Variable-temperature neutron diffraction studies of the short, strong hydrogen bonds in the crystal structure of pyridine-3,5-dicarboxylic acid

Pyridine-3,5-dicarboxylic acid has been studied by single-crystal neutron diffraction at 15 and 296 K. Pyridine-3,5-dicarboxylic acid, in which the carboxylic acid protons have been replaced by deuterons, has also been studied at 15, 150 and 296 K. The protonated structure contains a short N⋯H⋯O hydrogen bond [N⋯O 2.523 (2) A at 15 K]. Temperature-dependent proton migration occurs where the N—H distance in the hydrogen bond changes from 1.213 (4) A at 15 K to 1.308 (6) A at 300 K. In the deuterated structure the overall hydrogen-bond length increased [N⋯O 2.538 (3) A at 15 K] and the magnitude of the migration increased so that the N—D distance changes from 1.151 (3) A at 15 K to 1.457 (4) A at 300 K.