Rate of unimolecular reactions

Because of the relatively slow rates of unimolecular reactions of excited acetone in solution at room temperature, acetone makes a convenient solvent-sensitizer for photosensitizatioh studies, provided that the substrate does not undergo competing chemical reactions with triplet acetone. A recent study of the effects of high-energy radiation on dilute acetone solutions of polynuclear aromatic molecules revealed that the triplet states of these compounds were being formed at close to the diffusion-controlled rate by collision with some pre-... 

We have seen that neither the requirements for activation energy nor the fact that the rates of unimolecular reactions are independent of collision frequency can be explained on the basis of the simple collision hypothesis or the radiation hypothesis. The elaborated collision hypothesis is able to explain them on the assumption of a time-lag in complex molecules between activation and decomposition. In this way a single molecule can collect energy from many successive collisions and store up a sufficient amount for activation. Just because a given hypothesis accounts for the facts, is no reason to consider that the hypothesis has been proved. There may be other hypotheses which will account equally well for the facts. The hypothesis of chain reaction offers a competing hypothesis which up to the present time has been increasing in favor. 

On the basis of the RRK model, the over-all rate of unimolecular reaction can be written as... 

Guo, Y., Thompson, D. L. and Miller, W. (1999) Thermal and microcanonical rates of unimolecular reactions from an energy diffusion theory approach, J.Phys.Chem. A, 103, 10308-10311. 

II. Quantum Energy Flow, Localization, and Their Influence on Rates of Unimolecular Reactions... 

Section II provides a summary of Local Random Matrix Theory (LRMT) and its use in locating the quantum ergodicity transition, how this transition is approached, rates of energy transfer above the transition, and how we use this information to estimate rates of unimolecular reactions. As an illustration, we use LRMT to correct RRKM results for the rate of cyclohexane ring inversion in gas and liquid phases. Section III addresses thermal transport in clusters of water molecules and proteins. We present calculations of the coefficient of thermal conductivity and thermal diffusivity as a function of temperature for a cluster of glassy water and for the protein myoglobin. For the calculation of thermal transport coefficients in proteins, we build on and develop further the theory for thermal conduction in fractal objects of Alexander, Orbach, and coworkers [36,37] mentioned above. Part IV presents a summary. 

II. QUANTUM ENERGY FLOW, LOCALIZATION, AND THEIR INFLUENCE ON RATES OF UNIMOLECULAR REACTIONS... 

The concentration effect could explain the observed changes as well. If the concentration of molecules in the cavities decreases with decreasing Alp content, the rate of bimolecular reactions (disproportionation) should decrease faster than the rate of unimolecular reactions (isomerisation). 

Surprisingly, [l.l.ljpropellane is somewhat more stable to thermal decomposition than the next larger propellane, [2.1.1]propellane, indicating a reversal in the trend of increased reactivity with increased strain. To understand this observation, it is important to recognized that the energy of both the reactant and intermediate influence the rate of unimolecular reactions that lead to decomposition. In the case of propellanes, homolytic rupture of the central bond is expected to be the initial step in decomposition. This bond rupture is very endothermic for [l.l.ljpropellane. Because relatively less strain is released in the case of [l.l.ljpropellane than in the [2.1.1J- and [2.2.1J-homologs, [l.l.ljpropellane is kinetically most stable. ... 

With decrease in pressure, the chance that an activated molecule will lose its energy decreases more rapidly than the chance of the energy becoming so distributed as to allow a reaction to occur. Consequently, at low pressures the rates of unimolecular reactions cannot remain independent of the pressure. The activated molecule tends to become a Van t Hoff intermediate, and the rate to depend on the collision frequency. This corollary of Lindemann s theory has been... 

In bimolecular reactions all conditions must be satisfied at the moment of encounter in imimolecular reactions they may be satisfied at any time between the collision which imparts the activation energy and the next one— in which the energy is likely to be removed again. For this reason the absolute rates of unimolecular reactions tend, for a given value of E, to be much higher. 

Equation (4-17) is the rate of unimolecular reaction at high pressures for those molecules which possess vibrational energy between E/Nq and E/Nq + (IE/Nq. Integration of Eq. (4-17) yields... 

The qualitative features of reaction mechanisms in solutions are substantially different from those in gases. Unimolecular processes still occur via collisional activation. However, solvent molecules which can affect activation are always in high concentration. In terms of a rate law such as (5.22) the experiments are being carried out under conditions where kJ[M] kd or A [A] and A /[M] A +[A] here [M] represents the concentration of solvent, always in large excess. There is significant short-range order in liquids as solvent molecules are loosely bound to one another and form transient structures which reduce the mobility of the products of a decomposition. Thus the rate at which products separate by diffusion must limit the rate of unimolecular reaction in solution.Unimolecular decomposition may still be considered a two-step process ... 

Micelles of non-functional surfactants (detergents) can catalyse bimole-cular reactions by bringing reactants together in an environment conducive to reaction and they inhibit reactions by keeping reactants apart but they affect rates of unimolecular reactions by providing a submicroscopic medium. The relation between rate and surfactant concentration can be explained in terms of the distribution of reactants between the aqueous and micellar pseudophases which can also be perturbed by added solutes. Catalysis depends upon the charge type of the reaction and reactant hydrophobicity. ... 

Studies on the effects of mixed micelles on rates of unimolecular reactions appear to be nonexistent, whereas only a few papers on the effects of mixed micelles on the rates of solvolytic reactions seem to have appeared in the literature until the end of 2004. 

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