A modified 2D-SUPER technique is demonstrated to allow independent measurement of the principal values of the chemical-shift tensors of overlapping protonated and unprotonated carbons. The insertion of a dipolar-dephasing period into the sequence causes loss of signal from protonated carbons. The spectrum obtained with this modification allows one to determine the principal values of the unprotonated carbons with high precision. Subsequent fitting of the usual 2D-SUPER spectrum, with the chemical-shift parameters of the unprotonated carbons fixed, gives the parameters of the overlapped resonances of the protonated carbons. As an example, we report the determination of the 13C chemical-shift parameters of the carbons of form II of piroxicam. The experimental results are compared with those obtained from calculations using the DFT/GIAO method. Potential applications of this method are discussed.

The complete 13C NMR chemical-shift tensors for the carbon sites of the two polymorphic forms (PI and PII) and the monohydrate form (PM) of the analgesic drug, piroxicam, are reported. The NMR parameters (isotropic chemical shifts, chemical-shielding anisotropies and asymmetries, and dipolar couplings), X-ray powder diffraction, and density functional calculations of piroxicam are analyzed in terms of hydrogen bonding and structure. The integration of all the data gives an improved model of the local solid-state structures of the polymorphs. In particular, the solid-state NMR spectra demonstrate that the asymmetric unit of the monohydrate, PM, contains two zwitterionic piroxicam molecules.