Nuclear magnetic shielding anisotropies from proton multiple pulse spectra

Proton multiple pulse spectra of powder samples of anhydrous oxalic acid, oxalic acid dihydrate, malonic-, succinic-, phthalic-, maleic- and fumaric acid. succinic-, phthalic- and maleic acid anhydride, and ferrocene have been recorded at 90 MHz. The spectra are analysed in terms of the nuclear magnetic shielding anisotropy Δσ= σ⊥ - σ⊥. No evidence for non-axially symmetric shielding has been obtained. The shielding anisotropies seem to reflect at least qualitatively the immediate electronic environment of the protons under study. The anisotropies obtained for carboxylic, aliphatic, olefinic and aromatic protons are ≈ + 18, ≈ +10, ≈ ± 0 and ≈ −5 ppm with substantial variations among the various compounds studied