Spectator solvent molecules

The study of a reaction in solution involving small molecules (i.e., molecules having a size compatible with a full quantum chemical computation) can be performed by means of a QM/MM approach. The subsystem requiring a quantum mechanical treatment consists of the molecules that take part in the reaction, and all the spectator solvent molecules of the sample are represented by the classical force field. Therefore, the energy of the system can be written as follows ... 

Finally, as already mentioned, it seems clear that the solvent molecules can be involved in the charge-transfer process. Detailed discussions of charge-transfer spectra in transition metal complexes quite often label the corresponding bands quite separately, giving them the label CTTS—charge transfer to solvent. So, the fact that Fel3 has recently been prepared in non-aqueous media suggests that the solvent—water—is not always the mere spectator that it was implicitly assumed to be above. 

Sridharan and Mathai noticed that the transesterification of small esters under acid-catalyzed conditions was retarded by the presence of spectator polar compounds. " Thus, given that water can form water-rich clusters around protons (solvent-proton complexes) with less acid strength than methanol-only proton complexes, some catalyst deactivation may be expected with increased water concentrations. Also, water-rich methanol proton complexes should be less hydrophobic than methanol-only clusters, thus making it more difficult for the catalytic species (H" ) to approach the hydrophobic TG (and possibly DG) molecules and contributing to catalyst deactivation. Therefore, with water present in the feedstock or produced during the reaction in significant quantities, some catalyst deactivation can take place by hydration. 

However, the story is not finished yet. The contribution of the solvent dipole layer to the properties of the whole double layer has not been determined. One way to do it would be by comparing the calculated results of dipole w th the total potential drop of the double layer. In this way we could determine how much the characteristics of the double layer are affected by the water layer in the interface, that is, if the water molecules behave more like spectators or more like actors. Nonetheless, this is not the only way to proceed. A better course to follow would be to compare, instead of potentials, the capacity of the dipole layer, Cdipole, with the total capacity. The advantage of this comparison is that we are already familiar with the experimental data of the total capacity (see Fig. 6.65). The only thing we would have to do is to transform the dipole potential into a dipole capacity, aprocedure we know how to do (see Section 6.5.5). 

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