Arrhenius activation energy molecular interpretation

It is quite simple to say that this article deals with Chemical Dynamics. Unfortunately, the simplicity ends here. Indeed, although everybody feels that Chemical Dynamics lies somewhere between Chemical Kinetics and Molecular Dynamics, defining the boundaries between these different fields is generally based more on sur-misal than on knowledge. The main difference between Chemical Kinetics and Chemical Dynamics is that the former is more empirical and the latter essentially mechanical. For this reason, in the present article we do not deal with the details of kinetic theories. These are reviewed excellently elsewhere " The only basic idea which we retain is the reaction rate. Thus the purpose of Chemical Dynamics is to go beyond the definition of the reaction rate of Arrhenius (activation energy and frequency factor) for interpreting it in purely mechanical terms. 

Following the general trend of looldng for a molecular description of the properties of matter, self-diffusion in liquids has become a key quantity for interpretation and modeling of transport in liquids [5]. Self-diffusion coefficients can be combined with other data, such as viscosities, electrical conductivities, densities, etc., in order to evaluate and improve solvodynamic models such as the Stokes-Einstein type [6-9]. From temperature-dependent measurements, activation energies can be calculated by the Arrhenius or the Vogel-Tamman-Fulcher equation (VTF), in order to evaluate models that treat the diffusion process similarly to diffusion in the solid state with jump or hole models [1, 2, 7]. 

By assuming an Arrhenius type temperature relation for both the diffusional jumps and r, we can use the asymptotic behavior of /(to) and T, as a function of temperature to determine the activation energy of motion (an example is given in the next section). We furthermore note that the interpretation of an NMR experiment in terms of diffusional motion requires the assumption of a defined microscopic model of atomic motion (migration) in order to obtain the correct relationships between the ensemble average of the molecular motion of the nuclear magnetic dipoles and both the spectral density and the spin-lattice relaxation time Tt. There are other relaxation times, such as the spin-spin relaxation time T2, which describes the... 

In Chapter 7 we turn to the other basic type of elementary reaction, i.e., uni-molecular reactions, and discuss detailed reaction dynamics as well as transition-state theory for unimolecular reactions. In this chapter we also touch upon the question of the atomic-level detection and control of molecular dynamics. In the final chapter dealing with gas-phase reactions, Chapter 8, we consider unimolecular as well as bimolecular reactions and summarize the insights obtained concerning the microscopic interpretation of the Arrhenius parameters, i.e., the pre-exponential factor and the activation energy of the Arrhenius equation. 

Order of reaction. If the rate of reaction is described by the abovementioned functional form, the order of reaction is defined as 0 = a + b. t is important to note here that the unit of k depends on the order of the reaction. For example, for a zero-order reactimi, k is expressed in units of mol/m /s for a first-order reaction, k is expressed in units of 1/s and for a second-order reaction, k is expressed in units of m /mol/s. The rate cmistant is usually taken to vary with the activatimi energy ( 3) and temperature (T) by the Arrhenius law, as k = A oexp( a/BT). From molecular interpretations, the rate constant is the rate of successful collisions between the reacting molecules, the activation energy represents the minimum kinetic energy of the reactant molecules in order to form the products, and k correspraids to the rate at which these collisions occur. 

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