Deformation density negative

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. 

FIG. 10.11 Electron density in the metal-ligand plane of dichromium tetraacetate, (a) Molecular diagram, (b) deformation density through Cr—Cr and the acetyl group averaged over equivalent regions. Contours are at 0.10 eA-3. Negative contours are broken lines. Source Benard et al. (1980). 

FIG. 11.3 Comparison of the ab-initio local density functional deformation density for Si with the experimental static model deformation density. Contour interval is 0.025 e A-3. Negative contours are dashed lines. Source Lu and Zunger (1992), Lu et al. (1993). 

Fig. 6. Section of the X-X deformation density through the Mn-C(3)-Mn plane of (M-CH2)[(r)5-C5Hs)Mn(CO)2]I (3a see Fig. 1). Contour interval 0.1 eA-3. Dashed contours for negative, bold line for zero deformation density. Resolution 2 sin max/A = 1.41 A 1 U7max = 30°). Average e.s.d. of the observed density in general position is 0.07 eA 3 (127).

Fig. 1. Observed deformation density of KaNa[Co(N02)6l (591. (a) Deformation density in a section containing a Co—N bond and two C3 axes, whose plane is shown in (b). Contours are at intervals of 0.2 e A3. Negative contours are dotted, zero being chain dotted, (b) Molecular Structure of [Co(N02)fil3, showing the plane in (a), (c) Illustrative view of the arrangement of electron excess regions U2g>.

Figure 3. ORTEP view (a) and theoretical static deformation density of H3PO4 in the 0=P-0(H) plane (b). Contours interval 0.1 e A 3 (—) positive contours, ( ) negative contours zero contour omitted.

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. 

In terms of hybrid-bond theory it appears reasonable that the deformation density could be negative in some bonds. When a p-block atom with n valence electrons forms a bond, the valence shell is polarized into a tetrahedral distribution with n/4 electrons concentrated around each potential... 

Fig. 5.35. Electron-density distributions in borates — deformation density maps for corner-sharing BO, planar triangular units (a) calculated density map for planar H4B,0, (b) experimental density map for LiBOj (contour interval 0.1 electrons A dashed contour is zero negative contours are dotted) (after Zang et al., 1985 reproduced with the publisher s permission).

Fig. 7.8. Calculated (using ab initio SCF Hartree-Rock-Roothaan MO methods) deformation density maps for various Si202 ring-containing molecules (a) H4Si206 in the plane of the Si-O-Si-0 ring (b) H6Si20, in the Si-O-Si plane. The contour interval is 0.05 e A with negative contours dashed and the zero contour dotted (after O Keeffe and Gibbs, 1985 reproduced with the publisher s permission).

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