Hydrogen bond short

The equivalence of the donor and acceptor P-0 bond lengths (Table 13.2) suggests a symmetrical nature of many short ( 2.50 A) hydrogen bonds. This has been shown to be the case with CaHP04 for example [17] (Table 13.3). 

In addition, it is found that these bonds often lie across a crystallographic symmetry element such as a symmetry centre, which requires a symmetrical bond. 

TABLE 13.3 Some Phosphates Reported to Contain Short Hydrogen Bonds  

Crystallographic principles are not violated, however, if these structures contain a statistical distribution of unsymmetrical H bonds, with an equal number of bonds (linear or bent) containing H atoms on either side of the centre of symmetry  

Since thermal motion of the H atom along the bond (and perpendicular to it) has also to be taken into account, the distinction between truly symmetrical and statistically arranged slightly unsymmetrical bonds should be small. Neutron diffraction data have given a strong indication that some short H bonds, at least, are definitely unsymmetrical and the subject remains controversial. 

This almost pathological consequence of the fundamental difference between X-ray and neutron diffraction should be borne in mind in choosing between X-ray and neutron diffraction. The latter is not necessarily always the more favourable technique. 

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Speakman JC (1972) Acid Salts of Carboxylic Acids, Crystals with some Very Short Hydrogen Bonds. 12 141-199 Spirlet J-C, see Muller W (1985) 59/60 57-73... 

Anderson, S., S. Crosson, and K. Moffat (2004). Short hydrogen bonds in photoactive yellow protein. Acta Crystallogr 60 1008-1016. 

Another anion pair held together by a short hydrogen bond is [H V0(02)2(bipy) 2] [4] in which two peroxo complex anions are linked, with Rq-. q- 245.6 pm (Szentivanyi and Stomberg, 1984). An example of an uncharged system with a very short hydrogen bond is that of 2-(N,N-diethylamino-JV-oxymethyl)-4,6-dichlorophenol [5], with o = 240pm (Koll et al 1986). 

When the enol ring is adjacent to a cyclic moiety, then it is possible to achieve very short hydrogen bonds, as in the structure of usnic acid, a natural product found in lichens. A low-temperature X-ray diffraction analysis of this compound showed two enol moieties, one in which a carbon-carbon bond of the enol was part of a cyclohexenone ring, and this had... 

Buemi (1990) has also carried out semi-empirical AMI calculations on 3-(4 -biphenyl)pentane-2,4-dione whose molecular parameters have been fully determined by X-ray diffraction (Emsley et al., 1988b). The most stable configuration predicted by theory was not that found in the crystal structure. The calculated hydrogen-bond length (7 o o 275.9 pm) differed from the observed value (241.1pm). The conclusion was reached that the short hydrogen bond observed in the crystal is a peculiarity of the solid state. 

This reaction encompasses a number of interesting features (general Brpnsted acid/ Brpnsted base catalysis, bifunctional catalysis, enantioselective organocatalysis, very short hydrogen bonds, similarity to serine protease mechanism, oxyanion hole), and we were able to obtain a complete set of DFT based data for the entire reaction path, from the starting catalyst-substrate complex to the product complex. 

Oxalic acid dihydrate, studied by several laboratories as part of the HJCr oxalic acid project, contains a short hydrogen-bond of 2.481 AO O distance, linking the oxalic acid and water molecules. All experiments are in agreement that the lone-pair peak of the water-molecule oxygen atom is polarized into the short hydrogen bond. The deformation density in the plane perpendicular to the water-molecule plane, bisecting the H—O—H angle, for one of the experiments is shown in Fig. 12.6. 

Two recent studies show that this conclusion does not apply to very short hydrogen bonds. In methylammonium hydrogen succinate monohydrate, three normal hydrogen bonds, with O—H between 1.72 and 1.86 A exist, while a very... 

Morrison CA, Siddick MM, Camp PJ, Wilson CC (2005) Toward understanding mobile proton behavior from first principles calculation the short hydrogen bond in crystalline urea-phosphoric acid. J Am Chem Soc 127 4042 048... 

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