Hydrogen Activation by a σσ*-Carbene Through Quantum Tunneling.

Area of Science:

• Physical Chemistry
• Organic Chemistry
• Quantum Chemistry

Background:

• Carbene electronic structure is defined by σ and π frontier orbitals.
• Substituents can stabilize singlet carbene ground states by modifying frontier orbital energies.

Purpose of the Study:

• To characterize the electronic structure and reactivity of the N-iodo Hammick intermediate.
• To investigate the reaction mechanism of this novel carbene with molecular hydrogen and deuterium.

Main Methods:

• UV photolysis of 2-iodopyridine in solid neon at 4.4 K to generate the carbene.
• Reaction studies with molecular hydrogen and deuterium.
• Instanton theory computations to model reaction mechanisms.

Main Results:

• The N-iodo Hammick intermediate exhibits resonance between carbene σ and N-I bond σ* orbitals, defining its frontier orbitals.
• The singlet carbene reacts with molecular hydrogen, but not deuterium, via N-I bond cleavage facilitated by quantum tunneling.
• Computational studies revealed a shift in reaction mechanism from concerted hydrogen addition at higher temperatures to hydrogen atom abstraction at lower temperatures.

Conclusions:

• A new class of carbene, the N-iodo Hammick intermediate, has been identified.
• Quantum tunneling and temperature-dependent mechanisms govern the carbene's reactivity with hydrogen isotopes.
• This discovery offers novel opportunities for exploring carbene reactivity and electronic structure.