Flexible transition states

Quack M 1981 Faraday Discuss. Chem. Soc. 71 309-11, 325-6, 359-64 (Discussion contributions on flexible transition states and vibrationally adiabatic models statistical models in laser chemistry and spectroscopy normal, local, and global vibrational states)... 

The determination of the microcanonical rate coefficient k E) is the subject of active research. A number of techniques have been proposed, and include RRKM theory (discussed in more detail in Section 2.4.4) and the derivatives of this such as Flexible Transition State theory. Phase Space Theory and the Statistical Adiabatic Channel Model. All of these techniques require a detailed knowledge of the potential energy surface (PES) on which the reaction takes place, which for most reactions is not known. As a consequence much effort has been devoted to more approximate techniques which depend only on specific PES features such as reaction threshold energies. These techniques often have a number of parameters whose values are determined by calibration with experimental data. Thus the analysis of the experimental data then becomes an exercise in the optimization of these parameters so as to reproduce the experimental data as closely as possible. One such technique is based on Inverse Laplace Transforms (ILT). 

The two association reactions have been examined theoretically by Marcus, Wardlaw and co-workers [47-49, 69]. They treated these reactions using Flexible Transition State Theory (FTST), a variational derivative of transition state theory. The difficulty with association reactions such as reactions (31) and (32) is that there is no barrier to association and so there is no obvious location on the reaction coordinate for the transition state. Recent developments of TST place more emphasis in locating the molecular geometry for which the reactive flux is a minimum, and the transition state is associated with this geometry. 

RRKM theory to dissociation reactions having no barrier to the reverse process of association. Accordingly one must, in the general case, allow for the influence of exit channel couplings in order to predict the properties of separated products based on the transition state distributions embodied in N E, J). Two ways to accomplish this in approximate fashion are outlined below. One models the effect of exit channel dynamics within the framework of the flexible transition states discussed in chapter 7 and the other handles the exit channel dynamics explicitly using classical dynamics. The former model is referred to as variational RRKM theory with exact channel couplings (VRRKM/ECC). 

A simple formula for the canonical flexible transition state theory expression for the thermal reaction rate constant is derived that is exact in the limit of the reaction path being well approximated by the distance between the centers of mass of the reactants. This formula evaluates classically the contribution to the rate constant from transitional degrees of freedom (those that evolve from free rotations in the limit of infinite separation of the reactants). Three applications of this theory are carried out D + CH3, H + CH2, and F + CH3. The last reaction involves the influence of surface crossings on the reaction kinetics. 

Klippenstein SJ, Marcus RA. (1989) Application of unimolecular reaction rate theory for highly flexible transition states to the dissociation of CH2CO into CH2 and CO. J. Chem. Phys. 91 2280-2292. 

The most simple application of this methodology involves the use of a rigid transition state model. The atoms directly involved in the bond breaking/forming process are kept in fixed positions taken from a calculation of the transition state in a model system. In this case, the coordinates of the substituents are optimized at the MM level. A more sophisticated approach involves the use of a flexible transition state model. In this case, the position of all atoms are optimized. This approach requires the development of new MM parameters to describe the bond breaking/forming process. 

Application of the Canonical Flexible Transition State Theory to CH3, CF3, and CCI3 Recombination Reactions. 

S. Robertson, A. F. Wagner, and D. M. Wardlaw, /. Phys. Chem. A, 106, 2598 (2002). Flexible Transition State Theory for a Variable Reaction Coordinate Analytical Expressions and an Application. 

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