Solid-state 13C chemical shift tensors in terpenes.: 2.: NMR characterization of distinct molecules in the asymmetric unit and steric influences on shift in parthenolide

PHORMAT and FIREMAT solid-state NMR analyses provide all of the C-13 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 C-13 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-CH3 bond resulting from steric interactions.