Charge transfer states, electric moments

Actual electron transfer does occur in oxidation/reduction, or "redox", reactions. In this type of reaction, there is a change in the oxidation state of the adsorbate. A simple example is the chemisorption of an alkali atom, in which it becomes a 1+ ion, transferring its outer electron to empty electron orbitals of the substrate. It is the large electric dipole moment created by this charge transfer process that lowers the work function of surfaces on which alkali atoms are adsorbed (e.g., "cesiation") by up to several eV. This type of bonding is generally strong, and it can also be either molecular or dissociative. 

Calculations of eight frontier molecular orbitals for D2h dimers 9 = 90°), and six frontier orbitals for D2 dimers were carried out from the four frontier orbitals of the monomers. A simplified description of the frontier orbitals evolution is reproduced in Figure 13.22, along with a transition dipole moment representation in the orthogonal and oblique bis-porphyrins. Raman resonance spectroscopy of the excited states shows that some transitions involve charge transfer between the two subunits, and that the contribution of charge transfer increases with the degree of coplanarity of the dimers. This is consistent with previous electric dipole measurements in the excited states. 

Ab initio molecular dynamics simulations usually describe fluctuations of molecular electronic structures and can broaden knowledge of electric dipole moments, polarization processes, or possible charge-transfer effects, and complement classic MD methods. However, AIMD simulations are computationally demanding, and only relatively small systems of tens of ion pairs can be propagated over short time periods of tenths of picoseconds. These simulations typically begin from liquid state conflgurations that are equilibrated with classic MD using empirical interaction potentials. 

The tensor elements of the molecular quantity CD, cd) and, therefrom, the averaged quantity / may in principle be calculated quantum mechanically, may be expressed in terms of the energy levels of the molecule and the electric dipole transition moments between the corresponding quantum states. Exact (ab initio) calculations are very cumbersome, but a number of simplified procedures (semiempirical calculations) have been applied to this problem and their results allow a reasonably successful interpretation of the measured results, in particular where strong charge-transfer effects come into play. 

We now turn to another important influence on ET dynamics, electric fields. It is reasonable that the presence of a strong electric field should influence the dynamics of charge separation processes because the dipole moment associated with the newly formed CS state will interact with the field. This interaction will modify the barrier height for the charge separation process. Indeed, it has been postulated that electric field effects might be the cause of the observed directionality of electron transfer in the photosynthetic reaction centre (see Figure 37). 

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