Charge-separation model

FIGURE 17 Charge separation model for the observed hydrolysis of [Th(OH)2(H20)7] to [Th(0H)3(H20)3] -I- [(H30)(H20>2]. The structures were computed by DFT. Reproduced with permission from Rutkowski el al. (2013). Copyright 2013 American Chemical Society. 

Figure 4.16a illustrates the charge separation model from a thermodynamic perspective. As soon as the photoexcited electrons are trapped at the unoccupied d orbital states, the apparent Fermi level of the system will shift from Ep to Ep due to accumulation of electrons. This is the most recognised model to explain the role of promoters. It is therefore not difficult to predict that the metal NPs, which can trap the most electrons in quantity, win the competition based on this assumption. Unfortunately, experimental results disagree with this prediction, as Au NPs can trap more electrons than Pt but show relative poor performance compared to that of Pt in photoinduced reduction reactions. Apparently, a more precise model that takes the kinetics of the trapped electrons into cmisideration is needed, as shown in Fig. 4.16b. In this model the kinetics of trapped electrons for redox reaction (kred) and reverse trapping to the trap state of semiconductor (krev) were taken into account. These two rate constants can be extracted by in situ UV-vis spectrometry...

Fig. 4.16 (a) Charge separation model proposed (Reprinted with permission from Ref. [66] Copyright 2004, American Chemical Society), (b) Charge separation model with consid tion of kinetics of trapped electrons, (c) In situ UV-vis spectra for the determination of kr and k d of An and Pd supported on Ti02. (d) The derived and of Au-Pd alloys and core-shell-structured Au-Pd NPs as promoters (Reprinted with permission from Ref. [67] Copyright 2014, American Chemical Society)... 

The proposed scenario is mainly based on the molecular approach, which considers conjugated polymer films as an ensemble of short (molecular) segments. The main point in the model is that the nature of the electronic state is molecular, i.e. described by localized wavefunctions and discrete energy levels. In spite of the success of this model, in which disorder plays a fundamental role, the description of the basic intrachain properties remains unsatisfactory. The nature of the lowest excited state in m-LPPP is still elusive. Extrinsic dissociation mechanisms (such as charge transfer at accepting impurities) are not clearly distinguished from intrinsic ones, and the question of intrachain versus interchain charge separation is not yet answered. 

For S vl attack, considerable charge separation has taken place in the T.S. (cf. p. 81), and the ion pair intermediate to which it gives rise is therefore often taken as a model for it. As the above halide series is traversed, there is increasing stabilisation of the carbocation moiety of the ion pair, i.e. increasing rate of formation of the T.S. This increasing stabilisation arises from the operation of both an inductive effect,... 

Bimolecular reactions of aniline with /V-acyloxy-/V-alkoxyamides are model Sn2 processes in which reactivity is dictated by a transition state that resembles normal Sn2 processes at carbon. Electronic influences of substituents support a non-synchronous process which has strong charge separation at the transition state and which is subject to steric effects around the reactive centre, at the nucleophile but not on the leaving group. The sp3 character of nitrogen and disconnection between the amino group and the amide carbonyl renders these reactions analogous to the displacement of halides in a-haloketones. 

Figure 4. Calculated HAB values as a function of Fe -Fe separation, based on the structural model given in Figure 1 and the diabatic wavefunctions I/a and f/B. Curves 1 and 2 are based on separate models in which the inner-shell ligands are represented, respectively, by a point charge crystal field model [Fe(H20)62 -Fe(HsO)63 ] and by explicit quantum mechanical inclusion of their valence electrons [Fe(HgO)s2 -Fe(H20)s3+] (as defined by the dashed rectangle in Figure 1). The corresponding values of Kei, the electronic transmission factor, are displayed for various Fe-Fe separations of interest.

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