Hydrogen bond, three-center

There is also a fourth oxygen atom at 2.75 A from H(l), 2.60 A from H(2) and 2.55 A from H(3), making four-center configurations. 

Metrical Aspects of Three- and Four-Center Hydrogen Bonds 

The first report of a three-center (bifurcated) hydrogen bond was in 1939 in the 

X-ray crystal structure analysis of the amino acid glycine [63], and later confirmed by a neutron diffraction study [65]. The hydrogen-bond lengths and angles are given in Ihble 8.1, together with some other early studies based on neutron diffraction data. 

A very interesting example of three-center hydrogen bonding is provided by the neutron diffraction analysis in 1966 of the para- and ferroelectric crystals of ammonium sulfate [451]. The transition from one to the other state involves a change in hydrogen bonding in which three of the five three-center hydrogen bonds become two-centered (see Thble 8.2). 

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In certain cases. X-ray crystallography has shown that a single H—A can form simultaneous hydrogen bonds with two B atoms bifurcated or three-center hydrogen bonds). An example is an adduct (1) formed from pentane-2,4-dione (in its... 

Figure 10.3. Orbital interaction diagrams for a) symmetrical three-center hydrogen bonding and b) bifurcated hydrogen bonding.

For some other three-center hydrogen bonds, see Taylor Kennard Versichel J. Am. Chem. Soc. 1984, 106, 244 Jeffrey Mitra J. Am. Chem. Soc. 1984,106, 5546 Staab Elbl Krieger Tetrahedron Lett, 1986, 27, 5719. 

Scheme 3.7 Bimolecular complexes involving three-centered hydrogen bonds...

Parra RD, Bulusu S, Zeng XC (2003) Cooperative effects in one-dimensional chains of three-center hydrogen bonding interactions. J Chem Phys 118 3499—3509... 

In protophilic media, amides exist as monomeric H-complexes with a two-centered H-bond and 1 2 H-complexes of the open-chair dimer with a bifurcated (three-centered) hydrogen bond. The formation of a strong bifurcated H-bond weakens the bridging N-H...O=S bond. 

Hydrogen-Bond Configurations TWo- and Three-Center Hydrogen Bonds Bifurcated and Tandem Bonds... 

In crystal structure analyses where the hydrogen atoms are not directly observed, as in macromolecules and structures with disordered water molecules, the signature of the three-center hydrogen bond is a triangle of potential donor or acceptor nonhydrogen atoms at distances in the range observed for weak hydrogenbonding interactions, i.e., 2.8 to 3.5 A. 

The large variation in the X-H A angles and the inability to distinguish between two- and three-center hydrogen bonds tended to obscure many of the systematic features1 that became apparent when more accurate analyses were made accessible. 

Table 8.1. Some early examples of three-center hydrogen-bond geometries by neutron diffraction...

Three-center hydrogen bonds in the carbohydrate crystal structures. The first systematic study which suggested that three-center hydrogen bonds are more widespread than originally thought came from the examination of the results of the neutron diffraction crystal structures of the pyranose and pyranoside sugars referred to in Chapter 7 [58]. 

As the O-ft - O angle in a three-center hydrogen bond approaches 90°, the O - - O van der Waals repulsive force comes into play which gives rise to the excluded region described in Part I. The bond length of the minor component tends to increase. For an H O hydrogen bond of 2.5 A, a covalent O-H bond of... 

An example of a crystallographically symmetrical three-center hydrogen bond... 

Table 8.3. Geometries of three-center hydrogen bonds from the neutron diffraction analyses of carbohydrates ...

Table 8.4. Summary of three-center hydrogen-bond geometries in crystal structures of some small biological molecules...

Three-center hydrogen bonds in the nucleic acid constituents occur as frequently as in the carbohydrates (see Thble 2.3). Because there are virtually no neutron diffraction studies available for this class of compounds, the analysis of the three-center bonds has to rely on data where the X-H bond lengths are normalized and are therefore to some extent less reliable than those of the carbohydrates (Table 8.3) which are based on neutron diffraction data. 

Since nucleic acid constituents exhibit a wider range of donor and acceptor types, the three-center hydrogen bonds have more variation than in+the carbohydrates. It is clear from the data summarized in Thble 8.5 that - NH3 (and NH4), O—H and — NH2 groups tend to form more three-center bonds than OwH... 

Due to proton deficiency, crystal structures of amino acids display a much higher proportion of three-center hydrogen bonds. Their geometries, given in Th-bles 8.6 and 8.7, are based on neutron diffraction data, of which a relatively large number is available for this class of biological molecules. 

The three-center hydrogen bonds observed in the crystal structures of the amino acids are described in Thble 8.6. They range from symmetrical to unsymmet-rical, as found for the carbohydrates and nucleic acid constituents, but there is a significantly greater tendency to form more unsymmetrical bonds. A particular feature are the chelated three-center hydrogen bonds discussed below. 

Chelated three-center hydrogen bonds and chelated bifurcated hydrogen bonds involve two acceptors or two donors belonging to the same molecule. They are observed in the crystal structures of the amino acids and their salts (Thble 8.7), and in the crystal structures of the nucleic acid constituents (Thble 8.8) ... 

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