Crystal Data cell

As a result of having two chiral centers, four stereoisomers of ascorbic acid are possible (Table 1) (Fig. 2). Besides L-ascorbic acid (Activity = 1), only D-araboascorbic acid (erythorbic acid (9)) shows vitamin C activity (Activity = 0.025-0.05). The L-ascorbic acid stmcture (1) in solution and the soHd state are almost identical. Ascorbic acid crystallizes in the space group P2 with four molecules in the unit cell. The crystal data are summarized in Table 2. 

Currently, there are about 197,500 entries in the National Institute of Standards and Technology (NIST) Crystal Data File. An exhaustive search takes about one minute. Unit cell parameters are very definitive. Usually only one or a few hits are found and the appropriate Hterature reference(s) are Hsted. If no hits are found, the stmcture has not been previously reported. 

Indexings and Lattice Parameter Determination. From a powder pattern of a single component it is possible to determine the indices of many reflections. From this information and the 20-values for the reflections, it is possible to determine the unit cell parameters. As with single crystals this information can then be used to identify the material by searching the NIST Crystal Data File (see "SmaU Molecule Single Stmcture Determination" above). 

Real data is often available only for periodic systems, so only the density in the crystal unit cell need to be considered. Now the X-ray experiment gives structure factors Fh (along with errors at) which are related to the unit cell charge density via a Fourier transform,... 

X-ray structural analysis of the methylsulfate compound indicates the orthorhombic crystal unit cell contains two translationally inequivalent cations positioned on mirror planes and tilted at 3 ° relative to the two-fold screw (c) axis (23). This is a compromise orientation for simultaneously, rather than individually, maximizing x ccc and x /> bbc in this polar structure. This structure is therefore consistent with the extremely large SHG intensity reported in Table 1 while, also consistently, preliminary x-ray data show the perrhenate and tetrafluoroborate salts to be isostructural (23.). Details of the packing... 

In addition to the dynamic disorder caused by temperature-dependent vibration of atoms, protein crystals have static disorder due to the fact that molecules, or parts of molecules, do not occupy exactly the same position or do not have exactly the same orientation in the crystal unit cell. However, unless data are collected at different temperatures, one cannot distinguish between dynamic and static disorder. Because of protein crystal disorder, the diffraction pattern fades away at some diffraction angle 0max. The corresponding lattice distance <7mm is determined by Bragg s law as shown in equation 3.7 ... 

E. X-ray Crystal Data Search System. The National Bureau of Standards (NBS) has collected a file of data pertaining to some 24,000 crystals, including those in the Cambridge file described above (20). The data in the NBS file include the cell parameters, the number of molecules. Z. in the unit cell, the measured and calculated densities of the crystal and two determinative ratios, such as A/B and A/C. Every com-... 

Because of the very different cell dimensions and symmetries of the two guest structures, they could be distinguished trivially using single-crystal data, as their... 

Unit cell parameters, originally obtained photographically and later redetermined accurately from diffractometer settings, are listed with other relevant crystal data in Table II. X-ray diffraction data were collected on a Nonius CAD-3 diffractometer in the manner described in an earlier publication (34). Reflec-... 

Crystal data are available for NH4Ag(SCN)2142 and KAg(SCN)2.143 The symmetry of the ammonium salt was monoclinic, belonging to space group P2Jn. The unit cell contained four molecules. The crystal was found to be built up of AgSCN molecules, NH4 ions and SCN- ions... 

CRYSTAL DATA Cl 2/c 1 (No. 15 unique axis b, cell choice 1)... 

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