NMR study of solid lactic acid (2-hydroxypropanoic acid)

Polycrystalline l-lactic acid ((S)-2-hydroxypropanoic acid) has been investigated by pulsed proton magnetic resonance methods between 77 K and the melting point, 329 K, at 14 and 25 MHz. A reduction of dipolar second moment is ascribed to methyl group reorientation. This motion is also responsible for spin-lattice relaxation; its correlation time is characterized by an activation energy 12·2 ± 1 kJ mol-1 and time factor (3·5 ± 1) × 10-13 s. Below 100 K quantum-mechanical tunnelling contributes to reduction of the second moment and to spin-lattice relaxation, characterized by an excitation energy 6·2 ± 1 kJ mol-1.