Exploring tunneling ESEEM beyond methyl groups in nitroxides at low temperatures

Area of Science:

• Physical Chemistry
• Quantum Mechanics
• Spectroscopy

Background:

• Electron spin echo envelope modulation (ESEEM) at low temperatures is influenced by methyl rotor tunneling.
• This tunneling effect provides information about the local environment of methyl rotors in nitroxides.
• Existing models analyze methyl quantum rotor (MQR) behavior and rotation barrier distributions.

Purpose of the Study:

• To investigate and compare the tunneling dynamics of geminal methyl and ethyl group rotors in nitroxides.
• To extend the MQR model for analyzing tunneling ESEEM from multiple rotor types coupled to a single electron spin.
• To explore different theoretical levels for the MQR model and validate findings with DFT calculations.

Main Methods:

• Utilized the methyl quantum rotor (MQR) model, extended to accommodate multiple rotor types.
• Employed Monte-Carlo based fitting and identifiability analysis of the MQR model parameter space.
• Performed density functional theory (DFT) calculations to determine rotor rotation barriers.

Main Results:

• Ethyl groups in nitroxides exhibit stronger tunneling ESEEM contributions than methyl groups.
• The methyl rotors of ethyl and propyl groups show distributions at lower rotation barriers compared to geminal methyl groups.
• Conformational flexibility significantly impacts the hindrance of rotor rotation, as confirmed by DFT calculations.

Conclusions:

• The study successfully extracts rotation barrier distributions for individual rotor types in mixed-rotor nitroxides.
• Identified dominant rotor types contributing to the observed tunneling ESEEM in the Hahn echo decay signal.
• Demonstrated that ethyl groups provide more significant tunneling ESEEM signals due to favorable hyperfine coupling characteristics.