Reactions electronically adiabatic

This chapter concentrates on the experimental determination of product energy distributions and their interpretation in terms of the dynamics of collisions. In a volume concerned with excited states, it seemed appropriate to bias the article in favor of the spectroscopic methods and results, at the expense of the crossed-beam studies. Electronically adiabatic reactions, which pass smoothly from reactants to products in their electronic ground states, are emphasized, since the results of such processes may be compared with trajectories computed with the equations of classical, rather than wave, mechanics, and the effects of kinematic factors on the sharing of energy can be explored. 

Considerable use continues to be made of classical trajectory calculations in relating the experimentally determined attributes of electronically adiabatic reactions to the features in the potential energy surface that determine these properties. However, over the past 3 or 4 years, considerable progress has been made with semiclassical and quantum mechanical calculations with the result that it is now possible to predict with some degree of confidence the situations in which a purely classical approach to the collision dynamics will give acceptable results. Application of the semiclassical method, which utilises classical dynamics plus the superposition of probability amplitudes [456], has been pioneered by Marcus [457-466] and by Miller [456, 467-476],... 

For a collinear, electronically adiabatic reaction in the variables xAB, xBC, the procedure is to digitate these variables and number the finite-difference mesh so that the differential operator becomes a banded matrix. Then, the Schrodinger equation becomes... 

Ill) but does involve an energy exchange between the translation and vibration-rotation motions as far as the reaction coordinate in the transition region can be treated as a separable one Cosequently, then and only then for an electronically adiabatic reaction = 1 ... 

If the condition (72.Ill) of a very fast motion along the classical reaction path is realized, the factor 32 for an electronically adiabatic reaction is the actual tunneling correction, since the classical dynamical effects are then small (32 1).A dependence of 32 on... 

Por electronically adiabatic reactions, the isotopic ratio can be computed on the basis of either classical or quantum-mechanical methods for evaluating the transition probabilities. In the classical temperature range (T>2T ), = 1 and = 1 hence,... 

For an electronically adiabatic reaction (X= 1), this equation turns into the expression (24.IV) of the simple collision theory. In this case the "diatomic" model will be valid only if, after the very fast "non-adiabatic" collision of the radicals CH, the reaction is com-... 

An evaluation of factor aiders only non-adiabatic changes of the electronic state, is possible using the methods described in Sec.6.2.III. For electronically adiabatic reactions X electronically non-adiabatic reac-... 

The corresponding equations for electronically adiabatic reactions, obtained from (85.IV) and (88.IV), are... 

Eyring s equation t ac represent the real tunneling correction for an electronically adiabatic reaction only if = 1. ... 

The general treatment of the theory of chemical reactions presented in this book is based on the usual adiabatic separation of nuclear and electronic motions which permits a definition of the potential energy as a function of internuclear distances This approach proves to be very useful for the study of electronically adiabatic reactions, provided a separation of the rotation of the reacting system, treated as a supermolecule, is possible. In general, such a separation seems to be a bad approximation /10/. A consideration of the coupling of the overall rotation with the internal motions of the system means taking into account the possibility of non-adiabatic transitions from one to another potential energy surface. This is still an unsolved problem of theoretical chemistry which is open for discussion. 

The calculation of reaction rates is generally carried out in two steps. In the first step one calculates or models the potential energy surface, PES (or surfaces however in the present report we limit our attention to electronically adiabatic reactions for which only a single surface is involved). In the second step one calculates dynamical quantities, using the PES as given.It is becoming increasingly clear, however, that these two steps should not be performed independently. 

In an adiabatic reaction products will be in electronically excited state. According this criteria, the reactants and products both will be in electronically Adiabatic reaction. 

Thermal electronically adiabatic reactions (Section 4.2.1), which occur under the conditions of the Maxwell— Boltzmann distribution, compose the laigest class of bimolecular reactions. Diverse experimental studies of this class of reactions are resulted first by requests of practice. Information on elementary reactions is necessary for understanding complicated chemical reactions, which are important in the chemistry of atmosphere, combustion, for technologies, etc. 

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