Experimental charge density

Flensburg, C., Larsen, S. and Stewart, R.F. (1995) Experimental charge density study ofmethylam-monium hydrogen succinate monohydrate. A salt with a very short O-H-O hydrogen bond,. /. Phys. Chem., 99, 10130-10141. 

When observed structure factors are used, the thermally averaged deformation density, often labeled the dynamic deformation density, is obtained. An attractive alternative is to replace the observed structure factors in Eq. (5.8) by those calculated with the multipole model. The resulting dynamic model deformation map is model dependent, but any noise not fitted by the muitipole functions will be eliminated. It is also possible to plot the model density directly using the model functions and the experimental charge density parameters. In that case, thermal motion can be eliminated (subject to the approximations of the thermal motion formalism ), and an image of the static model deformation density is obtained, as discussed further in section 5.2.4. 

The moments of a charge distribution provide a concise summary of the nature of that distribution. They are suitable for quantitative comparison of experimental charge densities with theoretical results. As many of the moments can be obtained by spectroscopic and dielectric methods, the comparison between techniques can serve as a calibration of experimental and theoretical charge densities. Conversely, since the full charge density is not accessible by the other experimental methods, the comparison provides an interpretation of the results of the complementary physical techniques. The electrostatic moments are of practical importance, as they occur in the expressions for intermolecular interactions and the lattice energies of crystals. 

In this expression, the dipole dipole interactions are included in the electrostatic term rather than in the van der Waals interactions as in Eq. (9.43). Of the four contributions, the electrostatic energy can be derived directly from the charge distribution. As discussed in section 9.2, information on the nonelectrostatic terms can be deduced indirectly from the charge density. The polarizability a, which occurs in the expressions for the Debye and dispersion terms of Eqs. (9.41) and (9.42), can be expressed as a functional of the density (Matsuzawa and Dixon 1994), and also obtained from the quadrupole moments of the experimental charge density distribution (see section 12.3.2). However, most frequently, empirical atom-atom pair potential functions like Eqs. (9.45) and (9.46) are used in the calculation of the nonelectrostatic contributions to the intermolecular interactions. 

Spackman et al. (1988) have used experimental charge densities to sum... 

The first charge density observation of bond bending in cyclopropane was from the experimental charge densities of cts-l,2,3-tricyanocyclopropane (Hartman and Hirshfeld 1966) and 2,5-dimethyl-7,7-dicyanonorcaradiene (Fritchie 1966). It has been confirmed by a considerable number of other studies, including one on... 

In summary the discussion above appears to provide a crude and qualitative explanation of the experimental charge densities. For more detailed recent progress by computer simulations in this field the reader is referred to theoretical contributions in this volume. However, the determination of the effective charge densities remains a highly delicate issue and the discussion above may at most provide some qualitative trends which, however, are based on several assumptions. 

In this section, I have included a few of my papers dealing with synthesis, defects and certain properties of oxidic materials and fullerenes, besides a general article dealing with important directions in materials chemistry. There is also an article dealing with experimental charge densities in organic molecular crystals. These articles should indicate the diversity and breadth of coverage in materials chemistry. 

This work is to be followed by an analysis of an experimental set of intensity data obtained with both the X-ray and polarized neutron techniques. This will allow application of the formalisms described above, and a direct comparison of experimental charge density parameters with theoretical results. 

D. M. M. Jaradat, S. Mebs, L. Chgcihska, and P. Luger, Experimental charge density of sucrose at 20 K Bond topological, atomic, and intermolecular quantitative properties, Carbohydr. Res., 342 (2007) 1480-1489. 

Lecomte, C., Guillot, B., Jelsch, C., Podjamy, A. Frontier example in experimental charge density research experimental electrostatics of proteins, Int. 1. Quant. Chem. 101(5) (2005) 624-634. 

The above results are in line with the picture obtained from the experimental charge density studies on nitrobenzene [65] made by use of low temperature X-ray diffraction studies [64]. When the perpendicular sections are carried out through the CN and CC bonds in the ring in their centers, the picture obtained is as in Fig. 15. To make the view more clear the additional operation is made the difference between this map and the same map rotated by 90° is shown, indicating the 7t-bond ellipticity. 

The following conclusion may be drawn from the above results the CN bond in nitrobenzene is almost cylindrical indicating a very low contribution of the n -electron component. In contrast to them, the typical aromatic CC bonds in the ring are significantly elliptical - as expected from the chemical intuition and experience. Thus the results of experimental charge density studies are in line with the much simpler treatment based on the HOSE model and precisely measured bond lengths. 

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