Rate constant of deactivation

There are several guidelines that should be followed in order to increase the chemoselectivity of the monoadduct. Firstly, radical concentration must be low in order to suppress radical termination reactions (rate constant of activation [fcal and fca2] < < rate constant of deactivation kd t andfcd2]). Secondly, further activation of the monoadduct should be avoided ( al> >kd2). Lastly, formation of oligomers should be suppressed, indicating that the rate of deactivation (kd 2[Cu"LmX]) should be much larger than the rate of propagation ( [alkene]). Alkyl halides for copper-catalyzed ATRA are typically chosen such that if addition occurs, then the newly... 

Other methods for the deactivation of carbenium ions are based on the use of nucleophiles for example, sulfides act as deactivators in the polymerization of IBVE initiated by triflic acid [37,38]. In this case, the sulfides react with the carbenium ions to form sulfonium ions. Dynamic NMR measurements were used to estimate the rate of the exchange process and demonstrated that exchange occurs predominantly by dissociation of the sulfonium ions to carbenium ions rather than bimolecularly via exchange of the sulfonium ion with the sulfide [21], The rate constant of activation was kacl = 60 sec-1 and the rate constant of deactivation is d<=aci = 106 mol- -L-sec-1 at -30° C in CH2CI2. 

Figure 1. The dimer (ZnOEP)2Ph fluorescence intensity quenching with increasing amounts of extra-ligand H2P(p-Pyr)2 and the triad (ZnOEP)2Ph H2P(p-Pyr)2 formation. Schematic energy levels diagram for low-lying locally excited singlet states of the dimer (ZnOEP Ph, the extra-ligand H2P (Lig), locally excited triplet state of H2P (T), radical ion pair charge transfer state (CT), and rate constants of deactivation pathways (toluene, 295 K).

Thus, polydispersities decrease with conversion, p, the rate constant of deactivation, Ada. and also the concentration of deactivator, [XCu ]. They, however, increase with the propagation rate constant Ap and the concentration of initiator, [RX]q. This means that more uniform polymers are obtained at higher conversions, when the concentration of deactivator in solution is high and the concentration of initiator is low Also, more uniform polymers are formed when the deactivator is very reactive and monomer propagates slowly (styrene rather than acrylate). Polydispersities can also be expressed as a function of the reaction time t (243) ... 

For an irreversible reaction of reactants A and B [with the initial relative activity cat = 0) = 1], and assuming power law first-order kinetics regarding both reactants A and B, we obtain for this example of a catalytic reaction under deactivation by a sintering process the following expression (with k as the initial rate constant of the reaction and kdeact as the rate constant of deactivation) ... 

Fig. 2 Reversible and dynamic equilibrium between active radical growing species and dormant species (K, means rate constant of activation Kj means rate constant of deactivation Rp means rate constant of propargation M means monomer, P-X represents dormant polymer species represents reactive polymer radical species)...

The effective rate law correctly describes the pressure dependence of unimolecular reaction rates at least qualitatively. This is illustrated in figure A3,4,9. In the lunit of high pressures, i.e. large [M], becomes independent of [M] yielding the high-pressure rate constant of an effective first-order rate law. At very low pressures, product fonnation becomes much faster than deactivation. A j now depends linearly on [M]. This corresponds to an effective second-order rate law with the pseudo first-order rate constant Aq ... 

Figure 6. Jablonski diagram for the excited-state proton transfer and energy dissipation in TIN kSo s0> ks,s,-, kT,Tl- rate constants of proton-transfer processes in the ground state, first excited singlet state, and triplet state, respectively, and k,j rate constants of radiationless deactivations and k,- rate constants of intersyslem...

The reciprocal of the FRET-unperturbed donor lifetime, td, is given by the sum of all rate constants for deactivation. These parameters have been extensively discussed in earlier chapters. We note in passing that the constants with extreme values in Eq. (12.1) disappear if one expresses the absorption (excitation) spectrum of the acceptor in terms of the molecular absorption cross-section, o (2) = 1017ln[10] Njy x e (2)(nnr/moleculc). 

Several processes may compete with fluorescence for deactivation of the lowest excited singlet state. As a result only a fraction of the molecules formed in the lowest excited singlet state, < )/, actually fluoresce. <()/ is called the quantum yield or fluorescence efficiency. It is usually a fraction but may be unity in some exceptional cases and is related to the probabilities (rate constants) of fluorescence (kf) and competitive processes (kd) by... 

Here, Ox is the oxidant that reacts with the reductant L, kr is the pseudo-first-order rate constant causing the rise in P, the excited product that is the intermediate in the type II reaction, and ks is the rate constant for deactivation of A [fcs = kf + kd in Eq. (1)]. P will fall in concentration with rate constant ksT as it transfers electronic excitation to the acceptor A to form the excited acceptor A. ... 

Early studies have shown that tryptophan, tyrosine, histidine, methionine and cysteine, either as free amino acids or as components of peptides, are excellent substrates for O2 oxidation reactions. Usually, reaction of O2 with amino acids is mostly described in terms of chemical quenching with the exception of tryptophan, for which collisional deactivation as the result of physical quenching is not neghgible. The rate constants of O2 toward the main reactive amino acids that show a strong solvent dependence are reported in Table 2 for neutral aqueous solutions with values within the range 0.8-3.7... 

The dependence of IjG of the formation of products 56 and 19, plotted against the reciprocal concentration of 55, are differently curved, i.e., the ratio of the yields of 56 and 19 changes with the concentration of 55. This points out that at least two different excited states of the donor (benzene) are transferred, differing in k /k, values (km — rate constant of energy transfer from an excited benzene molecule to 55, kq = over-all constant of deactivation of excited benzene molecule by other pathways) which influence the reactions of the acceptor 55 in different ways. 

Consider the simple unimolecular reaction of Eq. (15.3), where the objective is to compute the forward rate constant. Transition-state theory supposes that the nature of the activated complex. A, is such that it represents a population of molecules in equilibrium with one another, and also in equilibrium with the reactant, A. That population partitions between an irreversible forward reaction to produce B, with an associated rate constant k, and deactivation back to A, with a (reverse) rate constant of kdeact- The rate at which molecules of A are activated to A is kact- This situation is illustrated schematically in Figure 15.1. Using the usual first-order kinetic equations for the rate at which B is produced, we see that... 

Rate constants of unimolecular processes can be obtained from spectral data and are useful parameters in photochemical kinetics. Even the nature of photoproducts may be different if these parameters change due to some perturbations. In the absence of bimolecular quenching and photochemical reactions, the following reaction steps are important in deactivating the excited molecule back to the ground state. 

Very recently, Findlay, Fortin, and Snelling122 have obtained confirmation for a rate constant of about 103 liter mole"1 sec"1 for the deactivation of C OA,) by 02. The decay of the A = 1.27 jj. band was followed, as a function of oxygen concentration, after cessation of irradiation of oxygen-benzene mixtures.121 A value of A 26b(M 02) = 1.4 + 0.3 x 103 liter mole"1 sec"1 is quoted, and the agreement with the value obtained by Clark and Wayne, with an entirely different experimental method, is surprisingly good. 

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