Carbon-13 chemical shielding in ammonium hydrogen oxalate hemihydrate

Abstract

We have measured the two 13C chemical shifttensors in single crystals of ammonium hydrogen oxalate hemihydrate, NH4HC2O4⋅ (1/2)H2O, using solid state double resonance techniques. One of the two types of tensors in the unit cell, the one belonging to the ionic carboxyl group, is oriented with its least shield component σ11 within 6° of the C–C bond, while the other tensor, belonging to the protonated carboxyl group, has its σ11 twisted 24° off this bond. Both tensors have σ33 perpendicular to the oxalate plane. Protonation of the carboxyl group does not affect the shifts of the σ11 and σ33tensor elements, but it does move the σ22 component in the direction of increased shielding. Implications of the orientation of these tensors to crystal and molecular structure determination are discussed.