PREDICTION OF CRYSTAL STRUCTURES OF TWO MONOSACCHARIDES
Marian Durik, Lubomir Smrcok, Daniel Tunega
Institute of Inorganic Chemistry, Slovak Academy of
Sciences, SK-842 36 Bratislava, Slovak Republic,
e-mail : uachdur@savba.sk
Crystal molecular structures of monosaccharides C13H20N2O5, P21 and P62, respectively, here called RKSA1 and RKSA5 for short) were simulated by potential energy minimization. Positions of all atoms within the molecules were derived from already known crystal structures [1,2], atomic (Mullikan) charges were obtained from an ab initio HF SCF calculation (3-21G basis set). To describe the interactions between molecules two force fields, Williams and Dreiding [3,4], were applied. Simulation were done with the cell parameters fixed at the values found for single crystals. Powder patterns of 4x20 generated structures were compared to the experimental powder patterns taken with Stoe STADI-P transmission difractometer by means of Rietveld refinement. At this stage the set of refined parameters included only a scale factor, overall temperature parameter, zero-point and W of FWHM parameters.
In all but one case (RKSA1 + Dreiding) the structure with the lowest energy has provided the best model for Rietveld refinement. In Fig.1 a,b dependence of Rwp on E is shown for both RKSA1 and RKSA5 generated using Williams force field, while in Fig.2 there is a graphic output from Rietveld refinement showing agreement of yo/yc for the best RKSA1 structure. Note, that in this case also the cell parameters were refined (Rw=15%, w=const.)
Computer programs used : Hardpack [5] for simulation, Gaussian 94 [6] for ab initio HF SCF and Fullprof [7] for Rietveld refinements.