Proton sharing

CS indicated that the enolate of acetyl-CoA is significantly more stable than the enol or a proton-sharing enolic form and thus do not support the proposal that a low barrier hydrogen bond is involved in catalysis in CS. This study demonstrates the practial application of high level QM-MM studies to the elucidation of mechanistic details of an enzymatic reaction that are otherwise unclear. 

Minkwitz and Schneider54 have been able to obtain protonated methanol in the form of hexafluorometalate salts (MeOH2+MF6, M = As, Sb), and these were characterized by X-ray diffraction. Protonated propanol and proton-bound dimers of methanol, ethanol, and propanol with the proton shared between two alcohol molecules (5) have also been characterized by infrared multiphoton dissociation... 

Since both cocrystals and salts are multicomponent crystalline forms, it is clear that the distinction between the two depends on the degree of proton transfer between the donor and the acceptor. In this view, salts would be characterized by effectively complete proton transfer, while cocrystals would exhibit proton sharing with little or no transfer [17,18]. In a survey study of over 80 salts and cocrystals prepared by the interaction of carboxylic acids and N-heterocyclic compounds, it was reported that structure prediction and targeted synthesis appeared to be more difficult for salts than for cocrystals [19]. 

We conclude that the LBHB bond is able to keep the proton-sharing oxygen atoms close to each other but the repulsion of the other oxygens of the carboxylates renders the... 

The dihydrate may be a chain. The hydrogen bond shown in the diagram differs from that between water molecules in having a proton sharing three electrons instead of four, one from an oxygen atom of the biradical form of the oxygen molecule and two from the O—H bond. 

This proton sharing between electronegative atoms is called the hydrogen bond. It occurs in polymers carrying amide (-CONH-), car-... 

Type of H-bond Strong (proton-shared) Moderate (medium) Weak (conventional)... 

Table 1 also contains X-Y distances, the FC terms, and 2hJx Y for selected complexes stabilized by traditional or proton-shared O-H-N, C-H-O, O-H-O, F-H-O, and C-H-F hydrogen bonds. The signs of the FC... 

X-H is a proton donor, particularly if the X-H bond is significantly stretched in a complex with a proton-shared hydrogen bond ... 

Table 4. Computed X-Y and Y-H distances (A), Fermi-contact terms and coupling constants lhJH-Y (Hz), and reduced coupling constants [lhKH-Y (x 1019) N A 2 m-3] for X-H-Y hydrogen bonds with significant proton-shared character.

How would this picture change when the hydrogen bond is a symmetric, proton-shared hydrogen bond Figure 12 illustrates three types of lower-energy excited-state triplet wavefunctions. Types a and... 

Fig. 13. The signs of 2hKx Y, KX-h, and lhKH-Y for complexes with traditional and proton-shared hydrogen bonds.

Figure 5.54. The absolute-value COSY-45 spectrum of 5.1 compared with the equivalent COSY-90 spectrum. The tilting apparent on some COSY-45 crosspeaks can indicate whether the active coupling arises from protons sharing geminal or vicinal relationships which produce peaks with positive and negative slopes respectively (see text).

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