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Abstract:
Nuclear magnetic shielding tensors have been determined for 1H in pyromellitic acid dihydrate (PMDH) using multiple-pulse NMR and for 13C in PMDH and malonic acid using cross polarization and proton decoupling. The total experimental shielding tensor σtotal, for the ring proton has been divided between intra- and intermolecular contributions on the basis of a simple model calculation for the latter. The resulting σintra conforms more closely to the local molecular symmetry than σtotal. The results for the carboxyl protons in PMDH are consistent with earlier results on hydrogen-bonded systems: an increasing shielding and anisotropy with decreasing hydrogen bond length. For the two protons in the water molecule only one shielding tensor is observed at room temperature. This is due to 180° flip motions of the molecule about its two-fold pseudosymmetry axis. The carbon shielding tensors show essentially the same features as earlier results on similar molecular fragments; e.g., for the ring carbons in PMDH and the carboxyl carbons in PMDH and malonic acid the most shielded direction is closely normal to the plane of the appropriate molecular fragment. At least one of the ring carbons, however, shows an unusually large deviation of 5° between the two directions. This is ascribed to the fact that the carboxyl groups in PMDH are strongly twisted with respect to the ring plane.
