Electron Transfer Mediated by Solvent Molecules

In thermospray interfaces, the column effluent is rapidly heated in a narrow bore capillary to allow partial evaporation of the solvent. Ionisation occurs by ion-evaporation or solvent-mediated chemical ionisation initiated by electrons from a heated filament or discharge electrode. In the particle beam interface the column effluent is pneumatically nebulised in an atmospheric pressure desolvation chamber this is connected to a momentum separator where the analyte is transferred to the MS ion source and solvent molecules are pumped away. Magi and Ianni (1998) used LC-MS with a particle beam interface for the determination of tributyl tin in the marine environment. Florencio et al. (1997) compared a wide range of mass spectrometry techniques including ICP-MS for the identification of arsenic species in estuarine waters. Applications of HPLC-MS for speciation studies are given in Table 4.3. 

As has been discussed above, molecular clusters produced in a supersonic expansion are preferred model systems to study solvation-mediated photoreactions from a molecular point of view. Under such conditions, intramolecular electron transfer reactions in D-A molecules, traditionally observed in solutions, are amenable to a detailed spectroscopic study. One should note, however, the difference between the possible energy dissipation processes in jet-cooled clusters and in solution. Since molecular clusters are produced in the gas phase under collision-free conditions, they are free of perturbations from many-body interactions or macro-molecular structures inherent for molecules in the condensed phase. In addition, they are frozen out in their minimum energy conformations which may differ from those relevant at room temperature. Another important aspect of the condensed phase is its role as a heat bath. Thus, excess energy in a molecule may be dissipated to the bulk on a picosecond time-scale. On the other hand, in a cluster excess energy may only be dissipated to a restricted number of oscillators and the cluster may fragment by losing solvent molecules. 

The authors proposed a mechanism based on a cage-mediated guest-to-host electron transfer (Fig. 9.30) in which the cage acted as a photosensitizing molecular flask. Excitement of the coordination cage, followed by electron transfer from alkyne to an electron-deficient cage and the reaction of a molecule of water (solvent) with the obtained phenyl alkyne radical cation, results in benzylic radicals and subsequently the anti-Markovnikov product. 

Figure 9.3 Mip-mediated surface water displacement from polystyrene (PS). PS beads were labeled with tethered nitroxide spin labels. Upon excitation during electron spin resonance, polarization was transferred to the surrounding water molecules within 15 A. Polarized water molecules present relaxation correlation times (t in Table inset) that reflect the local environment. Longer times denote more confinement and lower diffusivily. Only one adhesive protein, mfp-3s, is capable of increasing the surface water relaxation time, presumably by adsorbing directly onto the PS surface. Mfp-3s is notable for having a high hydrophobicity (inset table), as indicated by the negative hydropathy value, which denotes a normalized per average free energy of transfer from water to the non-polar solvent Data from ref. 22.

Returning to the resonance structures in Fig. 7.13, we inunediately notice that the zwitter ionic structure may be stabihzed far better than the keto structure by polar solvents. In our water-bridging system, a water molecule is aheady there to mediate proton transfer. After the proton transfer is completed, this water molecule can now serve as a stabihzing polar solvent as schematically drawn in Fig. 7.16. Such an effect should be much weaker for the keto form. Without the water molecule, the zwitter ionic structure is energetically higher than the keto form, but due to the stabilization, their difference turns out to be smaller. This is why the double bonds remain in the both sites of O — C and C — N, and the 7r-electron flux continues to fluctuate among these atoms. 

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