Strong H-bonds

Together with pyridones, the tautomerism of pyrazolones has been studied most intensely and serves as a model for other work on tautomerism (76AHC(Sl)l). 1-Substituted pyrazolin-5-ones (78) can exist in three tautomeric forms, classically known as CH (78a), (DH (78b) and NH (78c). In the vapour phase the CH tautomer predominates and in the solid state there is a strongly H-bonded mixture of OH and HN tautomers (Section 4.04.1.3.1). However, most studies of the tautomerism of pyrazolones correspond to the determination of equilibrium constants in solution (see Figure 20). 

In the solid state sulfamic acid forms a strongly H-bonded network which is best described in terms of zwitterion units +H3NSO3" rather than the more obvious formulation as aminosulfuric acid, H2NS02(0H). The zwitterion has the staggered configuration shown in Fig. 15.50a and the S-N distance is notably longer than in the sulfamate ion or sulfamide. 

HF is miscible with water in all proportions and the phase diagram (Fig. 17.4a) shows the presence of three compounds H2O.HF (mp — 35.5°), H2O.2HF (mp-75.5°) and H2O.4HF (mp — 100.4°, i.e. 17° below the mp of pure HF). Recent X-ray studies have confirmed earlier conjectures that these compounds are best formulated as H-bonded oxonium salts [HsOJF, [H30][HF2], and [H30][H3F4] with three very strong H bonds per oxonium ion and average O - - F distances of 246.7, 250.2... 

The dissociation constant for the first process is only 1.1 X 10 lmol at 25°C this corresponds to pKa 2.95 and indicates a rather small free hydrogen-ion concentration (cf. CICH2CO2H, p ffl 2.85) as a result of the strongly H-bonded, undissociated ion-pair [(H30)" F ]. By contrast, K2 = 2.6 X 10 lmol pK2 0.58), indicating that an appreciable number of the fluoride ions in the solution are coordinated by HF to give HF2 rather than by H2O despite the very much higher concentration of H2O molecules. 

Treatment of 2-amino-3-hydroxypyridine (185) with 2-chloro-3-nitropyridine (193) easily provided intermediate 194. Attempts to cyclize 194 with potassium hydroxide in aqueous ethanolic solutions failed, probably due to strong H-bonding. Similarly as with phenoxazines, the cyclization smoothly proceeded in DMSO to give low yield (31%) of 1,9-diazaphenoxazine (195) (Scheme 30) (74CC878, 76JHC107, 77H391). 

For microporous compounds with special compositions, calcination effects are even more severe. As compared with zeolites, these compounds have lower thermal stability. Strictly speaking, most of them are nonporous since removal of the occluded guest molecules by calcination usually results in collapse. This is due to strong H-bonds with the framework, coordination bonds, and sometimes the templating molecule is shared with the inorganic polyhedra. Relevant examples of low-stability microporous compounds with interesting structural features are zeolitic open-framework phosphates made of Ga [178], In [179], Zn [180], Fe [181],... 

DMSO and water form a solution with nonideal behavior, meaning that the properties of the solution are not predicted from the properties of the individual components adjusted for the molar ratios of the components. The strong H-bonding interaction between water and DMSO is nonideal and is the primary driver for the very hygroscopic behavior of DMSO. Even short exposure of DMSO to humid air results in significant water uptake. Water and DMSO nonideal behavior results in an increase in viscosity on mixing due to the extensive H-bond network. 

As an example of this nonlinear character we may consider two pairs of compounds, naphthalene versus quinoline and indole versus benzimidazole (Fig. 11.5). In both pairs of compounds the second differs from the first by a mutahon of an aromahc -CH group to an aromahc nitrogen, which introduces a strong H-bond acceptor into the molecule. In quinoline, which has no H-bond donor, the acceptor has no favorable interaction partner in the supercooled liquid or crystalline state, while it can make strong H-bonds with the solvent in water. Therefore, log Sw of quinoline is about 2 log units higher [35, 36] than that of naphthalene, i.e. the introduction of the H-bond acceptor strongly increases solubility in this... 

Figure 1-2. The two dominant conformers of prephenate in solution. Conformer (a) has both the OH and COO- groups solvated by the environment. Conformer (b) has a strong H-bond between the OH and the COO groups. Adapted from Ref. [18]...

Naturally selectivity in a several-component system is primarily influenced by rather strong effects such as the presence or absence of strong H-bonding, but possibly also by much weaker interactions (e.g. of C—H... O type). In this regard, it is interesting to note the similarity between the selectivity exerted by such simple inclusion hosts, e.g. /, and chiral recognition 103). In both cases, weak interactions are of decisive importance in the final outcome of the experiments. Entropic effects have been demonstrated to play a fundamental role in enzymatic reactions 102,107 >. Conceptual similarity of inclusion compounds to more complicated associates is underlined thereby. 

Strong H-bond acceptor (or strong electron donor)... 

Because water is highly polar and is capable of forming strong H-bonds, the dipole-ion attractive forces are also large. 

The coupling constants could be used to characterize H-bond networks in the active centers of enzymes and ribozymes. In particular, the proposed low-barrier H-bonds or short strong H-bonds should give rise to very strong coupling values. 

Class 1 Molecules, which may create a three-dimensional net of strong H-bondings H2O, polyols, aminoalcohols, oxy-acids, polyphenols, hydroxylamine etc. 

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