Solid-State¹³C Chemical Shift Tensors in Terpenes. 2. NMR Characterization of Distinct Molecules in the Asymmetric Unit and Steric Influences on Shift in Parthenolide

Abstract

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.

A new sensitive isotropic–anisotropic separation experiment—SPEED MAS

“…In addition, Hartmann-Hahn [9,10] and 1 H TPPM [11,12] decoupling conditions are affected by the spinning speed and thus MAT experiments require a different experimental setup than commonly run high-speed MAS experiments. Nevertheless, MAT techniques are widely applied in many studies [13][14][15][16][17].…”

Density functional theory study of N–H⋯O, O–H⋯O and C–H⋯O hydrogen-bonding effects on the 14N and 2H nuclear quadrupole coupling tensors of N-acetyl-valine

“…Therefore, hydrogen atom positions were optimized at the B3LYP/6-311G level of theory for both monomer and pentamer cluster. This level of theory has been previously shown to give hydrogen atom position comparable to neutron diffraction values for a variety of systems [16,[32][33][34].…”

A systematic study on hydrogen bond interactions in sulfabenzamide: DFT calculations of the N-14, O-17, and H-2 NQR parameters

“…Furthermore, fully optimization at the level of B3LYP/6-311 ++ G⁎⁎ was performed for an isolated gas-phase of sulfabenzamide, see Table 1. This level of theory has been previously shown to give hydrogen atom position comparable to neutron diffraction values for a variety of systems [55][56][57][58]. To have a comparison among the capabilities of various nuclei to contribute in the H-bonding interactions, and also to systematically investigate the H-bonding effects on the 14 N, 17 O and 2 H EFG tensors in sulfabenzamide crystalline structure, all of the EFG calculations were performed for three models of system; an isolated gasphase, the monomer and the heptameric cluster of sulfabenzamide; see Tables 2-4.…”