Hydrogen-atom tunneling reactions in solid para-hydrogen and their applications to astrochemistry

H-atom tunneling reactions play important roles in astrochemistry, but an understanding of these reactions is still in its infancy. The unique properties associated with quantum solid para-hydrogen provide an effective environment for the generation and reactions in situ of H atoms at low temperature. Several techniques have been employed to generate H atoms to study astrochemically relevant systems that provide significant insight into the formation of complex organic molecules (COM) and help to explain the relations between the abundance of some pairs of stable species. These results introduce new concepts in astrochemistry, including H-induced H abstraction, H-induced fragmentation, and H-induced uphill isomerization in darkness that have been overlooked previously. This mini-review summarizes the state of the art in this field, discussing fundamental understanding and techniques concerning H-atom generation, H-tunneling reactions, and their applications; the perspectives and open questions that await further exploration are discussed.