Matrix Confinement Governs Competing Heavy‑Atom and Hydrogen Tunneling Pathways

1-Methylcyclobutylfluorocarbene (MFC) has served as a benchmark for ground-state carbon tunneling, yet its conformer-dependent tunneling competition in the matrix has remained unresolved for two decades. Here, we constructed three independent, channel-specific, highly accurate machine-learning potential energy surfaces and employed full-dimensional ring-polymer instanton theory to compute gas-phase tunneling rates. Our calculations reveal that rapid exo↔endo equilibration strongly favors endo-MFC, which decays exclusively via hydrogen tunneling. This intrinsic kinetic branching ratio sharply contradicts matrix experiments, where only heavy-atom tunneling was detected and the predicted hydrogentunneling product was not reported. This discrepancy can be rationalized by the dual regulatory effects exerted by rigid matrix cages: they sterically block large-amplitude conformer interconversion to trap exo-MFC, while simultaneously distorting the endo-MFC geometry to suppress its hydrogen tunneling. Dedicated spectroscopic searches over an extended spectral range could test the predicted hydrogen-tunneling pathway. This computational framework quantitatively disentangles how matrix confinement tunes competing heavy-atom and hydrogen quantum tunneling, resolving the longstanding mechanistic ambiguity of the MFC benchmark carbene system.