The isotope exchange reactions H++D2⇄HD+D+ and D++H2⇄HD+H+ in the temperature range 200–300 K

Abstract

Nearresonant vibrational energy transfer in ozone. Doubleresonance measurements and calculations in the temperature range 200-300 K Forward and reverse rate coefficients have been measured in the temperature range 200-300 K for the two reactions H+ + D,-=HD + D+ and D+ + H,-=HD + H+. Equilibrium constants derived therefrom agree with theoretical van't Hoff plots calculated from statistical mechanics and confirm the temperature calibration of the SIFT apparatus used. It is suggested that these reactions can be used as kinetic thermometers to measure independently the temperature of ion-molecule reaction cells. The system provides a particularly clear example of the role of statistical factors in chemical kinetics, 1 for the forward reactions and 112 for their reverse reactions; and the system illustrates further the relationship between statistical factors in kinetics, symmetry numbers in statistical mechanics, and the corresponding thermodynamic entropy changes. Constraints upon the temperature dependence of the rate coefficients are derived from consideration of thermodynamics and collision dynamics, and the data are seen to conform to these over a limited temperature range. A further trend is suggested by the data, supporting previous observations of isotopeexchange reactions-the rate coefficients of the exoergic reactions decrease with increasing temperature-and may be described in terms of the partitioning of the system according to the number of states available to the products and the original reactants. It is suggested that this should be a general result for reactions where the exoergicity is comparable to the temperature of measurement.

Dynamics of the D+ + H2 → HD + H+ reaction at the low energy regime by means of a statistical quantum method

“…19 up to E c ∼ 0.18 eV, and within the experimental uncertainties of some other ion beam techniques for larger energies up to ∼1 eV. 47 The comparison of thermal rate constants between 100 and 500 K revealed that the statistical predictions are larger than both QM and QCT results, but are on the contrary, perhaps accidentally, the closest theoretical rates to the measurements by Henchman et al 18 Recently, some of us have carried out TIQM calculations on the D + +H 2 → HD+H + reaction to obtain ICSs and thermal rate constants at low temperature. 49 The comparison with previous experimental work 19,50 revealed that the calculation reproduced rate constants beyond a collision energy of 0.006 eV.…”

“…Some of the earliest experimental work on the exchange D + +H 2 →H + +HD reaction has been devoted to measure rate constants. [15][16][17][18][19] Holliday et al 20 reported integral cross sections (ICSs) and energy distributions obtained in a tandem mass spectrometer experiment. A great deal of the theoretical investigation on the title reaction has considered a large enough energy domain to cover the electronic crossing between the energy curves of the states correlating to H + +HD and HD + +H at about 1.6 eV above the H 2 well minimum.…”

Deuterium fractionation in gas-phase reactions measured in the laboratory

“…The values (GIB 92) have a weak maximum at 300 meV and fall slowly below 1:5 × 10 −9 cm 3 s −1 at the lowest nominal collision energy achieved, 25 meV. The two VT-SIFT results measured by Henchman et al (1981) are shown as solid triangles. They appear to indicate another temperature trend.…”

“…This simplest of the chemical reactions has been studied extensively both experimentally and theoretically. As a result the dynamics of H-D exchange processes at low energies is well understood, also for other isotopic modiÿcations (Henchman et al, 1981;Gerlich, 1982). A selection of experimental results obtained until 1990 have been summarized in a review by Gerlich (1992), see for example 75,76,85 and 87 therein.…”

“…Thermal rate coe cients for the indicated proton deuteron exchange reactions. The experimental points have been measured by Henchman et al (1981), the lines are the results of a statistical calculation by Gerlich (1982). Complete thermalization of the kinetic energy of the protons and deuterons and of the ortho-para ratios has been assumed.…”