Accuracy of trajectory calculations and transition state theory for thermal rate constants of atom transfer reactions

The reliability of several practical techniques for computing the equilibrium rate constant of elementary atom-transfer reactions is discussed. Conventional transition state theory and two generalizations, the canonical variational theory of reaction rates (also known as the method of free energy surfaces) and the adiabatic theory of reactions (also known as the microcanonical variational theory of reactions), are all considered. For these theories the transmission coefficient is set equal to unity as usual. In addition the quasi-classical trajectory method and three extensions, the quasi-classical trajectory method with quantum mechanical energetic threshold, the quasi-classical trajectory reverse histogram method, and the classical S matrix theory, are considered. Results of the application of these theories to compute thermal rate constants for collinear and three-dimensional systems where accurate quantal calculations are available are reviewed. The systems considered are H + H₂, Cl + H₂, H + Cl₂, F + H₂, H₂ + I, and isotopic analogues at 300-1500 K. The comparisons discussed should allow more realistic estimates to be made of the errors in using these approximate theories to calculate thermal rate constants, isotope effects, and activation energies for chemical reactions.