Symmetrical Hydrogen Bonds between Oxygen Atoms

Symmetrical Hydrogen Bonds between Oxygen Atoms.—It was 

The O—H bond length in water iB 0.96 A. The O—H distance for a symmetrical O—H—O hydrogen bond is 1.17 A the two oxygen atoms would then be 2.34 A apart. 

For most hydrogen bonds between oxygen atoms the O---O distance 

The N—C distance is 1.303 0.013 A and the C—O distance is 1.244 0.012 A. These bond lengths indicate that the two structures N—C=0 and N=C—O make nearly equal contributions N—0=0 about 57 percent and N=C—O about 43 percent. Hence the oxygen atom has about 0.43 of its valence unsatisfied. 

The difference 0.24 A between the O—H distance in this compound and in water corresponds to bond number 0.40 (Equation 7-7). Accordingly the formation of a symmetrical hydrogen bond permits the satisfaction of the valence of the oxygen atom. 

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The structure of KHgYj, where HgY = glutaric acid, contains infinite chains of unsymmetrical H Y anions linked by short — but not symmetrical — 0—H- -0 bonds. These chains are cross-linked through HgY molecules, bound at each end by longer hydrogen bonds between their hydroxylic oxygen atoms and the non-hydroxylic oxygen atoms of the chains (105). 

It was reported by Curran ef fll. [19] in 1994 that a symmetrical A, A -diarylurea effectively catalyses the allylation of cyclic sulfmyl radicals with allyltributylstannane (Table 9.5). High trans selectivity in the product was observed when a stoichiometric amount of urea catalyst was used (run 4), suggesting activation in the transition state by double hydrogen bonding between the urea hydrogen atoms and the sulfmyl oxygen atom. 

A typical example of a symmetric RAHB system is the O- H- -O bond in symmetrically substituted fS-diketones (see, for example, [5]). These compounds include a strong hydrogen bond between two close oxygen atoms. In addition to RAHB bonds (-0-H- -0=), the strong homonuclear hydrogen... 

More symmetric hydrogen bonds are kept linear by the strong repulsion between the oxygen atoms. Longer hydrogen bonds are usually bent since the increased... 

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