Photochemical decomposition mechanism

To improve the performance of photodegradable plastics, it is necessary to understand the photochemical decomposition mechanisms so that appropriate features and properties can be incorporated into the polymers to make... 

For operation at telecommunication wavelengths, one must be concerned with two-photon excitation processes leading to photochemical decomposition mechanisms. Femtosecond pulse techniques, such as those described by Stro-hkendl et al. [185], prove useful in evaluating two-photon excitation cross sections and excited state energy transfer processes relevant to photochemical decay pathways. 

Pacansky and J.R. Lyerla, Photochemical decomposition mechanisms for Az yype photore sists, IBMJ. Res. Dev. 23, 42 (1979). 

Some examples on photodegradative processes of different photochemical decomposition mechanisms of multicomponent based polymeric materials, either made photodegradable via incorporated specific entities or not, are further depicted in this chapter. 

The kinetics and mechanism of the thermal and photochemical decomposition of dialkyldiazenes (15) have been comprehensively reviewed by Engel. The use of these compounds as initiators of radical polymerization has been covered by Moad and Solomon2 and Sheppard.50 The general chemistry of azo-compounds has also been reviewed by Koga et cr/./11 Koenig,3 and Smith.3J... 

The photochemical decomposition of Mes3GeHgCl was studied by Castel and coworkers107. The main photoreaction is the ejection of Hg, and the formation of Mes3GeCl, presumably via an intramolecular mechanism. A number of other reactions are observed, however, all occurring via formation of Mes3Ge-. The photochemistry is summarized in Scheme 28, with the reactions being followed by E.S.R. and N.M.R. spectroscopies. 

Photochemical decomposition can also be carried out in the presence of a suspension of photoactive material such as Ti02 where substrate absorption onto the uv activated surface can initiate chemical reactions e. g. the oxidation of sulphides to sul-phones and sulphoxides [37]. This technology has been adapted to the destruction of polychlorobiphenyls (PCB s) in wastewater and is of considerable interest in environmental protection. Using pentachlorophenol as a model substrate in the presence of 0.2 % TiOj uv irradiation is relatively efficient in dechlorination (Tab. 4.5) [38]. When ultrasound is used in conjunction with photolysis, dechlorination is dramatically improved. This improvement is the result of three mechanical effects of sonochemistry namely surface cleaning, particle size reduction and increased mass transport to the powder surface. 

According to Bowden and Yoffe [116] other possible mechanisms should be considered, including direct photochemical decomposition. A number of experiments have been carried out by members of the Bowden school. 

These authors suggest the following mechanism for the initiation of photochemical decomposition of azides ... 

The activation energy was found to be 3.2 0.1 kcal/mole. The suggested mechanism of photochemical decomposition is as follows ... 

Diazomethane. Although CH2N2 photolysis has been widely used as a methylene source, the reaction of CH2 with CH2N2 has received little attention. The quantum yield in the photochemical decomposition of CH2N2 is about four at both 4360 A. and 3650 A.96 The mechanism is believed to involve a short energy chain, in addition to the simple process... 

Photochemical decomposition of ethyl azidoformate in cyclohexene gives 7-carbethoxy-7-azabicyclo[4.1.0]heptane (108) as main product (56% at 38° and 75% at —75°) together with the three isomeric cyclohexenylurethans.82>83M>8 A nitrene mechanism for the reaction is supported by the fact that the same products in almost the same ratio are obtained in the base-induced cleavage of N-p-nitrobenzenesulfonoxyurethan (107). All evidence indicates that the second reaction yields the carbethoxynitrene by an a-elimination mechanism. The aziridine isomerizes at 140° into 2-ethoxy-4,5-cy clo hexano-A2-oxazoline (109). 

The question as to whether the thermal azirine formation proceeds through a vinyl nitrene intermediate or by a concerted mechanism is not as yet resolved. A nitrene intermediate seems most probable on the basis of its similarity with cyclopropene formation from alkenylcarbenesM>> and because either thermal or photochemical decomposition of vinyl azides yields die azirines (Table XI). 

It would be of great interest to know whether the photochemical decomposition of ozone proceeds by the mechanism... 

It is often possible to describe the photoexcitation of a molecule in terms of a localized excitation, e.g., excitation of the 0=0 group of a ketone. In most photochemical decompositions the bond that breaks and leads to fragmentation is not the same as the site of localized excitation.165 Therefore, it is necessary to consider the mechanism of intramolecular energy transfer as part of the photochemical reaction. 

HgS Hgl2, Takei observed microscopic black spots which he attributed to colloidal mercury. Heating facilitates reversion of the crystal to the yellow form which is explained as a vaporization of the Hg deposits. If this mechanism proves to be correct, this process is merely a permanent photochemical decomposition of the complex and not a true photochromic phenomenon. 

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