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PHORMAT and FIREMAT solid-state NMR analyses provide all of the 13 C tensor principal values for the carbons in solid parthenolide which contains two molecules per asymmetric crystallographic unit and 15 carbons per molecule. Only one pair of the isotropic lines is degenerate, thus 29 different sets of principal values have been measured along with the 29 isotropic resonances. The FIREMAT signal-to-noise per unit time is significantly higher than PHORMAT thereby allowing a more accurate 13 C tensor analysis in an experiment taking much less time. This FIREMAT feature is especially beneficial for the inherently broader sp 2 carbon spectral bands. Comparison of experimental tensor principal values with those computed from X-ray coordinates provides shift assignments for all carbon pairs. This comparison also clearly differentiates between shifts arising from the two specific molecules in the asymmetric unit for 7 of the 15 pairs of carbons. Two additional pairs of carbons may be assigned to specific molecules in the asymmetric unit at lower confidence levels while the six remaining carbon pairs (1 sp 2 and 5 sp 3 ) are unassignable in the asymmetric unit with the present level of theoretical computations. Experimental sp 2 and sp 3 principal values are used to evaluate five different tensor computational methods. The B3PW91 method with the 6-31+G (2d, p) basis provides the most accurate tensors with a root-mean-square error of 2.3 and 3.2 ppm for sp 3 and sp 2 carbons, respectively. Repulsive steric interactions between methyl protons and proximate neighboring protons correlate with large upfield shifts in two of the three methyl carbon tensor principal components. These changes appear to correlate with variations in the electronic charge in the C-CH 3 bond resulting from steric interactions.

To investigate the origins of solid-state NMR shift differences in polymorphs, carbon NMR chemical shift tensors are measured for two forms of solid 10-deacetyl baccatin III: a dimethyl sulfoxide (DMSO) solvate and an unsolvated form. A comparison of ab initio computed tensors that includes and omits the DMSO molecules demonstrates that lattice interactions cannot fully account for the shift differences in the two forms. Instead, conformational differences in the cyclohexenyl, benzoyl, and acetyl moieties are postulated to create the differences observed. X-ray analysis of six baccatin III analogues supports the suggested changes in the cyclohexenyl and benzoyl systems. The close statistical match of the (13)C chemical shifts of both polymorphic forms with those calculated using the X-ray geometry of 10-deacetyl baccatin III supports the contention that the B, C, and D rings are fairly rigid. Therefore, the observed tensor differences appear to arise primarily from conformational variations in ring substituents and the cyclohexenyl ring.