1H NMR study of lithium D-lactate

Area of Science:

• Solid-state nuclear magnetic resonance spectroscopy
• Materials science
• Chemical physics

Background:

• Polycrystalline D-lactic acid lithium salt, also known as lithium D-lactate, is a material with potential applications in various chemical and material science fields.
• Understanding the molecular dynamics of such salts is crucial for optimizing their properties and performance.
• Proton magnetic resonance spectroscopy offers a powerful non-destructive technique to probe molecular motions in solids.

Purpose of the Study:

• To investigate the molecular dynamics of polycrystalline lithium D-lactate using pulsed proton magnetic resonance.
• To determine the activation energy and pre-exponential factor associated with methyl group rotation.
• To explore the influence of quantum mechanical tunneling on molecular motion at low temperatures.

Main Methods:

• Pulsed proton magnetic resonance spectroscopy was employed at 25 MHz across a temperature range of 77 K to 300 K.
• Analysis of spin-lattice relaxation times and dipolar second moments provided insights into molecular motion.
• Comparison with electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) data for related systems.

Main Results:

• Methyl group rotation was identified as the dominant relaxation mechanism in lithium D-lactate.
• The rotational motion is characterized by an activation energy (Ea) of 14.5 ± 0.5 kJ/mol and a time factor (τ0) of (1.5 ± 0.5) x 10⁻¹³ s.
• A reduction in the dipolar second moment below 100 K was attributed to quantum mechanical tunneling, with an excitation energy of 6.1 ± 1 kJ/mol.

Conclusions:

• The study elucidates the key molecular dynamics governing lithium D-lactate, with methyl rotation being the primary contributor to relaxation.
• The derived activation energy for methyl rotation is higher than that observed in related lactate radical systems.
• Quantum mechanical tunneling plays a significant role in the low-temperature dynamics, influencing the overall molecular behavior of lithium D-lactate.