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

摘要

Complete carbon-13 chemical shift tensors are measured in single crystals of methyl alpha-D-galactopyranoside monohydrate, methyl alpha-D-glucopyranoside, methyl alpha-D-mannopyranoside methyl beta-D-galactopyranoside, methyl beta-D-glucopyranoside hemihydrate, and methyl beta-D-xylopyranoside. The fits of the experimental data to the second rank form of shift tensors reflect the accuracy of the measured tensors and yield standard deviations that range between 0.27 and 0.75 ppm. Ab initio gauge-invariant atomic orbital (GIAO) computations using the D-95 double-zeta basis set are used to assign the experimental tensors to the carbons in the unit cell. The root-mean-square (rms) deviation of the diffraction-structure-based GIAO shieldings fitted to all of the experimental shifts is 4.99 ppm. By optimizing the ring and methyl proton positions with the Gaussian-92 program and repeating the CIAO computations, the root-mean-square deviation is reduced to 2.40 ppm. These results illustrate that complete C-13 chemical shift tensors measured in single crystals and interpreted with quantum-chemical computations can be used to evaluate differences between crystal structures obtained with X-ray diffraction, neutron diffraction, and structural optimization methods.