Unimolecular process gases

Detailed reaction dynamics not only require that reagents be simple but also that these remain isolated from random external perturbations. Theory can accommodate that condition easily. Experiments have used one of three strategies. (/) Molecules ia a gas at low pressure can be taken to be isolated for the short time between coUisions. Unimolecular reactions such as photodissociation or isomerization iaduced by photon absorption can sometimes be studied between coUisions. (2) Molecular beams can be produced so that motion is not random. Molecules have a nonzero velocity ia one direction and almost zero velocity ia perpendicular directions. Not only does this reduce coUisions, it also aUows bimolecular iateractions to be studied ia intersecting beams and iacreases the detail with which unimolecular processes that can be studied, because beams facUitate dozens of refined measurement techniques. (J) Means have been found to trap molecules, isolate them, and keep them motionless at a predetermined position ia space (11). Thus far, effort has been directed toward just manipulating the molecules, but the future is bright for exploiting the isolated molecules for kinetic and dynamic studies. 

In a gas phase, the loss of energy requires collisions, whereas in a condensed phase, it can be considered a unimolecular process.)... 

By inspection alone, we can guess that AY will be negative for bimolecular reactions, since two components associate to form one (the TS). The value of AY is positive for unimolecular processes, such as gas-phase dissociation. 

Both the collision and activated complex theories predict a mild dependence of Z on T, and the latter also predicts a mild dependence of E on T. In practice, over the limited temp ranges of the usual exptl conditions, these mild dependencies are rarely observed. Both theories also predict that normal values of Z should be 1013 to 1014 sec"1 for unimolecular processes. This agrees with many exptl observations. In many cases, however, because of steric effects, Z can be much smaller than normal . Benson (Ref 12) presents evidence that Z for certain unimolecular gas reactions producing two free radicals, or for reactions involving the opening of a small carbon ring, is larger than normal and is of the order 1016 sec"1... 

Tetramethylethylene. Very recently, product studies on the 03 + tetramethylethylene (TME) reaction have been made by the authors group in attempts to probe various reaction channels operative for the dimethyl-substituted Criegee intermediate (CH3)2C—OO under atmospheric conditions [130]. Among the potentially important reaction channels suggested by previous theoretical studies [131] and experimental results in the gas- and solution-phases [122,132] are the bimolecular reaction with aldehydes (reaction (46c)) and the following unimolecular processes ... 

Balog R, Langer J, Gohlke S et al (2004) Low energy electron driven reactions in free and bound molecules from unimolecular processes in the gas phase to complex reactions in a condensed environment. Int J Mass Spectrom 233 267-291... 

In analyzing the origin of enzyme catalysis, Warshel and others have advocated the importance of comparing the enzymatic reaction with a reference reaction in water [32]. In addition, it is also necessary to study the reference reaction in the gas phase in order to understand the intrinsic reactivity and the effect of solvation. Thus, to understand enzyme catalysis fully, we must compare results for the same reaction in the gas phase (intrinsic reactivity), in aqueous solution (solvation effects), and in the enzyme (catalysis). This is not possible when there is no model reaction for the uncatalyzed process in the gas phase and in water, or if the uncatalyzed reaction is a bimolecular process as opposed to a unimolecular reaction in the enzyme active site. None of these problems apply to the ODCase reaction. Furthermore, OMP decarboxylation is a unimolecular process, both in water and the enzyme, providing an excellent opportunity to compare directly the computed free energies of activation [1] this is the approach that we have undertaken [16]. Warshel et al. used an ammonium ion-orotate ion pair fixed at distances of 2.8 or 3.5 A as the reference reaction in water to mimic an active site lysine residue [32]. 

Physical Properties.—Further Z-ray studies of methyl thio- and dithio-esters have confirmed previous findings (see Vol. 5, p. 180) that the ester group is almost planar, with a Z conformation, Kinetic studies of the gas-phase pyrolysis of dithioacetates have shown that the reaction is a concerted unimolecular process, consistent with the mechanism shown in Scheme 10. °... 

Room-temperature gas-phase recombination of CH2F and CD2CI radicals has been reported to give CH2FCD2CI. Three unimolecular processes are in competition with collisional deactivation of CH2FCD2CI HCl and DF elimination give CHF=CD2 and... 

This type of catalysis occurs at a surface, usually a solid surface. Recent developments in nanotechnology have produced nanometer-size solid particles that act as efficient catalysts. A solid catalyst acts by adsorbing molecules from a gas or liquid phase onto its surface, where they react. Consider a unimolecular process in the gas phase. 

R. Balog, J. Langer, S. Gohlke, M. Stano, H. Abdoul-Carime, and E. Illenberger, Int. J. Mass Spectrom., 233, 267-291 (2004). Low Energy Electron Driven Reactions in Free and Bound Molecules From Unimolecular Processes in the Gas Phase to Complex Reactions in a Condensed Environment. 

The Langmuir-Hinshelwood picture is essentially that of Fig. XVIII-14. If the process is unimolecular, the species meanders around on the surface until it receives the activation energy to go over to product(s), which then desorb. If the process is bimolecular, two species diffuse around until a reactive encounter occurs. The reaction will be diffusion controlled if it occurs on every encounter (see Ref. 211) the theory of surface diffusional encounters has been treated (see Ref. 212) the subject may also be approached by means of Monte Carlo/molecular dynamics techniques [213]. In the case of activated bimolecular reactions, however, there will in general be many encounters before the reactive one, and the rate law for the surface reaction is generally written by analogy to the mass action law for solutions. That is, for a bimolecular process, the rate is taken to be proportional to the product of the two surface concentrations. It is interesting, however, that essentially the same rate law is obtained if the adsorption is strictly localized and species react only if they happen to adsorb on adjacent sites (note Ref. 214). (The apparent rate law, that is, the rate law in terms of gas pressures, depends on the form of the adsorption isotherm, as discussed in the next section.)... 

The thermal decomposition of diacyl peroxides has been the most frequently employed process for the generation of alkyl radicals. The rate and products of the unimolecular decomposition of acetyl peroxide have been the subject of many studies. Acetyl peroxide decomposes at a convenient rate at 70-80°C both in the solution and in the gas... 

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