Carbon-13 Chemical Shift Tensors in Methyl Glycosides, Comparing Diffraction and Optimized Structures with Single-Crystal NMR

Abstract

Complete carbon-13 chemical shift tensors are measured in single crystals of methyl α-d-galactopyranoside monohydrate, methyl α-d-glucopyranoside, methyl α-d-mannopyranoside, methyl β-d-galactopyranoside, methyl β-d-glucopyranoside hemihydrate, and methyl β-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-ζ 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 GIAO computations, the root-mean-square deviation is reduced to 2.40 ppm. These results illustrate that complete 13C chemical shift tensors meas...