Proton magnetic shielding in malonic acid by multiple pulse techniques

The proton NMR in single crystals of malonic acid has been studied by multiple pulse line narrowing techniques. The nuclear magnetic shielding tensors σ(i) of all protons in malonic acid could be determined from the spectra. There are two magnetically distinct carboxyl protons. The principal components of their shielding tensors are found to be σ(1)ZZ= −0.8 ppm, σ(1)YY = −19.2 ppm, σ(1)XX = −21.8 ppm, and σ(2)ZZ = −1.0 ppm, σ(2)ZZ = −21.3 ppm relative to adamantane. The error limits are estimated to be ± 1 ppm. The most shielded directions lie along the hydrogen bond directions to within 8 degrees. The least shielded directions are essentially perpendicular to the plane of the carboxyl groups. Within experimental accuracy the shielding of the aliphatic protons is axially symmetric about the CH bond axes. The anisotropy Δσ = σ⌈ − σ⊥ is (4 ± 1) ppm. The gross features of the anisotropy of the carboxyl protons are shown to be governed by the diamagnetic effect