Proton shielding in single crystals of calcium and lead formate

Abstract

The magnetic shielding tensors of the protons in single crystals of Ca(HCOO) 2 and Pb(HCOO) 2 have been measured by multiple-pulse techniques. Both salts contain two inequivalent formate ion sites; the shielding of the proton of the formate ion is thus measured in four different environments. It is found to be substantially different in the four sites: the orientation of the principal axis system of the proton shielding tensor relative to the respective formate ion varies from site to site; likewise, the shielding anisotropies are different, ranging from −5.0 to −9.2 ppm. An attempt is made to assess the intermolecular shielding contributions σ inter on the basis of a simple magnetostatic model. Subtraction of σ inter from the total shielding σ- results in an intramolecular shielding tensor σ intra whose orientation relative to the formate ion is similar in all four sites. We propose that σ intra can be identified with the shielding of the proton in an isolated formate ion. Multiple-pulse spectra and rotation patterns from single crystals of Cd(HCOO) 2 have also been recorded. Severe line broadening caused by the magnetic 111Cd and 113Cd isotopes prevented extraction of the full proton shielding tensors from the data, but the rotation patterns can be compared meaningfully with computer-generated rotation patterns based on breaking σ inter down into σ intra σ HCOO − inter, and σ Cd 2+ inter Complete consistency between experimental and calculated rotation patterns is found.