Crystal structure 2192 hydrogen atom positions

P212121 Z = 8 Dx = 1.419 R = 0.068 for 1,373 intensities. The crystal structure contains two symmetry-independent molecules having slightly different conformations. The pyranoside conformations are 1C4 with Q = 57,60 pm, 0=173,177°. The nitro and acetate groups are oriented approximately normal to the mean plane of the pyranoside ring. The atomic coordinates refer to the d enantiomer. The hydrogen-atom positions were not reported. 

Most of the crystal-structure analyses report hydrogen-atom positions these are less accurate than those of the non-hydrogen atoms by a factor of ten, except in the case of neutron diffraction, where the accuracies are comparable. 

Because the electron density describing the hydrogen atom positions could not be observed in those early X-ray crystal structure analyses for technical reasons, the myth persists that hydrogen atoms cannot be seen by X-ray crystal diffraction. This led to the unfortunate association of the descriptor hydrogen bond length with the separation between the two hydrogen-bonded nonhydrogen atoms. 

The number of crystal structure data correlates with the number of statistical analyses of hydrogen-bond geometries. As the amount of crystal structure data increased, so did the efforts to use it as a basis for statistical analyses of the geometrical properties of hydrogen bonds. Because there was only a limited amount of neutron data available and the X-ray analyses did not have the accuracy to provide reliable hydrogen atom positions, most of the earlier analyses focused on the nonhydrogen donor-acceptor distances, i.e., O- O for O-H - O bonds and N 0 or N- -N for N—H 0 = C or N-H- -N bonds [33, 53, 381-384]. 

The results of a survey of 120 crystal structures which provided reasonably reliable hydrogen atom positions [84] are shown in Thble 7.13. Only hydrogen bonds of type C-OH Ow, C-0H 0=C, C-OH ()< , and... 

Fig. 6. Parts of the crystal structures of acidic aluminum orthophosphates (polyhedral representation), (a) Al(Fl2P04)3-C (b) Al(Fl2P04)3-A. One structural unit of Al(Fl2P04)3-C is indicated by shading. The hydrogen atom positions are shown by wedges [reprinted with permission from Kniep (14) copyright 1986 Wiley-VCFI].

Single-crystal x-ray studies of the contact-ion pair complexes shown in Table II have been completed. The data were measured with a Picker automated diffractometer, and the structures were solved by direct methods. Hydrogen atom positions were included in all the structures but usually not refined. Refinements of the structures were made using a full-matrix, least-squares technique with neutral-atom scattering factors. 

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