Assigning carbon-13 NMR spectra to crystal structures by the INADEQUATE pulse sequence and first principles computation: a case study of two forms of testosterone
A C-13 CPMAS NMR experiment at high field (11.7 T) has produced significantly improved dispersion for the a form of testosterone, allowing revisions and extensions to be made to the assignments. Correlations shown by an INADEQUATE two-dimensional spectrum, recorded at 16.5 T, have allowed the components of most of the doublet signals to be grouped into two sets (for the two crystallographically independent molecules). First-principles computations, employing a fully solid-state approach, have been used to obtain values for the crystallographic splittings, which are discussed in relation to the experimental values. This procedure enables assignments to the two groups to be suggested for all but one of the remaining doublet signals. It also allows the two sets of signals to be identified specifically to the two independent molecules in the crystal structure. Computations were also carried out for the beta form of testosterone (a dihydrate). The shift differences between the alpha and beta forms were compared with the experimental data, with encouraging results. Comparisons were also made between computed and experimental shielding anisotropies and asymmetries for three of the carbons of the alpha form. The methodology has a high potential for future applications, though more examples need to be evaluated before general conclusions can be drawn.