X — N deformation density

A first map of this kind is shown in Fig. 5.8. The X-N deformation density is thermally averaged, and has limited resolution as the summation in Eq. (5.17) is truncated at the limit of the experimental observations. Since both Fobs and Fcalc are complex for an acentric structure, the structure factor phases are continuously variable, and must be considered. Expression (5.17) can be rewritten as... 

The X-N deformation densities are important for the study of the charge density distribution in and around hydrogen atoms. Without the extra effort required for a neutron experiment, assumptions on the hydrogen atom location and vibrations must be made which introduce a considerable uncertainty in the results. 

Hsu B, Schlemper EO (1980) X-N Deformation density studies of the hydrogen maleate ion and the imidazolium ion. Acta Cryst B 36 3017-3023... 

For the 2-cyanoguanidine molecule6 the static deformation density has been mapped by least-square refinement against low-temperature X-ray data in order to explain the fact that the C—N bonds around the C atom are almost identical and the fact that a large negative charge (—0.2 e) is on the N(3) atom. Hence one must take all the resonance forms (2) into consideration. 

Figure 13. Dynamic deformation density on a pyrrole plane (a), in the (x,y) four nitrogen plane (b) and in the (y,z) N-Fe-O (THF) plane (c) in TPPFedHFte corresponding theoretical density in the (x,y) plane for the 5E2g state of PFe(H20)2 [33] d). Contours as in Figure 6 (reproduced from [34]).

Deformation densities defined in this way typically show density accumulation in the bonds and lone pair regions. Exceptions were first observed [181] in standard X-ray electron-density mapping of a polycyclic molecule containing C, H, N and O atoms. A steady decrease in the order C-N > C-0 > N-N > O-O, of deformation densities in bonds, was observed. The density along the 0-0 bond was found to be negative throughout. 

An excellent example in which the information obtained from X-X/,0 studies of deformation density maps has been compared with that from theoretical calculations is given by tetrafluoroterephthalonitrile. This compound is ideal for these studies because it contains no hydrogen atoms, and all the atoms have similar scattering power (6 to 9 electrons for C, N, and F). The molecule has high symmetry which simplifies the number of parameters to be refined. A very precise set of X-ray Bragg reflections were measured at a low temperature (98 K) and to sin0/A = 1.15 Least-squares refinement based on the high-order data... 

Fig. 3.7. (a) Ab initio self-consistent Held calculated electron deformation density distribution of trans [(it -CjHslFelCOljlj in the plane containing the iron-iron axis and the bridging carbonyl ligand. The contour interval is -200e/nm and the dashed contours correspond to negative deformation and the solid contours correspond to zero and positive deformation density (b) experimental electron deformation density distribution derived from X-N calculations. The contour interval is — lOOe/nm . (From Bbnard, 1982.)... 

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