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

13C chemical shielding tensors in three oxalates were investigated in order to determine the effe~ts of protonation and conformation changes on the tensor elements and their orientations. The expenmental results. together with those in the accompanying paper [R. G. Griffin and D. J. Ruben, J. Chern. ~hys. 63, 1272 (1975)], indicate that, while neither of these factors seems to seriously effect the ma~mt~des U" and U" they do perturb U" and the tensor orientation. Protonation produces an upfield shift (Increased II 33' 12 h' h . shielding) of U" and in one case a concurrent rotation of the tensor about U" 33, the element w IC IS perpendicular t~ the COO plane. On the other hand, the simple conformational change of the twisti~g of carboxyl groups relative to one another, which occurs in diammonium oxalate ~onohy~rat~, results In small rotation of the tensor around U"II and also may contribute to a decrease In the shielding of the U"n component in this molecule. The orientation of the 13C tensor in oxalic acid di~ydrate is differen.t from that observed in other oxalate ions and carboxyl groups. The influence of a change In hydrogen bonding strength on tensor elements is discussed.