Charge resonance coupling

We start by recalling that the framework of diahatic states depicts a competition in solution between the electronic resonance coupling [5 — which tends to delocalize the solute electronic charge — and the solvent polarization — which tries to localize it, to better solvate the reaction sys-... 

Hence, one should expect that, along the ESP at sufficiently large inter-nuclear separation, the solvent will overcome the delocalizing effects of the electronic resonance coupling, and localize the solute charge distribution... 

The first cyanine dye was made in 1856 by Greville Williams. Thus the blue charge-resonance system 6.216 was produced when oxidative coupling took place between N-... 

The third radical cation structure type is the cyclohexane-1,4-diyl radical cation (22 +) derived from 1,5-hexadiene. The free electron spin is shared between two carbons, which may explain the blue color of the species ( charge resonance). Four axial p and two a protons are strongly coupled (a = 1.19 mT, 6H). + ... 

The third radical cation structure type for hexadiene systems is formed by radical cation addition without fragmentation. Two hexadiene derivatives were mentioned earlier in this review, allylcyclopropene (Sect. 4.4) [245] and dicyclopropenyl (Sect. 5.3) [369], The products formed upon electron transfer from either substrate can be rationalized via an intramolecular cycloaddition reaction which is arrested after the first step (e.g. -> 133). Recent ESR observations on the parent hexadiene system indicated the formation of a cyclohexane-1,4-diyl radical cation (141). The spectrum shows six nuclei with identical couplings of 11.9G, assigned to four axial p- and two a-protons (Fig. 29) [397-399]. The free electron spin is shared between two carbons, which may explain the blue color of the sample ( charge resonance). At temperatures above 90 K, cyclohexane-1,4-diyl radical cation is converted to that of cyclohexene thus, the ESR results do not support a radical cation Cope rearrangement. 

Zhuo investigated O chemical shifts of o-hydroxy Schiff bases. These systems are in some instances tantomeric. As described previonsly O chemical shifts are very good indicators of tantomerism (see Section n.K.l). Provided that good reference values for the two tautomeric states exist, the equilibrium constant can be determined. Zhuo used the values for simple Schiff bases as models for the phenolic form (48). For the form 48B a value from a simple enamine was chosen. This, however, is not a very appropriate choice, as it does not at all take into account the charged resonance form (48C). The equilibrium constant determined for Af-(2-hydroxy-l-naphthalenyhnethylene) amine is quite different from that derived by /(N,H) coupling constants. ... 

The only way the electron can be locahsed on this model acceptor site is if some perturbation makes it lower in energy than the resonantly coupled atomic sites of the lattice. This perturbation is the nuclear fluctuation of the dielectric medium that displace the nuclear configurations towards position C in Fig. 2.24 where the charge-... 

In many ways the electronic coupling in both the neutral and the mixed-valence compounds can be reduced to the simple orbital diagram depicted in Fig. 7 [26]. In the neutral complex only the MLCT transition is observed, but for the singly oxidized radical cations three electronic transitions are possible the LMCT, the IVCT (in a fully delocalized compound this is better described as charge resonance transition), and an LMCT. For carboxylate linkers the LMCT is not observed because the CO2 Tt orbitals are too low in energy, but substitution of O by S or NR raises the energy of this filled n orbital. 

Electron spin resonance coupling constants to hydrogen nuclei can be calculated using a neural net technique that relies on the UHF, spin-annihilated UHF, and AUHF spin densities, charge and bond order descriptors to calculate the coupling constants to within about 0.5 G for neutral radicals and radical cations. 

Wang C, Mohney B K, Williams R, Hupp J T and Walker G C 1998 Solvent control of vibronic coupling upon intervalence charge transfer excitation of (NC)gFeCNRu(NH3)g- as revealed by resonance Raman and near-infrared absorption spectroscopies J. Am. Chem. Soc. 120 5848-9... 

The ability to create and observe coherent dynamics in heterostructures offers the intriguing possibility to control the dynamics of the charge carriers. Recent experiments have shown that control in such systems is indeed possible. For example, phase-locked laser pulses can be used to coherently amplify or suppress THz radiation in a coupled quantum well [5]. The direction of a photocurrent can be controlled by exciting a structure with a laser field and its second harmonic, and then varying the phase difference between the two fields [8,9]. Phase-locked pulses tuned to excitonic resonances allow population control and coherent destruction of heavy hole wave packets [10]. Complex filters can be designed to enhance specific characteristics of the THz emission [11,12]. These experiments are impressive demonstrations of the ability to control the microscopic and macroscopic dynamics of solid-state systems. 

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