Proton magnetic shielding in single crystals. Evidence of intermolecular shielding contributions

Abstract

Pyromellitic acid dianhydride (PMDA), Ca(HCOO)₂ and Pb(HCOO)₂ were studied by multiple pulse techniques in single crystals. The shielding tensors σ of the respective protons are presented. The orientations of the proton shielding principal axes systems do not conform to molecular symmetry and for the formate ion Δσ is found to vary drastically from one crystal site to another. These findings are shown to be evidence of intermolecular shielding contributions. A simple method of accounting for these contributions is presented.