Fast Heavy-Atom Tunneling in Trifluoroacetyl Nitrene

Area of Science:

• Chemical Physics
• Quantum Mechanics
• Spectroscopy

Background:

• Quantum mechanical tunneling (QMT) is a significant phenomenon in chemical reactions, though typically observed with light atoms like hydrogen.
• Heavy atom tunneling is comparatively rare and less understood in chemical and biological systems.

Purpose of the Study:

• To investigate and report evidence of heavy atom tunneling in trifluoroacetyl nitrene (CF3C(O)N).
• To characterize the tunneling behavior and kinetics of CF3C(O)N under cryogenic conditions.

Main Methods:

• Generation of trifluoroacetyl nitrene (CF3C(O)N) in cryogenic matrices (Ar, Ne, N2) via laser photolysis of trifluoroacetyl azide (CF3C(O)N3).
• Characterization using Infrared (IR) and Electron Paramagnetic Resonance (EPR) spectroscopy.
• Measurement of kinetic isotope effects (14N/15N) and half-life times at low temperatures (2.8 K and 5 K).

Main Results:

• Observed rapid rearrangement of CF3C(O)N to CF3NCO with half-lives under 10 minutes, even at 2.8 K.
• Detected unprecedentedly large 14N/15N kinetic isotope effects (1.18-1.33), indicating significant tunneling.
• Demonstrated that tunneling disappearance slows considerably in chemically active solvents like toluene and 2-methyltetrahydrofuran at 5 K.

Conclusions:

• Confirmed heavy atom tunneling in trifluoroacetyl nitrene, contradicting theoretical activation barriers.
• Highlighted the significant role of QMT in the rearrangement of CF3C(O)N, even for heavy atoms.
• Showcased the influence of the surrounding matrix environment on tunneling rates.