Competing quantum tunneling processes of heavy and light particles in isocyanic acid radical anions

JJ Tang and WB Fan and LA Wang and X Jiang and BB Sun and YY Zhou and W Fang and XQ Zeng and MF Zhou, NATURE COMMUNICATIONS, 16, 10687 (2025).

DOI: 10.1038/s41467-025-65720-0

Quantum tunneling is known to be mass dependent. Here we report an unexpected mass effect on tunneling reactions in isocyanic acid radical anions (HNCO center dot-), which exhibit negative electron affinities. The cis- and trans-isomers of HNCO center dot- were generated in a solid neon matrix and their interconversion and electron-detachment tunneling kinetics were investigated using infrared absorption spectroscopy. The results reveal that electron tunneling, which involves some degree of nuclear motion of HNCO center dot- to neutral HNCO, occurs at a slower rate than the cis-to-trans isomerization of HNCO center dot-, with the latter following a distinct bond angle inversion pathway driven by the heavy carbon atom. This contrasts to the typical dominance of light hydrogen atom tunneling in cis-trans isomerization systems. These findings are rationalized by instanton theory calculations. Our model, which explicitly includes the neon matrix via a QM/MM approach, reveals that the carbon-driven pathway is favored by a lower barrier and shorter tunneling distance.

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