AB-INITIO CALCULATION OF H-2 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 H- 2 quadrupole coupling constants (DQCCs) in hydrogen bonded molecular c rystals has been assessed for the alpha and beta polymorphs of oxalic acid dihydrate. Results demonstrate the importance of considering the full periodic structure, rather than a single molecule or a small clus ter of molecules. Intramolecular contributions to the DQCCs and contri butions arising from the first shell of neighbouring molecules and fro m outer shells are quantified. The influence of the size of basis set on the calculated DQCCs is investigated; the 6-31G* basis set is foun d to give good agreement with experiment. Correlations between the cal culated DQCCs and geometrical parameters an investigated. (C) 1997 Els evier Science B.V.