Proton bifurcation, hydrogen bonds

From the first transition state (TSl, Fig. 1), the reaction path leads to the tetrahedral intermediate 1 (INTI). In the latter, the proton transfer from methanol to the tertiary amine function is completed (from 1.183 to 1.059 A), and the negative charge at the former carbonyl oxygen atom reaches its maximum. This charge is compensated by a further shortening of the bifurcated hydrogen bonds to 2.040 A (-0.103 A) and 1.765 A (-0.096 A) (Fig. 1). The thiourea moiety thus forms an oxyanion hole similar to the amide groups of the serine protease backbone [41]. 

The largest number of hydrogen bonds in crystal structures of alkyl hydroperoxides refer to intermolecular bonds between the hydroperoxide proton and functionalities of the type 0=X, where X denotes a sulfur (e.g. 27), carbon (e.g. 30) or a phosphorous atom (e.g. 32, Figure 14, Table 7)93,108,115 geometry of [l,2-bis(diphenylphosphinoyl)ethane] bis(2,2-dihydroperoxypropane) (32) in the solid state is a rare example of a bifurcated hydrogen bond between an OOH donor and an 0=X proton acceptor. 

In another study of the AA/Et3N system28 , this time also using 13C and low temperature NMR, the 1 1 complex was postulated to involve a bifurcated hydrogen bond, Fig. 3(1), rather than proton transfer to the amine. On the other hand the complex formed between AA and Et2NH, although still with a bifurcated hydrogen bond, did involve proton transfer, Fig. 3(H)1. 

The gas phase basicities and pKa values of tris(phosphazeno) substituted azacalix[3](2,6) pyridine in acetonitrile and some related compounds were examined by the density functional theory (DFT) computational method. It was shown that the hexakis(phospha-zeno) derivative of azacalx[3](2,6)pyridine is a hyperstrong neutral base, as evidenced by the absolute proton affinity of 314.6kcal/mol and pKa (MeCN) of 37.3 units. It is a consequence of the very strong bifurcated hydrogen bond (32kcal/mol) and substantial cationic resonance effect [14]. 

Now let us take two interacting water molecules. First, let us ask how many minima we can find on the electronic ground-state energy hypersurface. Detailed calculations have shown that there are two such minima (see Fig. 7.9). The global minimum corresponds to the configuration characteristic for the hydrogen bond (cf. p. 863). One of the molecules is a donor, and the other is an acceptor of a proton (Fig. 7.9a). A local minimum of smaller stability appears when one of the water molecules serves as a donor of two protons, while the other serves as an acceptor of them called the bifurcated hydrogen bond," (Fig. 7.9b). 

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