Quantum yields of reactions

If kiac and kd are much larger in 5-bromoacenaphthylene than in acenaphthylene and both rate constants are relatively large in the former, the quantum yield of reaction would be expected to be less sensitive to small increases in klsc, due to external heavy-atom perturbation, and more sensitive to increases in kd resulting from external heavy atoms. 

Using light of 3660 A the quantum yield of reaction (1), zero time. At any other time some light is absorbed by M(CO)5NEt3 giving an apparent value of < 1. In the absence of Et3N quantitative recombination of M(CO)5 and CO occur when the irradiation is stopped9. 

Equation (10) allows one to calculate a by determining the quantum yield of reaction as a function of sensitizer concentration. However, it should also be remembered that if the substrate absorbs part of the light, m may need to be corrected for changes in the amount of light absorbed by the sensitizer. 

When the rate constants are such that the last term in Eq. (15) is not negligible, determination of the quantum yield of reaction as a function of sensitizer concentration will reveal back transfer this experiment also tests for sensitizer self-quenching (Section III.B). In the most frequently encountered case, forward transfer is much faster than back transfer (i.e., ke k.e) so that this term is small. The dependence of m on sensitizer concentration disappears and Eq. (15) reduces to the usual expression for quantum yield as a function of [A]. 

The efficiency of a photochemical reaction is defined (Section 1.2) in terms of quantum yield of reaction < r,... 

If the quantum yield of reaction is unity and the reaction is simple... 

Figure 4.25 Jablonski diagram of the heavy atom effect on photochemical reactivity. If excitation to S2 (hv2) is followed by intersystem crossing (isc) to T2, the quantum yield of reaction R decreases at higher excitation energies, ic = internal conversion, a = absorption, f = fluorescence, p = phosphorescence...

Quenching is observed as the lowering of a quantum yield of reaction with increasing concentrations of an additive Q. In the first place, this is simply an experimental observation which implies no particular quenching mechanism. Before the actual quenching of the reactive state by energy transfer can be established, it is necessary to eliminate certain apparent quenching effects ... 

They estimate that the quantum yield of reaction 61 if 0.8 while that of reaction 62 is C.2. The fraction of available energy... 

Now to return to the measurement of the quantum yield of reaction (96). Each primary dissociation introduces two radicals into the system and (97), (98), and (99) each removes two radicals from the system. Hence the primary quantum yield may be found from the relationship... 

According to mechanism (III) even if the concentration of A in one of the two sites (7) is much smaller than the amount in site X, appreciable photoreaction from AY may occur through AxA Y energy transfer as outlined in Sch. 21 or if the quantum yield of reaction from A Y is much larger than that from A x. An example in which situation (III) operates is the photodimerization of 9-cyanoanthracene in the crystalline state presented above (Sch. 23) [138,139]. On a statistical basis, many more molecules within the crystalline bulk phase are expected to be excited than those at defect sites. However, the reaction cavities capable of supporting reaction are specific to defect sites. Efficient photodimerization is believed to occur from exciton migration from the inert bulk sites to the defect sites. 

The quantum yield of reaction 17 varies with the irradiation wavelength, from about 0.07 near 300 nm to 0.025 at 355 nm down to 0.015 at 371 nm [35,40]. Differently from nitrate, nitrite is a source but also a sink for the hydroxyl radical, the latter reaction yielding nitrogen dioxide [35,37-40,43] ... 

Obviously, reactions 23, 24 play a role only if the quantum yield of reaction 19 is relatively near its upper limit. The following reactions can also take place [35,45-51], but kinetic analysis indicates that they are very likely to play a secondary role in the system [44] ... 

Triplet sensitization did not lead to intramolecular adducts for higher alkanes but resulted in polymer formation (vide infra) a reaction scheme including excited complex formation in a primary step was proposed. The rate constant for excited complex formation, k, could be calculated as a function of the number of methylene units and diminished by lengthening of the chain (Table 8). The quantum yield of intersystem crossing was much lower (Table 8) than in the isolated chromophore, N-butylamalelmlde ( isc = 0.25), and substantially lower than the quantum yield of reaction, (Table 8). 

A mathematical model for the measurement of pseudo first order rate constants in laser flash photolysis has been put forward. Another data treatment provides a method for determining quantum yields of reactions of the type A(+hv) = B(+hv,A)... 

A convenient, if somewhat less accurate, method for measuring a series of quantum yields of reaction is the merry-go-round setup (Figure 3.30). It holds about 20 sample and actinometer solutions with equal absorbance in cylindrical cells (test-tubes), which are... 

The conditions necessary to observe a quantum chain process (a quantum gain) in solid matrices using the domino mechanism were reported by Ebbesen et al. to find new application of the amplification of the effect of photon in molecular optics [99]. The quantum yield of reaction from A to B (Oq) in solution is described as Eq. (4), where O is the quantum yield of production of the reactive excited state and k, and ki are the rate constants of energy transfer and unimolecu-lar deactivation, respectively. Since the molecular diffusion is slow in the solid matrices, the equation is modified by using Perrin s equation as shown in Eq. 

The wavelength-dependent quantum yield of reaction Eq. (23) has been explained by a wavelength-dependent probability of Eq. (25) versus Eq. (26) 478>, possibly due to a hot ground-state molecule 4>. 

By combining the two-step excitation TG technique with the population grating (Section II.C), the rates and the quantum yield of reactions from higher excited states were determined for dimethyl-s-tetrazine [35] (Scheme 10). 

The quantum yield of Reaction 1 in our 185-200-nm. wavelength range is unknown, but it is sufficiently high with our 100-joule pulse to provide an initial 10 7M e m concentration. A loss of e aq or, which is equivalent under our conditions, a loss of H or OH radicals, can therefore only occur in second-order reactions such as 5 and 6. 

Time-resolved photoacoustic calorimetiy has been used by a number of research groups to determine the enthalpies and quantum yields of reaction in inorganic and organic systems (1-7). Our experiment technique is similar to those previously described in the literature Q). 

Real Time (RT-) FTIR spectroscopy pormits not only to follow quantitatively the polymerization by monitoring the disappearance of the IR absorption characteristic of the polymerizable reactive groups (acrylates, methacrylates, epoxy rings, vinyl ether double bonds, thiol groups etc.) but also to determine at any moment the actual degree of conversion and hence the residual imreacted groups content. This analytical method has proved extremely valuable for measuring the polymerization rates and quantum yields of reactions that develop in the millisecond time scale. 

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