Crystal molecular coordination sphere

In typical organic crystals, molecular pairs are easily sorted out and ab initio methods that work for gas-phase dimers can be applied to the analysis of molecular dimers in the crystal coordination sphere. The entire lattice energy can then be approximated as a sum of pairwise molecule-molecule interactions examples are crystals of benzene [40], alloxan [41], and of more complex aziridine molecules [42]. This obviously neglects cooperative and, in general, many-body effects, which seem less important in hard closed-shell systems. The positive side of this approach is that molecular coordination spheres in crystals can be dissected and bonding factors can be better analyzed, as examples in the next few sections will show. 

Each molecule in the model crystal cluster is identified by a symmetry operator, O(y ), connecting it to the reference one, by a distance, R(J) (usually between centers of mass), and by E J), as described above. For comparisons, E(J) is usually expressed as a percent of the total (PPE) the assembly of molecules with (y)>0.02 (PE) is taken as the molecular coordination sphere (see 12.4.3). 

As indicated in 12.5.1, it is difficult to compare two crystal structures. Structures with different space groups may actually be quite similar, and the comparison of spatial symmetry and even of cell parameters can be quite misleading. A much safer comparison is between the molecular coordination spheres, using 0(J)> U) nd E(j) (see 12.3.2.4). The similarity of 0 j) and R(j) may be less compelling, but the partitioning of the energy over molecule-molecule interactions usually provides a very strict test of similarity. 1 vo crystal structures with the same E(J) fo " few nearest neighbours are likely to turn out to be the same. 

The X-ray diffraction analysis of these complexes (4 -Ua--i -Ue) revealed atom positions and connectivities of one molecule of alkane in the coordination sphere of the uranium(III) center and a second molecule of cycloalkane co-crystallized in the lattice. Molecules 4 -Ua—4 "-Ue are isostructural and isomorphous. The molecular... 

First of all, electronic structure of nanoparticles was discussed. The influence of the size of particle on its electronic structure is determined by the nature of bonds in the particle lattice. In the lattice of molecular crystal intermolecular bonds cause only minor alterations in an electronic structure of molecules and are localized between the nearest neighbors in such lattice. In the lattice of inorganic crystal with purely ionic bonds the interaction of ion with medium is also localized in small space of the several coordination spheres surrounding an ion in the lattice. The transition of ion in the excited state gives essential disturbance of ionic lattice only in this space. 

A crystal structure usually is described by the unit cell dimensions, space group and coordinates of the atoms (or orientation and position of the molecules) in the asymmetric unit. This, in fact, is the order in which the information is obtained when a crystal structure is determined by X-ray or neutron diffraction experiments. However, an equivalent way to describe a structure is to place the center of a molecule at the origin of an orthogonal coordinate system and to specify its molecular surroundings. This alternative is especially powerful in crystals with one molecule per asymmetric unit because the orientations of the surrounding molecules are related to the central molecule by crystallographic symmetry. The coordination sphere or environment of the structure then is defined as those surrounding molecules which are in van der Waals contact, or nearly in contact, with the central molecule. 

Therefore, the first step in developing a method for predicting crystal structures was to characterize the coordination spheres found in known structures. This was limited to compounds of moderate molecular weight... 

Photochemical behaviour of coordination compounds described in previous chapters results mainly from electronic interactions between the central metal atom or ion and ligands in the hrst coordination sphere. An increased size of molecular systems to clusters and nanosized crystals expands the possibility of photoinduced electron transfer between the discrete electronic states to excitation within bands. Furthermore, interactions of nanoparticles with molecules yield unique materials, combining structural versatility of molecular species with collective properties of solids. 

The reaction of RhCls with an excess of the anion of TIPT in MeCN also generated a dinuclear complex in high yield, in which the bridging aromatic thiolate also binds via an arene substituent, but in this case the two rhodium atoms are not equivalent (76). A representation of the overall crystal and molecular structure is shown in Fig. 18. A simplified view of the coordination sphere of one rhodium atom is shown in Fig. 19. The structure comprises two distinct metal atoms, with... 

I THF crystallizes from hexane as a dimer. The central four membered ring is formed by the two lithium atoms and two metallated carbon atoms. The carbon atoms show two lithium contacts. Similarly, the lithium atoms have two carbon contacts. An intramolecular coordination of the piperidino nitrogen and the oxygen of the THF molecule complete the coordination sphere of lithium. A ball and stick drawing of the molecular structure of I THF and selected bond distances and angles are given in Fig. 1. 

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