Controlling factors of tunneling reactions in solid hydrogen at very low temperature

Investigating H-atom reactions in small PAHs with imperfect aromaticity: A combined experimental and computational study of indene (C9H8) and indane (C9H10)

Polycyclic aromatic hydrocarbons (PAHs) are widely recognized as catalysts for interstellar H2 formation. Extensive exploration into the catalytic potential of various PAHs has encompassed both theoretical investigations and experimental studies. In the present study, we focused on studying the reactivity of an imperfect aromatic molecule, indene (C9H8), and its hydrogenated counterpart, indane (C9H10), as potential catalysts for H2 formation within the interstellar medium. The reactions of these molecules with H atoms at 3.1 K were investigated experimentally using the para-H2 matrix isolation technique. Our experimental results demonstrate that both indene and indane are reactive toward H atoms. Indene can participate in H-atom-abstraction and H-atom-addition reactions, whereas indane primarily undergoes H-atom-abstraction reactions. The H-atom-abstraction reaction of indene results in the formation of the 1-indenyl radical (R1) (C9H7) and H2 molecule. Simultaneously, an H-atom-addition reaction forms the 1,2-dihydro-indene-3-yl radical (R2) (C9H9). Experiments also reveal that the H-atom-abstraction reaction of indane also produces the R2 radical. To the best of our knowledge, this study represents the first reporting of the infrared spectra of R1 and R2 radicals. The experimental results, combined with theoretical findings, suggest that indane and indene may play a role in the catalytic formation of interstellar H2. Furthermore, these results imply a quasi-equilibrium between the investigated molecules and the formed radicals via H-atom-addition and H-atom-abstraction reactions.

Hydrogen atom quantum diffusion in solid parahydrogen: The H + N2O → cis-HNNO → trans-HNNO reaction

The diffusion and reactivity of hydrogen atoms in solid parahydrogen at temperatures between 1.5 K and 4.3 K are investigated by high-resolution infrared spectroscopy. Hydrogen atoms are produced within solid parahydrogen as the by-products of the 193 nm in situ photolysis of N₂O, which induces a two-step tunneling reaction, H + N₂O → cis-HNNO → trans-HNNO. The second-order rate constant for the first step to form cis-HNNO is found to be inversely proportional to the N₂O concentration after photolysis, indicating that the hydrogen atoms move through solid parahydrogen via quantum diffusion. This reaction only readily occurs at temperatures below 2.8 K, not due to an increased rate constant for the first reaction step at low temperatures but rather due to an increased selectivity to the reaction. The rate constant for the second step of the reaction mechanism involving unimolecular isomerization is shown to be independent of the N₂O concentration as expected. The inverse concentration dependence of the rate constant for the reaction step that involves the hydrogen atom demonstrates clearly that quantum diffusion influences the reactivity of the hydrogen atoms in solid parahydrogen, which does not have an analogy in classical reaction kinetics.

Signatures of a quantum diffusion limited hydrogen atom tunneling reaction

We are studying the details of hydrogen atom (H atom) quantum diffusion in parahydrogen quantum solids in an effort to better understand H atom transport and reactivity under these conditions.