Electron density studies

Experimental electron density studies are a recent development in the structural characterization of metallotetrapyrroles. It is hoped that these studies will provide an additional 

A final question concerns the unusual core conformation of all of the four- and five-coordinate rt-cation radical species listed in Table XI. All these species have an unusual saddle-shaped conformation. Possible reasons for this are to be described in the conformation section. 

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Espinosa, E., Molins, E. and Lecomte, C. (1997), Electron density study of the one dimensional organic metal bis(thiodimethylene)-tetrathiofulvalene tetracyanoquinodimethane, Phys. Rev. B, 56(4), 1820-1833. 

Using the refinement parameters, electron density maps were calculated. Figure 1 shows an example derived from CU96. According to all refinement results the intensities of the strong reflections were too low. Therefore, they were omitted for the electron density studies. The problem is currently under study. Additionally, low-temperature measurements are planned for the near future. 

Electron Density Studies of Octahedral and Distorted Octahedral Complexes... 

Electron Density Studies of Molecular Crystals 281 table 12.3 Topological Analysis of Theoretical Densities on Strained Ring Molecules... 

We do not discuss here the very many applications of electron density studies which are summarized in many recent books on the subject [93-96]. 

It is not our intention in this article to give a comprehensive review of electron-density studies of inorganic compounds but to describe the characteristic behavior of 3d electrons in molecules and to illustrate how the nature of the bonding in inorganic compounds is reflected in experimental densities. 

III. Some Applications of Electron Density Studies in Molecular Compounds. . . 282... 

Because electron density is a local property, electron density studies of the peptide-like molecules show that the nonspherical part of the deformation density (i.e the P]m parameters of Eq. 8) remain essentially the same for a given atom in the same environment (the peptide residue, a phenyl ring, a methyl group...) [29], The same observation was made for porphyrin ligands [30] and by Brock, Dunitz, and Hirshfeld [37] for naphthalene and anthracene type molecules. All these observations suggest that the multipole parameters are highly transferrable from one atom to a chemically similar atom in different molecules and crystals. A key question is is it possible to determine for each chemical type of a given atom a small set of pseudoatom multipole parameters, and can such parameters be used to calculate electrostatic properties of new molecules To answer this question [29], two accurate but low resolution X-ray data sets (sin 0/Xmax = 0.65 A-1) were... 

III. SOME APPLICATIONS OF ELECTRON DENSITY STUDIES IN MOLECULAR COMPOUNDS... 

Both methods are successful only when highly accurate data can be obtained. The second method also implies that data can be collected up to large sin(6)/ values (typically up to 1.2 A"1) despite the decrease in diffraction intensity due to the Debye-Waller factor and to the falloff in the atomic scattering factor. As far as we know, only two examples of electron density studies performed on organic conductors can be found in the literature. 

In view of the results above we may ask Why are there so few electron density studies of organic conductors Clearly, even if the required ex-... 

Hamzaoui, F., Baert, F. and Wojcik, G. (1996). Electron-density study of m-nitrophenol in the orthorhombic structure. Acta Crystallogr. B 52, 159-64. [81, 212]... 

This was reported to provide a better normalization when applied to singlecrystal data collected for electron density studies of CeBg. In Equation (70), Hg, yip and yiy represent the linear absorption coefficients of the excitation, X-ray and emitted light within the image plate layer. The corresponding incident angles are v, k and P. The vector of the incident X-ray is given by z. 

Workstations for protein crystallography have been set up by the Departments of Chemistry and Biology at the Brookhaven National Laboratory. The optical system design for the Chemistry beam line is based on a spherical collimating mirror, followed by a two-crystal ( +, —) monochromator and finally by a bent cylindrical mirror (Hastings et al 1983). The instrument combines a liquid helium cryostat Huber diffractometer, for work on valence electron density studies of small molecules, with an oscillation camera for protein crystallography. The diffractometer may be upgraded to include a multichannel electronic area detector. 

Transition Metal Compounds. - Macchi et al. performed the first experimental electron density study of a 7t-ligand -coordinated to a metal atom.156 They considered this work as an experimental test of the Dewar-Chatt-Duncanson (DCD) bonding formalism and expected that their work would provide information concerning the 7r-complex versus the metallacycle dichotomy. The authors claim that the successful application of AIM to the experimentally determined p has been the most important step in the coupling of X-ray studies and theoretical chemistry. Their paper reports on the determination of an accurate electron density of crystalline bis(l,5-cyclooctadiene)-nickel, Ni(COD)2 (Figure 11) by X-ray diffraction at 125 K. 

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