End-to-end correlation for a C-12 hydrocarbon chain.

Area of Science:

• Physical Chemistry
• Chemical Physics
• Materials Science

Background:

• Nuclear spin-lattice relaxation provides insights into molecular motion.
• Magnetic Relaxation Dispersion (MRD) profiles probe spectral density functions related to molecular dynamics.
• Modeling molecular dynamics requires accurate theoretical frameworks.

Purpose of the Study:

• To investigate the end-to-end correlation function of a doubly labeled molecule using 19F NMR.
• To compare experimental MRD data with various theoretical models for molecular dynamics.
• To identify the most suitable model for describing restricted translational diffusion in a polymer chain.

Main Methods:

• 19F nuclear spin-lattice relaxation rate constants were measured across a range of magnetic field strengths.
• The doubly labeled molecule, 1,12-diaminododecane, featured a nitroxide radical and a trifluoromethyl group.
• Intermolecular relaxation contributions were eliminated by extrapolation to zero concentration.

Main Results:

• The rotational model (Solomon-Bloembergen-Morgan) inadequately described the experimental MRD profile.
• Freed's model for freely diffusing solutes was not suitable due to the tethered paramagnet.
• A generalized Torrey model, accounting for chain length restrictions, accurately described the data.
• A modified Hwang-Freed model also provided a good approximation for long chains.

Conclusions:

• The generalized Torrey model effectively captures the complex molecular dynamics of the doubly labeled molecule.
• The study highlights the importance of considering restricted diffusion and finite chain length in relaxation studies.
• Accurate modeling is crucial for interpreting NMR relaxation data in complex molecular systems.