Trimolecular rate constant

Rate Constants and Activation Energies of the Trimolecular Reaction 2RH + 02 —> Free Radicals (Experimental Data)... 

TABLE 4.5 Rate Constants of the Trimolecular Reaction RCH=CH2 RCHCH2OOCH2CHR + o2 + ch2=chr ... 

The values of the rate constants of the trimolecular reactions 2RCH=CH2 + 02 are collected in Table 4.5. 

The preference of the trimolecular reaction over bimolecular RH + 02 is the result of weak C—H bond in diacetals and a polar media (see Chapter 4). The last factor is important for the energy of formation of very polar TS of the trimolecular reaction. The rate constants of trimolecular reactions are presented in Table 7.14. 

Rate Constants of Radical Generation by Trimolecular Reaction of Dioxygen with Ethers [68]... 

Free radical formation in oxidized organic compounds occurs through a few reactions of oxygen bimolecular and trimolecular reactions with the weakest C—H bond and double bond (see Chapter 4). The study of free radical generation in polymers (PE, PP) proved that free radicals are produced by the reaction with dioxigen. The rate of initiation was found to be proportional to the partial pressure of oxygen [6,97]. This rate in a polymer solution is proportional to the product [PH] x [02]. The values of the apparent rate constants (/ti0) of free radical formation by the reaction of dioxygen (v 0 = k 0[PH][O2]) are collected in Table 13.8. 

This result is quite clear in view of the importance of the proton donor compounds and the fact that the observed trimolecular rate constant of the catalytic reaction is higher by a factor of 40-50 than that of the noncatalytic reaction 29,30). 

The trimolecular rate constants were determined in earlier works with no account of any donor-acceptor interactions in the system 26,50), but later on an attempt has been made to consider such reactions I4,16,17,29 30). The effective kinetic and thermodynamic parameters that are independent of conversion are useful in describing the kinetic curve (such as that in Fig. 5), although the interpretation of the physical meaning of these constants is still very tentative. Such a situation seems to be typical... 

Fig. 6. Dependence of the effective value of the trimolecular rate constant for the reaction of diglycidyl ether of resorcinol with 2,6-diaminopyridine on conversion at different temperatures79. (1) 323 K (2) 333 K. (3) 343 K. (4) 353 K...

Table 11 presents one more result important for the chemistry of epoxy compounds, namely within the experimental error the rate constant of the free ion is the same for all counterions. This means that such strong nucleophilic particles as carbanions (and evidently alkoxy anions) are capable of opening the epoxy ring without additional electrophilic activation. This result explains the apparently contradictory results that, depending on the reaction conditions, either tri-140 144,166-I71) or bimolecular kinetics 175-I79> is observed. The bimolecular kinetics also can be explained in terms of the trimolecular mechanism, since proton-donor additives play a dual role. 

We have introduced the notation k loc o to denote the low-pressure (subscript 0 ) rate constant for the association reaction of A and B to form C, which exhibits trimolecular kinetics (superscript (3) )- Thus, at very low pressures, the kassoc becomes directly proportional to [M]. 

For each set of constant input and output concentration constraints a system of linear chemical reactions has a unique steady state. For a network of nonlinear biochemical reactions, however, there could be several steady states compatible with a given set of constraints. The number and character of these steady states are determined by the structure of the network including the extent of nonlinearity, the number and connectivity of the individual chemical reactions and the values of the reaction rate constants and the concentrations of the reactants. The higher the order of a chemical reaction, the more steady states may be compatible with a given set of chemical constraints. The simple trimolecular reaction system of Schlogl [13] illustrates how a third-order chemical reaction can have two stable steady states compatible with a single set of chemical constraints ... 

Unlike proteins, which exert control over transfer distances by positioning amino acid residues according to the tertiary structure, most ternary PCET reactions studied in model systems to date are trimolecular reactions. This complicates kinetics measurements and analysis, and can mask the underlying physics. The PCET yield depends on both the association constant (K soc) lo form the PCET precursor complex, and the subsequent pseudo-bimolecular PCET rate constant (kpcET)- fi is imperative to decouple the measurement of K soc kpcET io... 

The interpretation of these reactions was a considerable triumph for conventional transition-state theory. Simple collision theory proved unsatisfactory for trimolecular reactions, owing to the difficulty of defining a collision between three molecules, and usually led to very serious overestimations (by several powers of ten) of the rate constants. Similar difficulties are encountered with dynamical treatments, and these have still not been satisfactorily resolved. Conventional transition-state theory, by regarding the activated complex as being in equilibrium with the reactants, leads to a very simple formulation of the rate constant and to values in good agreement with experiment. It also very neatly explains the rather marked negative temperature dependence of the pre-exponential factors for these reactions. 

With rising pressure, the elementary rate constants of mono- and trimolecular reaction increase, tending to the high-pressure limit. 

After considering the TST, the major contributions of it to the field of chemical kinetics can be briefly summarized. It gave the possibility to calculate the rate, knowing characteristics of reactants and provided a link between geometry of reactants (or their configuration) and kinetics, as well as between the structure of the matter and kinetics. The notion of reaction coordinates was introduced and it became possible to calculate rather accurately the values of rate constants for mono-, bi-, and trimolecular reactions and to predict the temperature dependence for these reactions (Table 3.1). 

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