Mechanism Lindemann

The system of coupled differential equations that result from a compound reaction mechanism consists of several different (reversible) elementary steps. The kinetics are described by a system of coupled differential equations rather than a single rate law. This system can sometimes be decoupled by assuming that the concentrations of the intennediate species are small and quasi-stationary. The Lindemann mechanism of thermal unimolecular reactions [18,19] affords an instructive example for the application of such approximations. This mechanism is based on the idea that a molecule A has to pick up sufficient energy... 

This approxunation is generally valid if For the Lindemann mechanism of unimolecular reactions... 

Thus, the competition between deactivation of the intermediate A and product formation is given in terms of the ratio a = Id lk, . When the second-order rate constants k, k2, and ki are set for the system, the ratio a is directly proportional to the pressure [M], since a = ( 2/ 3)[M]. Thus, the effect of varying [M], the variable in the Lindemann mechanism that defines the pressure, can... 

C. A. Hollingsworth, P. G. Seybold, L. B. Kier, and C.-K. Cheng, First-order stochastic cellular automata simulations of the Lindemann mechanism. Int. J. Chem. Kinet. 2004, 36, 230-237. 

Lindemann Mechanism of unimolecular reactions — activation by collisions... 

According to the Lindemann-Christiansen hypothesis, formulated independently by both scientists in 1921, all molecules acquire and lose energy by collisions with surrounding molecules. This is expressed in the simplified form of the Lindemann mechanism, in which we use an asterisk to indicate a highly energetic or activated molecule, which has sufficient energy to cross the barrier towards the product side, and M is a molecule from the surroundings M may be from the same type as A ... 

If the stoichiometric equation for unimolecular reaction is A -> B + C, and if the energized molecules are denoted by A, the Lindemann mechanism consists of the following sequence of events. 

Thus, according to this (Lindemann) mechanism, a unimolecular reaction is first-order at relatively high concentration (cM) and second-order at low concentration. There is a... 

A test of the Lindemann mechanism is normally applied to observed apparent first-order kinetics for a reaction involving a single reactant, as in A - P. The test may be used in either a differential or an integral manner, most conveniently by using results obtained by varying the initial concentration, cAo (or partial pressure for a gas-phase reaction). In the differential test, from equations 6.4-20 and -20a, we obtain, for an initial concentration cAo = cM, corresponding to the initial rate rPo,... 

As carried out above for the Lindemann mechanism, application of the steady-state approximation gives the apparent unimolecular rate constant in Equation (24) where [Av] represents the IR photon density. Again two limits may be considered. 

The isomerization of cyclopropane follows the Lindemann mechanism and is found to be unimolecular. The rate constant at high pressure is 1.5 x 10- s- and that at low pressure is 6 X 10- torr- s-K The pressure of cyclopropane at which the reaction changes its order, found out ... 

The following Lindemann mechanism for the unimolecular decomposition of a molecule A in the presence of a species Y (which may be any molecule such as inert gas like Helium or even A itself)y considered ... 

Lindemann mechanism, Free radical chain mechanism, Enzyme kinetics, Surface chemistry... 

Use the following data for a unimolecular decomposition to determine k and k2 which appear in the simple Lindemann mechanism assume that k has a value of 5.0 x 1010 mol 1 dm3 s From this determine the mean lifetime of the activated molecule. Comment on the results. 

The graph of 1 / versus 1 / [A] is linear, and the data fits the simple Lindemann mechanism, giving... 

The same result is traditionally derived from the high-pressure limit of the Hinshelwood-Lindemann mechanism see Section 7.4.1. 

The activation and its interplay with chemical reaction is often described by the so-called (generalized) Hinshelwood Lindemann mechanism ... 

The Lindemann mechanism of thermal collisional activation describes these systems ... 

Thermal Activation. A slightly different formulation must be employed for excitation of M by a thermal activation process since the stabilization products cannot be measured directly and the rate of activation is related to a rate of collision. The usual Lindemann mechanism... 

Pressure-dependent rate constants for the syn-anti conformational process in larger alkyl nitrites provide a further test of the ability of RRKM theory to successfully model the kinetics of the internal rotation process in these molecules. Solution of the Lindemann mechanism shows that at the pressure where the rate constant is one-half of its limiting high-pressure value, Pm, the frequency of deactivating collisions is comparable to , the average rate that critically... 

Rabinovitch and co-workers found that the Lindemann mechanism is adequate for modeling the pressure dependence of bimolecular region unimolecular rate constants for extracting collision efficiencies for the methyl isocyanide isomerization [122]. For the conformer conversion of molecule A at constant temperature, it can be written as,... 

In the Lindemann mechanism, a time lag exists between the energisation of A to A and the decomposition (or isomerisation) of A to products. During this time lag, A can be de-energised back to A. According to the steady-state approximation (s.s.a), whenever a reactive (i.e., short-lived) species is produced as an intermediate in a chemical reaction, its rate of formation is equal to its rate of decomposition. Here, the energised species A is short-lived. Its rate of formation = kJAp and its rate of decomposition k t [A][A ] + k2[A ]. Thus,... 

Note The Lindemann mechanism was also suggested independently by Christiansen. Hence, it is also sometimes referred to as the Lindemann-Christiansen mechanism. The theory of unimolecular reactions was further developed by Hinshelwood and refined by Rice, Rampsberger, Kassel and Marcus. 

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