Quadrupole bonds

A yellow crystalline product, Cr2(MeNCPhNMe)4, has been isolated96 by the reaction of Cr2(02CMe)4 with Li(MeNCPhNMe) in hexane. The molecule has virtual D4h symmetry and there is no axial coordination, since the four methyl groups on each end screen the axial positions very effectively (33). The Cr—Cr distance is 1.843 (2) A, which represents one in the shorter range of the super short Cr=Cr quadrupole bonds. 

I is the bond length. The experimental quadrupole moment is consistent with a charge, q, of approximately 0.5e. In fact, a better representation of the electrostatic potential around the nitrogen molecule is obtained using the five-charge model shown in Figure 4.20. 

The range of systems that have been studied by force field methods is extremely varied. Some force fields liave been developed to study just one atomic or molecular sp>ecies under a wider range of conditions. For example, the chlorine model of Rodger, Stone and TUdesley [Rodger et al 1988] can be used to study the solid, liquid and gaseous phases. This is an anisotropic site model, in which the interaction between a pair of sites on two molecules dep>ends not only upon the separation between the sites (as in an isotropic model such as the Lennard-Jones model) but also upon the orientation of the site-site vector with resp>ect to the bond vectors of the two molecules. The model includes an electrostatic component which contciins dipwle-dipole, dipole-quadrupole and quadrupole-quadrupole terms, and the van der Waals contribution is modelled using a Buckingham-like function. 

The reduction of the C— Br and C—1 group moments from 1.10 and 0.90 in bromo- and iodo-benzene to about 0.80 and 0.50 in 2-bromo- and 2-iodo-thiophene has been ascribed to the larger weight of resonance forms such as (8) and (9) in the thiophene series. The chlorine, nuclear, quadrupole, resonance frequencies of chloro-substituted thiophenes are much higher than those of the corresponding benzene derivatives. This has been ascribed to a relayed inductive effect originating in the polarity of the C—S o-bond in thiophenes. The refractive indices, densities, and surface tension of thiophene, alkyl- and halo-thiophenes, and of some other derivatives have been... 

Table 1. The 72-atom model examined by different theoretical methods. The energy differences (AE in kcal/mol) are calculated with respect to the lowest SCF energy. q(Fe) stands for Mulliken population charges on the Fe atoms q(S) and SS(b.i.) are the Mulliken population charges and the bond index for the bridging S atoms, respectively AEq is the calculated Mossbauer quadrupole splitting constant [mm/sec]. The PUHF spin states are those projected from the UHF wavefunction with 5 = 5,. 

In a conventional Fe Mossbauer experiment with a powder sample, one would observe a so-called quadrupole doublet with two resonance lines of equal intensities. The separation of the lines, as given by (4.36), represents the quadrupole splitting The parameter Afg is of immense importance for chemical applications of the Mossbauer effect. It provides information about bond properties and local symmetry of the iron site. Since the quadrupole interaction does not alter the mean energy of the nuclear ground and excited states, the isomer shift S can also be derived from the spectrum it is given by the shift of the center of the quadrupole spectrum from zero velocity. 

Quadrupole splittings are often interpreted from ligand field models with simple rules for the contributions from each occupied f-orbital (see discussion above). However, these models fail even qualitatively in the case of more covalent metal-ligand bonds. An example concerns the quadrupole spUttings of Fe(IV)-oxo sites in their 5 = 1 or 5 = 2 spin states. Here, ligand field considerations do not even provide the correct sign of the quadrupole splitting [60]. 

Ni(niox)2] Quadrupole splitting of a square-planar nickel complex, the sign of which demonstrates that the EFG of the nonbonding electrons outweighs that of the bonding electrons donated by the ligands... 

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