Exploring Nuclear Spin Conservation in the CH₂ + H₂ Reaction

Nuclear spin angular momentum in molecules with identical nuclei is conserved in many physical and chemical processes, but its role in chemical reactions involving atomic rearrangements remains underexplored. In this study, we investigate the nuclear spin selection rules in the reaction between methylene (CH₂) and hydrogen (H₂) using high-resolution infrared spectroscopy in quantum solid parahydrogen. Our results show that in the triplet methylene (³CH₂) reaction, the nuclear spin distribution of the products matches perfectly with the expected nuclear spin selection rules for the stepwise reaction mechanism. In contrast, in the excited singlet methylene (¹CH₂) reaction, the nuclear spin states of the methane product deviate from those predicted with a simple direct insertion mechanism. This deviation likely results from hydrogen atom dissociation due to the excess energy of the excited methane product or from a competitive process between relaxation to triplet methylene and stabilization of the methane product. Notably, our findings confirm that nuclear spin conservation is maintained even in reactions involving intermediates with unpaired electron spins such as methylene. This demonstrates the importance of detecting nuclear spin states as a powerful tool for revealing the details of reaction mechanisms, particularly in processes related to hydrocarbon combustion and planetary atmospheric chemistry.