Ab initio calculation of 2H quadrupole coupling constants in molecular crystals: application to polymorphs of oxalic acid dihydrate

The use of ab initio Hartree-Fock techniques for the calculation of 2H quadrupole coupling constants (DQCCs) in hydrogen bonded molecular crystals has been assessed for the α and β polymorphs of oxalic acid dihydrate. Results demonstrate the importance of considering the full periodic structure, rather than a single molecule or a small cluster of molecules. Intramolecular contributions to the DQCCs and contributions arising from the first shell of neighbouring molecules and from outer shells are quantified. The influence of the size of basis set on the calculated DQCCs is investigated; the 6–31G∗∗ basis set is found to give good agreement with experiment. Correlations between the calculated DQCCs and geometrical parameters are investigated.