New Rate Constants for D + H₂and H + D₂Between ∼1150 and 2100 K

The shock tube technique with D- and H-atom atomic resonance absorption spectrometric (ARAS) detection has been used to measure rate constants for two isotopic modifications of the most fundamental chemical reaction, H + H 2 → H 2 + H: D + H 2 → HD + H (1) and H + D 2 → HD + D (2). Hydrogen atoms were produced from the thermal decomposition of either C 2 D 5 I or C 2 H 5 I. Ethyl iodide decomposition above ∼1150 K is fast, and the product ethyl radicals decompose even faster, giving ethylene and hydrogen atoms. This clean source of atoms then allows for first-order analysis of both reactant and product hydrogen atoms for determining rate constants. The rate constant results can be described by the Arrhenius expressions k 1 = 3.17 × 10 -10 exp(−5207 K / T ) cm 3 molecule -1 s -1, over the temperature range 1166−2112 K, and k 2 = 2.67 × 10 -10 exp(−5945 K / T ) cm 3 molecule -1 s -1, over the temperature range 1132−2082 K. These new results are compared to earlier results and supply additional values for evaluating the rate behavior for both reactions over the very large temperature range ∼200−2200 K. These evaluations are then compared to recent quantum mechanical scattering calculations of the thermal rate behavior that are based on a new and quite accurate potential energy surface (i.e., globally accurate to ∼0.01 kcal mol -1 ). Within experimental error, there is now complete convergence between the experimental evaluation and the new theory, bringing to completion a 75-year effort in chemical kinetics and dynamics. This is the first completely solved problem in chemical kinetics.