Volume changes, with electronic transition

MnAs exhibits this behavior. It has the NiAs structure at temperatures exceeding 125 °C. When cooled, a second-order phase transition takes place at 125 °C, resulting in the MnP type (cf. Fig. 18.4, p. 218). This is a normal behavior, as shown by many other substances. Unusual, however, is the reappearance of the higher symmetrical NiAs structure at lower temperatures after a second phase transition has taken place at 45 °C. This second transformation is of first order, with a discontinuous volume change AV and with enthalpy of transformation AH. In addition, a reorientation of the electronic spins occurs from a low-spin to a high-spin state. The high-spin structure (< 45°C) is ferromagnetic,... 

In addition to the described above methods, there are computational QM-MM (quantum mechanics-classic mechanics) methods in progress of development. They allow prediction and understanding of solvatochromism and fluorescence characteristics of dyes that are situated in various molecular structures changing electrical properties on nanoscale. Their electronic transitions and according microscopic structures are calculated using QM coupled to the point charges with Coulombic potentials. It is very important that in typical QM-MM simulations, no dielectric constant is involved Orientational dielectric effects come naturally from reorientation and translation of the elements of the system on the pathway of attaining the equilibrium. Dynamics of such complex systems as proteins embedded in natural environment may be revealed with femtosecond time resolution. In more detail, this topic is analyzed in this volume [76]. 

For the mechanistic interpretation of activation volume data for nonsymmetrical electron-transfer reactions, it is essential to have information on the overall volume change that can occur during such a process. This can be calculated from the partial molar volumes of reactant and product species, when these are available, or can be determined from density measurements. Efforts have in recent years focused on the electrochemical determination of reaction volume data from the pressure dependence of the redox potential. Tregloan and coworkers (139, 140) have demonstrated how such techniques can reveal information on the magnitude of intrinsic and solvational volume changes associated with electron-transfer reactions of transition... 

Correspondingly, low-symmetry atomic or molecular arrangements comprising different types of chemical bonds normally exhibit pronounced anisotropy of the compressibility. Structural reorganizations due to pressure-induced phase transitions are associated with discontinuous volume decreases and normally increasing coordination numbers. These structural changes not only modify the coordination environment in the crystal structure but frequently also the electronic properties of the solid. 

Oxidative quenching by dimethylviologen (MV " ") (Eq. 6.36, kq = 9.5 x 10 in aqueous solution) has been reported to exhibit a modestly negative AV (—2.6 cm mol ) [24]. Interestingly, recent PAG experiments [75] indicate that the overall volume change for Eq. 6.36 is positive. Thus, it appears that the transition state of the electron transfer reaction is dominated by formation of an encounter complex with a smaller volume than the separated reactants most likely due to solvation effects. Separation of the encounter complex into products has a large, positive AV (Fig. 6.15). 

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