Analysis of conformational polymorphism in pharmaceutical solids using solid-state NMR and electronic structure calculations.

A detailed analysis of molecular structure in three polymorphic forms of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile is made using a combination of multidimensional solid-state NMR (SSNMR) experiments and molecular modeling via electronic structure calculations. These compounds, collectively referred to as ROY because of their red, orange, and yellow colors, share a similar molecular structure with the exception of the dihedral angle between the phenyl and thiophene rings. The ROY materials make it possible to study the influence of nearly a single degree of freedom on the associated NMR spectra. Using the 2D PASS (Antzutkin et al. J. Magn. Reson. A 1995, 115, 7) experiment, spectral editing techniques, and DFT-based calculations of the local fields, an analysis is made of the sensitivity of all carbon and nitrogen sites to changing molecular conformation. Chemical shift and dipolar coupling information obtained from these experiments vary noticeably between forms and are subsequently used to quantitatively determine aspects of molecular structure in these materials, including the coplanar angle between the phenyl and thiophene rings. The influence of motion on the methyl and nitro chemical shifts is also investigated. The accuracy of the information obtained from local field analysis and the model structure calculation demonstrates the capabilities of SSNMR as a quantitative structural method.