Redetermination of the solvent-free crystal structure of l-proline

The title compound, (S)-pyrrolidine-2-carboxylic acid (C5H9NO2), commonly known as l-proline, crystallized without the inclusion of any solvent or water molecules through the slow diffusion of diethyl ether into a saturated solution of l-proline in ethanol. l-Proline crystallized in its zwitterionic form and the molecules are linked via N—H...O hydrogen bonds, resulting in a two-dimensional network. In comparison to the only other publication of a single-crystal structure of l-proline without inclusions [Kayushina & Vainshtein (1965). Kristallografiya, 10, 833–844], the R1 value is significantly improved (0.039 versus 0.169) and thus, our data provides higher precision structural information