Trimolecular kinetics

When the perturbation is small, the reaction system is always close to equilibrium. Therefore, the relaxation follows generalized first-order kinetics, even if bi- or trimolecular steps are involved (see chapter A3.41. Take, for example, the reversible bimolecular step... 

ACTIN ASSEMBLY KINETICS MICROTUBULE ASSEMBLY KINETICS Triglyceride synthesis kinetics (hepatic), LIRID TRACER KINETICS 2,4,5-Trihydroxyphenylalanine quinone, REDOX-ACTIVE AMINO ACIDS TRIMOLECULAR MOLECUARITY ORDER... 

A kinetic study of the reaction between phosphine and formaldehyde showed that it is of the first order with respect to phosphine and to the aldehyde. It is catalysed by HCl. At hydrogen chloride concentrations of less than 0.2 mol/1 the rate of reaction is proportional to the HCl-content of the solution, at higher concentrations the rate is independent of the HCl-content The dependencies found can be accounted for by both bi- and trimolecular mechanisms. In the latter case, a simultaneous interaction between one molecule of aldehyde, one of phosphine and a proton must be assumed ... 

At this point it is necessary to consider several possible sources of ambiguity in interpretation of the l3 data. First, it should be noted that I2 may form an addition complex (2, 9) with Co(CN)5l 3, just as it does with alkyl iodides in nonaqueous solution. (Experiments bearing on this question are in progress.) Fortunately, the presence of such a complex would not alter the numerical values of the kinetic parameters. Secondly, there is the possibility that Reaction 5 is a trimolecular process, with the I2 and 1 reactants adding to Co(CN)5-2 at separate stages of the reactant. 

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... 

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]. 

Even in the days of van t Hoff many such cases were well known— multimolecular reactions which were kinetically unimolecular or bimolecu-lar or (rarely) trimolecular. But it was relatively recently, mainly in the second decade of this century, that chemists began to realize that some reactions may very well be kinetically of a mixed type. To quote a case where the mechanism was complete, mention may be made of the... 

Elementary reactions are individual reaction steps that are caused by collisions of molecules. The collision can occur in a more or less homogeneous reaction medium or at the reaction sites on a catalyst surface. Only three elementary kinetic processes exist mono-, bi-, and trimolecular processes. Of these, trimolecular processes are rarely found, because the chance of three molecules colliding at the same time is very small. Each elementary reaction consists of an activation of the reactants, followed by a transition state and decomposition of the latter into reaction products ... 

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... 

Elementary reactions are characterized by their molecularity, to be clearly distinguished from the reaction order. We distinguish uni- (or mono-), bi-, and trimolecular reactions depending on the number of particles involved in the essential step of the reaction. There is some looseness in what is to be considered essential , but in gas kinetics the definitions usually are clearcut through the number of particles involved in a reactive collision plus, perhaps, an additional convention as is customary in unimolecular reactions. 

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

Sorption. The implementation rate of the second group of reactions depends on the composition and properties of geologic medium (biocenoses, homogenous, heterogeneous and biochemical processes during water mixing). Many processes include bi- or trimolecular reactions with complex kinetic equations. The total effect of all these processes is represented by a sum whose each addend describes their contribution in the mass-exchange balance ... 

Chemical kinetics studies the rate of chemical reactions and factors that influence them. A chemical process can be divided into a sequence of one or more elementary reactions that typically involve collisions between two molecules (bimolecular reaction) or dissociation/isomerization of a reagent molecule (uni-molecular reaction). A trimolecular reaction, which is the collision of three reactant molecules, occurs less frequently. Usually, the mechanisms are described in a single global step, but in reahty they occur in a series of elanentary steps [1]. 

We have shown using a Galerkin type of resolution scheme that when the kinetic constant of the trimolecular step in (1) fluctuates, new pure noise induced transitions become possible. For the sake of clarity let us recall for example that when A = 2, the extrema of the probability density behave as represented in figure 1. u is plotted as a function of a and for the values of e indicated. The curves labelled 1 correspond to the situation typical above the Hopf bifurcation. For small intensities (curve in the lower left corner) one sees that the noise suppresses the extremum corresponding to the usual limit cycle this is the same behavior as in 2.2. Its amplitude which is equal to one (in normalizing with respect to the deterministic limit cycle, i.e. 

Then, the resulting tin(II) mono- and/or dialkoxide initiates polymerization in the same manner as the other metal alkox-ides. However, there was, at that time, no direct proof of such a mechanism and several other mechanisms have been pro-posed. " The most often cited was the trimolecular mechanism in which first the catalyst-monomer complex is formed. This mechanism has conclusively been shown not to operate since it excludes the presence of Sn atoms covalently bonded to the growing macromolecules. The matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectral measurements of the cyclic ester/ROH/Sn(Oct)2 system revealed the presence of tin(II) alkoxides in the growing polyester chains. Moreover, the kinetic studies also clearly supported this sequence of the exchange reactions. ... 

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