Multiple pulse study of the proton shielding in single crystals of maleic acid

Abstract

The proton NMR in single crystals of maleic acid has been studied by means of multiple pulse line-narrowing techniques. The magnetic shielding tensors of the four magnetically inequivalent protons forming hydrogen bonds could be determined independently. The transferability of the magnetic shielding tensor is put forward as a physical index of the hydrogen bond. The connection between the principal directions of the shielding tensors and the bond directions is demonstrated and used as a means to obtain refined hydrogen positions in the crystal structure. Only one composite line of the olefinic protons could be observed. Its analysis in terms of the individual tensors of the olefinic protons suggests that the double bond principal directions are reflected in those of the shielding tensors in contradistinction to the single bond systems. The origin of this effect is discussed.