Barrier height atomic reaction systems

We start with the reaction of abstraction of a hydrogen atom by a CH3 radical from molecules of different matrices (see, e.g., Le Roy et al. [1980], Pacey [1979]). These systems were the first to display the need to go beyond the one-dimensional consideration. The experimental data are presented in table 2 together with the barrier heights and widths calculated so as to fit the theoretical dependence (2.1) with a symmetric gaussian barrier. 

It is seen from this figure that the barrier height for the reactants is equal to about 6.2 kcal/mol, while the reaction energy calculated as the difference of the products minus the energy of the reactants is equal to about —15.2 kcal/mol (an exothermic reaction). What happens to the atoms when the system moves along the reaction path This is shown in Fig. 14.9. 

The idea of benchmarking quantum chemical methods by introducing databases covering a wide variety of different properties, for example, atomization energies, spectroscopic properties, barrier heights and reaction energies of diverse reactions, proton affinities, interaction energies of noncovalent bond systems, transition metal systems, and catalytic processes, was extended by Truhlar and coworkers [51]. They were the first to carry out overall statistical analyses of combinations of different test sets to obtain an overall mean absolute deviation (MAD) number for each tested quantum chemical method, which made a comparison with other approaches more feasible. 

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