Radical absorption

Figure B2.5.7 shows the absorption traces of the methyl radical absorption as a fiinction of tune. At the time resolution considered, the appearance of CFt is practically instantaneous. Subsequently, CFl disappears by recombination (equation B2.5.28). At temperatures below 1500 K, the equilibrium concentration of CFt is negligible compared witli (left-hand trace) the recombination is complete. At temperatures above 1500 K (right-hand trace) the equilibrium concentration of CFt is appreciable, and thus the teclmique allows the detennination of botli the equilibrium constant and the recombination rate [54, M]. This experiment resolved a famous controversy on the temperature dependence of the recombination rate of methyl radicals. Wliile standard RRKM theories [, ] predicted an increase of the high-pressure recombination rate coefficient /r (7) by a factor of 10-30 between 300 K and 1400 K, the statistical-adiabatic-chaunel model predicts a...

Table B2.5.5. The photochemical decomposition of methyl radicals (UV excitation at 216 nm). ris tire wavenumber linewidth of the methyl radical absorption and /ris the effective first-order decay constant [54].

Various kinetic models on particle formation were proposed by different researchers. These may be classified as follows (1) radical absorption mechanisms by Gardon [28-34] and Fisch and Tsai [13], (2) micellar nucleation newer models by Nomura et al. [35,36] and by Hansen and Ugelstad [37], (3) homogeneous nucleation by Fistch and coworkers [13,38,39]. 

Although iron deficiency is a common problem, about 10% of the population are genetically at risk of iron overload (hemochromatosis), and elemental iron can lead to nonen2ymic generation of free radicals. Absorption of iron is stricdy regulated. Inorganic iron is accumulated in intestinal mucosal cells bound to an intracellular protein, ferritin. Once the ferritin in the cell is saturated with iron, no more can enter. Iron can only leave the mucosal cell if there is transferrin in plasma to bind to. Once transferrin is saturated with iron, any that has accumulated in the mucosal cells will be lost when the cells are shed. As a result of this mucosal barrier, only about 10% of dietary iron is normally absorbed and only 1-5% from many plant foods. 

Hayashi et al., 1989], involving the addition of monomer and initiator to a previously prepared emulsion of polymer particles, is especially useful for this purpose since it allows the variation of certain reaction parameters while holding N constant. Thus, h in seeded styrene polymerization drops from 0.5 to 0.2 when the initiator concentration decreases from 10-2 to 1CT5 M. At sufficiently low Ru the rate of radical absorption is not sufficiently high to counterbalance the rate of desorption. One also observes that above a particular initiation rate ([I] = lO-2 M in this case), the system maintains case 2 behavior with h constant at 0.5 and Rp independent of Ri. A change in Ri simply results in an increased rate of alternation of activity and inactivity in each polymer particle. Similar experiments show that h drops below 0.5 for styrene when the particle size becomes sufficiently small. The extent of radical desorption increases with decreasing particle size since the travel distance for radical diffusion from a particle decreases. 

Figures 5a),b) show the diminution of the thiocyanate radical absorption at 480 nm (A) as increasing amounts of each added competitor substance were added for the substrates added. Using equation (1), plots of Aq/A vs [s]/[SCN ] were made, the slopes of which yielded k(OH + s)/k(OH + SCN), where s is...

With the use of lock-in detection, the measured EPR signal corresponds to the first derivative of the radical absorption spectrum. The 1st integral of a measured EPR spectrum as a function of the magnetic field is computed according to... 

The extinction coefficients for the T-T neutral aminyl, and cation radical absorptions of 97 were used to calculate the quantum efficiencies for N—H cleavage and photoionization. The results indicate that in cyclohexane, the efficiency of cleavage is ca. 90%. Thus, roughly 90% of those upper triplet states that do not relax to T undergo cleavage. In acetonitrile on the other hand, the efficiencies for neutral and cation radical production are 0.53 and 0.45, respectively. In other words, of the upper triplets that do not regenerate Tv half decay to neutral radical and the other half to cation radical. It should be noted that the actual proportion of direct cleavage events may be smaller than indicated from the efficiencies because one of the cation radical decay routes is deprotonation to form the neutral radical. 

Copolymerization of styrene and butyl acrylate was successfully carried out by Huang et al. using the redox initiator system (NH SjOg/NaHSC at lower temperature [68]. The rate of the miniemulsion polymerization increases with increasing butyl acrylate concentration and decreases with increasing styrene concentration. This was attributed to differences in the water solubility. The lower water solubility of styrene either increases the desorption rate of the radicals or reduces the radical absorption of the monomer droplet [81]. 

In eqns. (66) and (67), Mt>injl and pt>init designate initiation rates in the monomer and polymer-rich phases, respectively, k.d and kd are the rate constants of radical absorption and desorption in solid phase particles, and MV and PV are the total volumes of the monomer-rich phase and of the precipitated polymer particles, respectively. 

There are many cases when radical absorption in the UV region overlaps with a tail of absorption of a photoinitiator or other species produced by photolysis. Optical detection may become less sensitive or impossible. Also, a transient radical may have low absorption (extinction coefficient) in the UV-vis region. For example, a... 

The concept of radical capture efficiency was further elaborated on by Hansen et al. [15-17]. By applying the theory of mass transfer with simultaneous chemical reactions, they proposed the following expression to represent the net rate of radical absorption by a particle, introducing an ""absorption efficiency factor F into Eq. 8... 

Unzueta et al. [18] derived a kinetic model for the emulsion copolymerization of methyl methacrylate (MMA) and butyl acrylate (BA) employing both the micellar and homogeneous nucleation mechanisms and introducing the radical absorption efficiency factor for micelles, F, and that for particles, Fp. They compared experimental results with model predictions, where they employed the values of Fp=10 and Fn,=10", respectively, as adjustable parameters. However, they did not explain the reason why the value of Fp, is an order of magnitude smaller than the value of Fp. Sayer et al. [19] proposed a kinetic model for continuous vinyl acetate (VAc) emulsion polymerization in a pulsed... 

Fig. 10. Absorption spectra of four l-alkyl-4-carbomethoxy-pyridinyl radicals, (R), in thin films at 77 K. The three main bands are at 6S0 nm, 400 nm and 300 nm. The 650 nm band (charge-transfer absorption of pimer) shifts from 623 nm for I-CH3 to 675 nm for l-lCHsfsC, along with a two-fold intensity decrease. The 400 nm band is composed of the pyridinyl radical absorption of the pimer, and absorptions of the two ester conformational isomers of the pyridinyl radical monomer. The 300 nm band varies with the different relative contributions of the pimer and the monomer...

Fig. 18. The spectrum of the l-methyI-2-carbomethoxypyridinyl radical, 2, at 77 K is the initial radical film . Pimer (2, 2 ) (i omplex) absorption at 645 nm overlies the weak monomeric radical absorption in the same wavelength region. The radical bands are lost at high temperatures, with formation of the radical dimer (2-2). Successive spectra for —140, —100 and —90 °C are shown...

Fig. 21. Spectroscopic changes on warming a thin fdm of l-methyl-4-carbomethoxypyridinyl 4 ). At —60 °C, the radical absorption is replaced by a very broad band in the 400 nm region along with absorptions near 270 nm and 230 nm. Recooling to 77 K and irradiation (350-500 nm) regenerates the radical with about 50% return of the pimer...

Fig. 22. Spectroscopic changes on warming l-methyl-4-carbo-t-butoxypyridinyl radical. Pimer absorption is tost completely brfore loss of all radical absorption in the 400 nm region, and parallels the decrease in the 380 nm absorption. At —65 °C, very little pyridinyl radical is present. The long wavelength absorption (rar wj transition in the dimer) extends from 350 nm past 500 nm...

Thus, while 10 - 10 particles/1iter is typical for styrene emulsions, particle numbers as high as 10 particles/1iter are common with vinyl chloride. As a result, the rate of radical absorption per particle tends to be low relative to the rate of initiation while desorption per particle tends to be enhanced. This effect is reflected in a low value for a and, more significantly. 

The combined effect of these special characteristics is to (1) enhance the rate of particle phase termination relative to the rate of radical absorption and desorption (i.e. reduce m and o ) (2) shift the equilibrium between aqueous and particle phase radicals by reducing the level of aqueous radical activity. The result, we believe, can be depicted by an ir vs. o relationship such as curve 2 in Figure 11. The situation is analogous to the styrene example where r < 1/2 except that conditions for Case II are never met. 

In polymerizations where desorption is very high, all micelles grow simultaneously. This is because radical absorption is followed almost iimiediately by desorption with only a brief micellar growth period. As a result, all micelles experience an equal but intermittant growth. The number of latex particles is determined solely by the number of micelles initially present. The number of micelles, m, is given by the expression... 

The copper(I) alkynyls displayed rich photochemistry and particularly strong photoreducing properties. The transient absorption difference spectrum of [Cu3(dppm)3(/X3-) -C=CPh)2]+ and the electron acceptor 4-(methoxycarbonyl)-A-methylpyridinium ion showed an intense characteristic pyridinyl radical absorption band at ca. 400 nm. An additional broad near-infrared absorption band was also observed and it was assigned as an intervalence-transfer transition of the mixed-valence transient species [Cu Cu Cu (dppm)3(/x3- -C=CPh)2] +. The interesting photophysical and photochemical properties of other copper(I) alkynyl complexes such as [Cu(BTA)(hfac)], 2 [Cui6(hfac)8(C=C Bu)8], and [Cn2o(hfac)8(CsCCH2Ph)i2] have also been studied. 

The difficulties associated with free radical absorption spectroscopy are almost entirely experimental ones and in the first part of this paper a brief account is given of the limitations of the existing methods with particular reference to the flash technique developed by the author. The second part deals with a few of the radical spectra which have been obtained by this method. 

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