Deuteron NMR on single crystals. EFGs, signs of quadrupole coupling constants, and assignment of σ tensors in malonic acid

Abstract

The high-field NMR of two dipolar-coupled deuterons experiencing different EFGs is investigated theoretically and shown to be solvable analytically. The determination of the signs of quadrupole coupling constants from the spectral appearance is discussed. Room-temperature FT deuteron spectra of deuterated crystals of malonic acid were recorded at 54.7 MHz. The dipolar fine structure resulting from the coupling of the deuterons of the CD 2 group could be resolved. From the rotation patterns of the line splittings, the EFGs at the sites of the four inequivalent deuterons are determined and compared with the previous work of Derbyshire et al.. The results confirm the conclusion, drawn in a previous paper on the proton NMR of malonic acid, that the methylene deuteron internuclear vector and the normal of the CCC backbone of the molecule subtend an angle of about 6°. The signs of the quadrupole coupling constants of the methylene deuterons are shown to be positive. The correlation of chemical shifts with quadrupole splittings allowed to assign directly the chemical shift tensors measured previously by multiple-pulse proton NMR to the methylene hydrogen sites.