Hopping timescale

A number of different techniques have been applied to test the distance and orientation dependence of ET reactions (Closs and Miller, 1988 Closs et al, 1989 Liang et al., 1990 Reimers and Hush, 1990 Fox and Chanon, 1988 Wasielewski, 1989 Paddon Row and Jordan, 1988 Joachim et al, 1990 McConnell, 1961). Our method of analysing the mode of charge distribution in charged species is esr spectroscopy, which defines the timescale of the detectable dynamic species (Gerson, 1967 Kurreck et al, 1988 Wertz and Bolton, 1972). If an electron transfer is slow relative to the esr timescale (<10 7s) the spectrum corresponds to that of monomeric model compounds with a single electrophore. If the hopping process is rapid on the esr timescale, one will detect an effective delocalization. 

Thus, in the present approach, the major focus is on the question of how we can influence the external parameters like solvent and counterion and the intrinsic structural parameters within the systems A-l-A to force the electron-hopping process into the timescale of the experiment, or at least to establish clearly the borderline cases. That we are still looking at an electron-hopping process in the case of effective charge delocalization over the entire molecule and not at a pure resonance phenomenon may be reassured by VIS/NIR spectroscopy of the neutral and charged species the absorption of a single chromophore should be detected unless a very fast process > 1012 Hz is taking place. 

The same control mechanism can be put to work for the doubly layered electrophores [14]. From esr measurements it appears that a change only of the ion pairing brings about a different hopping rate and creates a different spin-density distribution within the timescale of the experiment. 

How well do these quantum-semiclassical methods work in describing the dynamics of non-adiabatic systems There are two sources of errors, one due to the approximations in the methods themselves, and the other due to errors in their application, for example, lack of convergence. For example, an obvious source of error in surface hopping and Ehrenfest dynamics is that coherence effects due to the phases of the nuclear wavepackets on the different surfaces are not included. This information is important for the description of short-time (few femtoseconds) quantum mechanical effects. For longer timescales, however, this loss of information should be less of a problem as dephasing washes out this information. Note that surface hopping should be run in an adiabatic representation, whereas the other methods show no preference for diabatic or adiabatic. 

ESR and l70 NMR spectra of le -reduced SiW O o demonstrate that the unpaired electron is weakly trapped on a W atom at low temperatures but undergoes rapid hopping (intramolecular electron transfer) at room temperature (Section II). Anions generated by 2e (and 4e -) reduction are ESR-silent, but 170 and 183W NMR spectra show that the additional electrons are fully delocalized (on the NMR timescale) at room temperature and generate ring currents analogous to those produced by the 7i-electrons of benzene. In contrast, in the case of le -reduced PMoW iO, the electron is localized on a more reducible Mo atom at room temperature (251). 

Although these NMR data clearly support a dynamical model for disorder in P-cristobalite, they are not sensitive to whether the motions of adjacent oxygens are correlated (as required for a model of re-orienting twin domains), or, whether the motion is continuous or a hopping between discrete positions they indicate only that the path of each oxygen traces a pattern with 3-fold or higher symmetry over times of the order 4.7-10 s. Thus, these results cannot discriminate between models based on RUMs or dynamical twin domains, and place only a lower limit on the timescale of the motions. A tighter restriction... 

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