Electronically distorted structures

In the bond valence model quantum effects are treated classically by including them in the interatomic repulsion described by eqn (3.1) or (3.2). There are, however, a number of cases where quantum effects are directly responsible for deviations from the higher symmetry that would otherwise be expected. Such electronically distorted structures were discussed in Chapter 8. 

The structure of MnP is a distorted variant of the NiAs type the metal atoms also have close contacts with each other in zigzag lines parallel to the a-b plane, which amounts to a total of four close metal atoms (Fig. 17.5). Simultaneously, the P atoms have moved up to a zigzag line this can be interpreted as a (P-) chain in the same manner as in Zintl phases. In NiP the distortion is different, allowing for the presence of P2 pairs (P ). These distortions are to be taken as Peierls distortions. Calculations of the electronic band structures can be summarized in short 9-10 valence electrons per metal atom favor the NiAs structure, 11-14 the MnP structure, and more than 14 the NiP structure (phosphorus contributes 5 valence electrons per metal atom) this is valid for phosphides. Arsenides and especially antimonides prefer the NiAs structure also for the larger electron counts. 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

In the X-ray structures of both 3 and 4, the tetrahedral distortion is greater than that found in model A. Since there is an electronic preference for a square planar coordination, the pendant phenyl substituents in 3 and 4 likely result in further steric crowding and therefore in more distorted structures compared to model A. In the QM/MM model B an optimized 0 angle of 30° is found, close to the 34° angle of the X-ray structure. Since the phenyl and trimethyl phenyl groups are accounted for on a steric basis only in model B, the result supports the notion that the severely distorted coordination of the Pd center in 3 is due to a steric effect. 

Up to now, the dynamical approach developed by Takagi is applied in electron diffraction structure analysis for accurate diagnosis of a thick distorted nanocrystals. Within this approach, the amplitudes of the transmitted and diffracted waves i given by the following Takagi-... 

Figure 6. On the right, the n-system ofTMMand the electronic configuration of the ground state are shown ( 2 labels are used). The left panel presents electronic states of TMM at the ground state equilibrium D h geometry, and at the two Jan-Teller 2 distorted structures (equilibrium geometries of the 1 B and 1 i states). The corresponding adiabatic singlet-triplet gaps are also shown.

La2Cu04 has a structure and crystal chemistry virtually identical to that of La2Ni04 with a couple of important exceptions. Firstly, all octahedrally coordinated Cu compounds show a spontaneous electronic distortion (the Jahn-Teller distortion described in Section 8.3.1) by which the two axial bonds become longer and the four equatorial bonds become shorter. The distortion observed in La2Cu04 is usually attributed to this effect, but the observation of the same distortion in La2Ni04 shows that the driving force in both compounds... 

Small oscillatory torsions of molecular units within the segment produce a de-localization of the electronic density according to the Debye-Waller effect . In a very distorted structure as in the melt its smooth intensity distribution is not affected to a measurable extent. Rotational isomers, on the other hand, induce remarkable changes of the ru involved. Moreover, it is easily shown that the Boltzmann weighting factor of the molecular conformation ensemble also has a substantia] influence on the ru. It is strictly not possible to calculate the... 

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