Primary hydrogen bonds

B- HCCH involving weak primary hydrogen bonds Z- HCCH have revealed large non-linearities, but with an angle 0 that remains reasonably close to those predicted by the rules. Figure 22 illustrates this result through the experimentally determined geometries for the cases when B is 2,5-dihydrofuran [200], oxirane [201], formaldehyde [202], thiirane [203], and vinyl fluoride [204], On the other hand, as noted in Sect. 3.1.3, both... 

Primary hydrogen bond Secondary hydrogen bond ... 

Scheme 2.5.11 The primary hydrogen bonds in co-crystals of 2-aminopyrimidine and dicarboxylic acids.

Scheme 2.5.20 The primary hydrogen-bond motifs in the three known polymorphs of maleic hydrazide display the same connectivity.

Even if similar primary hydrogen-bond strengths are found in these complexes, such a correlation is surprising because it seemingly requires a linear correlation between the complexation entropy and the number of... 

Synthetic systems with three primary hydrogen bonds and four repulsive secondary interactions, ADA DAD motifs in the supramoiecular vernacular, are among the best known of all noncovalent host-guest systems. Although not generally as stable as the previously mentioned DDA AAD system (because they have four repulsive... 

Figure 21 Primary hydrogen bonding assembly tuned by peripheral jr-interactions.

Scheme 21 The primary hydrogen bond in the saccharin/4-methyl p)ridine Af-oxide cocrystal.

Scheme 22 The primary hydrogen-bond interaction in the 1 1 cociystal benzoic acid carbamazepine.

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

Figure 2 A schematic representation of Jorgensen s secondary hydrogen-bonding hypothesis for complexes containing one to four primary hydrogen bonds (adapted from W.L. Jorgensen and J. Pranata J. Am. Chem. Soc. 112 2008-2010,1990 and J. Pranata. S.G.Wierschke, and W.L Jorgensen. J.Am. Chem. Soc. 114 4010 011, 1991.)...

An extensive search of the small-molecule crystal structure database reveals several clear examples of amino groups of aniline as acceptors while only few such cases were reported for nucleobases [39]. The relatively infrequent occurrence of amino - amino contacts in crystals of DNA constituents is not surprising. Amino groups are mostly involved in primary hydrogen bonds where the amino group serves as an H-donor. The amino group is expected to act as the acceptor when it is not fiilly involved in the primary H-bonds. This is more likely to occur in crystals of aniline and less likely in crystals of nucleobases which have a much higher capability to form primary H-bonds. 

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