Resonance charge transfer

Topsom, 1976) and to treat them separately. In this review we will be concerned solely with polar or electronic substituent effects. Although it is possible to define a number of different electronic effects (field effects, CT-inductive effects, jt-inductive effects, Jt-field effects, resonance effects), it is customary to use a dual substituent parameter scale, in which one parameter describes the polarity of a substituent and the other the charge transfer (resonance) (Topsom, 1976). In terms of molecular orbital theory, particularly in the form of perturbation theory, this corresponds to a separate evaluation of charge (inductive) and overlap (resonance) effects. This is reflected in the Klopman-Salem theory (Devaquet and Salem, 1969 Klop-man, 1968 Salem, 1968) and in our theory (Sustmann and Binsch, 1971, 1972 Sustmann and Vahrenholt, 1973). A related treatment of substituent effects has been proposed by Godfrey (Duerden and Godfrey, 1980). 

In summary, as shown by different theoretical approaches (charge transfer, resonance energies and NLMO delocalization) resonance effects occur in all three classes of methanides. However, the magnitude of such effects strongly differs depending on the degree of substitution. 

The first term on the right, y, is the electronic contribution of 7° to the polarization at 2o) and the second term the contribution from pz. Note that pz cannot be determined from this experiment without a knowledge of the dipole moment. In compounds exhibiting significant charge transfer resonance pzPz y and the contribution of y is often ignored. 

Some interesting points can be made from Figure 6.3, for which the data of Docherty et al. (17) have been used and which shows the plot of the contributions of various excited states in the sum over states indicated in equation 7 to P ec for 4-amino-4 -nitro-frans-stilbene. First, most of the nonlinearity occurs because of the charge-transfer resonance associated with the lowest optical transition in the molecule. Second, the seventh excited state drastically reduces the nonlinearity of the molecule. Careful examination of the matrix elements is required to determine whether this reduction is caused by reverse charge transfer or by an unfavorable transition moment between two of the low-lying excited states. Third, the calculation converges with inclusion of only a fraction of the total number of excited states included in the calculation. Judicious use of the last observation could save tremendous amounts of computer time in evaluating classes of similar molecules. 

By carrying this observation to its extreme, a relatively simple two-level model can be developed that is useful for conceptualizing trends that influence p. If we assume a model with a nonpolar ground state and a polar lowest excited state coupled by charge-transfer resonance (structure 6.2), equation 6.7 simplifies to... 

Kramers doublet of 0, and the excited charge-transfer resonant level to when 488.0 nm laser excitation... 

The kinetics and mechanism of the reduction of Mn at a platinum electrode in perchlorate media have been reported and the new value derived for the redox potential, e = 1.535 0.003 V, is closely similar to that obtained by other workers. Previously, hydrolytic equilibria of these highly charged aquo-ions have been interpreted in terras of a M -OH" M OH charge-transfer resonance consistent with a correlation with reduction potentials. A corresponding extent of charge transfer is inferred in the interpretation of the electrode mechanism. 

Figure 3. Two limiting charge transfer resonance forms of a donor-acceptor polyene molecule the neutral form a) and the charge separated form b).

A nearly universal feature of EDA complexation is the presence of new absorption bands in the electronic spectrum of the complex that are not found in the spectrum of uncomplexed donor or acceptor [137-140]. These spectral bands are observed even in cases where no other evidence of complexation exists, i.e., where Keda is too small to measure. The charge-transfer resonance theory of Mulliken [141] was originally formulated to account for these striking spectral features. According to Mulliken, the ground-state wave function for the complex can be formulated as... 

I. Smova-Sloufova, B. Vlckova, T.L. Snoeck, D.J. Stufkens, P. Matejka, Surface-enhanced Raman scattering and Surface-enhanced resonance Raman scattering excitation profiles of Ag-2,2 -bipyridine surface eomplexes and of [Ru(bpy)3] on Ag colloidal surfaces manifestations of the charge-transfer resonance contributions to the overall surface enhancement of Raman scattering. Inorg. Chem. 39, 3551 (2000)... 

A wide variety of molecules and ions have shown SERS (more than 80 different species have been observed to give SERS in a electrochemical cell ), and nearly all of these species can enter into a bonding relationship with the metal surface. It would appear, then, that for the 10 to 10 enhancements in the electrochemical environment, the formation of a weak chemical bond with the active site is a necessity. The total enhancement can be attributed to both a classical electromagnetic enhancement, which does not require a surface bond, and a chemical enhancement, most likely a charge transfer resonance Raman enhancement, which would require the surface-molecule interaction. 

We will define SERS as the case where the major enhancement contributions come from EM effects and an adsorption-induced charge transfer (CT) resonances. In this case, the contribution to enhanced scattering of molecules not adsorbed on the metal surface should be small with respect to those in the first layer since, firstly, the EM enhancement factors are larger on the surface and, secondly, the charge transfer resonance can only occur at the surface, i.e., for a chemisorbed species. The CT polarizability can be 10 to 10 times larger than the NR polarizability. Accordingly, we write the SERS scattering intensity equation as... 

Charge Transfer Resonance Dependence on Potential and Excitation Frequency... 

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