Chemical reaction rate theory, relaxation kinetics

With the advent of picosecond and subsequently femosecond laser techniques, it became possible to study increasingly fast chemical reactions, as well as related rapid solvent relaxation processes. In 1940, the famous Dutch physicist, Kramers [40], published an article on frictional effects on chemical reaction rates. Although the article was occasionally cited in chemical kinetic texts, it was largely ignored by chemists until about 1980. This neglect was perhaps due mostly to the absence or sparsity of experimental data to test the theory. Even computer simulation experiments for testing the theory were absent for most of the intervening period. 

The competition between intramolecular vibrational relaxation and chemical reaction has been discussed in terms of the applicability of transition state theory to the kinetic analysis [6], If the environment functions mainly as a heat bath to ensure thermalization among the vibrational modes in the excited complex, then transition state theory is a good approximation. On the other hand, when the reaction is too fast for thermalization to occur the rate can depend upon the initial vibronic state. Prompt reaction and prompt intersystem crossing are, by definition, examples of the latter limit. 

Our approach to the study of the departure from equilibrium in chemical reactions and of the "microscopic theory of chemical kinetics is a discrete quantum-mechanical analog of the Kramers-Brownian-motion model. It is most specifically applicable to a study of the energy-level distribution function and of the rate of activation in unimolecular (dissociation Reactions. Our model is an extension of one which we used in a discussion of the relaxation of vibrational nonequilibrium distributions.14 18 20... 

The immediate relation to the relaxation concept of enzyme catalysis [3,4, 20, 21] is the theory of rate processes that has been developed by Fain [35]. In this paper. Fain states that the conventional approach to the kinetics of chemical reactions does not hold true for highly ordered macromolecular structures. The traditional approach to the problem implies that all vibrational modes undergo fast relaxation to thermal equilibrium. In the conventional approach to chemical kinetics, the changes in electronic states and nuclear vibration amplitudes were considered separately. Fain has proposed the self-consistent description of simultaneous changes taking place in the... 

There were many attempts at a theoretical description of the relaxational motion of the enzyme molecule. In order to describe the behavior of an individual enzyme complex, we have to discard the usual approach of the transition-state theory. According to this theory [33], an elementary act of chemical transformation proceeds via a so-called activated complex which is in a state of thermodynamic quasi-equilibrium. The classical chemical kinetics always consider the equilibrium state as a dynamic process. At equilibrium, the rates of direct and reverse reactions are equal but both processes never stop. In a mechanical process, equilibrium means the end of any movement. 

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