270 MHz multiple pulse study of the olefinic protons in potassium hydrogen maleate

The nuclear magnetic shielding tensors of the protons in potassium hydrogen maleate (KHM) have been determined in single crystals by means of multiple pulse line narrowing techniques at 270 MHz. The increase of resolution in solid state NMR spectra on switching the spectrometer frequency from 90 MHz to 270 MHz is demonstrated. The results for the carboxylic protons in KHM agree fully with those of a previous 90 MHz study. Unlike at 90 MHz separate lines from individual olefinic proton sites could be resolved at 270 MHz allowing a straightforward determination of the corresponding shielding tensors. The principal shielding components found are 0.9, −2.0, −3.1 ± 0.2 ppm relative to liquid water. An assignment of the four experimental shielding tensors to the four olefinic sites in the crystal is proposed on the basis of molecular and local symmetry. According to this assignment the most shielded direction of the olefinic protons is in the molecular plane of the maleate anion and perpendicular to the CH bond axis