Carbon-13 chemical shielding tensors in ammonium hydrogen malonate

Abstract

Chemical shielding tensors have been determined in single crystal ammonium hydrogen malonate by proton‐enhanced ¹³C NMR. The average principal values of the carboxyl tensors are σ₁₁ = −112.8±2, σ₂₂ = −45.8±3, and σ₃₃ = 25.6±2 ppm relative to benzene. Using the crystal structure determined in the preceding paper by Chapuis et al., the orientations of the tensor principal axes have been determined and assigned to specific carbon sites. The carboxyl tensor is oriented with the most shielded element perpendicular to the plane of the carboxyl group and the least shielded element σ₁₁ in the plane of the carboxyl group making an angle of 25 ° relative to the bisector of the CO bonds, rotated towards the long CO bond, displaying the effects of hydrogen bonding. The principal values of the methylene tensors are σ₁₁ = 65.6±2, σ₂₂ = 76.3±2, σ₃₃ = 110.2±2 ppm. The more or less unique, most shielded element, is in the C–C–C plane and is nearly along the C–C bond adjacent to the out‐of‐plane carboxyl group.