Rate constant, specific

Single-photon vibrational excitation of CH3NC at 726.5 nm in Reddy and Berry s experiments produced molecules with 39.3 kcal/mole internal energy. The Stern-Volmer plot was linear. The assumption is that strong collisions gave a value of k e) = 2 x 10 s , only slightly larger than the 

Although their results can be matched satisfactorily by the simplest theoretical model of rapid /w r molecular and facile /w ermolecular energy transfer, Reddy and Berry caution against accepting their results at face value. They point out that collisions, rather than an intramolecular process, could transfer molecules from the prepared state to others of essentially the same energy. This mechanism could also explain their results and would be more consistent with trajectory calculations. Despite the elegance of Reddy and Berry s experiment, it appears that still more sophisticated studies are required to resolve this question. 

Especially detailed photoactivation studies have been carried out on the dissociation of NO2 and on the isomerization of cycloheptatriene. As well as the fluorescence experiments referred to in Section 1.3.4, the photodissociation of NO2 has been studied at selected wavelengths below the photochemical threshold at 397.9 nm. The photochemistry of NO2 is complicated by the profusion of low-lying excited states. It appears that it is a state that is primarily excited, but that these molecules rapidly decay into a dense manifold of levels which are strongly mixed with the ground state.Although this complicates some issues, it means that photochemical and thermal dissociation can be directly compared, and the system serves as a prototype of a small molecule decomposing into two free-radical fragments. 

Plots of against [M] are quite linear but the slopes show a marked dependence on the identity of M. With CHT + CHT collisions assumed to be strong, k e) was calculated for several different initial energies. The values obtained exceed the RRKM predictions, especially at the highest initial energies where 4.5 x A rrkm( )- In such a large molecule, 

Most of the numerous chemical activation studies on unimolecular reactions were carried out prior to 1972, when they were authoritatively reviewed by Setser. Experiments on halocarbons chemically activated by a recombination process, such as (1.63a), are particularly valuable. Although for most species only one activation route has been used, results for different 

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A bimoleciilar reaction can be regarded as a reactive collision with a reaction cross section a that depends on the relative translational energy of the reactant molecules A and B (masses and m ). The specific rate constant k(E ) can thus fonnally be written in tenns of an effective reaction cross section o, multiplied by the relative centre of mass velocity... 

Quack M and Troe J 1974 Specific rate constants of unimoiecuiar processes ii. Adiabatic channei modei Ber. Bunsenges. Phys. Chem. 78 240-52... 

If all the resonance states which fomi a microcanonical ensemble have random i, and are thus intrinsically unassignable, a situation arises which is caWtA. statistical state-specific behaviour [95]. Since the wavefunction coefficients of the i / are Gaussian random variables when projected onto (]). basis fiinctions for any zero-order representation [96], the distribution of the state-specific rate constants will be as statistical as possible. If these within the energy interval E E+ AE fomi a conthuious distribution, Levine [97] has argued that the probability of a particular k is given by the Porter-Thomas [98] distribution... 

If the state-specific rate constants are assumed continuous, equation (A3.12.65) can be written as [103]... 

Troe J 1983 Specific rate constants k(E, J) for unimolecular bond fissions J. Chem. Phys. 79 6017-29... 

In equation (A3.13.24), /c. is the specific rate constant for reaction from level j, and are energy transfer... 

Cobos 0 J, Hippier H and Tree J 1985 High-pressure falloff curves and specific rate constants for the reactions J. Phys. Chem. 89 342-9... 

The power law developed above uses the ratio of the two different shear rates as the variable in terms of which changes in 17 are expressed. Suppose that instead of some reference shear rate, values of 7 were expressed relative to some other rate, something characteristic of the flow process itself. In that case Eq. (2.14) or its equivalent would take on a more fundamental significance. In the model we shall examine, the rate of flow is compared to the rate of a chemical reaction. The latter is characterized by a specific rate constant we shall see that such a constant can also be visualized for the flow process. Accordingly, we anticipate that the molecular theory we develop will replace the variable 7/7. by a similar variable 7/kj, where kj is the rate constant for the flow process. 

Kinetics. Details of the kinetics of polymerization of THF have been reviewed (6,148). There are five main conclusions. (/) Macroions are the principal propagating species in all systems. (2) With stable complex anions, such as PF , SbF , and AsF , the polymerization is living under normal polymerization conditions. When initia tion is fast, kinetics of polymerizations in bulk can be closely approximated by equation 2, where/ is the specific rate constant of propagation /is time [I q is the initiator concentration at t = 0 and [M q, [M and [M are the monomer concentrations at t = 0, at equiHbrium, and at time /, respectively. 

Butylene isomers also can be expected to show significant differences in reaction rates for metaHation reactions such as hydroboration and hydroformylation (addition of HCo(CO). For example, the rate of addition of di(j -isoamyl)borane to cis-2-huX.en.e is about six times that for addition to trans-2-huX.en.e (15). For hydroformylation of typical 1-olefins, 2-olefins, and 2-methyl-l-olefins, specific rate constants are in the ratio 100 31 1, respectively. 

Hydrolysis to Glycols. Ethylene chlorohydrin and propylene chlorohydrin may be hydrolyzed ia the presence of such bases as alkaU metal bicarbonates sodium hydroxide, and sodium carbonate (31—33). In water at 97°C, l-chloro-2-propanol forms acid, acetone, and propylene glycol [57-55-6] simultaneously the kinetics of production are first order ia each case, and the specific rate constants are nearly equal. The relative rates of solvolysis of... 

The rate constants reported in Table 4 are double the specific rate constant for a given site when two identical sites are present, so that the actual position reactivity is half of the level indicated. Since the rates are reported as seeond order, the... 

Specific Rate Constants for the Reaction of Chloboazines with Various Amines... 

Photoinitiation is an excellent method for studying the pre- and posteffects of free radical polymerization, and from the ratio of the specific rate constant (kx) in non-steady-state conditions, together with steady-state kinetics, the absolute values of propagation (kp) and termination (k,) rate constants for radical polymerization can be obtained. 

Table 5 Specific Rate Constant kr x 10 min ) of the Resin Catalyzed Hydrolysis of Ethyl Propionate in Aqueous Solvents at 25°C...

In a chemostat and biostat or turbidostat, even with differences in the supply of nutrients and/or fresh media, constant cell density is obtained. The utilisation of substrate and the kinetic expressions for all the fermentation vessels are quite similar. It is possibile to have slight differences in the kinetic constants and the specific rate constants.3,4 Figure 5.9 shows a turbidostat with light sources. The system can be adapted for photosynthetic bacteria. 

This paper also reported results for three permanent dipole molecules, HC1, H20, and NH3, which exhibited an energy dependence of the specific rate constant in this energy range. The data for ammonia are typical and are presented in Figure 7. The points are experimental results,... 

If the intermediate compound XZ is very unstable, Z cannot serve as a catalyst, while if it is very stable then the reaction stops. The intermediate compound XZ must have the right degree of stability for the catalyst to be effective. It must be borne in mind that the catalyst will accelerate the forward as well as the reverse reactions to the same extent, so that the ratio of the specific rate constants for the forward (kf) the backward (kb) reactions will not be affected. As an example a gaseous reaction between sulfur dioxide and oxygen to yield sulfur trioxide may be considered. The reaction, which can be represented by the equation... 

If the activity coefficients are assumed to be unity, the specific rate constant k is then identical to k K. It can be shown that... 

Equations (2.10) and (2.12) are identical except for the substitution of the equilibrium dissociation constant Ks in Equation (2.10) by the kinetic constant Ku in Equation (2.12). This substitution is necessary because in the steady state treatment, rapid equilibrium assumptions no longer holds. A detailed description of the meaning of Ku, in terms of specific rate constants can be found in the texts by Copeland (2000) and Fersht (1999) and elsewhere. For our purposes it suffices to say that while Ku is not a true equilibrium constant, it can nevertheless be viewed as a measure of the relative affinity of the ES encounter complex under steady state conditions. Thus in all of the equations presented in this chapter we must substitute Ku for Ks when dealing with steady state measurements of enzyme reactions. 

It is the specific rate constant k that is a direct reflection of the reactivity of a molecule toward a particular process, not the product yield or quantum yield. 

In addition, it was observed that the sensitized photolysis produced the same distribution of products with the same efficiency (fingerprint characteristic of the triplet state). From quenching studies the specific rate constant for the rearrangement could be obtained. Phenyl migration rearrangement is of intermediate efficiency, interposed between the more efficient and less efficient type A processes (Table 7.4). The type of mechanism proposed for this transformation is as follows ... 

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