13C chemical shielding in oxalic acid, oxalic acid dihydrate, and diammonium oxalate

摘要

13C chemical shielding tensors in three oxalates were investigated in order to determine the effects of protonation and conformation changes on the tensor elements and their orientations. The experimental results, together with those in the accompanying paper lsqb;R. G. Griffin and D. J. Ruben, J. Chem. Phys. elding) of sgr;22and in one case a concurrent rotation of the tensor about sgr;33, the element which is perpendicular to the COO plane. On the other hand, the simple conformational change of the twisting of carboxyl groups relative to one another, which occurs in diammonium oxalate monohydrate, results in small rotation of the tensor around sgr;11and also may contribute to a decrease in the shielding of the sgr;22component in this molecule. The orientation of the13C tensor in oxalic acid dihydrate is different from that observed in other oxalate ions and carboxyl groups. The influence of a change in hydrogen bonding strength on tensor elements is discussed.