C-13 CHEMICAL-SHIFT TENSORS IN METHYL GLYCOSIDES, COMPARING DIFFRACTION AND OPTIMIZED STRUCTURES WITH SINGLE-CRYSTAL NMR

Complete carbon-13 chemical shift tensors are measured in single cryst als of methyl alpha-D-galactopyranoside monohydrate, methyl alpha-D-gl ucopyranoside, methyl alpha-D-mannopyranoside methyl beta-D-galactopyr anoside, methyl beta-D-glucopyranoside hemihydrate, and methyl beta-D- xylopyranoside. The fits of the experimental data to the second rank f orm of shift tensors reflect the accuracy of the measured tensors and yield standard deviations that range between 0.27 and 0.75 ppm. Ab ini tio gauge-invariant atomic orbital (GIAO) computations using the D-95 double-zeta basis set are used to assign the experimental tensors to t he carbons in the unit cell. The root-mean-square (rms) deviation of t he diffraction-structure-based GIAO shieldings fitted to all of the ex perimental shifts is 4.99 ppm. By optimizing the ring and methyl proto n positions with the Gaussian-92 program and repeating the CIAO comput ations, the root-mean-square deviation is reduced to 2.40 ppm. These r esults illustrate that complete C-13 chemical shift tensors measured i n single crystals and interpreted with quantum-chemical computations c an be used to evaluate differences between crystal structures obtained with X-ray diffraction, neutron diffraction, and structural optimizat ion methods.