Unimolecular reaction—solution

Schroeder J 1996 The role of solute-solvent interactions in the dynamics of unimolecular reactions in compressed solvents J. Phys. Condens. Matters 9379... 

Troe J 1977 Theory of thermal unimolecular reactions at low pressures. I. Solutions of the master equation J. Chem. Phys. 66 4745-57... 

The thermal decompositions described above are unimolecular reactions that should exhibit first-order kinetics. Under many conditions, peroxides decompose at rates faster than expected for unimolecular thermal decomposition and with more complicated kinetics. This behavior is known as induced decomposition and occurs when part of the peroxide decomposition is the result of bimolecular reactions with radicals present in solution, as illustrated below specifically for diethyl peroxide. 

On a phenomenological level, transitions between long-lived stable states can be described in terms of reaction rate constants. Consider, for instance a solution of two well-defined chemical species s3 and 38 that can interconvert through the unimolecular reaction... 

My idea that in bulk monomer and in some solutions the propagation is a unimolecular reaction is supported strongly by the way in which the DP depends on the monomer concentration. At low m, 1/DP increases rectilinearly with 1/m, as demanded by the conventional Mayo equation for bimolecular propagation but at high m, 1/DP increases rectilinearly with m, as is required for unimolecular propagation (see Section 5). 

The experiments show that the dilution of all the monomers leads to a change of rate, and I contend that at the earliest stage of dilution the polymerizations are still mainly unimolecular and I offer an explanation for the effects of solvents on the rate of the unimolecular reactions. Since the rate constants, k, are defined by (4.1) and (4.14) they can only be calculated if [P+ M] is known. As explained in Section 3b, there are reasons for believing that for cyclopentadiene and for isobutene [P+ M = c, but for the former there are no results for solutions, and for the latter no c values are available, so that for these monomers could only be calculated for the bulk polymerizations. 

If rates in solution and in gas phase are to be equal, the activity coefficient factor, i.e./A/B//x must be equal to unity. For unimolecular reactions, where the reactant and activated complex have similar structures and/A and /x do not differ widely, the rate of reaction in solution will be quite similar to that in the gas phase. 

Figure 3.2 An enthalpy profile for a unimolecular reaction in solution, involving the formation of a radical pair inside a solvent cage. Adapted from [61],...

Now, as for the unimolecular reaction, we could also write the mass-balance equations for 6a, 9b, and 9( and solve the algebraic equations for the rate in terms of partial pressures. However, for these expressions we obtain a quadratic equation whose solution is not very instructive. 

In this part of the chapter, we will briefly outline the main types of CL reactions which can be functionally classified by the nature of the excitation process that leads to the formation of the electronically excited state of the light-emitting species. Direct chemiluminescence is the term employed for a reaction in which the excited product is formed directly from the unimolecular reaction of a high-energy intermediate that has been formed in prior reaction steps. The simplest example of this type of CL is the unimolecular decomposition of 1,2-dioxetanes, which are isolated HEI. Thermal decomposition of 1,2-dioxetanes leads mainly to the formation of triplet-excited carbonyl compounds. Although singlet-excited carbonyl compounds are produced in much lower yields, their fluorescence emission constitutes the direct chemiluminescence emission observed in these transformations under normal conditions in aerated solutions ... 

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-... 

From a structural point of view, mechanism in a single crystal can be much closer to a set of identical atomic trajectories than to the kind of fuzzy statistical average with which one must be content in solution. It is not surprising that with this kind of structural uniformity the site problems that plague kinetic studies in rigid glasses disappear. Adherence to first-order rate laws can be as close in single crystals as it is in fluids, and equally valid activation parameters can be obtained for thermal unimolecular reactions of reaction intermediates [12]. 

The knowledge of the two-minima energy surface is sufficient theoretically to determine the microscopic and static rate of reaction of a charge transfer in relation to a geometric variation of the molecule. In practice, the experimental study of the charge-transfer reactions in solution leads to a macroscopic reaction rate that characterizes the dynamics of the intramolecular motion of the solute molecule within the environment of the solvent molecules. Stochastic chemical reaction models restricted to the one-dimensional case are commonly used to establish the dynamical description. Therefore, it is of importance to recall (1) the fundamental properties of the stochastic processes under the Markov assumption that found the analysis of the unimolecular reaction dynamics and the Langevin-Fokker-Planck method, (2) the conditions of validity of the well-known Kramers results and their extension to the non-Markovian effects, and (3) the situation of a reaction in the absence of a potential barrier. 

In addition to unimolecular reactions, the excited state may participate in several bimolecular processes. At high concentrations, dimer formation, excimer formation, exciplex formation, solute-solvent complexation, energy transfer, and collosional deactivation may occur. The high-concentration conditions are often experienced when the guest molecules are loaded onto the layered materials with high coverages and specific examples will be provided shortly. 

Rates of reactions in solution and unimolecular reactions in the gas phase are dependent on pressure. 

Although the specific decomposition rate is much lower in nitric acid solution the decomposition still follows the first order law because the number of simple molecules which decompose is always proportional to the total concentration of nitrogen pentoxide at any time, and this is the criterion of a unimolecular reaction. 

In our discussion of the reactions of acid chlorides, we deduced that a unimolecular reaction to give a cation must be happening. This cation cannot be detected under these conditions as it reacts too quickly with nucleophiles. If we remove reactive nucleophiles from solution, the cation is still too unstable to be isolated at room temperature. But if we go down to -120°C we can keep the cation alive long enough to run its NMR spectrum. 

The hydration of hexametaphosphate, (NaP03)6, also proceeded as a unimolecular reaction. In neutral or alkaline solution ortho-... 

In the case of 16a and 16b, it has been demonstrated that the rate constants are almost independent of concentration and solvent polarity, which indicates that the racemization is a unimolecular reaction and does not involve ionic species in the rate-controlling step. The value of k2 is typically about 4.2 x 10 s at 25 °C. Activation parameters have been calculated from the rate constants measured in a temperature range from 20.4 to 39.8°C, revealing the values for A// = 24.3 kcalmol , and AA = —2.0calK . The same authors also reported thermal isomerization in solution between diastereomers of 20 accompanied by decomposition. Gradual isomerization in solution of m-9 into trans- obeying the first-order kinetics (k = 4.0 x 10 s , = 0.966) has also been reported <2005T6693>. As in the case of 16... 

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