Hydrogen-Atom-Assisted Uphill Isomerization of N-Methylformamide in Darkness

N-Methylformamide, HC(O)NH(CH₃), is the smallest amide detected in the interstellar medium that can exist as cis and trans isomers. We performed reactions of H atoms with trans-NMF in solid para-hydrogen at 3.3 K and found that the cis-NMF isomer, which has higher energy, increased continuously in darkness, demonstrating a previously overlooked and seemingly unlikely isomerization of prebiotic molecules through H-atom tunneling reactions in the absence of light. Infrared spectra of radical intermediates trans-•C(O)NH(CH₃) and trans-HC(O)NH(•CH₂) were identified. Further H addition and H abstraction enhanced the formation of CH₃NCO, HNCO, and CH₂NH in the H-rich experiments. These results indicate that, unlike the dual cycle of H-abstraction and H-addition channels chemically linking formamide and HNCO, the H addition to CH₃NCO produced only cis-radicals that led to cis-NMF. Furthermore, H-atom-induced fragmentation by breaking the C-C bond provides links between NMF and HCNO/CH₂NH. These endothermic isomerization/decomposition reactions become possible through the coupling with H + H → H₂.