Donor-acceptor overlap

Finally, Fig. 5.13 shows the adiabatic bending potential for nonlinear deformations. The angular dependence appears similar to a cosine-like (or dipole-dipole) behavior near equilibrium, but departs conspicuously from this mathematical form at larger deformation angles. The potential shows the strong propensity for linear F H F H-bonding arrangements consistent with maximization of n-o donor-acceptor overlap. 

There are several fundamental reasons why the GMH and adiabatic formulations are to be preferred over the traditionally employed diabatic formulation. The definition of the diabatic basis set is straightforward for intermolecular ET reactions when the donor and acceptor units are separated before the reaction and form a donor-acceptor complex in the course of diffusion in a liquid solvent. The diabatic states are then defined as those of separate donor and acceptor units. The current trend in experimental design of donor-acceptor systems, however, has focused more attention on intramolecular reactions where the donor and acceptor units are coupled in one molecule by a bridge.The direct donor-acceptor overlap and the mixing to bridge states both lead to electronic delocalization, with the result that the centers of electronic localization and localized diabatic states are ill-defined. It is then more appropriate to use either the GMH or adiabatic formulation. 

The barrier crossing frequency described by Eq. (36) is dependent not only on the barrier height, but also on its shape. Equation (36) was derived under the assumption that there is no donor-acceptor overlap and the barrier has a cusp-like shape. 

The synclinal conformation (sc) is appropriate for overlap of an oxygen nonbonded pair with the a C—Cl orbital. The preferred ap relationship, requires an antiperiplanar alignment of a lone-pair orbital with the bond to the electronegative substituent. Because of the donor-acceptor nature of the interaction it is enhanced in the order F < O < N for the donor (D) atom and N < O < F for the acceptor (A) atom. 

The above relationships between the thiiranes (20) and their dioxides (17) are reminiscent of those between cyclopropane and cyclopropanone. The entire phenomena of the C—C bond lengthening and the concomitant C—S bond shortening in the three-membered ring sulfones and sulfoxides can be accounted for in terms of the sulfur 3d-orbital participation and the variation in the donor-acceptor capacities of the S, SO and S02 . The variations of the calculated valence-state orbital energies, together with the corresponding variations of the C—C overlap populations, can be used to understand the discontinuous variations of the C—C and the C—S bond lengths in the series thiiranes -... 

What is the donor/acceptor ratio in a given cell Again, this ratio cannot be directly derived because it concerns two quantities that stem from fluorophores with different properties (absorption coefficient, quantum yield, spectra) and that emit into two channels differing in gain, filters, and excitation intensity. Thus, the (overlap corrected) intensity of acceptors in channel A will be a factor k times that of donors in D, at equimolar concentrations,3 or ... 

For a first approximation, k can be simply calculated by dividing by I for a donor-acceptor fusion construct, because both quantities are corrected for overlap and FRET. Note however that this requires acceptor concentration, we simply find ... 

The emission spectrum of the donor must overlap with the excitation spectrum of the acceptor. 

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-... 

In many cases, the interactions are directed in accordance with the VSEPR rules, confirming the concept of (donor) —> a (acceptor) overlap. The above collection of structural data reveals that in most cases particular supramolecular networks were observed , not designed . 

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