The current density isosurface is shown for the lattice structure of methyl-D-glucopyranoside when the magnetic field is perpendicular to the plane of the paper. Carbon and oxygen are represented as black and magenta spheres, respectively. Hydrogen atoms are not shown in the image. The shielding contribution of the current density (isosurface set to in GAUSSVIEW) to the nucleus is color mapped onto the isosurface. The magnitude of the shielding is indicated by color (blue deshielding and red shielding), which ranges from −0.05 to 0.05 ppm. While the intramolecular contributions dominate, it is visually evident that the oxygen atoms on neighboring molecules play a non-negligible role to the overall shielding.
The linear correlation between the magnetic shielding and chemical shift tensors is plotted for all 14 organic compounds using the (a) GIAO method and (b) the lattice-including GIPAW PBE/Fine method. The atomic positions in the lattice are further refined for the GIPAW method while the geometries for the calculations were taken from neutron diffraction data without modification for the GIAO method. Color distinguishes the tensors associated with the carbohydrate molecules (red) and the aromatic molecules (blue). The plot scatter (RMSD) is reduced when including the lattice for GIPAW. Unexpectedly, the carbohydrate molecules follow a separate trend line from the aromatic molecules, which is clearly revealed by the structure of the residuals in (b).
Refinement of the neutron diffraction structures can lead to an improved shielding-shift relationship. Geometry optimized structures when the lattice structure is included reduce the plot scatter and slope deviation from −1. The GIPAW/PBE magnetic shielding tensor components using (a) the neutron diffraction and (b) geometry-optimized structures are plotted here for carbon nuclides in the carbohydrate molecules within our test database. The methoxy groups (red), which have the most significant change in diffraction geometry upon GIPAW optimizations, show a systematic error in the trend line that is corrected by the optimization.
Shielding anisotropy ellipsoids, where the axis length corresponds to the difference in principal components and the isotropic value graphically display the shielding tensors. The ellipsoids for acenaphthene demonstrate how the relative magnitude and orientation of the shielding tensors appear in the molecule. For aromatic tensors, errors are largest in , which corresponds to the smallest axis and is oriented nearly perpendicular to the carbon-hydrogen bond.
Different methods were employed to predict the shift result and result in unique trend lines for the shielding-shift correlation plots. The linear fit parameters can deviate significantly from their ideal values (solid black line) of −1 for slope and 188 ppm for the shielding of TMS reference for the different levels of theory and can be dependent on the bonding type of the molecules. While the shielding-shift correlation for the lattice-including GIPAW PBE/Fine (blue and magenta) method shows an improved scatter over the GIAO method (cyan and red), the divergent trend lines for the carbohydrate and aromatic tensors are apparent.
RMSD and regression parameters for shielding-shift correlation.
Magnetic-shielding distance for organic crystals.
Error in predicted isotropic shift (ppm).
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