Porphyrins, deformation density

The distinction between the two states is readily illustrated in a deformation density section perpendicular to the porphyrin plane through the iron atom. Because of the transfer of an electron into the d2z orbital in going from 3EgA, and to 3A2g, the former configuration shows a deficiency and the latter an excess of density above and below the nitrogen atom. This is confirmed by theoretical deformation density maps (Fig. 10.10) (Rohmer 1985). The experimental map for FePc shows the deficiency along the z axis, as expected from the 4-orbital populations listed in Table 10.10. 

Irngartinger et al. 1977), and the hydro-bis(squarate) anion (Lin et al. 1994). Even in the deformation density maps of the five-membered pyrrole rings in transition metal tetraphenyl porphyrins (chapter 10), bond bending is visible. 

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... 

Figure 1b, Deformation electron density in the porphyrin plane in bis(pyridine)(meso-tetraphenylporphinato)iron(II). Contours...

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