Carbon dioxide, quantum yield

DPA) in dimethylphthalate at about 70°, yields a relatively strong blue Umax =435 nm) chemiluminescence the quantum yield is about 7% that of luminol 64>. The emission spectrum matches that of DPA fluorescence so that the available excitation energy is more than 70 kcal/mole. Energy transfer was observed on other fluorescers, e.g. rubrene and fluorescein. The mechansim of the phthaloyl peroxide/fluorescer chemiluminescence reaction very probably involves radicals. Luminol also chemiluminesces when heated with phthaloyl peroxide but only in the presence of base, which suggests another mechanism. The products of phthaloyl peroxide thermolysis are carbon dioxide, benzoic acid, phthalic anhydride, o-phenyl benzoic acid and some other compounds 65>66>. It is not yet known which of them is the key intermediate which transfers its excitation energy to the fluorescer. 

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

Titanium dioxide suspended in an aqueous solution and irradiated with UV light X = 365 nm) converted benzene to carbon dioxide at a significant rate (Matthews, 1986). Irradiation of benzene in an aqueous solution yields mucondialdehyde. Photolysis of benzene vapor at 1849-2000 A yields ethylene, hydrogen, methane, ethane, toluene, and a polymer resembling cuprene. Other photolysis products reported under different conditions include fulvene, acetylene, substituted trienes (Howard, 1990), phenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,6-dinitro-phenol, nitrobenzene, formic acid, and peroxyacetyl nitrate (Calvert and Pitts, 1966). Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of phenol and nitrobenzene (Atkinson, 1990). Schwarz and Wasik (1976) reported a fluorescence quantum yield of 5.3 x 10" for benzene in water. 

Photolytic. Dalapon (free acid) is subject to photodegradation. When an aqueous solution (0.25 M) was irradiated with UV light at 253.7 nm at 49 °C, 70% degraded in 7 h. Pyruvic acid is formed which is subsequently decarboxylated to acetaldehyde, carbon dioxide, and small quantities of 1,1-dichloroethane (2-4%) and a water-insoluble polymer (Kenaga, 1974). The photolysis of an aqueous solution of dalapon (free acid) by UV light (X = 2537 A) yielded chloride ions, carbon dioxide, carbon monoxide, and methyl chloride at quantum yields of 0.29, 0.10, 0.02, and 0.02, respectively (Baxter and Johnston, 1968). 

The photolysis of trifluoroacetone with light of wavelength 3130 A. has been studied by Sieger and Calvert.48 The products of decomposition were shown to be carbon monoxide, methane, ethane, 1,1,1-trifluoro-ethane, and hexafluoroethane. There was no indication of the presence of carbon tetrafluoride or methyl fluoride. The quantum yield of all products was low at low temperature and it is assumed that the excited molecule of trifluoroacetone has an appreciable lifetime and may be deactivated by collision before decomposition can occur. This contention is supported by the decrease in the quantum yields observed when foreign gases such as carbon dioxide are added, and by the fall in quantum yields with increase in trifluoroacetone concentration. 

The photochemical processes of triatomic molecules have been extensively studied in recent years, particularly those of water, carbon dioxide, nitrous oxide, nitrogen dioxide, ozone, and sulfur dioxide, as they are important minor constituents of the earth s atmosphere. (Probably more than 200 papers on ozone photolysis alone have been published in the last decade.) Carbon dioxide is the major component of the Mars and Venus atmospheres. The primary photofragments produced and their subsequent reactions are well understood for the above-mentioned six triatomic molecules as the photodissociation involves only two bonds to be ruptured and two fragments formed in various electronic states. The photochemical processes of these six molecules are discussed in detail in the following sections. They illustrate how the knowledge of primary products and their subsequent reactions have aided in interpreting the results obtained by the traditional end product analysis and quantum yield measurements. 

Selective carbon dioxide reduction to CO has been accomplished in a non-aqueous medium that includes tricarbonyl (2,2 -bipyridinium) rhenium , /ac-Re(bpy) (CO)3X (X=Cl, Br) as light-active component and homogeneous catalyst for C02 reduction [183-185]. In dimethylformamide solutions that include TEOA as sacrificial electron donor, photosensitized reduction of C02 to CO proceeds with a quantum efficiency of

Mechanistic investigations have revealed that reductive ET quenching of the rhenium complex (Eq. (54)) yields the catalytic intermediate active in deoxygenation of C02. It has been suggested that carbon... 

In Fig. 30 it is seen that the effect with carbon dioxide is nearly proportional to the pressure. The reduction in quantum yield is due to the removal by collision of the excess energy of the excited nitrogen dioxide molecules before they can collide with other molecules of nitrogen dioxide and effect a chemical reaction. The energy is removed in the form of extra energy given to the colliding molecules, and thus converted into heat. 

Fig. 30.—Effect of carbon dioxide in reducing the quantum yield < N2°6 (corrected for screening by nitrogen tetroxide) A—4,050, O—3660, 0-3,130 A.

After an investigation of several years with the same type of algae and approximately the same intensity of monochromatic light as used by Warburg we are forced to conclude that the photosynthetic process is considerably less efficient than the 0.2 or 0.3 which is now accepted.63 With the monochromator already described and with direct, microchemical analysis of the gas or by titration for oxygen we find a quantum yield of about 0.05 changing somewhat with the conditions. Under our conditions ten to twenty quanta instead of the classical four seem to be necessary to convert one molecule of carbon dioxide into plant material. 

Coating the vessel with potassium chloride eliminated the chain reactions and simplified the kinetics. It was found that the quantum yields of hydrogen, carbon monoxide, and formic acid decreased with an increase in oxygen pressure in both coated and clean vessels. The quantum yield of carbon dioxide was large, ca. 3.0, variable (and, therefore, presumably heterogeneous) in the coated vessel, but in the clean vessel it increased with the oxygen pressure (Fig. 7). 

Hoare68 using 3130 A. at 120 and 200°C showed that the quantum yield of carbon monoxide decreased and that of carbon dioxide increased with the oxygen pressure. The presence of 130 mm. of inert gas had little effect on the photooxidation. 

The photooxidation has been studied by Blacet,17 who found at room temperature using 3130 A. radiation and oxygen pressures of 10 to 100 mm. that the products were carbon monoxide, carbon dioxide, water, acetaldehyde, ethanol, and propionic acid. The quantum yields of water, acetaldehyde, and ethanol were 9,3, and 3, respectively. 

Capture of light energy. Nature captures light energy and uses it for the reduction of carbon dioxide on a colossal scale, albeit with a rather low quantum yield. It is an important objective to find simple chemical systems that can perform similarly one way to imitate Nature in this respect is to utilize... 

Reduction in the quantum yield for the formation of methane resulting from the addition of inert foreign gases such as helium, neon, argon, nitrogen and carbon dioxide supports this suggestion of a hot radical mechanism, as does the observation by Harris and Willard that methane formation is enhanced at short wavelengths (1849 A). Souffle et have also proposed some ethane formation from the reaction of hot radicals by... 

In a preliminary study (284) on the photolysis of the t-butyl methyl ether-02 CT-complex some products have been identified and their quantum yields determined. They are peroxidic compounds (4i = 0.15), t-butyl formate (cj) = 0.21), t-butanol ((j> = 0.035), 2-methoxy-2-methylpropionaldehyde ( = 0.03), formaldehyde ((j> = 0.04), water (iji = 0.3), and carbon dioxide (cj) = 0.007). It was noted (284) that deprotonation of the t-butyl methyl ether radical cation, although it largely occurs at the methyl group next to the oxygen (product, t-butyl formate), is also possible at the 8-posltlon (product, 2-methoxy-2-methylpropionaldehyde). 

The challenging photochemical reduction of carbon dioxide to formate is catalyzed by Ru" [111] (cf. Section 3.3.4). For example, with the 2,2 -bipyridine-ruthenium(II) complex the active species is formed by photolabilization. Water renders the system more efficient with quantum yields up to 15%. Methanol is the photoproduct when CO2 is reduced with Ti02 in propene carbonate/2-propanol... 

The exact mechanism of activation of carbon dioxide is still unrevealed. Great research efforts are needed to overcome this bottleneck problem in terms of both experimental and computational research. Quantum mechanical modelling gives a better insight to the activation mechanism. Once one identifies the appropriate material which will adsorb even gaseous carbon dioxide and transfer it into the corresponding radical anion, yield products with greater than 10% silver bottom efficiency, in a similar way of ammonia synthesis, then the rest becomes history. 

The conversion of naphthalene to 2-naphthoic acids by irradiation with carbon dioxide and electron donors (e.g. amines or dimethoxybenzene) has been further investigated and the quantum yields of the reaction measured for different solvents and donors. Electron transfer also occurs in the photochemical phosphonation of naphthalene and phenanthrene achieved by irradiation with trialkyl-phosphites and electron acceptors such as 1,3-dicyanobenzene. The photonitration of phenol in aqueous solutions of nitrate ion has been reported and phenols have been prepared by irradiation of substituted benzenes with the aromatic N-oxide (132). ... 

Hydrogen is evolved in the photolysis of poly-a-methylstyrene with a quantum yield of 1.7 x 10-2, together with some carbon dioxide and carbon monoxide. This indicates that there are some bonded oxygen atoms in the polymer and it has been suggested that degradation could be due to the scission of a few weak bonds situated at random in the polymer chain [10]. No mention is made of the evolution of methane as a consequence of methyl group abstraction. It has been noticed that a yellow colour is produced on long exposure and that this remains even when the polymer has been reprecipitated [10]. 

Volatile products are evolved during the photolysis of polymethylmethacrylate. By far the greatest proportion of low molecular weight fragments consists of carbon monoxide, methyl formate and methanol [12, 72]. The last two products are formed with quantum yields of 0.14 and 0.48, respectively [12]. Methane, hydrogen and carbon dioxide are also produced [12]. Except for hydrogen, all the volatiles have been... 

The photochemistry of polyethylacrylate irradiated in vacuo at 253.7 nm is similar to that of polymethylacrylate crosslinking and scission of the polymer chains occur simultaneously [84]. Although absolute values of the quantum yields of these reactions have not been determined, solubility data indicate that both processes occur with the same quantum yield, as was observed for the photolysis of polymethylacrylate. Hydrogen, methane, carbon monoxide and dioxide are produced in the photolysis [85]. 

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