Weak hydrogen bonds between

Collagen forms a triple helix, where three chains of connected amino acids form weak hydrogen bonds between the double-bonded oxygen atoms and the hydrogen atoms attached to the adjacent chain s nitrogens. The three chains then twist together like three cords in a rope. 

It is well accepted that the high diastereospecificify of aldehyde allylboration reactions is a consequence of the compact cyclic transition structure. Theoretical calculations have shown that the chairlike transition structure shown in Scheme 1 and Fig. 1 is the lowest in energy relative to other possibilities such as the twist-boat conformation. With boronate reagents, it has also been suggested that a weak hydrogen bond between the axial boronate oxygen and the hydrogen of the polarized formyl unit contribntes to the preference for the transition structme with the aldehyde substituent in the psendo-eqnatorial position. ... 

The same type of Au- -N interaction between adjacent [Au(CN)4] anions is found in [Cu(dien)][Au(CN)4]2 and [Cu(en)2][Au(CN)4]2 (dien = diethylenetriamine en = ethylenediamine) [48], although in these examples the copper cations act as bridges between neighboring chains generating 2-D sheets, as shown in Figure 5.26. The Au- -N separations, of 3.001 and 3.137 A in the triamine complex and 3.035(8) A in the diamine one are similar to those in other tetracyanoaurate(III) derivatives. Additional weak hydrogen bonds between amino hydrogen and N(cyano) atoms increase the dimensionality of the supramolecular structure. 

In N,C-linked biheteroaryls, e.g. (186)-(188), two rings are virtually coplanar, the conformation in such molecules is controlled by weak hydrogen bonding between an acidic a-hydrogen of one ring and the lone pair of a pyridine type nitrogen in another ring. 

In addition, there is weak hydrogen bonding between the F lone pairs and one of the H s on N favoring the conformer on the right. 

Most common and practically important miscible polymers owe their miscibility to a specific interaction of some sort between the two polymers. The commonest of these is the postulated weak hydrogen bond between halogen containing polymers and oxygen contaning polymers, e.g. 

The carbonyl species may interact with acidic OH groups (3650cm ) inside the zeolite framework. This characteristic band is seen to shift to 3550 cm" and this was attributed to weak hydrogen bonding between a CO ligand and the OH group. Air oxidation of (CO)gCo3C—CH3 trapped in NaY cavities (241) leads to Co accompanied by CO2, C03 , and CO. 

However, the reaction field treatment cannot account for the third low-energy conformer (fgg). This is within 0.1 kJ/mol of ttt in the gas phase as found by ab initio calculations, the stabilisation being ascribed to an intramolecular weak hydrogen bond between a methyl hydrogen and an ether oxygen at the other end of the molecule. Because of the large dipole moment (1.7 D), reaction-field treatments predict this conformer to be by far the most stable in solution (5), completely at odds with experiment where this conformer is never observed. On the other hand, the presence or absence of this conformer has serious... 

Figure 2.1.26 Crystal packing of 40 (P21/c) [65] along [100] showing the (010) cleavage plane with weak hydrogen bonds between the adjacent alternating monolayers hydrogen bonds are indicated.

Some evidence for a weak hydrogen bond between an O-H- -F—C or N-H- -F—C system would be a short H- -F distance and/or O- -F or N- -F distance, and an estimated negative charge distribution on the fluorine atom. A spectral analysis of 2-fluorophenol revealed that the s-els conformer is a stable form in which the distance of OH- -F (2.235 A) is shorter than the sum of the van der Waals radii of fluorine and hydrogen [16]. The distance of OH- -F in tetrafluoro-l,4-benzhydroquinone is much shorter (2.02 A), where the hydroxyl group is more acidic. 

Left) The double helix is stabilized by weak hydrogen bonds between the A and T bases and between the C and G bases. (Right) During replication, the two strands are unwound and used... 

Chromosomes, when observed under a microscope, are fatty, tubular-looking substances. They are, in fact, made up of a series of genes, which are tightly twisted double strands of DNA molecules (Figure 25.2). The two strands are held together by weak hydrogen bonding between pairs of nucleotide bases on each strand, which may be viewed as steps in the DNA ladder. ... 

---