [58] that dealt with 30 b and listed 32 further diacetylenes. First order reaction was claimed in order to try to substantiate topochem-ical behavior [58]. However, inspection of the published kinetic curves indicates zero order up to 90 % conversion after an induction period instead of first order for... 

As was discussed earlier, ozone plays an important part in the chemistry of the troposphere, where its excess is harmful, and in stratospheric chemistry, where its shortage is also detrimental. Ozone can decompose by several mechanisms thermal, photochemical, homogeneous catalysis reactions and under the action of solid surfaces. In the laboratory, the latter effect can be controlled by a suitable treatment of the reactor walls, as well as by a study of the rate of reaction as a function of the surface/volume ratio. In order to eliminate photochemical and homogeneous catalysis reactions, the chemical reaction must be carried out in the absence of radiations and catalytic additives, such as halogenated substances. The mechanism put forward to interpret the thermal reaction, can be written as follows ... 

Fig. 7-11 Compilation of the most important photochemical processes in the atmosphere, including estimates of flux rates expressed in moles per year between the earth s surface and the atmosphere and within the atmosphere. (Modified with permission from P. J. Crutzen, Atmospheric interactions - homogeneous gas reactions of C, N, and S containing compounds. In B. Bolin and R. Cook (1983). "The Major Biogeochemical Cycles and Their Interactions," pp. 67-112, John Wiley, Chichester.)...

Garratt and Thompson [J. Chem. Soc., 1934, 524, 1817, 1822] have studied the photochemical and thermal decomposition of nickel tetra-carbonyl. Later work by Day, Pearson and Basolo [J. Am. Chem. Soc., 90 (6933), 1968] confirmed that the rate law postulated by Garratt and Thompson was obeyed for the homogeneous process. The mechanism postulated by both groups is ... 

A rough, but useful, comparison between typical sonochemical and photochemical efficiencies is shown in Table I. As shown, homogeneous sonochemistry is typically more efficient than photochemistry, and heterogeneous sonochemistry is several orders of magnitude better. Unlike photochemistry, whose energy inefficiency is inherent in the production of photons, ultrasound can be produced with nearly perfect efficiency from electric power. Still, a primary limitation of sonochemistry remains its... 

In 1981, the first report on the sonochemistry of discrete organometallic complexes demonstrated the effect of ultrasound on iron carbonyls in alkane solutions (174). The transition metal carbonyls were chosen for these initial studies because their thermal and photochemical reactivities have been well characterized. The comparison among the thermal, photochemical, and sonochemical reactions of Fe(CO)5 provides an excellent example of the unique chemistry which homogeneous cavitation can... 

CH ligands, (2) to initiate homogeneous catalytic reactions such as hydrogenation, hydroformylation, and the water gas shift reaction, and (3) to study the mechanism of thermal reactions by the photochemical preparation of possible intermediates. 

Nickel(II) acetylaeetonate has been recommended as a very efficient homogeneous catalyst for intramolecular cyclopropanations for unsaturated diazoketones 171 The yields were better than with activated CuO as catalyst (see Table 10 for examples). The authors of this study seem to combine routinely thermocatalytic with photochemical (tungsten lamp) decomposition of the diazoketones. The benefit of this procedure (higher yields, shorter reaction times) has been communicated in the CuO case, but not for the Ni(acac)2-eatalyzed reaction. 

One possible strategy in the development of low-overpotential methods for the electroreduction of C02 is to employ a catalyst in solution in the electrochemical cell, A few systems are known that employ homogeneous catalysts and these are based primarily on transition metal complexes. A particularly efficient catalyst is (Bipy)Re[CO]3Cl, where Bipy is 2,2 bipyridine, which was first reported as such by Hawecker et al. in 1983. In fact, this first report concerned the photochemical reduction of C02 to CO. However, they reasoned correctly that the complex should also be capable of catalysing the electrochemical reduction reaction. In 1984, the same authors reported that (Bipy)Re[C013CI catalysed the reduction of C02 to CO in DMF/water/ tetraalkylammonium chloride or perchlorate with an average current efficiency of >90% at —1.25 V vs. NHE (c. —1.5V vs. SCE). The product analysis was performed by gas chromatography and 13C nmr and showed no other products. 

It has been our goal for some time to run photochemical energy storage reactions without relay molecules or separate catalysts. We have concentrated on the photochemistry of polynuclear metal complexes in homogeneous solutions, because we believe it should be possible to facilitate multielectron transfer processes at the available coordination sites of such cluster species. 

Photochemical reactions are usually run in homogeneous solutions notwithstanding it is also possible to irradiate solid compounds directly. Examples of such reactions on a preparative scale 705) as well as a discussion on crystal lattice control on photoreactions 706) are found in the literature. Finally, specific effects of a micellar environement is also being used in photochemical reactions of preparative purposes707). 

The kinetics of the thermally induced homogeneous decomposition of phosphine (PH3) have not yet been studied. The species PH2, PH and P2 are formed on flash photolysis of PH3 and could be identified by their absorption spectra63. There are proposals as to the mechanism of the consecutive process after the photochemical primary step, but nothing is known about the kinetic parameters of these reactions. With arsine and antimony hydride only the heterogeneous decomposition has been studied64,65. 

The preceding sections dealt with thermal heterogeneous and homogeneous reductive and oxidative cleavages. These reactions may also be triggered photochemically by means of a sentitizer as pictured in Scheme 3.7 in the... 

The rate expression given by Eq. (10.10) applies generally for homogeneous and for heterogeneous photochemical reactions, e.g. for heterogeneous photoredox... 

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