ANALYZING AND ASSIGNING C-13 CHEMICAL-SHIFT TENSORS IN FRUCTOSE, SORBOSE, AND XYLOSE SINGLE-CRYSTALS

Complete carbon-13 chemical-shift tensors are measured in single cryst als of the monosaccharides beta-D-fructopyranose [57-48-7], alpha-L-so rbopyranose [87-79-6], and alpha-D-xylopyranose [58-86-6] using the tw o-dimensional chemical-shift correlation technique with a multiple-axi s sample-reorientation mechanism. Symmetry-constrained fitting of the experimental data extracts the tensors for the four equivalent molecul es in the P2(1)2(1)2(1) unit cells and the directions of the three two fold axes in the orthorhombic crystals. The standard deviations of the symmetry fits to the data are 0.30, 0.40, and 0.29 ppm for the three monosaccharide molecules, respectively. Gauge-invariant atomic orbital (CIAO) computations using the D-95 double-zeta basis set are then use d to assign the experimental tensors to the carbons in the molecules a nd to the molecules in the unit cell. All assignment permutations and combinations are explored, and that which minimizes the RMS deviation between the experimental and computed tensors is accepted as the best assignment. The GIAO results also identify the experimental twofold ax es with the crystallographic axes. Because the hydroxymethyl hydroxyl group in sorbose jumps between two positions, it is necessary to compu te the tensors for both molecular configurations and then to take a we ighted average of the results for comparison with the experimental dat a. The D-95 GIAO results have RMS deviations from the experimental ten sors of 1.90 ppm for fructose, 2.61 ppm for sorbose, and 2.66 ppm for xylose. Similar computations confirm a previous assignment of the carb on-13 tensors in sucrose and yield a 2.85 ppm RMS deviation. (C) 1996 Academic Press, Inc.