Use of Buckingham potentials in engineering approximations for chemical kinetics

Previous investigators have often used the Polanyi relationship to correlate activation barriers for series of reactions. The correlations often work, but a different Polanyi relationship is needed for each class of reactions. As a result, the correlations are difficult to extrapolate to new situations. In this article, we suggest new correlations that show greater capability to correlate rate data. Quantum mechanical calculations are done which indicate that the main barriers to ligand exchange reactions are associated with getting the reactants close enough to react. The energy to get the reactants close enough to react can be calculated from a united-atom Buckingham potential and correlated to rate data. The additional correlating parameter allows the activation energies for a series of hydrocarbon reactions to be collapsed onto a single line. The root-mean-square error of the line is 2 kcal/mol compared to 7 kcal/mol with the Polanyi relationship alone. This shows that our new correlating parameter will be useful in engineering approximations in kinetics.