Electron transfer spectra, effect

The radical cation of 1 (T ) is produced by a photo-induced electron transfer reaction with an excited electron acceptor, chloranil. The major product observed in the CIDNP spectrum is the regenerated electron donor, 1. The parameters for Kaptein s net effect rule in this case are that the RP is from a triplet precursor (p. is +), the recombination product is that which is under consideration (e is +) and Ag is negative. This leaves the sign of the hyperfine coupling constant as the only unknown in the expression for the polarization phase. Roth et aJ [10] used the phase and intensity of each signal to detemiine the relative signs and magnitudes of the... 

While being very similar in the general description, the RLT and electron-transfer processes differ in the vibration types they involve. In the first case, those are the high-frequency intramolecular modes, while in the second case the major role is played by the continuous spectrum of polarization phonons in condensed 3D media [Dogonadze and Kuznetsov 1975]. The localization effects mentioned in the previous section, connected with the low-frequency part of the phonon spectrum, still do not show up in electron-transfer reactions because of the asymmetry of the potential. 

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. 

Electron transfer [Eq. (1)] would occur at a rate near the diffusion limit if it were exothermic. However, a close estimate of the energetics including solvation effects has not been made yet. Recent support of the intermediacy of a charge transfer complex such as [Ph—NOf, CP] comes from the observation of a transient (Amax f 440 nm, t =2.7 0.5 ms) upon flashing (80 J, 40 ps pulse) a degassed solution (50% 2-propanol in water, 4 X 10 4 M in nitrobenzene, 6 moles 1 HCl) 15). The absorption spectrum of the transient is in satisfactory agreement with that of Ph—NO2H, which in turn arises from rapid protonation of Ph—NOf under the reaction conditions ... 

In summary, preliminary experiments have demonstrated that the efficiency and outcome of electron ionization is influenced by molecular orientation. That is, the magnitude of the electron impact ionization cross section depends on the spatial orientation of the molecule widi respect to the electron projectile. The ionization efficiency is lowest for electron impact on the negative end of the molecular dipole. In addition, the mass spectrum is orientation-dependent for example, in the ionization of CH3CI the ratio CHjCriCHj depends on the molecular orientation. There are both similarities in and differences between the effect of orientation on electron transfer (as an elementary step in the harpoon mechanism) and electron impact ionization, but there is a substantial effect in both cases. It seems likely that other types of particle interactions, for example, free-radical chemistry and ion-molecule chemistry, may also exhibit a dependence on relative spatial orientation. The information emerging from these studies should contribute one more perspective to our view of particle interactions and eventually to a deeper understanding of complex chemical and biological reaction mechanisms. 

On the other hand, some of Suhrmann s electrical resistance measurements on nickel and platinum films and Eischens observations, of the effect of Hj on the infrared spectrum of chemisorbed CO on platinum, referred to earlier, suggest electron-transfer from hydrogen to the metal, i.e., adsorption of positive ions. 

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