Atom—Molecule Kinetics Using ESR Detection. II. Results for D+H2→HD+H and H+D2→HD+D

Abstract

The wide temperature range, fast flow kinetic system with ESR atom detection described recently has been used to measure rate constants for the isotopic reactions $$\mathrm{D}+{\mathrm{H}}_2\to \mathrm{HD}+\mathrm{H},$$ $$\mathrm{H}+{\mathrm{D}}_2\to \mathrm{HD}+\mathrm{D},$$ over the range 250°—750°K and 300°—750°K, respectively. The precision of the data at a given temperature is about ±5%, except at the lowest temperature in each case where it is ±8%. It is believed that this also represents the accuracy. In the range 450°—750°K both sets of data are Arrhenius linear and obey (cubic centimeters mole⁻¹·second⁻¹) $$k_1{\text{=4.4×10}}^{13}\exp {\text{(−7610/RT);\hspace{0.17em}\hspace{0.17em}k}}_2{\text{=4.9×10}}^{13}\exp \text{(−9390/RT).}$$ Interpretation of the linear portion is made in terms of absolute rate theory with no tunneling. A potential‐energy surface constructed by fitting the experimental activation energies by the Sato method as described by Weston is in essential agreement with the latter's results for the H₃ complex. The properties of the complex give pre‐exponential factors in satisfactory agreement with experiment. The experimental ratio k₁/k₂ in the linear region is closely predicted simply by the limiting form using the ratio of collision frequencies and the zero‐point energy difference between H₂ and D₂. It is shown that the data in the nonlinear region below 450°K give apparent ``tunnel corrections'' τ for the two reactions which are qualitatively of the form τ₁<τ₂, while all theories predict τ₁>τ₂.