Competing Processes

Again, it is noteworthy that 4-substituted 5-hydrdxythiazoles (24) react like 5-hydroxy-THISs with alkynes to give pyrroles and sometimes with alkenes to give exo-cycloadducts (Scheme 22). In the latter case other processes compete with the cycloaddition, becoming dominant when 24 is treated with azo-compounds, enamines, or heterocumulenes (31). 

The most efficient intramolecular secondary processes competing with the acyl-alkyl diradical recombination in five-membered and larger cyclic ketones are hydrogen shifts resulting in the disproportionation of the diradical to either ketenes or unsaturated aldehydes [cf. (5) (4) (6)]. 

These processes compete with radiation-induced crosslinking, scission and, for case (c), polymerization. 

Thus, the process of hydride ion abstraction from a primary position is approximately thermoneutral, and hence we must conclude that it is an energetically allowed process, although possibly with a relatively small reaction rate. A process competing with primary H abstraction (Reaction 13) is methide ion abstraction (Reaction 11, loss of CH4 from the... 

The process competes with the traditional method of fiber production in which the precursor material is melted, usually in an arc furnace, then drawn through spinnerets and spun or impinged by high pressure air. The melt-spin process is not well suited to materials with high melting points such as zirconia, silicon carbide, or pure alumina. 

On the other hand. Type II process competes efficiently with the electron-transfer pathway in aerobic environments where the concentration of ground triplet state molecular oxygen is relatively high ( 0.27 mM), and singlet molecular oxygen (1O2) is the most abimdant ROS generated under these conditions, with a quantum yield 0.48 (Valle et al., 2011), eqn. 8. It is also possible an electron-transfer reaction from 3RF to 02 to form anion superoxide, but this reaction occurs with very low efficiency <0.1% (Lu et al., 2000). 

Iron(II) alkyl anions fFe(Por)R (R = Me, t-Bu) do not insert CO directly, but do upon one-electron oxidation to Fe(Por)R to give the acyl species Fe(Por)C(0)R, which can in turn be reduced to the iron(II) acyl Fe(Por)C(0)R]. This process competes with homolysis of Fe(Por)R, and the resulting iron(II) porphyrin is stabilized by formation of the carbonyl complex Fe(Por)(CO). Benzyl and phenyl iron(III) complexes do not insert CO, with the former undergoing decomposition and the latter forming a six-coordinate adduct, [Fe(Por)(Ph)(CO) upon reduction to iron(ll). The failure of Fe(Por)Ph to insert CO was attributed to the stronger Fe—C bond in the aryl complexes. The electrochemistry of the iron(lll) acyl complexes Fe(Por)C(0)R was investigated as part of this study, and showed two reversible reductions (to Fe(ll) and Fe(l) acyl complexes, formally) and one irreversible oxidation process."" ... 

The successful use of XV or XVII in polymer synthesis requires that elimination of Et2NH via 1,4- processes competes effectively with 1,2- processes. It is known that partially alkylated diamines... 

In the absence of triplet quenchers two processes compete for triplet deactivation radiative (phosphorescence) and nonradiative decay of the triplet to the ground state ... 

The efficiency of these radiative processes often increase at low temperatures or in solvents of high viscosity. Consequently emission spectra are generally run in a low-temperature matrix (glass) or in a rigid polymer at room temperature. The variation in efficiency of these processes as a function of temperature and viscosity of the medium indicates that collisional processes compete with radiative and unimolecular nonradiative processes for deactivation of the lowest singlet and triplet states. 

While for thermal reactions one usually does not correlate the energy input with the amount of product formed, electrochemists and photochemists are certainly more energy-minded . The first ones use the current yield to define the amount of product formed per electrons consumed. The latter ones use the so called quantum yield which is defined as the ratio of number of molecules undergoing a particular process from an excited state over moles of photons absorbed by the system, or in other words, the ratio of the rate constant for the process defined over the sum of all rate constants for all possible processes from this excited state (1.4). Thus, if for every photon absorbed, a molecule undergoes only one chemical process, the quantum yield for this process is unity if other processes compete it will be less than unity. 

Thus, the greater the numbers or rates of processes competing with fluorescence for deactivation of the lowest excited singlet state, the lower the value of <()/. The quantum yield of fluorescence is important in determining how intense chemiluminescence can be for a particular reaction. 

Can the costs for the overall manufacturing process compete with alternative routes ... 

The fluorescence spectra measured just upon ablation are given in Figure 2A as a function of laser fluence. The contribution below 370 nm was suppressed, as a Hoya L37 filter was used in order to cut off the laser pulse. Fluorescence spectra of this polymer film consist of sandwich (max. 420 nm, lifetime 35 ns) and partial overlap (max. 370 nm, lifetime 16 ns) excimers (20). The latter excimer is produced from the initially excited monomer state, while the sandwich excimer from the partial overlap excimer and the monomer excited states. Since these processes compete with efficient interactions between identical and different excimers (Si - Si annihilation) (12), the sandwich excimer is quenched to a greater extent compared to the partial overlap one under a high excitation. Actually the fluence-dependent spectral change around the threshold can be interpreted in terms of Si - Si annihilation. 

A few chapters of this book are devoted to the use of glycerol and alternatives to transesterification. These two processes compete with the food chain. Whereas in the short term these are viable, be it mainly through government subsidies, on the longer term these processes will have to be replaced by more amply available and non-food competing processes. 

Electron-poor olefins with higher oxidation potentials may decrease the rate of electron transfer and other processes competing for deactivation of the iminium salt excited states may increase. Alternate reaction pathways involving olefin-arene 2 + 2 cycloaddition may take place in the photochemistry of 133 with electron-poor olefins (equation 62)120,121. 

Mechanistic studies also indicate that 4-nitroveratrole (equation 20) and 4,5-dinitroveratrole (equation 21) undergo both singlet and triplet nucleophilic aromatic substitution with ethyl glycinate23. An electron transfer process competes against the nucleophilic aromatic photosubstitution for singlet excited 4-nitroveratrole, causing a decreased product yield in equation 20. 

Clearly this is a very interesting problem and of great practical relevance, very well suited to Monte Carlo simulation. At the same time, simulations of such problems have just only begun. In the context of crystal growth kinetics, models where evaporation-condensation processes compete with surface diffusion processes have occasionally been considered before . But many related processes can be envisaged which have not yet been studied at all. 

The photocatalytic system is shown in Scheme 5, where BNAH is oxidized by the ZnP + moiety in the radical ion pair ZaP -Ceo (ki) produced upon photoirradiation of ZnP-Ceo, whereas HV " is reduced to HV by the Ceo" moiety of ZnP +-C6o ki). These individual electron-transfer processes compete, however, with the BET in the radical ion pair (/cbet)- This pathway was experimentally confirmed by photolysis of the ZnP-Ceo/BNAH/HV and ZnP-H2P-C6o/BNAH/HV + systems with visible light (433 nm) in deoxyge-nated PhCN [70], For instance. Fig. 4 depicts the steady-state photolysis in deoxy-genated PhCN, in which the HV absorption band (X ax = 402 and 615 nm) increases progressively with irradiation time. By contrast, no reaction occurs in the dark or in the absence of the photocatalyst (i.e., ZnP-Ceo or ZnP-H2P-C6o) under photoirradiation [70]. Once HV+ is generated in the photochemical reaction, it was found to be stable in deoxygenated PhCN. The stoichiometry of the reaction is established as given by Eq. (3), where BNAH acts as a two-electron donor to reduce two equivalents of HV [70] ... 

Radical intermediates are also trapped by intramolecular reaction with an alkene or alkyne bond. At a mercury cathode this process competes with formation of the dialkylmercury [51], At a reticulated vitreous carbon cathode, this intramolecular cyclization of radicals generated by reduction of iodo compounds is an important process. Reduction of l-iododec-5-yne 5 at vitreous carbon gives the cyclopentane... 

The shape of this wave and the variation with pH are both consistent with fast equ-librium reactions In the pH region lower than the value of pK, for the hydroxyl radical, the reactions of this hydroxyl radical dominate the electrochemical process. Controlled potential reduction at the potential of this first wave indicates a IF process and the principal products are dimers of the hydroxyl radical. The second wave in this acidic region is due to addition of an electron and a proton to the neutral radical. This process competes with dimerization in the mid-pH range where the two polarographic waves merge. Over the pH range 7-9, monohydric alcohol is the principal product. At pH <3 or >12, pinacols are the main products. Unsymmet-rical carbonyl compounds afford mixtures of ( )- and meso-pinacols. Data (Table 10.3) for the ( ) / meso isomer ratio for pinacols from acetophenone and propio-phenone indicate different dimerization mechanisms in acid and in alkaline solutions. 

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