Application of variational transition-state theory and the unified statistical model to H + Cl₂ → HCl + Cl

Microcanonical variational theory with a semiclassical adiabatic ground-state transmission coefficient predicts the thermal rate constant for collinear H + Cl₂ within 6% for 200-1000 K and the kinetic isotope effect k_H/k_D within 6 or 9% (depending on the detailed assumptions in the transmission coefficient) over the same temperature range. Other versions of variational transition-state theory and even conventional transition-state theory with semiclassical or quantum mechanical adiabatic ground-state transmission coefficients are also tested and show similar accuracy. The variational calculations predict a greater breakdown of conventional transition-state theory at higher temperatures and for three-dimensional reactions. The unified statistical theory is less accurate than variational transition-state theory for the magnitudes of the collinear rate constants, but more accurate for collinear kinetic isotope effects.