Proton Transfers in an H-Bond Network

Beyond these very particular symmetrical cases, we may wonder whether such transfers of protons are of practical importance in chemistry, biology or physics. The object of this section is to show that the answer to such a question is, yes they are important, and we often have to deal with them, without however always recognizing them, hi order to support such an assertion, we shall examine some well-known mechanisms in which they are central and which clearly show that they may even be fundamental and it will also clearly appear that we still have a lot to learn about them. In reality, they are not found in such simple systems we have described in the preceding section. They scarcely occur in single H-bonds and most of the time they occur in systems that display a well-developed H-bond network , such as liquid water or aqueous media, which are more complex systems with an appreciable number of closely lying and interacting H-bonds. 

Dissociation of an acid or a base in water is certainly one of the most spectacular manifestations of proton transfers and is largely documented in classical chemistry. We thus very well know that on mixing hydrogen chloride with liquid water, we completely dissociate 

HCl molecules into Cl and H3O+ ions strongly diluted among H2O molecules, which clearly indicates that protons have been transferred from HCl molecules to water molecules. In this case, the H-bond network is exceptionally dense (liquid water is characterized (Ch. 9) by a number of H-bonds as great as that of covalent bonds), and the whole reaction that leads to dissociation of HCl requires at least two steps, with two different kinds of transfer of protons ionization and diffusion of ions. In this subsection, we examine the first step, which is ionization, before examining diffusion in the next step, and keeping in mind that recent experiments based on nonlinear time-resolved IR spectroscopy have even shown the existence of three steps for this process (5). 

Such a result points to the high sensitivity of the proton position in these H-bonded dimers to the surrounding. On the other hand, theory suggests that the addition of one to two HjO molecules to this CFH NHj dimer is sufficient for a complete proton transfer to occur (12,13). It also stresses the sensitivity of transfers of protons to the enviromnent, and particularly to the presence of other H-bonds or, as we have put it at the beginning of this section. 

Incidentally, let us note that these questions are not purely academic in the sense that experiments and analyses described above have immediate implications the answers to the questions raised on a possible proton transfer are precious for our knowledge of the reactions 

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