Termolecular

I h c effect of temperature in Monte Carlo simulations is primarily to modulate the strength of in termolecular in teraelion s, since temperature en Lers the simulation on ly th rough the Ho I Urn an n factor exp(-.-Ab7k r), where. AH represents a difference in potential... 

J G 1994. Extended Electron Distributions Applied to the Molecular Mechanics of Some termolecular Interactions. Journal of Computer-Aided Molecular Design 8 653-668. el A and M Karplus 1972. Calculation of Ground and Excited State Potential Surfaces of anjugated Molecules. 1. Formulation and Parameterisation. Journal of the American Chemical Society 1 5612-5622. 

Fig. 8. Rephcation. The amino adenosine X and the pentafluorophenyl ester Y form a hydrogen-bonded dimer XY, prior to reaction between the amine and the activated ester groups (shown in the circle). The reaction product is a <7 -amide conformer cis-Z that isomeri2es to the more stable trans- acnide Z. The rephcative process is cataly2ed by the reaction product Z (also referred to as the template). First, a termolecular complex XYZ is formed from X, Y, and Z.

Numerous other reactions of this type, including a sequence involving HO2, exist and the relative importance of each reaction depends on the mixture composition. The rates of these termolecular processes increase with increasing pressure but have Httle or no temperature dependence. 

The third-order process presumably involves reaction of a complex formed between the alkene and hydrogen halide with the second hydrogen halide molecule, since there is little likelihood of productive termolecular collisions. 

Even though the rearrangements suggest that discrete carbocation intermediates are involved, these reactions frequently show kinetics consistent with the presence of at least two hydrogen chloride molecules in the rate-determining transition state. A termolecular mechanism in which the second Itydrogen chloride molecule assists in the ionization of the electrophile has been suggested. ... 

The three basic mechanisms that have been considered to be involved in electrophilic additions to alkynes are shown below. The first involves a discrete vinyl cation. In general, it can lead to either of the two stereoisomeric addition products. The second mechanism is a termolecular process which would be expected to lead to stereospecific anti addition. The... 

Lastly, a termolecular cydization of unknown mechanism provides a useful synthesis of tetrakis(trifluoromethyl)thiophene 60 (equation 60)... 

We conclude that the rds transition state includes the elements of one cinnamoyl-imidazole and two butylamine molecules, but we do not know anything about their assembly. However, because a termolecular collision is very improbable, we are justified in supposing that this is a complex reaction, the three molecules having been brought together in stepwise fashion. 

Swain and Eddy have queried the wide applicability of the S l and Sif2 mechanisms and favored a push-pull termolecular process for the reaction of pyridine with methyl bromide in benzene solution for example, they have suggested that the effects observed on the addition of methanol, phenol, p-nitrophenol, and mercuric bromide to the reaction mixture can be explained by an intermediate of type 168. ... 

As for compounds 37, the rearrangements of 39 are considered to occur by a mechanism involving heterolytic N-C bond cleavage followed by in-termolecular recombination of the carbenium cation and benzotriazolyl anion so formed. 

Type 125 0x0 forms are characteristic for 5-hydroxy-l,2,4-triazoles [76AHC(S1), pp. 379, 388], These forms are additionally stabilized by an electron-withdrawing substituent, R = NO2 (98MRC343). Both hydroxy and 0x0 tautomers are capable of forming stable dimers owing to the in-termolecular hydrogen bonds (126 and 127 [76AHC(S1), pp. 377,379). 

This need not necessarily involve a termolecular collision, since it may be preceded by the equilibrium... 

Elementary reactions are generally unimolecular or bimolecular, depending on whether they entail the reaction of one species or two. Occasionally a termolecular step occurs, particularly between atoms or small molecules in the gas phase. Solution reactions that might appear to be termolecular usually prove really to be a succession of two simpler ones. 

The step is an example of a termolecular reaction, an elementary reaction requiring the simultaneous collision of three molecules. Termolecular reactions are uncommon, because it is very unlikely that three molecules will collide simultaneously with one another under normal conditions. 

Figure 6. Plot providing data for termolecular rate constant of reaction Ar+ + 2Ar Ar2 + + Ar...

The following abbreviations are used salenH2 = bis(salicylalde-hyde)ethylenediimine, salH = salicylaldehyde, SAB = 2-hydroxy-iV-(2-hydroxybenzylidine)aniline dianion, SAT = 2-(o-hydroxy-phenyDbenzothiazoline dianion, SAP = iV-(2-hydroxyphenyl)salicyl-adldimine dianion, and HMPT = hexamethylphosphoric triamide , and structures marked with an asterisk ( ) are polymeric, usually by in-termolecular association. 

Three basic types of fundamental processes are recognized unimolecular, bimolecular and termolecular. Unimolecular processes are reactions involving only one reactant molecule. Radioactive decay is an example of a unimolecular process ... 

Termolecular processes are common when two reactant molecules combine to form a single small molecule. 

In the atmosphere, [AT] is usually assumed to be atmospheric pressure at the altitude of interest. Therefore, unlike unimolecular and bimolecular processes, termolecular processes are pressure dependent. The units for the termolecular rate constant are cm /molecule s. 

If this is so, the protonated derivatives B and C would not appear at all. This conclusion stems from a value of w (see p. 335) of 5, indicating that water acts as a proton donor here as well as a nucleophile." " Termolecular processes are rare, but in this case the two water molecules are already connected by a hydrogen bond. (A similar mechanism, called Bac3, also involving two molecules of water, has been found for esters that hydrolyze without a catalyst. Such esters are mostly those containing halogen atoms in the R group.)... 

This mechanism, called the AdnS mechanism (termolecular addition, lUPAC AnAe)," has the disadvantage that three molecules must come together in the transition state. However, it is the reverse of the E2 mechanism for elimination, for which the transition state is known to possess this geometry (p. 1300). 

In contrast to the behavior of 3-hexyne in trifluoroacetic acid, addition of HCl in acetic acid yields essentially rra s-3-chloro-3-hexene (48%) and 3-hexanone (52%) as products, with less than 1% of the cis chloride (31,42,43). The 3-hexanone has been shown to arise from an intermediate vinyl acetate. The kinetics are complicated, but they seem to be of first order in substrate and second order in HCl. Added tetramethylammonium chloride increases the rate of product formation and changes the product composition to >95% trans-3-chloro-3-hexene and <5% 3-hexanone. A termolecular electrophilic addition via an intermediate such as 14 has been proposed (31,42) to account for these data. 

A mechanism is a description of the actual molecular events that occur during a chemical reaction. Each such event is an elementary reaction. Elementary reactions involve one, two, or occasionally three reactant molecules or atoms. In other words, elementary reactions can be unimolecular, bimolecular, or termolecular. A typical mechanism consists of a sequence of elementary reactions. Although an overall reaction describes the starting materials and final products, it usually is not elementary because it does not represent the individual steps by which the reaction occurs. 

In a termolecular reaction, three chemical species collide simultaneously. Termolecular reactions are rare because they require a collision of three species at the same time and in exactly the right orientation to form products. The odds against such a simultaneous three-body collision are high. Instead, processes involving three species usually occur in two-step sequences. In the first step, two molecules collide and form a collision complex. In a second step, a third molecule collides with the complex before it breaks apart. Most chemical reactions, including all those introduced in this book, can be described at the molecular level as sequences of bimolecular and unimolecular elementary reactions. 

The mechanism is one or more elementary reactions describing how the chemical reaction occurs. These elementary reactions may be unimolecular, bimolecular, or (rarely) termolecular. 

C15-0045. The reaction of NO with CI2 is 2 NO + CI2 2 NOCl Use reactant molecules to write appropriate elementary reactions that satisfy the following criteria (a) a unimolecular decomposition that generates Cl (b) a bimolecular collision in which a Cl atom is transferred between reactants and (c) a termolecular collision leading to the observed products. 

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