Proton transfer, hydrogen bonds switching

Figure 14.21 Proton transfer and hydrogen bond switch in phenol-carboxylate/ phenolate/carboxylic acid complexes 36 as PYP model systems. Counter cation tetrabuty-lammonium.

The question studied was the problem of whether there is a tautomerization, that is, proton transfer between two tautomers a and b (situations Hi to v in Figure 14.1) or whether there is a series of asymmetric low-barrier hydrogen bonds vi in Figure 14.1) as was illustrated in Figure 14.2. In addition, we note that fast hydrogen bond switches can occur, which effectively transfer the proton between the two oxygen atoms as illustrated in Figure 14.21. This process corresponds formally to the lUPAC definition. We will come back to this question in the next section. 

Traditionally also systems are considered as tautomeric when a bifunctional catalyst takes up a proton at one atomic site and transfers another proton to any other site (Figure 14.35c). Another important mechanism, that is, when a catalyst X takes up a proton from a nuclear site A and carries it via a hydrogen bond switch to another nucleus has received little attention (Figure 14.35d). 

A similar ECE two-electron reduction was also observed for a ubiquinone-thiourea model system, where the complexing component not only serves as a proton source, but also enables specific recognition, which in turn provides direct control of the switching between one- and two-electron reactions. In these host-guest complexes, hydrogen bonding to the quinone allows direct two-electron reduction to occur via a facilitated proton transfer. In this system, however, it appears to be difficult to determine whether proton transfer follows or precedes the second electron transfer. Preliminary studies suggest a pathway between a pre- or post-reduction proton transfer. 

Aviram also proposed an intramolecular hydrogen atom transfer switch, based on H bonding in ortho-quinone-cathecol systems [15,16]. It has been confirmed that there is interamolecular H atom transfer in such a system [ ]. Mitani is working toward a proton transfer switch [17]. 

Catalan, J., J.C. del Valle, J. Palomar, C. Diaz, and J.L.G. de Paz (1999), The six-membered intramolecular hydrogen bond position as a switch for inducing an excited state intramolecular proton transfer (ESIPT) in esters of o-hydroxynaphthoic acids, J. Phys. Chem. A, 103, 10921-10934. 

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