Pyromellitic acid dianhydride: crystal structure and anisotropic proton magnetic shielding

The nuclear magnetic shielding tensors of the protons in pyromellitic acid dianhydride were measured in single crystals by means of high resolution solid state multiple pulse N.M.R. techniques. In order to correlate the N.M.R. results to the molecules, the molecular and crystal structure of the compound was determined by direct X-ray methods. The molecules have approximately D 2h symmetry. There are Z = 4 molecules in the unit cell, they are all crystallographically equivalent and the two protons in each molecule are magnetically equivalent. The assignment of the measured proton shielding tensors to the various proton sites is based on linewidth differences in multiple pulse spectra which can be traced back to differences in dipolar interactions, and on local symmetry arguments. The least shielded direction of the protons is close to the normal of the aromatic ring, but the other two principal shielding directions do not conform to the D 2h symmetry of the molecule. The deviations are ascribed to intermolecular shielding contributions. These are calculated on the basis of a magnetic dipole model. The application of this model for assessing inter-molecular shielding contributions is discussed. The difference between the measured shielding tensor and the calculated intermolecular contribution conforms well to the D 2h symmetry of the molecule. We believe, therefore, that this difference can be identified with good accuracy with the proton shielding of an isolated pyromellitic acid dianhydride molecule. The results imply that the direction perpendicular to the bond within the molecular plane is the most shielded direction of the proton, whereas the bond direction itself is the inter-mediate principal shielding direction. It is believed that these features are characteristic for the shielding of aromatic protons in general