C-H hydrogen-bond donors

Amendola, V., Boiocchi, M., Colasson, B., Fabbrizzi, L., Douton, M.-J. R., Ugozzoli, F., A Metal based tris-imidazolium cage that provides six C-H hydrogen bond donor fragments and includes halides and pseudohalides. Angew. Chem. Int. Ed. 2006, 45, 6920-6924. 

In terms of supramolecular chemistry, imidazolium type ionic liquids are interesting because they are excellent C-H hydrogen bond donors. The X-ray crystal structure of bmim Cl shown this property clearly with C-Cl 3.39 A, Figure 13.24. Interestingly, two different polymorphic forms of this material are known depending on whether it is crystallised from the molten ionic liquid for from solution,... 

Two early examples of CTV being incorporated into network structures involves CTV acting as a hydrogen bond acceptor when the hydrogen bond donor is the acidic C-H groups of o-carborane [79]. Carboranes form C-H hydrogen bonds of this type in a number of other instances [80], and may also act as spherical guest molecules for CTV [77]. 

The main purpose of Section III is to establish the position of phenols on the scales of hydrogen-bond acidity, either solute (log X", a" logXa) or solvent (Ej(30)) scales. Here, the ability of phenols to act as hydrogen-bond donors will be compared to that of other O—H (water, alcohols, carboxylic acids), N—H, S—H and C—H hydrogen-bond... 

Figure 1.12 Stereo-defined structure of first turn of an a-helix to demonstrate the atom separation between N—H hydrogen bond donors and C=0 hydrogen bond acceptors. The C=0 acceptor of each n-th residue forms a hydrogen bond link with the N—H bond donor of the (n -I- 4)-th residue defining an atom separation of 13 between acceptor 0-atom and donor H-atom. Peptide backbone bonds are colour coded in the same way as in Figs. 1.6 and 1.8.

Figure 1.100 Illustration of water structure in solid (ice) and liquid state, (a) water has two 0—H bonds and two lone pairs each water molecule may form up to four short hydrogen bonds (b), two from 0—H hydrogen bond donors d and two from 0 lone pair hydrogen bond acceptors a (c) together these water molecules create "adamantane-like" Ih hydrogen bonded structures (d).

In contrast to the above described behavior of the basic component of an amphoteric metal oxide catalyst, the electrophilic function interacts with methane in a different manner. Here, the relatively electron rich C-H hydrogen bonds serve as electron donors to the metal center, resulting in a weakening of these bonds and methane activation, leading to formation of ethane or even direct formation of ethylene. 

U-pin is most comparable to a six-ring hairpin polyamide, likely due to a loss of two hydrogen bond donors upon removal of the y-turn element. Thus, the dimeric Py-lm U-turn element may be thought of as a C,G-specific replacement for the y-tum (Fig. 3.4). In combination with removal of the /9-Ala tail (see below), H-pin and U-pin polyamides could potentially bind purely G,C sites, a sequence type that has been difficult to target with other polyamide motifs. 

The H-bonding in the anhydrous 1 Im (Table 24) has topologic properties (Fig. 46) similar to those in the alcohol coordinatoclathrates of 1 with 1 2 host guest stoichiometry (cf. Fig. 17 a). Assuming a perfectly ordered crystal lattice, the resulting central loop of H-bonds should appear to have homodromic directionality with the donor/acceptor functions separated in space. This contrasts to the behavior in the dihydrated l Im where no such characteristic loops are formed. Involvement of the C—H hydrogen atoms of the imidazole molecule, however, is similar in both cases. 

SHwHBd Weak hydrogen bond donor index, sum of C—H hydrogen E-state values for hydrogen atoms on a C to which a F and/or Cl are also bonded... 

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