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

Quantum tunneling is known to be mass dependent. Here we report an unexpected mass effect on tunneling reactions in isocyanic acid radical anions (HNCO^•-), which exhibit negative electron affinities. The cis- and trans-isomers of HNCO^•- 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^•- to neutral HNCO, occurs at a slower rate than the cis-to-trans isomerization of HNCO^•-, 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.