Secondary hydrogen bonds

Other structural analyses of crystals in which the bifluoride is present are listed in Table 7. One compound, p-toluidinium fluoride [C7H,oN ][HF2 ], is worthy of further comment. The first X-ray diffraction study reported a symmetrical anion (Denne and MacKay, 1971), but a later analysis showed that the proton was not centred between the two fluorines and 7 f h values were 102.5 and 123.5 pm (Williams and Schneemeyer, 1973). This can be explained not by a double minimum potential energy well but by asymmetry due to other forces, such as secondary hydrogen bonding between one end of the bifluoride anion and the N—H group of the cation. An alternative explanation attributes the asymmetry of the bifluoride hydrogen bond to an unsymmetrical crystal field caused by the cation (Ostlund and Bellenger, 1975). 

Figure 1.18 (a) Primary and secondary hydrogen bond interactions between guanine and cytosine... 

The chromatographic separation of positional isomers (26-31), geometrical isomers (27,32-36) and enantiomers (37-49) has been achieved by utilizing the concerted action of inclusion complex formation, additional primary and secondary hydrogen-bond formation and steric hindrance effects between the solutes and the cyclodextrins (11,12,14-23,50). There is an abundant literature on the analytical applications of cyclodextrin-silicas (13-50), but not on their preparative chromatographic use. 

Fig. 1 Examples of positive and negative secondary hydrogen bond interactions on the binding affinity between triple donor D and acceptor A hydrogen bond partners...

Primary hydrogen bond Secondary hydrogen bond ... 

The complex is bound by weak secondary hydrogen bonds. Each monomer acts as a proton acceptor and a donor simultaneously, forming two six- or five-membered intermolecnlar hydrogen-bonded rings. Three homochiral (out of six predicted), and three heterochiral (out of six predicted) conformers were detected in the microwave spectra. 

Fig. 3 Secondary hydrogen-bonding interactions. Attractive interactions are marked green, and repulsive interactions are marked red. 

The model in Termonia and Smith (1986) is for ideal fibres made of a perfectly oriented array of fully extended macromolecules, with no other defects than chain ends resulting from finite molecular weight . Parameters defining the primary covalent bonding and the secondary hydrogen bonding are applied to a sy.stem modelled as a three-dimensional array of bonds which are viewed as coupled oscillators in a state of constant thermal vibration . I have no difficulty in accepting this as a reasonable... 

Mechanochemical degradation in solution of the dezoxiribo-nu-cleic acid, ADN, by capillaries shear, had as effect the transversal splitting of ADN macromolecule, with a double helix conformation, and not the scission of the secondary hydrogen bonds that assure the maintaining of this conformation in solution [1034-1037]. 

As already noted, hydrogen bonds are very sensitive to their context. Solvent and electronegativity effects likely play the largest roles in modulating their strength. However, several other factors can be identified as major contributors. The most frequently cited factors are resonance and polarization enhancement, although more recently another factor called "secondary hydrogen bonds" has found wide acceptance. 

Primary hydrogen bonds (im) Secondary hydrogen bonds (—) Repulsive interactions (—)... 

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