Hydrogen Activation by a σσ*-Carbene Through Quantum Tunneling

15.07.2025 | 09:00

The electronic structure of carbenes arises from the occupation of a σ and a π frontier orbital. While parent methylene possesses a triplet ground state (σ1π1), substituents are capable of stabilizing the singlet as the ground state (σ2π0 or σ0π2) by altering the frontier orbital energies. Here, we reveal that the 1,2[I]-shift isomer of 2-iodopyridine, the N-iodo Hammick intermediate, features a resonance between its carbene σ and N–I bond σ* orbitals, rendering them frontier orbitals. This singlet carbene is efficiently generated via UV photolysis of 2-iodopyridine in solid neon at 4.4 K and reacts with molecular hydrogen – but not deuterium – via N–I bond cleavage enabled by quantum tunneling. Instanton theory computations demonstrate the preference for a concerted hydrogen addition mechanism at elevated temperatures, while hydrogen atom abstraction dominates below 100 K despite a higher kinetic barrier for this process. Our findings introduce an unprecedented carbene class, unlocking new opportunities for reactivity and electronic structure explorations.

The electronic structure of carbenes arises from the occupation of a σ and a π frontier orbital. While parent methylene possesses a triplet ground state (σ¹π¹), substituents are capable of stabilizing the singlet as the ground state (σ²π⁰ or σ⁰π²) by altering the frontier orbital energies. Here, we reveal that the 1,2[I]-shift isomer of 2-iodopyridine, the N-iodo Hammick intermediate, features a resonance between its carbene σ and N–I bond σ* orbitals, rendering them frontier orbitals. This singlet carbene is efficiently generated via UV photolysis of 2-iodopyridine in solid neon at 4.4 K and reacts with molecular hydrogen – but not deuterium – via N–I bond cleavage enabled by quantum tunneling. Instanton theory computations demonstrate the preference for a concerted hydrogen addition mechanism at elevated temperatures, while hydrogen atom abstraction dominates below 100 K despite a higher kinetic barrier for this process. Our findings introduce an unprecedented carbene class, unlocking new opportunities for reactivity and electronic structure explorations.