Quantum free-radical formation

Aliphatic sulfides can be efficient co-initiators for the photoinduced polymerization induced by benzophenone [185, 186]. An exceptionally strong effect was observed for 2,4,6-trimethyl-1,3,5-trithiane (TMT). A model reaction for free-radical formation during photoreduction of an initiator triplet state by a sulfide is the photoreduction of benzophenone by dimethyl sulfide [171, 187-189]. In this process it was established that electron transfer from the sulfur atom to the triplet state of the benzophenone is a primary photochemical step. In this step, radical ions are formed. The overall quantum yields of photoproducts (ketyl radicals and radical anions) are low (Ed) 0.26) in aqueous solution, in the range 0.16-0.20 in mixed water-acetonitrile solution and less then 0.01 in pure acetonitrile. These results suggest that, in organic solvents, back electron transfer within the radical-ion pair to regenerate the reactants is the dominant process. 

Equations (54) and (55) clearly show that the rate of polymerization depends on the monomer concentration, the quantum yield of free-radical formation, and the reactivity of the free radicals produced after the photoinduced electron transfer process. 

There is another interesting feature of Eqs. (54) and (55). They also show that the rate of photoinitiated polymerization is proportional to the square root of the quantum yield of free-radical formation (<1>r )- It is evident from Scheme 21 that there are two types of processes that could initiate polymerization. The quantum yield of a-CBR radical production can be taken to be equal to the quantum yield of CO2. The quantum yield of a-SR radical could be estimated by... 

Figure41. Relationship between the rate of polymerization of acrylamide and quantum yield of free radicals formation for 4-carboxybenrophenone (CB)-sulfur-containing carboxylic acids (SCCA) initiating system. SCCA listed in Table 11.

On the basis of experiments carried out with mixtures of acetaldehyde and acetone, Grahame and Rollefson have determined relative quantum efficiencies of the free radical formation at 3130 and 2652 A. The results are in reasonable agreement with their earlier findings. 

As stated above, the thermochemistry of free radicals can also be estimated by the group additivity method, if group values are available. With the exception of a few cases reported in Benson (1976), however, such information presently does not exist. Therefore, we rely on the model compound approach (for S and Cp) and bond dissociation energy (BDE) considerations and computational quantum mechanics for the determination of the heats of formation of radicals. 

Once the actinic fluxes, quantum yields, and absorption cross sections have been summarized as in Table 3.19, the individual products < .,v(A)wavelength interval can be calculated and summed to give kp. Note that the individual reaction channels (9a) and (9b) are calculated separately and then added to get the total photolysis rate constant for the photolysis of acetaldehyde. However, the rate constants for the individual channels are also useful in that (9a) produces free radicals that will participate directly in the NO to N02 conversion and hence in the formation of 03, etc., while (9b) produces relatively unreactive stable products. 

Intramolecular bond formations include (net) [2 + 2] cycloadditions for example, diolefin 52, containing two double bonds in close proximity, forms the cage structure 53. This intramolecular bond formation is a notable reversal of the more general cycloreversion of cyclobutane type olefin dimers (e.g., 15 + to 16 +). The cycloaddition occurs only in polar solvents and has a quantum yield greater than unity. In analogy to several cycloreversions these results were interpreted in terms of a free radical cation chain mechanism. 

These photoinitiation processes which depend on the formation of free radicals in some photochemical reaction lead to chain reactions, since each molecule of initiator can promote the addition of many monomer units to a polymer chain. The quantum yield of monomer addition can therefore be much larger than unity, but it cannot be controlled since the growth of a polymer chain is then limited by termination reactions in which two free radicals react to produce closed-shell molecules. 

Block DA, Armstrong DA, Rauk A (1999) Gas phase free energies of formation and free energies of solution of C-centered free radicals from alcohols a quantum mechanical-Monte Carlo study. J Phys Chem A 103 3562-3568... 

Remember that a powerful lamp will have a greater photon flux than a weaker lamp. Further, photons enter a system one photon at a time. Thus every photon absorbed does not result in bond breakage or other possible measurable effect. The quantum yield, < >, is a measure of the effectiveness for effecting the desired outcome, possibly bond breakage and formation of free radicals. 

It should be noted here that thymine photodimerization may occur by a non-concerted mechanism, involving free radical intermediates. Indeed, photoproducts other than cis-syn dimer, such as the next most abundant thymine dimer, so-called 6 4 adduct, were observed in irradiated DNA. However, the quantum yield of cis-syn photodimer formation (r/j 0.02) is more than an order of magnitude higher than that of the 6 4 adduct ( 0.0013) which in turn is an order of magnitude higher than the quantum yields for other thymine isomers [68]. This specificity can lead to the conclusion that the thymine photodimerization occurs predominantly via concerted 2 + 2 cycloaddition mechanism. A time-resolved study of thymine dimer formation demonstrated that thymine cyclobutane dimers are formed on a timescale of less than 200 nsec, while the 6 4 adduct is formed on a timescale of few milliseconds [69]. The delay in the formation of the latter was attributed to the mechanism of its formation through a reactive intermediate. 

In fact, alkylated succinamides were isolated in some cases, though in very poor yields, and result from radical combination, which is a chain termination step. The experimental observations, i.e. the formation of (a) 1 1 adducts, (b) telomeric products, (c) alkylated succinamides, and (d) oxamide (when an olefin is absent), are consistent with a free radical mechanism. The telomeric products obtained support the assumption that we deal here with a chain reaction, because they are characteristic products of this type of reaction. Another proof for the chain reaction mechanism is the fact that when benzophenone is used as a photoinitiator (vide infra), the amount of benzpinacol formed is smaller than the amount of the 1 1 addition product of formamide and olefin (16). Quantum yield determinations will supply extra evidence for the validity of a chain reaction mechanism for this photoaddition reaction. 

As acceptors, they used 9,10-dicyanoanthracene (DCA) or 2,6,9,10-tetracyanoan-thracene (TCA), the donors were methylated benzenes or naphthalenes. The D/A pair was designed to give radical ion pairs on irradiation. In order to determine sep, they monitored the quantum yield of dimethoxystilbene radical cation formation [70], which intercepts the free radical cation of the donor exclusively. Assuming a constant k, from earlier studies [66b], they indirectly obtained k e, according to Eq. (11) ... 

The quantum yield of the formation of free radicals 0[Pg.250]

These molecules have a high quantum yield for the formation of their triplet state and usually rapidly dissociate into free radicals in the triplet state undergoing... 

---