Photochemical reactions quantum yield

Table 4.24. Absorption Spectra Data and Photochemical Reaction Quantum Yield of Fulgenates, Fulgenolides, and 54a in Flexane...

The availability of modem chromatographic and spectroscopic techniques makes possible to discover (and sometime rediscover) many photochemical reactions quantum yields are routinely determined. 

Kari FG, S Hilger, S Canonica (1995) Determination of the reaction quantum yield for the photochemical degradation of Fe(III)—EDTA implications for the environmental fate of EDTA in surface waters. 

The rate of photolytic transformations in aquatic systems also depends on the intensity and spectral distribution of light in the medium (24). Light intensity decreases exponentially with depth. This fact, known as the Beer-Lambert law, can be stated mathematically as d(Eo)/dZ = -K(Eo), where Eo = photon scalar irradiance (photons/cm2/sec), Z = depth (m), and K = diffuse attenuation coefficient for irradiance (/m). The product of light intensity, chemical absorptivity, and reaction quantum yield, when integrated across the solar spectrum, yields a pseudo-first-order photochemical transformation rate constant. 

While yields greater than unity provide evidence for chain reactions, yields less than unity do not indicate the absence of a chain reaction. Quantum yields as high as 106 have been observed in the photochemical reaction between H2 and Cl2. 

Stern-volmer kinetic relationships This term apphes broadly to variations of quantum yields of photophysical processes e.g., fluorescence or phosphorescence) or photochemical reaction (usually reaction quantum yield) with the concentration of a given reagent which may be a substrate or a quencher. In the simplest case, a plot of (or /M for emission) vs. concentration of quencher, [Q], is linear, obeying the equation... 

The energy of the MLCT excited state (F)oo( MLCT)) can be evaluated from the emission spectrum. Emission peak wavelength (le), emission quantum yields (0e), emission lifetimes (le), and reaction quantum yields of the photochemical ligand substitution reactions ( r) are summarized in Table III. The modification of the bipyridine ligand caused changes in. Boo( MLCT) as large as 2400 cm. ... 

Sande (98) reported that temperature is not expected to affect the absorption of photons as such, and no additional energy is needed for the reaction to take place. However, the temperature will affect subsequent chemical degradative reactions in the usual manner as described by the Arrhenius equation. If a secondary thermal reaction is involved, a temperature effect on the overall reaction quantum yield would be expected. A change in the viscosity of the liquid as a result of increase of room temperature can influence the rate of photochemical degradation. 

There are some differences in the photochemical reactions of 91a and 91b and alkenes when the acetonitrile solvent is replaced by other solvents such as alcohols, ethyl acetate and chlorinated hydrocarbons. These include (1) a change in the product ratio, (2) a change in the reaction quantum yield and (3) the formation of new products by trapping solvent molecules. For example, irradiation of 91b with 1-hexene in methanol gives the methoxyalkyl)anilines 104 and 105 (equation 37) rather than 100b and 101b. It should be noted that with all of the alcohols tested (i.e. methanol, 2-propanol, terf-butyl alcohol and TFE) the alkoxyaniline 106 is formed at most in trace amounts. 

Quantum yield data for organic chemicals are rather scarce. Selected data are shown in Table 6.2. Clearly, photochemical reactions have the potential to rapidly remove some compounds from water, and even compounds with low reaction quantum yields can disappear rapidly if they absorb sunlight efficiently. 

Aromatic nitriles are strong oxidants in their excited states (see Table I). Since they fluoresce strongly, the involvement of the singlet states can be easily proved by application of fluorescence quenching techniques. In all of the tested cases, it has been found that the Stern Volmer constant obtained from fluorescence analysis and that obtained from the double reciprocal plots of reaction quantum yield vs. quencher concentration are nearly equal, thus proving that the singlet stale is actually involved In the photochemical reaction. Actually it has been observed that a AG < 0 and polar solvents are necessary (although not sufficient, see Section 3) conditions for the photochemical proc-... 

Abnormally high quantum yields may occur in photochemical reactions. Einstein s law of photochemical equivalence is the principle that light is absorbed by molecules in discrete amounts as an individual molecular process (i.e., one molecule absorbs one photon at a time). From optical measurements it is possible to determine quantitatively the number of photons absorbed in the course of a reaction and, from analyses of the product mixture, it is possible to determine the number of molecules that have reacted. The quantum yield is defined as the ratio of the number of molecules reacting to the number of photons absorbed. If this quantity exceeds unity, it provides unambiguous evidence for the existence of secondary processes and thus indicates the presence of unstable intermediates. While yields greater than unity provide evidence for chain reactions, yields less than unity do not indicate the absence of a chain reaction. Quantum yields as high as 10 have been observed in the photochemical reaction between Hj and CI2. 

Efficiency of Photochemical Processes Quantum Yield of Photochemical Reaction... 

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. 

Fig. 3. Photochemical and thermal reactions of previtamin D2 where the quantum yields for photochemical reactions are given by the arrow. R is as shown...

Fig. 3. Photochemical and thermal isomerization products of vitamin D manufacture (49). The quantum yields of the reactions ate hsted beside the arrows...

Fig. 13.11. A schematic drawing of the potential energy surfaces for the photochemical reactions of stilbene. Approximate branching ratios and quantum yields for the important processes are indicated. In this figure, the ground- and excited-state barrier heights are drawn to scale representing the best available values, as are the relative energies of the ground states of Z- and E -stilbene 4a,4b-dihydrophenanthrene (DHP). [Reproduced from R. J. Sension, S. T. Repinec, A. Z. Szarka, and R. M. Hochstrasser, J. Chem. Phys. 98 6291 (1993) by permission of the American Institute of Physics.]...

A simple aliphatic ketone such as acetone, when promoted to its n,n excited state, undergoes a single unimolecular photochemical reaction in high quantum yield namely a-cleavage giving a methyl and acetyl radical which react further in secondary dark processes. In general, competition... 

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