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) x 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.