Aggregate hydrogen-bonded

Three types of carbohydrate sample have been reported to give data above mass 4000, namely, permethylated polysaccharides, permethylated glycosphingolipids, and naturally acylated forms of a mycobacterial O-methyl-D-glucose polysaccharide. All are hydrophobic, and desorption is probably facilitated by their inability to form strongly hydrogen-bonded aggregates, either with themselves or with the matrix. 

Simultaneous infrared absorption in a mixture of CO2 and H2O the role of hydrogen-bonded aggregates. J. Quant. Spectroscopy and Rad. Transfer, 52 295, 1994. 

Not all clathrates are hydrates. Other well-known examples have host lattices formed from hydrogen bonded aggregates of hydroquinone, phenol, and similar organic compounds. Non-hydrogen bonded host structures are also known. One example is a cyclotriphosphazene. (C6H402PN). that traps molecules such as benzene in tunnels in the crystal.2 In addition, coordination polymers are formed by ambidentate ligands, such as CN and SCN, which coordinate to metal ions at both ends (Chapter 12). Perhaps the best known of this type of compound is the series of Ni(CN)2NHj M compounds, where M may be benzene, thiophene, furon. pyrrole, aniline, or phenol. 

Fig. 48 (a) Janus-type discotic hydrogen bonded aggregate 178 and (b) possible hexagonal superlattice LC phase (cross section through the structure viewed along the column axis, gray = regions of the RF-chains) [300]... 

Dannenberg J J. 2002. Coopertivity in hydrogen bonded aggregates. Models for crystals and peptides. J Mol Struct 615 219-226. 

D. N. Chin, J. A. Zerkowski, J. C. MacDonald and G M. Whitesides, Strategies for the design and assembly of hydrogen-bonded aggregates in the solid state, in J. K. Whitesell (ed.), Organised Molecular Assemblies in the Solid State, Wiley, Chichester, 1999. 

One of the earliest examples of chiral melamine-cyanurate was reported by Whitesides and collaborators, who studied the hydrogen-bonded aggregate between three molecules of chiral cyanurate and a complementary achiral fr/ s-melaminc unit based on a 1,3,5 trisubstituted benzene with a melamine unit appended at each arm (Fig. 5) [20]. The fris-mclaminc unit is not chiral itself, but in the complex with cyanurate forms a tightly bonded hydrogen bond platform with C3 symmetry which is chiral at a supramolecular level due to the clockwise (P) and counterclockwise (M) conformation of the pendants (Fig. 5). 

The increase in B is what might be expected for a solvent of higher dielectric constant. The most significant point in these results is that there is no evidence of a decrease in molecular weight. It is, therefore, unlikely that large hydrogen-bonded aggregates are present in the solutions. 

In low-temperature, solid inert matrixes (argon, xenon 7 = 9 K) pyrrole forms hydrogen-bonded aggregates that are predicted to be mainly cyclic trimers and tetramers, based on DFT analysis, with a significant cooperativity effect <2004PCA6953>. 

The synthesis of noncovalent hydrogen-bonded aggregates can often be accomplished simply by mixing the components in the correct molar ratio in an appropriate solvent (usually chlorinated hydrocarbons such as chloroform). In some cases, one of the components (usually the cyanuric acids) may be poorly soluble in chloroform in these cases it may be useful to dissolve the components in a more polar solvent or solvent mixture (e.g. chloroform-methanol), then remove this solvent and redissolve the residue in chloroform. This procedure can overcome kinetic limitations to formation of aggregates associated with solubilities. 

Nuclear magnetic resonance Several aspects of the NMR spectra of these hydrogen-bonded aggregates are noteworthy. These features are discussed here for the specific case of 6a (Hub(M)3 3 neo-hexyl (CA) (Figure 6A) [40]. 

NMR other aspects Two other aspects of the H NMR spectrum of hydrogen bonded aggregates are useful in determining structure. Variable-temperature NMR can reveal dynamic processes that are fast (or slow) on the NMR time scale at room temperature [51]. Nuclear Overhauser effect (nOe) studies can be used to determine relative proximity of the hydrogen bonded protons and the diastereotopic methylene protons [40,43,46]. H NMR competition studies where two different hubs (for example, Hub(M)3 and Flex(M)3) compete for only three equivalents of CA allows direct assessment of the relative stability of the competing aggregates [45,55]. 

Ludlam-Brown, 1. and York, P. (1993). The crystalline modification of succinic acid by variations in crystallization conditions. / Phys. D Appl. Phys., 26, B60-B65. [92] MacDonald, J. C. (1993). Hydrogen-bonded aggregates imidazole as a hydrogen-bond director with applications toward the design of solid-state materials. Ph.D. Thesis, University of Minnesota, Minneapolis, Minnesota. [58]... 

---