Slow dynamics in a liquid crystal: 1H and 19F NMR relaxometry

Area of Science:

• Materials Science
• Physical Chemistry
• Nuclear Magnetic Resonance Spectroscopy

Background:

• Nuclear spin-lattice relaxation is crucial for understanding molecular dynamics in liquid crystals.
• Proton (1H) relaxation typically involves dipolar interactions, while fluorine-19 (19F) relaxation can be influenced by other mechanisms.

Purpose of the Study:

• To investigate the spin-lattice relaxation rates of 1H and 19F in the isotropic phase of 4'-butoxy-3'-fluoro-4-isothiocyanatotolane (4OFTOL).
• To elucidate the distinct relaxation mechanisms and molecular dynamics governing these two nuclear species across a wide Larmor frequency range.

Main Methods:

• Measurements of spin-lattice relaxation rates (R(1H) and R(1F)) were performed at various temperatures in the isotropic phase of 4OFTOL.
• The experiments covered a broad Larmor frequency range from 10 kHz to 50 MHz.

Main Results:

• 19F relaxation rates (R(1F)) exhibited significant dispersion (over two orders of magnitude) across the entire isotropic phase, unlike 1H relaxation.
• The observed R(1F) dispersion at low frequencies is attributed to spin-rotation coupling influenced by slow molecular torques from collective modes.
• Cross-relaxation between 1H and 19F nuclei becomes significant below ~400 kHz, contributing to the overall relaxation.

Conclusions:

• The distinct relaxation behaviors of 1H and 19F highlight different underlying molecular dynamics in liquid crystals.
• Slowly relaxing local structures are proposed as the mechanism responsible for the ultra-slow modes affecting 19F relaxation.
• The study provides insights into the complex interplay of relaxation mechanisms and collective dynamics in liquid crystalline systems.