Charge migration

Figure 9-23. Schematic diagram ol the EL processes in an electrochemical cell, reproduced from Ref. 1481. (a) Cell before applying a voltage, (b) doping opposite site as n- and p-lype, and (c) charge migration and radiative decay where Mu M2—electrodes O---oxidized (p lype doped) species . ..reduced (n-lype doped) species . ..electron-hole pair.

The approach discussed above can provide a qualitative description of the effect of external fields on bond-breaking processes. For example, consider the H2 molecule (HA — HB) in the presence of an Li+ ion 3 A away from HB on the A-B axis. To study this problem, we assume that there is no charge migration to the Li location (so that Pc = 0) and that fiAC = pBC = 0 since the Li+ ion is sufficiently far from HA and HB. In this case, we can write the H matrix as... 

Therefore, analysis of the efficiency and pattern of strand cleavage provides information on the relative rate of radical cation migration through different DNA sequences. This is powerful information for analysis of the charge migration mechanism. 

In alicyclic hydrocarbon solvents with aromatic solutes, energy transfer (vide infra) is unimportant and probably all excited solute states are formed on neutralization of solute cations with solute anions, which are formed in the first place by charge migration and scavenging in competition with electron solvent-cation recombination. The yields of naphthalene singlet and triplet excited states at 10 mM concentration solution are comparable and increase in the order cyclopentane, cyclohexane, cyclooctane, and decalin as solvents. Further, the yields of these... 

The second reactions represented in Schemes 5.15 and 5.16 deal with the same initial ion and the same products. The essence consists in the question, which of two complementary particles retains the charge To estimate the intensity of the alternative peaks one should use Stevenson s mle (Section 5.4.1.2). To propose a mechanism of the process (in terms of the described concept) it is necessary to take into account that stabilization of the charge is more important than stabilization of the radical (see above). As the reactions initiated by the charge involve charge migration, they are usually less favorable in comparison with the alternative reactions initiated by the radical center. 

Halle, B. and Karlstrom, G. 1983a. Prototropic charge migration in water. 1. Rate constants in light and heavy water and in salt solutions from oxygen-17 spin relaxation. J. Chem. Soc. Faraday Trans. 1179, 1031-1046. 

Figure 9.2 Formation of an electrical double layer responsible for electroendosmotic flow in an uncoated fused-silica capillary. The negative charges on the surface of the capillary are neutralized by positive charges of cations present in the buffer, which form an electrical layer near the surface of the capillary. When the electric held is apphed, the positive charges migrate toward the negative electrode, generating a bulk flow of the solution contained within the column. Electroosmosis exhibits a flat prohle, in contrast to hydraulic flow, which is parabolic.

Example The El mass spectmm of butanal mainly shows carbenium fragment ions due to simple bond cleavage that can be easily recognized from their odd-numbered m/z values. However, the base peak is represented by a [M-28] ion, m/z 44, obviously resulting from rearrangement. A closer look reveals that the charge-migration product, m/z 28, is also present in the spectrum. 

The McLafferty rearrangement itself proceeds via charge retention, i.e., as alkene loss from the molecular ion, but depending on the relative ionization energies of the respective enol and alkene products, the charge migration product, i.e., the corresponding alkene molecular ion is also observed. This is in accordance with Stevenson s rule (Chap. 6.2.2). 

Studying the competition of McLafferty rearrangement either with charge retention or charge migration and double hydrogen transfer has revealed that ion-neutral complex intermediates (Chap. 6.12) can also play a role for the latter two processes. [102]... 

Fig. 6.31. El mass spectrum of 3,5,5-trimethylcyclohexene. The molecular ion undergoes isobutene loss, 56 u, via RDA reaction yielding the base peak at m/z 68, which is also the only significant peak at even-numbered m/z. The charge migration product at m/z 42 is of low intensity. Spectrum used by permission of NIST. NIST 2002.

In a quite different context, the metal complexes with DNA will certainly find interesting applications in supramolecular (photo)chemistry [11], In this regard, DNA may be used as an interesting molecular scaffolding for the study of DNA-mediated electron transfer processes and charge migrations through the DNA double helix. 

Electrophoresis An electrochemical process in which macromolecules or colloidal particles with a net electric charge migrate in a solution under the influence of an electric current. 

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