Hydrogen‐Atom Tunneling Could Contribute to H₂ Formation in Space

Quantum tunneling observed without its characteristic large kinetic isotope effects

“…Hence, reaction R1 mainly proceeds on the surface, not in the bulk. The LH mechanism at low temperatures (10-50 K) also indicates that the atomic H and D additions require tunneling (11). Our previous study also found that the reactivity of C 6 H 6 with H atoms is inhomogeneous across its amorphous surface (8).…”

“…Given the activation barriers and low temperatures, these reactions proceeded via tunneling (8,11); however, we observed only small KIEs. The ratio of the hydrogenation and deuteration rates (H/D) was 1-1.5 at 15-25 K, whereas deuteration by tunneling typically occurs at a rate more than two orders of magnitude smaller than that of the comparable hydrogenation (11). This indicates that the isotopically insensitive surface processes of the atoms physisorbed on solid C 6 H 6 masked the tunneling KIE, despite tunneling's providing a classically anomalous reaction efficiency.…”

“…At 30-50 K, P H was two to five times larger than P D . Values of ΔC 6 H 6_H were also two to four times larger than those of ΔC 6 H 6_D at 30-50 K. P H and ΔC 6 H 6_H were greatest at 20 K. Remarkably, both P H /P D and ΔC 6 H 6_H /ΔC 6 H 6_D lay between 1.0-1.5 at 15-25 K despite the intrinsic H/D KIE associated with tunneling (J100) (11). These small differences between addition by H or D tunneling clearly show that isotopically insensitive surface processes strongly contributed to the outcome of these surface reactions.…”

Cooperative Interplay of van der Waals Forces and Quantum Nuclear Effects on Adsorption: H at Graphene and at Coronene

“…This large increase in the rate constant for H chemisorption at graphene is comparable with the increase in rate constant observed in related studies on the edge sites of PAHs. 28,29 We note that the precise value for the rate constant of chemisorption depends also on the exchange-correlation functional used. However, as we show in the supporting information several vdW inclusive DFT approaches have been tested 1 For the potential energy barriers ∆E we use the expression R phys-chem = A exp(−∆E/k B T ) where the prefactor A (∼ 10 13 s −1 ) is computed by taking a ratio of the product of frequencies at the initial (physisorbed) state with the product of frequencies at the transition state, within the harmonic approximation.…”

“…Hydrogen adsorption on carbon based materials such as graphite and graphene is relevant to hydrogen storage, 9 band gap engineering, [10][11][12][13] and potentially as the first step in H 2 formation in the interstellar medium. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Although there is enormous interest in H adsorption on carbonaceous surfaces, with graphene, graphite and polycyclic aromatic hydrocarbons (PAHs) being the most widely studied model systems, we still don't fully understand the seemingly simple process of how a single H atom adsorbs on the surface.…”