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Abstract:
Single crystals of malonic acid were investigated by Fourier transform “wide-line” and multiple-pulse “high-resolution” techniques. Crystal spheres were used to avoid warping of the applied field by bulk susceptibility shape effects. The analysis of the wide-line spectra revealed asymmetric hydrogen bonds in malonic acid and a twist of about 6° of the methylene proton internuclear vector with respect to the normal of the CCC backbone of the molecule. All four chemically shifted lines possible by crystal symmetry could be resolved in the high-resolution spectra. The analysis of these spectra yielded the nuclear magnetic shielding tensors of all the protons in malonic acid. Particular emphasis is laid on the methylene proton shielding tensors which had not been determined previously. They turned out not to be fully congruent within experimental accuracy. Their anisotropies, 4.85 ± 0.3 and 3.80 ± 0.3 ppm, respectively, and their asymmetry parameters η 0.4 and 0.8, respectively, are compared with the corresponding parameters of methylene proton shielding tensors in a variety of compounds of which powder spectra have been recorded. The shielding anisotropy of the methylene protons in malonic acid appears to be unusually small. The problem of assigning shielding tensors to proton sites is solved in a purely experimental manner by analyzing the widths of the resonance lines in multiple-pulse spectra.
