Carbon dioxide Charge-transfer complexes

It is assumed that an excited state charge transfer complex is formed between the nitroaromatic in its first triplet state and the respective substrate. Internal proton transfer is immediately followed by hberation of carbon dioxide. Finally hydrolysis of the hemiacetal Ar —X—CH2OH (X = NH or S) leads to 2-chloro-aniline or thiophenol, respectively. In the decarboxylation of a-phenylthio-acetic acid, some methyl-phenylsulfide is also formed. (7t,7r )-nitroaromatics are more reactive than nitro compounds with lowest (n,7t )-triplets iso). 

Treatment of thietane with electron acceptors, such as tetracyanoquino-dimethane (TCNQ), tetracyanoethylene (TCNE), maleic anhydride, or tetra-nitromethane, induces polymerization, believed to occur through an intermediate charge-transfer complex. The reaction with tetranitromethane is unusual because nitric oxide, nitrogen dioxide, carbon monoxide, ethylene, ethane, and propane are... 

The involvement of 1 1 complexes in propagation can occur in many ways. Combinations of an electron donor monomer, such as a vinyl ether or an olefin, and an electron-accepting monomer, such as maleic anhydride, carbon dioxide or sulphur dioxide, are thought to give rise, sometimes spontaneously, to alternating copolymers via a binary charge-transfer complex (CT) intermediate (3). 

Maleic anhydride and hexamethylbenzene form a 1 1 charge-transfer complex in methylcyclohexane solvent with a UV maximum at 340 tx. Ultraviolet irradiation of these solutions brings about the formation of carbon dioxide, pentamethylbenzylsuccinic anhydride, and resinous products (see Chapter 6). Analytical data suggest the polymeric material has a decarboxylated MA building block. Charge-transfer complexes were not detected in solid-state mixtures of MA and pentamethyl-benzene. Ultraviolet irradiation of these solid mixtures gives cyclic dimer, 1,2,3,4-cyclobutanetetracarboxylic anhydride 2, and resinous products. Pentamethylbenzylsuccinic anhydride, due to the possible absence of charge-transfer complexes, was not isolated from the solid-state photoreaction. 

There are various potential applications of photophysical phenomena in analytical chemistry. The relatively short lifetimes of most excited states, however, is a serious drawback to the construction of practical devices but studies which focus on finding ways to extend triplet lifetimes have now been described by Harriman et al. Kneas et al. have examined new types of luminescent sensor on polymer supports, and both Neurauter et al. and Marazuela et al. have designed sensors based on the ruthenium(II) polypyridine complex for the detection of carbon dioxide. A system, based on the formation of twisted intramolecular charge transfer states, has been devised for measuring the molecular weight of polymeric matrices (Al-Hassan et a/.), and the chemical reactivity at the interface of self-assembled monolayers has been assessed using fluorescence spectroscopy (Fox et al). 

There has been a growing interest in the utilization of CO2 as a potential Cl source for chemicals and fuels to cope with the predictable oil shortage in the near future. Insertion reactions of CO2 into M-H, M-0, M-N, and M-C bonds are well documented, where these reactions are explained in terms of the electrophilicity of CO2 il, 2). Catalytic syntheses of lactones (3-9) and pyrones (10-16) are also established by incorporation of CO2 into dienes and alkynes activated on low-valent metal complexes. Carbon dioxide shows only an electrophilicity under usual reaction conditions, but it exhibits a nucleophilicity upon coordination to low-valent metals because of the intramolecular charge transfer from metals to CO2. Metal-C02 formation may be the key species in electro- and photochemical CO2 reductions. Since the first characterization of [Ni(PCy3)2(T) (C,0)-C02)] (17), a variety of metal... 

A complex interplay of elementary surface processes includes (i) molecular adsorption, (ii) surface diffusion, (iii) charge transfer, (iv) recombination of adsorbed species, and (v) desorption of reaction products. These processes determine observable rates of reactions in PEFCs, that is, reduction of oxygen and oxidation of hydrogen, methanol, or carbon dioxide. 

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