Hydrogen bonds switching

Fig. 12.5 Twelve topological sets A,B, L made of symmetry-equivalent atoms of C3v C6QH18(a) hydride eight sets A,B,D,F,G,I,K,L have sjx no(jes each, whereas C,E,H,J include three nodes 13C-NMR theoretical spectrum in fact presents 12 lines with relative intensities 8(6) 4(3). Sets B(6) C(3) D(6) E(3) correspond to C-H bonds originating -NMR theoretical spectrum with four lines with relative intensities 2(6) 2(3). C3v C60H18(P) hydride has similar resonance patterns due to the fact that 12 hydrogen bonds switch from sets B and D to F and G (see Fig. 12.4)...

Unimolecular dynamics of smaller clusters has also been studied. The HF dimer provides a particularly interesting system because it involves a highly quantal degenerate rearrangement consisting of a concerted double hydrogen-bond switch (Quack and Suhm 1991 Truhlar 1990). 

SCHEME 4 SPO platinum catalysts. Top row stoichiometric hydroformylation with 1 middle row, catalyst 4 and hydrogen bond switching in 6 bottom row heterolytic cleavage of dihydrogen (69-71). (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

Therefore, in the first part of this section, intramolecular hydrogen transfers or intermolecular hydrogen transfers in preformed hydrogen bonded complexes in the solid state which are coupled only to minor heavy atom motions are discussed. H-transfers coupled to major heavy atom motions will then be treated in the second part they include pre-equilibria, hydrogen bond switches, conformational changes, solvent motions etc. 

Kooijman EE, Tieleman DP, Testerink C, et al. An electrostatic/hydrogen bond switch as the basis for the specific interaction of phosphatidic acid with proteins. / Biol Chem. 2007 282(15) 11356-11364. 

Y. Ni, S.M. Gruenbaum, J.L. Skinner, Slow hydrogen-bond switching dynamics at the water surface revealed by theoretical two-dimensional sum-frequency spectroscopy. Proc. Nat. Academy. Sci. USA. 110(6), 1992-1998 (2013)... 

But generally, tautomerizations will either constitute or be part of very complex processes, requiring hydrogen bond switching, counterion motions, solvent reorganization, or possibly a catalyst. 

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). 

It follows that there may be several elementary steps that constitute a tautomerization. But on the other hand, tautomerizations are coupled to a number of other phenomena, in particular hydrogen bond switches, molecular motions, and solvent reorganization. Even tautomerizations may constitute one of many elementary steps of complex molecular reactions, in particular of enzyme reactions. By using combinations of various spectroscopic and computational methods it will be interesting to extend the research area of tautomerism to more and more complex systems. 

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