Primary decomposition quantum yields

It is well-known that the overall quantum yield is near unity at 2537 A however, at 3130 A, it is less than 1 at low temperatures and approaches unity only above 100 C The overall quantum yields at 3130 A and 100 torr acetone pressure are 0.5, 0.7 and 1.0 at 0°, 25°, and 120 °C, respectively. (See further data of Heicklen and Noyes as well as those of Cundall and Davies .) The primary quantum yields can be determined, in principle, by studying the photolysis in the presence of radical scavengers. For such a purpose iodine has been mainly used, as it is known to decrease the value of 4 co to eliminate the formation of bi- 

Gorin ° determined the values of 1.0 (2537 A) and 0.85 (3130 A) for Pc 3i at 80-90 °C he assumed these to be the measure of the primary quantum yield j i. Later investigations also confirmed the value at 2537 A however, a considerably lower value, ch3I 0-2, was reported -at 3130 A and at temperatures of 100 °C and above. At 3130 A, the primary quantum yield is temperature dependent in the presence and absence of iodine Since the primary decomposition quantum yield is known to be near unity at 3130 A and above 100 °c - , one is inclined to interpret the low quantum yields, determined in the presence of iodine, as indications of the quenching effect of iodine. It is the triplet state of acetone which is likely to be quenched by Ij. Some of the results are, however, inconsistent with such a conclusion nevertheless, the quantum yields, determined in the iodine inhibition experiments, should be accepted with reserve. 

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At exciting wavelengths of 4358 A and 4047 A, the luminescence quantum yields as well as the primary decomposition quantum yields are... 

At 3650 A, the efficiency of luminescence increases with increasing biacetyl pressure . This is just the opposite of what would be expected on the basis of the Stern-Volmer relation. Moreover, it was established that the primary decomposition quantum yield decreases with increasing pressure (the plot of Ijcf) versus biacetyl concentration gives a straight line). The results were explained by the assumption that the molecules absorbing radiation of 3650 A are excited to high vibrational levels (of the upper singlet state) from which dissociation can occur, but luminescence cannot. Luminescence can only occur if vibrational excitation is removed by collision. 

After the primary step in a photochemical reaction, the secondary processes may be quite complicated, e.g. when atoms and free radicals are fcrnied. Consequently the quantum yield, i.e. the number of molecules which are caused to react for a single quantum of light absorbed, is only exceptionally equal to exactly unity. E.g. the quantum yield of the decomposition of methyl iodide by u.v. light is only about 10" because some of the free radicals formed re-combine. The quantum yield of the reaction of H2 -f- CI2 is 10 to 10 (and the mixture may explode) because this is a chain reaction. 

The quantum yield of ozone decomposition at 334 nm (L2) is 4, indicating that one of the products must be an excited species capable of decomposing 0 further. The primary process of the 0 photolysis at 334 nm occurs according to the reactions ... 

The choice of new complexes was guided by some simple considerations. The overall eel efficiency of any compound is the product of the photoluminescence quantum yield and the efficiency of excited state formation. This latter parameter is difficult to evaluate. It may be very small depending on many factors. An irreversible decomposition of the primary redox pair can compete with back electron transfer. This back electron transfer could favor the formation of ground state products even if excited state formation is energy sufficient (13,14,38,39). Taking into account these possibilities we selected complexes which show an intense photoluminescence (0 > 0.01) in order to increase the probability for detection of eel. In addition, the choice of suitable complexes was also based on the expectation that reduction and oxidation would occur in an appropriate potential range. 

Preparative photolysis of AETSAPPE (0.25 M aqueous solution) at 254 nm (Rayonet reactor) resulted in the formation of the disulfide product 2-amino(2-hydroxy-3-(phenyl ether) propyl) ether disulfide (AHPEPED) as the primary photoproduct Photolysis of AETSAPPE at 254 nm (isolated line of medium pressure mercury lamp) resulted in rapid initial loss of starting material accompanied by formation (analyzed by HPLC) of AHPEPED (Figure 12a and 12b) (Scheme IV). Similar results were obtained for photolysis- at 280 nm. Quantum yields for disappearance of AETSAPPE and formation of AHPEPED at 254 nm and 280 nm are given in Table I. The photolytic decomposition of AETSAPPE in water was also accomplished by sensitization ( x =366 nm) with (4-benzoylbenzyl) trimethylammonium chloride (BTC), a water soluble benzophenone type triplet sensitizer. The quantum yield for the sensitized disappearance (Table I) is comparable to the results for direct photolysis (unfortunately, due to experimental complications we did not measure the quantum yield for AHPEPED formation). These results indicate that direct photolysis of AETSAPPE probably proceeds from a triplet state. 

A kinetic study of the photolysis at low conversions by Berces and Forgeteg305 leads to the conclusion that OH is the sole reactive species formed in the primary process. At 2650 A and 2537 A, respectively, the quantum yields are 0.1 and 0.3. Secondary reactions are expected to be similar to those involved in the thermal decomposition of HN03, as discussed in the preceding section. 

In all cases when photolytic isomerization into aliphatic alkanes was observed it was found that the quantum yield of primary decomposition calculated from the measured end products is slightly smaller than unity. 

Similarly to the fluorescence quantum yields, the yields of individual primary decomposition steps generally show considerable excitation energy dependence the yields of the unimolecular H2 and alkane eliminations and also those of the radical-type decompositions show a continuous variation with photon energy [27,39,42,107,115]. In cyclohexane photolysis the sum of the quantum yields of the two primary decompositions described by Reactions (5) and (6) is practically unity between photon energies 7.6 and 11.6 eV yield decreases with the energy, [Pg.382]

Although the picture of the photochemical primary processes in cyclopentanone which has been presented seems self-consistent, a number of minor points still have to be explained. These are (a) the dependence of the ratio of ethylene to cyclobutane on the geometry of the system (6) the puzzling fact that a constant fraction, between 2/10 and 3/10, of the initially excited molecules seem to return to the ground state without decomposition, by a route that is virtually unaffected by pressure. Before this can be explained it is necessary to confirm the value for the quantum yield for decomposition and (c) the fact that 2.5 kcal./mole of energy affects the reaction path profoundly. In the ground state the enthalpies of 2 and 3 differ by 19 kcal./mole at 25° while 3 and 4 may be estimated to differ by 15 to 20 kcal./mole. This phenomenon may be explained when a clear understanding of the excited state of the molecule is obtained. 

The primary quantum yield was then shown to decrease from 0.59 to a steady value of 0.20 for 50 mm. acetone as up to 0.5 mm. biaeet.yl was added. The quantum yield of carbon monoxide decreased correspondingly from 0.16 to 0.05. It is clear that under such conditions deactivation of the triplet state by biacetyl causes a major decrease in the decomposition of acetone. The ratio of the total acetone emission intensity to the absorbed intensity is small (0.02 at 40°C.5li). 

When the linear flow rate was high or at the early stages of decomposition, nitrogen was not found. Since the concentration of hydrazine under these conditions was very small, nitrogen must have been produced by the decomposition of hydrazine either by radical attack or by photolysis. The observation that the quantum yield of ammonia decomposition approaches unity at low pressure in the flow system signifies that the quantum yield for the primary dissociation into H + NH2 is unity. The lower experimental quantum yields then arise from ammonia reforming steps. Many reactions have been proposed among radicals and hydrazine, namely ... 

Weeks JL, Metheson MS (1956) The Primary Quantum Yield of Hydrogen Peroxide Decomposition, J. Am. Chem. Soc. 78 ... 

In nearly all of the other recent studies of formaldehyde photodecomposition (55,115,157,168,235), the quantum yields of H2 (or HD or D2) and/or CO were measured as the stable photoproducts. From these values, the quantum yields of primary processes 22 and 23, 4>j and 4>xi, respectively, were deduced on the basis of certain mechanistic assumptions which will be presented below. It turns out that both H2 and CO are not exclusively formed from the molecular elimination process (eq. 23). They are also produced from the radical decomposition process (eq. 

Photodecomposition. Since the last review of photochemistry of HFA (61), there has been a great deal of effort expended in the study of the primary processes and decomposition modes of HFA. The photodecomposition products observed appear to be carbon monoxide and hexafluoroethane exclusively. The trifluoroacetyl radical, CF3CO, must be very unstable. As in acetone, it has been proposed that the decomposition processes must overcome an energy barrier, as temperature-dependent quantum yields were observed (252). A detailed mechanism that takes into account a vibrational deactivation cascade has been proposed by several authors (34,35,97,252). 

Such information can be found in the work of Jaffe and Klein on the photolysis of NO2 in the presence of SO2. They measured the quantum yield of nitrogen dioxide decomposition by in situ NO2 absorptiometry. In the absence of SOj the quantum yield is 2, since each atom of oxygen formed in the primary photolytic process can react with another molecule of NO2... 

The quantum yield for the disappearance of the diazirine was found to be 2 0.5. The formation of the diazomethane corresponded to that of about one-tenth of the decomposition of the diazirine but leveled off during the photolysis. The authors su ested that this leveling off may have been due to secondary photolysis of the diazomethane by scattered radiation. These results set a lower limit for 0.2 for k /ki, i.e., at least 20% of the primary decomposition of the diazirine is an isomerization to diazomethane. Very surprisingly, no appreciable effect on the yield of diazomethane was found when nitrogen was added. 

Sieger and Calvert reported the photolysis products of 1,1,1-trifluoroacetone at A 3130 A to be carbon monoxide, methane, ethane, 1,1,1-trifluoroethane, and hexafluoroethane. A low quantum yield for decomposition near room temperature may be explained in terms of the excited trifluoroacetone having an appreciable lifetime and therefore suffering possible collisional deactivation before decomposition can occur. Two possible primary stepts of Type 1 have been proposed... 

The photochemical decompositions of perfluoroazomethane - and per-fluoroazoethane closely resemble those of the parent azohydrocarbons. Thus perfluoroazomethane when irradiated in the near ultraviolet, decomposes to nitrogen and perfluoroethane via a primary dissociation step in which trifluoro-methyl radicals are formed. Quantum yields of about 0.25 in the gas phase at room temperature - may be the result of collisional deactivation of excited perfluoroazomethane molecules. 

A range of a-chloronitrosoalkanes has been investigated, most decompositions being carried out in methanol solution. Saturated oximes and hydrogen chloride have been identified among the products, and quantum yields for the decomposition of the nitroso-compounds are near unity. Possible primary steps are (1) direct molecular elimination of hydrogen chloride and (2) homolytic rupture of the carbon-chloride bond... 

The primary quantum yields, based on the results of Blacet et al. are summarized in Table 8. (When calculating the numerical values, it has been assumed that 01 = 0CH3i nnd 011 = 0cH< in the presence of sufficient iodine.) These results show that the quantum yield of the decomposition into stable molecules increases as the absorbed energy increases. Dissociation into radicals is more dominant at the longer wavelengths, while the efficiencies of the two primary processes are commensurable at shorter wavelengths. 

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