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

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Abstract
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 [R. G. Griffin and D. J. Ruben, J. Chem. Phys. elding) of σ22 and in one case a concurrent rotation of the tensor about σ33, 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 σ11 and also may contribute to a decrease in the shielding of the σ22 component in this molecule. The orientation of the 13C 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.