| The search for a method to calculate the rates of reactions vs. temperature has been a long-standing endeavor. This document outlines concepts that bring us closer to this goal.; A new computational procedure for the characterization of transition states for chemical reactions is proposed and tested. If we search the “inexpensive” intrinsic reaction coordinate (IRC) for the maximum of Energy[Method(1)] along this reaction path, the resulting “IRCMax method”, Max{lcub}Energy[Method(1)]{rcub}//IRC{lcub}Geom[Method(2)]{rcub}, reduces errors in transition state geometries by a factor of four to five, and reduces errors in classical barrier heights by as much as a factor of ten. When applied to the CBS-4, G2(MP2), G2, CBS-Q, and CBS-QCI/APNO model chemistries, the IRCMax method reduces to the standard model for the reactants and products, and gives RMS errors in the classical barrier heights for ten atom exchange reactions of 1.3, 1.2, 1.0, 0.6, and 0.3 kcal/mol respectively.; We have reexamined several high accuracy methods for computational thermochemistry: G2, G2(MP2), CBS-Q, G2(MP2,SVP), CBS-q, CBS-4, and B3LYP/6-311+ G(3df,2p). We have employed ΔfH0298 for the “extended G2 neutral test set” for this comparison. Experimental spin-orbit interactions have been included in all calculated atomic energies. We determine “isodesmic bond additivity corrections” (BACs) for several types of bonds by least-squares fits to the heats of formation for 76 organic species with up to 10 carbons and a variety of heteroatoms. The mean absolute deviations are reduced from: 1.49, 1.93, 1.22, 1.53, 2.28, 3.09 and 3.45 kcal/mol to: 0.55, 0.57, 0.77, 0.63, 1.03, 0.98, and 1.16 kcal/mol. The maximum errors are reduced to about 3 kcal/mol for all but the DFT method (4.2 kcal/mol). The BACs are especially useful for larger molecules with many similar bonds. |
