Tunneling chemical reactions D+H2→DH+H and D+DH→D2+H in solid D2–H2 and HD–H2 mixtures: An electron-spin-resonance study

Abstract

Tunneling chemical reactions D + H2 --> DH + H and D + DH --> D2 + H in solid HD-H2 and D2-H2 mixtures were studied in the temperature range between 4 and 8 K. These reactions were initiated by UV photolysis of DI molecules doped in these solids for 30 s and followed by measuring the time course of electron-spin-resonance (ESR) intensities of D and H atoms. ESR intensity of D atoms produced by the photolysis decreases but that of H atoms increases with time. Time course of the D and H intensities has the fast and slow processes. The fast process, which finishes within approximately 300 s after the photolysis, is assigned to the reaction of D atom with one of its nearest-neighboring H2 molecules, D(H2)n(HD)(12-n) --> H(H2)(n-1)(HD)(13-n) or D(H2)n(D2)(12-n) --> H(HD)(H2)(n-1)(D2)(12-n) for 12 > or = n > or = 1. Rate constant for the D + H2 reaction between neighboring D atom-H2 molecule pair is determined to be (7.5 +/- 0.7) x 10(-3) s(-1) in solid HD-H2 and (1.3+/-0.3) x 10(-2) s(-1) in D2-H2 at 4.1 K, which is very close to that calculated based on the theory of chemical reaction in gas phase by Hancock et al. [J. Chem. Phys. 91, 3492 (1989)] and Takayanagi and Sato [J. Chem. Phys. 92, 2862 (1990)]. This rate constant was found to be independent of temperature up to 7 K within experimental error of +/-30%. The slow process is assigned to the reaction of D atom produced in a cage fully surrounded by HD or D2 molecules, D(HD)12 or D(D2)12. This D atom undergoes the D + DH reaction with one of its nearest-neighboring HD molecules in solid HD-H2 or diffuses to the neighbor of H2 molecules to allow the D + H2 reaction in solid HD-H2 and D2-H2. The former is the main channel in solid HD-H2 below 6 K where D atoms diffuse very slowly, whereas the latter dominates over the former above 6 K. Rate for the reactions in the slow process is independent of temperature below 6 K but increases with the increase in temperature above 6 K. We found that the increase is due to the increase in hopping rate of D atoms to the neighbor of H2 molecules. Rate constant for the D + DH reaction was found to be independent of temperature up to 7 K as well.

Reactions of Atomic Hydrogen with Formic Acid and Carbon Monoxide in Solid Parahydrogen II: Deuterated Reaction Studies

“…We only observe growth of species after photolysis that can be attributed to H atom reactions with DCOOD, DCOOH, and CO. Our results are therefore consistent with the earlier H atom reaction studies that showed that D atoms are not mobile in solid H 2 due to the unique manner in which these species diffuse through the solid. The D atoms cannot move via exchange tunneling because the D atom gets trapped in an HD molecule after the first D + H 2 → HD + H exchange reaction. This isotopic variant of the H atom exchange reaction is exothermic (−260 cm –1 ) due to the lower zero-point vibrational energy of HD compared to H 2 and readily occurs with a time constant of 77 s at 4.2 K in solid D 2 -H 2 mixtures .…”

“…We speculate that DCO produced during photolysis of DCOOD undergoes efficient secondary photolysis such that under these conditions no peaks due DCO are observed. We only observe growth of species after photolysis that can be attributed to H atom reactions with DCOOD, DCOOH, and CO. Our results are therefore consistent with the earlier H atom reaction studies that showed that D atoms are not mobile in solid H 2 due to the unique manner in which these species diffuse through the solid. The D atoms cannot move via exchange tunneling because the D atom gets trapped in an HD molecule after the first D + H 2 → HD + H exchange reaction.…”

“…One of the first research groups to study pure tunneling reactions focused on creating H atoms in solid molecular hydrogen (SMH). − These are fascinating chemical systems due to the importance of nuclear quantum effects; the light mass of the H atom results in a large de Broglie wavelength, and the H 2 molecule undergoes both large amplitude translational zero-point motions and free rotation within SMH . Through a series of radiolysis and photolysis studies of solid H 2 –D 2 mixtures and solid HD, Miyazaki and his group (followed by Kumada) used electron spin resonance (ESR) spectroscopy to demonstrate a number of nonstandard chemical phenomena: (1) H atoms readily diffuse through SMH even at temperatures below 5 K by repeated H + H 2 → H 2 + H tunneling exchange reactions, (2) D atoms produced in solid H 2 rapidly react with the host thereby trapping the D atom in HD and creating mobile H atoms, and (3) H atoms in solid H 2 are remarkably stable on the time scale of hours due presumably to inefficient H + H → H 2 recombination in enriched parahydrogen crystals. − Proof of the importance of tunneling in the measured reaction dynamics is provided by comparison of the rate constant for the D + HD → H + D 2 reaction estimated experimentally to be 3.5 × 10 –27 cm molecule –1 s –1 at 4.2 K with the thermally activated rate constant calculated to be 10 –400 cm 3 molecule –1 s –1 . Further proof the reaction occurs exclusively through tunneling is provided by measuring the rate constants for different isotopic variants of this reaction.…”

Tunneling chemical exchange reaction $\textrm{D}+\textrm{HD}\rightarrow\textrm{D}_{2}+\textrm{H}$ in solid HD and D$_2$ at temperatures below 1$\,$K

“…The exchange reaction (3) is expected to proceed two orders of magnitude faster than the reaction (4) and cannot be measured in a pure H 2 matrix with the accumulation technique used in our experiments. Measurements of samples with a small fraction of H 2 are diffusion limited and therefore do not represent the true reaction rate [9]. But such measurements will yield information on the diffusion of D if the reaction (3) remains fast at ultralow temperatures.…”

“…The rate of the reaction (4) was measured experimentally in the temperature range 1.9-6.5 K in a series of works by Miyazaki and Kumada [7,8]. Reaction (3) proceeds two orders of magnitude faster andits rate was measured only recently by Kumada [9].…”