Dimer molecular

The structure of AICI3 is similarly revealing. The crystalline solid has a layer lattice with 6-coordinate Al but at the mp 192.4° the stmcture changes to a 4-coordinate molecular dimer Al2Clg as a result there is a dramatic increase in volume (by 85%) and an even more dramatic drop in electrical conductivity almost to zero. The mp therefore represents a substantial change in the nature of the bonding. The covalently bonded... 

FIGURE 62. Molecular dimeric structure of Sn[(TeSi(SiMe3)3]2- Reprinted with permission from Reference 218. Copyright (1993) American Chemical Society... 

Figure 1.3 Molecular structure of Sulforhodamine B and exciton band energy diagram for molecular dimers with oblique and coplanar inclined transition dipoles. (Reproduced from ref. 10, with permission.)...

Molecular dimerization, as in the transition from Hansa Yellow to disazo yellow pigments, increases the color strength considerably. A comparison of a Naph-thol AS/disazo condensation pigment pair gives similar results ... 

Acylation of 1-aminoanthraquinone affords 1-acylaminoanthraquinones. The carbonamide function renders these compounds very fast to migration. The migration fastness may be improved further by introducing substituents which carry additional carbonamide groups, or by molecular dimerization using aromatic dichlorides. 

Madjet Mel A, Muh F, Renger T (2009) Deciphering the influence of short-range electronic couplings on optical properties of molecular dimers application to special pairs in photosynthesis. J Phys Chem B 113 12603-14... 

Molecular dimer geometries, magnesium porphine-porphine, 24-28 Molecular orbital diagram, porphine, 254,255f... 

Das AK, Banerjee A, Drew MGB, Haidar D, Baneijee A. Stepwise self-assembly of a tripeptide from molecular dimers to supramolecular /8-sheets in crystals and amyloid-like fibrlis in the solid state. Supramol Chem 2004 16 331-335. 

The single crystal structure of benzoic acid has been reported [52]. In its crystal, the molecule is approximately planar, but distances of0.042 A and 0.068 A displace the carboxylate carbon and one of its oxygen atoms, respectively, from the mean plane. The structure is characterized by the existence of hydrogen-bonded molecular dimers in the unit cell. 

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. 

The PIXEL code and documentation (OPiX package) can be downloaded at the author s site http //users.unimi.it/gavezzot. Computing times on an ordinary PC are of the order of a few seconds for a molecular dimer, to a few minutes for a complete crystal structure. PIXEL has been repeatedly shown to give results of accuracy comparable to ab initio MO methods [51]. For an example. Fig. 5 [52] shows a comparison between MP2, PIXEL, uncorrected and corrected periodic-orbital... 

Fig. 9 The calculated energy profile (relative energies, MP2/6-31G counterpoise corrected) for the direct approach of bromine and nitrogen in a bromobenzene-pyridine molecular dimer. The absolute well depth is 8.8 kJ mol ...

Derivatives of the linear polyacenes, naphthalene,46 anthracene,41,90,93 naphthacene,91,92 and pentacene,92 form stable photodimers M2 when irradiated in concentrated Oa-free solution or (exceptionally) in the crystalline state.94,95 Transannular a-bonding of the molecular dimer components results in a folding of the aromatic planes about the bonded atoms and a reduced -electron delocalization reflected in a shift of the absorption spectrum to much higher frequencies.46,92,96... 

Such a model can clearly account for the drop in the magnetic susceptibility. The drop in the conductivity is to be expected because the molecular dimer should be considerably less mobile than the monomer, the solvation should be stronger, and it will probably be more difficult for the NH3 molecule to drift across the pair of cavities. The alternative explanation is of course the formation of neutral complexes with the cations but if the cations retain their original mobility, this seems ruled out. 

The transition between the metallic and non-metallic regions is, as we have stated, in many ways similar to that in doped semiconductors, but there are certain differences, which we believe are due to the pairing of cavities to form what we have called molecular dimers . This intermediate region has two parts. 

From 3 to (say) 0.3 MPM the conductivity is not in the metallic range, but we believe that the material behaves like an intrinsic semiconductor, current being carried by electrons in the upper band. This, as we have seen, is not a Hubbard band, but the band formed from molecular orbitals for an extra electron on molecular dimers. In this range d In a jdT... 

Apart from the drop in the susceptibility as we approach the transition, the increase in the Hall-coefficient ratio (Fig. 10.18) is further evidence for the model as we have seen, evidence from doped semiconductors shows that RH does not deviate from the free-electron form when EF lies in the pseudogap for current carriers in a highly correlated gas (unless antiferromagnetic order sets in). Also, the susceptibility increases with temperature this may be because the molecular dimers dissociate. 

An exception to the rule that lowering the Iciiiperaiure cutises transitions to phases with increased order sometimes occurs for polar compounds w hich form the smectic l. phase (a layered structure formed by molecular dimers). Decreasing the temperature cutises u transition front nematic to smectic. 1,.,. but a further lowering of the temperature produces J transition back to the nematic phase tcalled the reentrant nematic phase). Electric or magnetic fields also may induce mesomorphic phase transitions. 

Fig. 2 Relative placement of HOMO and LUMO energy levels in a molecular dimer. The dashed lines indicate the energies for the homo-dimer. For the heterodimer, these levels are shifted up or down by e so that the band off-set is 2e. In the discussion, we assume that AEa = AEb. Shown also is the electron/hole configuration corresponding to state 3 = — .

Fig. 32. Crystal packing diagram showing the extended chain network of c/oso-l-Pb-2,3- Si(CH,)i -2,3-C2B4H4 2 molecular dimers with benzene molecules of crystallization.

The question of inter- vs, mtra-molecular dimerization in the octaanion of 46 was investigated by diffusion measurements72 and by conducting competition studies, i.e. aggregation with free 434. Both experiments pointed to the formation of iwfra-molecular dimers. 

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