Effects on Electronic Transitions

A stress applied to a crystal results in a strain. A phenomenological description of the electron energy levels under elastic strain was developed by Bardeen and Shockley [12]. It is referred to as the deformation potential approximation (DPA), in which the one-electron Hamiltonian is developed in a Taylor s series of the strain components The perturbation is written in cartesian coordinates, for a linear order in strain, as  

We consider first the effect of a uniaxial stress on the EM-like electronic spectra of donors with CB degeneracy and then the situation for acceptors. 

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Near the TS things change. The rapid evolution of the light components of the system (electrons and H atoms involved in a transfer process) makes the adiabatic approximation questionable. Also the sudden time dependent perturbation we introduced in Section 1.1.3 to describe solvent effects on electronic transitions is not suitable. We are considering here an intermediate case for which the time dependent perturbation theory does not provide simple formulae to support our intuitive considerations. Other descriptions have to be defined. 

In this section we present results of the application of the ASEP/MD method to the study of solvent effects on electron transitions in p-coumaric acid derivatives. This system was chosen because it is an example of electron excitations that promote internal rotation around formal double bonds. Internal rotations are characterized by large flows of charge and, consequently, important solvent effects. Furthermore, they are involved in very interesting phenomena, as are dual fluorescence in push-pull chromophores [33-35] or cA-trans photoisomerization reactions [36-38]. An adequate description of the excited states involved in these processes demand state of the art quanmm calculations including both static and dynamic electron correlation contributions. Furthermore, in many cases the solute is stabilized by hydrogen bonds, consequently, it is compulsory to use a microscopic description of the solvent in order to account for specific interactions. In these conditions ASEP/MD becomes a good alternative to other methods and it can help to shed light on these processes [39 2]. 

In order to illustrate how the general model presented in the previous paragraphs can be applied to the study of real molecular systems, two test cases will be presented focusing on the different aspects of the solvation effects on electronic transitions. 

The colors obtained depend primarily on the oxidation state and coordination number of the coloring ion (3). Table 1 Hsts the solution colors of several ions in glass. AH of these ions are transition metals some rare-earth ions show similar effects. The electronic transitions within the partially filled d andy shells of these ions are of such frequency that they fall in that narrow band of frequencies from 400 to 700 nm, which constitutes the visible spectmm (4). Hence, they are suitable for producing color (qv). 

In seeking an explanation for the implied changeover in mechanistic pathway we need to consider, in each case, the effect on the transition state of both electronic and steric factors. For SN2 attack, the enhanced inductive effect of an increasing number of methyl groups, as we go across the series, might be expected to make the carbon atom that... 

There are two simple types of geometric isomerism possible for octahedral complexes. The first exists for complexes of the type MA2B4 in which the A ligands may be either next to each other (Fig. 12.18s) or on opposite apexes of the octahedron (Fig. 12.18b). Complexes of this type were studied by Wemer, who showed that the proiro and video complexes of tetraamminedichlorocobalt([Il) were of this type (see Chapter 11). A very large number of these complexes is known, and classically they provided a fertile area for the study of structural effects. More recently there has been renewed interest in them as indicators of the effects of lowered symmetry on electronic transition spectra. 

R. Bonaccorsi, C. Ghio and J. Tomasi, The effect of the solvent on electronic transitions and other properties of molecular solutes, in R. Carbo (ed.), Current Aspects of Quantum Chemistry, Elsevier, Amsterdam, 1982, p. 407. 

In contrast to these nonpolar compounds, very dramatic solvent effects on UV/ Vis spectra have been observed for dipolar meropolymethine dyes, especially mero-cyanines, due mainly to the change in their dipole moments on electronic transition. An example is the following negatively solvatochromic pyridinium V-phenolate betaine, which exhibits one of the largest solvatochromic effects ever observed cf. Fig. 6-2 [10, 29]). 

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