3,4-Pyridinedicarboxylic acid is a compound commonly used as a multifunctional ligand in coordination chemistry. The crystal structure of its orthorhombic Form I has been known for over 30 years, while a new Form II has only been reported recently as crystallizing in the monoclinic space groupC2/c. The present work shows that, in its entire temperature stability range, Form II is in fact primitive monoclinic, with space groupP21/c. At 120 K, the unit cell parameters are a = 7.594(1) Å, b = 7.332(1) Å, c = 12.113(2) Å, β = 104.339(4)° and V = 653.4(2) Å3. In order to characterize the temperature dependent behaviour of 3,4-pyridinedicarboxylic acid, both polymorphs were studied by single crystal X-ray diffraction at 120 and 400 K, variable temperature powder X-ray diffraction between room temperature and 600 K, solid state NMR and computational methods. Short O–H⋯O hydrogen bonds are found in both forms of 3,4-pyridinedicarboxylic acid, but there appears to be no temperature-induced proton migration along these bonds.
