Short-range Electron-exchange Energy Transfer

SHORT-RANGE ELECTRON-EXCHANGE ENERGY TRANSFER... 

David L. Dexter proposed a theory of energy transfer between donor and acceptor molecules from their close approach (within 10 A), so that their electron orbitals can overlap to exchange the electrons between them. This theory is sometimes called short-range electron exchange or collisional energy transfer theory [14]. The distance that makes the energy transfer to occur between molecules D and A is almost comparable to their collisional diameter. For this reason, this theory is referred to coUisional energy transfer theory. 

In contrast to the dipole-dipole interaction, the electron-exchange interaction is short ranged its rate decreases exponentially with the donor-acceptor distance (Dexter, 1953). This is expected since, for the electron exchange between D and A, respective orbital overlap would be needed. If the energy transfer is envisaged via an intermediate collision complex or an exciplex, D + A—(D-------A)- D + A, then Wigner s rule applies there must be a spin com-... 

Figure 6.15 Electron movements occurring in short-range triplet-triplet energy transfer by the exchange mechanism. Note that an electron initially on D moves to A and an electron initially on A moves to D ...

Short range transfer by exchange mechanism occurs when donor and aceer. electronic wavefunctions spatially overlap. The rate follows diflusion-contro",. kinetics if donor and acceptor energy levels are in near resonance. Transit forbidden by dipole-dipole mechanism may occur by exchange mechanism, e.g... 

Higher multipole-multipole interaction terms decrease at higher inverse powers of the intermolecular separation, but become important when the dipole-dipole interaction is symmetry forbidden, e.g., in benzene where the octupole-octupole interaction is dominant [161]. The electron-exchange interaction requires overlap of the electronic wave functions of M d and Ma, and it is therefore of short range (<1.5 nm). Due to an exponential decrease in the overlap of electronic wave functions with intersite distance, the energy transfer rate is expected to decrease more rapidly and, in fact, it can be expressed as (see e.g., Ref. 162)... 

The basic concepts of the one-electron Kohn-Sham theory have been presented and the structure, properties and approximations of the Kohn-Sham exchange-correlation potential have been overviewed. The discussion has been focused on the most recent developments in the theory, such as the construction of from the correlated densities, the methods to obtain total energy and energy differences from the potential, and the orbital dependent approximations to v. The recent achievements in analysis of the atomic shell and molecular bond midpoint structure of have been summarized. The consistent formulation of the discontinuous dependence of on the particle number and its effect on the spatial form of and charge transfer within the system have been presented. The recently developed direct approximations of the long- and short-range components of have been overviewed. 

Finally we note the existence of repulsive exchange forces which arise when the electron clouds of the two molecules begin to overlap. These occur because of the exclusion principle which forbids two electrons to occupy the same orbital unless their spins are antiparallel. Since all orbitals are already occupied by electron pairs, any overlap means that some electrons must be transferred to anti-bonding orbitals of higher energy, which implies a repulsive interaction. This interaction, being of very short range, depends upon the exact relative positions of the two molecules and it is not worth while to consider the almost endless possibilities involved until we can deal with molecules fixed in a crystal lattice. 

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