Electronic structure theory channels

Fig. 14A The reaction H2 + OH H2O + H eneigy profile V )(s) for —00 < s < 00. The value of the reaction coordinate i = —00 corresponds to the reactants, while = 00 correspraids to the products. It turns out that the product energy (shown by a small square on the right) is lower than the energy of the reactants (i.e., the reaction is exothermic). The barrier height in the entrance channel calculated as the difference of the top of the barrier and the lowest point of the entrance channel amounts to 6.2 kcal/mol. Source T. Dunning, Jr. and E. Kraka. from At/wo/rccs in Molecular Electronic Structure Theory, ed. T. Dunning, Jr., JAI Press, Greenwich, CT (1989), courtesy of the authors.

Direct dynamics simulations based on a high level of electronic structure theory, may be performed to study chemical events that occur in a short time. Thus, though a large amount of compute time is required for each integration step, only a small number of integration steps are required. High-level direct dynamics is practical for simulating the exit-channel dynamics of a chemical reaction from the transition state to products, since this is usually a direct... 

To strike a balance, structural information will be presented only for those cases of direct relevance to the photoinitiated reactions, namely (i) a few examples of CO2-HX and N2O-HX spectra in which large, qualitative differences can be seen, (ii) a compilation of all experimentally derived distances and angles, and (iii) comparisons to calculated values, when a combination of experiment and theory is needed to draw conclusions. What has been rather absent so far in studies of weakly bonded complexes, is information about systems involving heavy atoms. It is true that species like iodine and bromine do not necessarily bring joy to the lives of electronic structure theorists, but these are the systems most amenable to the experimental photoinitiated reaction studies. Also, the possibility of marked differences for seemingly homologous series makes quantitative evaluations imperative. Our discovery of the qualitative structural difference between CO2-HCI and C02-HBr underscores this point [34]. As pragmatists, we were able to quickly exploit this difference, but more importantly, it may be possible in the future to systematically use such differences to examine different photochemical entrance channels. 

The determination of the timescales and of the several channels of deactivation of the excited molecule is essential. For this reason, a strong interplay between experiments and theory is mandatory. The first role of the theoretical study is to elucidate the electronic structure of the molecules, to determine the low-lying electronic transitions, and to assign... 

Also, it should be re-emphasized that the model assumes that the only condition to be satisfied for a reaction to occur is one of closeness of approach, and does not mention anything about the structures of reactants and products. Thus the model is not able to predict any possible dependence of a on the internal energy nor what fraction of the products is produced in excited vibrational and electronic states. In particular, there is no way that this theory can predict relative probability C, with which each exit channel i from the complex is used. In the cases of multi-channel reactions, eqn. (53) is better written as... 

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