(334d) Elucidation of Structure-Reactivity Relationships Using Quantum Chemistry

Over the past ten years, the use of quantum chemistry has become widespread for estimation of kinetic and thermodynamic quantities. The advent of comprehensive, user-friendly software such as Gaussian and GAMESS and the immense popularity of density functional theory have greatly increased the number of practitioners in the field of quantum chemistry. One area in which quantum chemistry has not been widely exploited is in the derivation of quantitative structure-reactivity relationships or QSARs. Such relationships, such as the popular Evans-Polanyì relationship, are widely used in kinetic modeling to estimate the rate constants for large numbers of reactions where experimental data are questionable or absent. This talk reviews advances we have made in deriving QSARs for a number of important reaction classes in oxidation chemistry as well as in developing a consistent algorithmic approach for elucidation of other relationships. Two important factors that must be considered when kinetic and thermodynamic data are calculated for the purpose of elucidating a QSAR are: (1) the choice of method and basis set and (2) the model used to calculate vibrational frequencies. While the choice of method and basis set greatly influence the barrier and heat of reaction for a single reaction, it is not clear to what extent, for example, the size of basis set will affect parameters regressed from a typical Evans-Polanyì plot. Additionally, the treatment of low-frequency vibrations as harmonic oscillators is known to have a strong impact on the Arrhenius pre-exponential factor whereas the effect of this assumption upon the other important quantities is more subtle. Several examples involving hydrogen transfer reactions will be used to show the influence of both factors on the final results obtained. We have used our approach to study four different reaction families germane to condensed-phase, free radical oxidation of hydrocarbons. Beta scission of alkoxy radicals, intramolecular hydrogen transfer of alkylperoxy radicals, intermolecular hydrogen transfer between alkoxy/alkylperoxy radicals, and reactions in the scavenging mechanism of several hindered phenol antioxidants have been explored and QSARs have been developed in order to provide quick and reliable estimates of the rate constants for reactions in these families. For the intramolecular hydrogen transfer reaction family, we show two examples that completely fail to follow typical Evans-Polanyì behavior. Features of these QSARs will be reviewed as well as our recommended approach for obtaining new QSARs using quantum chemistry and transition state theory.