Rate constant unimolecular homogenous

The unimolecular quenching rate constant, k, defines the reaction efficiency when the probe and quencher are in the supramolecular structure. The value for the effective quenching rate constant can be obtained from conventional steady-state or time-resolved quenching experiments, provided all of the excited probe molecules are complexed to the supramolecular structure. This rate constant is an easy parameter to measure experimentally, but it only provides information on accessibility when compared to the quenching rate constant in homogeneous solution. This comparison assumes that the intrinsic reactivity (i.e., the reactivity after diffusion) is the same in the homogeneous solution and in the supramolecular system. This is a fair assumption when the... 

Central to catalysis is the notion of the catalytic site. It is defined as the catalytic center involved in the reaction steps, and, in Figure 8.1, is the molybdenum atom where the reactions take place. Since all catalytic centers are the same for molecular catalysts, the elementary steps are bimolecular or unimolecular steps with the same rate laws which characterize the homogeneous reactions in Chapter 7. However, if the reaction takes place in solution, the individual rate constants may depend on the nonreactive ligands and the solution composition in addition to temperature. 

The first indication that A-acyloxy-A-alkoxyamidcs reacted by an acid-catalysed process came from preliminary H NMR investigations in a homogeneous D20/ CD3CN mixture, which indicated that A-acetoxy-A-butoxybenzamide 25c reacted slowly in aqueous acetonitrile by an autocatalytic process according to Scheme 4 (.k is the unimolecular or pseudo unimolecular rate constant, K the dissociation constant of acetic acid and K the pre-equilibrium constant for protonation of 25c).38... 

The decomposition is homogeneous and probably unimolecular, with rate constants given by... 

Fig. 9. Log-log plot of the homogeneous unimolecular rate constant vs. pressure in the decomposition of C2F40 at 126°C. The solid lines represent theoretical curves from Kassel theory with 8 and 9 effective oscillators. From Lenzi and Mele106 with permission of the American Institute of Physics.

After in the foregoing chapter thermodynamic properties at high pressure were considered, in this chapter other fundamental problems, namely the influence of pressure on the kinetic of chemical reactions and on transport properties, is discussed. For this purpose first the molecular theory of the reaction rate constant is considered. The key parameter is the activation volume Av which describes the influence of the pressure on the rate constant. The evaluation of Av from measurement of reaction rates is therefor outlined in detail together with theoretical prediction. Typical value of the activation volume of different single reactions, like unimolecular dissociation, Diels-Alder-, rearrangement-, polymerization- and Menshutkin-reactions but also on complex homogeneous and heterogeneous catalytic reactions are presented and discussed. 

Figure 4.12 Variation of reverse transition time rr with the time of forward electrolysis tx. The quantity k represents the unimolecular homogeneous rate constant for the conversion of the product of the forward electrolysis to a nonelectroactive species (in terms of the reverse electrolysis).

For homogeneous unimolecular reactions, the reaction rate is proportional to the first power of the concentration of the species reacting. For a homogeneous bimolecular process, the observed reaction rate is the rate constant multiplied by the concentration of the activated complex, which is, in turn, proportional to the product of the partial pressures of the two reacting species. 

Due to geometric differences, homogeneous coUisional activation and heterogeneous activation by the walls of a VLPP reactor have to be treated in a slightly different way. This has been investigated for a spherical vessel by Barker. He finds that the corresponding unimolecular rate constant can be expressed as ... 

Rate constants for unimolecular homogeneous PH3 decomposition were calculated by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory and by the use of estimated values for the activation energies. Rate constants at the high-pressure limit for reaction (5), log(k/s)= 14.18-11 610/T [5] or 14.00-12610/T [4], include activation energies of 222 or 241 kJ/mol, respectively. Calculated rate constants for reaction (6) are log(k/s)=15.74-18 040/T with an activation energy of 345 kJ/mol. At 900 K PH formation is thus predicted to exceed PH2 formation by a factor -10. Calculated fall-off pressures for both reactions which indicate the onset of second-order decomposition, are quite high, about 10 Torr in an H2 bath gas [5]. 

For reactions treated in this work, where one observes second-order intermolecular ET between freely diffusing small donor and acceptor molecules in homogeneous solution, ET is assumed to be preceded by the diffusion-controlled formation of a donor-acceptor collision complex. When formation and disassociation occur at or near diffusion control and are rapid relative to ET, the observed second-order ET rate constant, fc(obsvd)> is the product of the equilibrium constant for collision complex formation, K, and the unimolecular rate constant from Eq. (10.4) for ET from within the precursor complex (Eq. (10.6) [32, 33b] ... 

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