Variable temperature NMR characterization of α-glycine

Abstract

Proton NMR spin–lattice relaxation times in the laboratory frame ( T 1) and in the rotating frame ( T 1 ρ ) were measured as a function of temperature for a static sample of α-glycine. Both T 1 and T 1 ρ data can be fit quantitatively by a single thermally-activated motion (the modulation of the dipolar coupling by random hopping about the threefold axis of the –NH 3 group), with no addition of other mechanisms at any temperature between 173 and 415 K. An activation energy of 21.7 ± 1 kJ/mol was extracted and is compared with previously reported values for both α- and γ-glycine. Such comparisons allow the correction of glycine polymorphs misidentified in the literature. The minimum in T 1 at 325 K corresponds to a correlation time of 0.53 ns. Chemical shifts as a function of temperature were measured by 1H CRAMPS and by 13C and 15N CP/MAS experiments. These results are discussed relative to a previous report of anomalous electrical behavior in α-glycine within this temperature range.