The reaction H2+D2⇄2HD. A long history of erroneous interpretation of shock tube results

Abstract

Rate constants for the reaction, H+D2→HD+D, over the temperature range, 724-2061 K, by the flash photolysis shock tube technique An ultraclean 2 in. Ld. single pulse shock tube coupled to an atomic resonance absorption system was constructed in order to determine hydrogen atom concentration by Lyman-a absorption. The tube was baked to 300·C and pumped down to _10-7 Torr. Ultrapure argon could be shock heated to -2500 K with no spurious H atom absorption. The system was constructed in order to study the kinetics of chemical reactions which are strongly catalyzed by H atoms, under the conditions where no such atoms are present. Specifically, the role of H atoms in the H2 + D2---+2HD exchange reaction was studied. Mixtures of hydrogen and deuterium diluted in argon were shock heated to 1375-1760 K; samples were then taken from the tube and analyzed mass spectrometrically for the ratio [HD]/[D2J. 1400 K was the highest temperature at which no spurious H atom absorption was observed in a shocked mixture of 1% H2-1 % D 2 . At 1400 K, under the conditions of no absorption, no, or s; I %HD conversion was obtained. At higher temperatures Lyman-a absorption was detected and more HD conversion was observed. A comparison between these results and results obtained previously in conventional systems suggests that the high HD conversion observed in the past was caused by hydrogen atoms generated from impurities. The existence of a molecular mechanism in the H2-D2 exchange reaction is highly doubtful.2742

Four-center reactions: A quantal model for H4

“…24 Silver and Stevens 24 investigated the largest variety of nuclear geometries and found a path on the H 4 PES for the exchange reaction leading from trapezoidal to linear structures for energies just above the dissociation energy of the H 2 molecule. No reaction path requiring less energy than a single H 2 dissociation was found, which is in accordance with the later experiment of Lifshitz et al 23 Brown and Silver carried out quasiclassical trajectory calculations on different London-type and valence-bond surfaces finding that the branching ratios for the inelastic, exchange reaction and dissociative processes are extremely sensitive to the details of the PES. 25 Schwenke 19 carried out quasiclassical trajectories calculations on the three-body recombination rates of H 2 in the presence of H 2 , allowing the possibility of the exchange process.…”

“…Hydrogen atoms generated by the impurities would initiate the chain reactions HϩD 2 →HDϩD,DϩH 2 →HDϩH; hence the percentage of HD arised from the 4C reaction was difficult to ascertain. More recently, Lifshitz and co-workers 23 carried out experiments with an ultraclean shock tube, where the concentration of H atoms could be obtained from Lyman-␣ absorption. They found that, when no H atoms are detected, no ͑or р1%͒ HD products are found.…”

Global nine-dimensional potential energy surface for the H5 system. I. Ab initio multiple reference single and double excitation configuration interaction computations

“…7 This reaction has been the subject of great controversy on its mechanism. 8 Large discrepancies exist between experimentally derived activation energies ͑about 40 kcal/mol 7 or a lower bound of 70 kcal/mol 9 on the bimolecular energy barrier or 45 kcal/mol 9 on the termolecular energy barrier͒ and very accurate theoretical calculations of barrier heights on the H 4 PES ͑about 110 kcal/mol 10,11 ͒. The role of low levels of impurities is a very important experimental problem.…”

“…Experiments carried out with an ultraclean shock tube, under the conditions where no H atoms are present, found that no HD products of the reaction H 2 ϩD 2 →2HD are obtained. 8 Lifshitz et al consider that this reaction is strongly catalized by H atoms, that are present in the experiments formed from the absorbed pump oil impurities from the shock tube walls. 8 They have proposed the following chain reaction mechanism for the homogeneous H atom catalysis:…”

Laser-induced addition of hydrogen fluoride to unsaturated molecules: the HF + CH2CF2 system

“…atom concentrations in shock-heated H2/D2 mixtures, Lifschitz and Frenklach (12) found that only a small proportion of their HD yield arose from H atom reactions, and that the remainder originated from a 4-centre bimolecular reaction with an activation energy of 38 + 5 kcal mol-'. However, since then, the experiments have been repeated in an ultra-clean shock tube (13) and the results have disproved the 4-centre mechanism. It was shown that the previous spurious results were due to minute impurities (probably hydrocarbons) leading to false interpretations of the role of H atoms in the mechanism.…”