Complexes Formed by Hydrogen Bonding

The refractive index for a series of homogeneous polyelectrolyte complexes in the range of 40-80% gel water content at 22 °C is given by 

A hydrogen bond is formed through intermolecular interaction between an electron-deficient hydrogen and a region of high electron density. Its fundamental role in the structure of DNA and the secondary and tertiary structures of proteins is known. The specific properties and structures of water are also caused by hydrogen bonds. 

Proton-accepting polymers and proton-donating polymers typically interact with each other in aqueous medium and organic solvents almost stoichiometri-cally. This complex formation is affected by temperature, polymer structure, polymer concentration, solvent and other interaction forces, e.g. hydrophobic interactions. In general, the ratio [proton-accepting polymer units]/[proton-donating polymer units] in mol/1 of the complex is almost unity in dilute solu- 

The existence of a certain number of undissociated carboxy groups is necessary for PMAA and PEO to form a stable complex through hydrogen bonds. If this condition is satisfied at a certain pH (so-called critical pH ), a stable complex is formed irreversibly. In such complex formation, dissociated carboxy groups are influenced by the complexation and become undissociated by the extrae- 

1 Tsuchida et al.223 reported that in aqueous solution, the ratio of the components of the system of PMAA-PVPo is 1 1 whereas Bekturov et al.223) found a ratio of 3 2. This discrepancy may be caused by the influence of the concentration, pH and molecular weight of individual polymer components 

---

IPNs composed of PAAc and PAAm may shrink at a low temperature because of interpolymer complexes formed by hydrogen bonding as shown in Fig. 9. The complexes dissociate at higher temperatures due to breaking of hydrogen bonds, and IPNs swell rapidly above a critical transition temperature [40]. 

In the absence of catalysts, HNF2 adds reversibly to cyclic and noncyclic aliphatic aldehydes and ketones to produce a-difluoro-aminoalcohols R COR" + HNF2 R C(OH)NF2R". These alcohols are prepared by mixing the components at room temperature or below (122) in the presence or absence of solvents (78). The formation of a-difluoroaminoalcohols follows that of the complex formed by hydrogen bonding. 

Recently, the use of sulfolane solvent allowed better kinetic control of the oxidation chain, with an increase of the selectivity to 80% or greater, at ca 8% benzene conversion. The by-products were catechol (7%), hydroquinone (4%), 1,4-benzo-quinone (1%) and tar (5%) [53, 54]. According to these authors, a rather stable complex, formed by hydrogen bonding with sulfolane, promoted desorption and hindered the re-adsorption of phenol, protecting it from consecutive oxidation (Equation 18.7). Actually, the rate of oxidation of phenol in the presence of sulfolane was only 1.6 times that of benzene, while it was 10 times higher in the presence of acetone. 

Complexes formed by hydrogen bonding are mixtures for TSCA purposes, and their components must be on the Inventory. ... 

Interaction of complexes formed by hydrogen bonding with a solvent 20.5... 

Figure 3.46 Calculated structure of 23a-(.S ,, S)-40 complex formed through hydrogen bondings. (a) View perpendicular to helix axis and (b) along helix axis. (Reprinted with permission from Ref. 211. Copyright 1999 by the Chemical Society of Japan.)...

Clathrate hydrates discussed in Section 8.3.3 also provide exciting examples of dynamic complexes. The cages formed by hydrogen bonded water molecules in these systems are constantly decomposed and reformed, but they are stabilized by appropriate guests [58]. If the latter are too small to fill the cage they, in turn, move inside it. 

Recently, we [53] and others [54] simultaneously reported an example of a complex in which the transition metal dictates the coordination mode, viz. urea-functionalized phosphine 20, which forms a trans palladium complex, complemented by hydrogen bonding ofthe urea fragments (Figure 10.6). Bear in mind that any monophosphine, and even wide bite angle diphosphines, give trans complexes with a hydrocarbyl palladium halide. More interestingly, the urea moieties can function as a host for another halide ion. 

Fig. 20 Sequence of LC phases formed by hydrogen bonded complexes between substituted 1,3,5-triazines with complimentary benzoic acids depending on the number of chains and degree of fluorination (R1-R5 = ORF, ORjj, H) [153]...

Chiral recognition in complexes, linked by hydrogen bonds, has been studied experimentally and theoretically. In some cases, chiral systems can aggregate and form long chains or helix shape structures. The subsequent chemical processes along the chains can invert the chirality of the molecules producing what we have called racemization waves [37]. The control and rationalization of these processes are of the utmost importance in the development of novel molecules designed as switches. 

Thus, the alternating copolymers even possessing a considerable content of the second (nonreacting) component (up to 40 mol-%) may form interpolymer complexes stabilized by hydrogen bonds. 

In the triazine and 9EA MIP model systems predictions may be made based on previous studies of closely related solution complexes. Of particular relevance is the work by Welhouse and Bleam [101] on solution complexes of ATR formed by hydrogen bonding with various small molecules and the work by Lancelot on the... 

In Scheme 16, D and A are proton donor molecules and proton acceptor molecules, respectively. The quantity K represents the stability of the complex DH A, formed by hydrogen bonding interactions in the ground state. For the 2-naphthylamine/pyridine system, the K value in hexane (obtained by fluorescence quenching measures) is 0.6 dm3 mol-1. This value is reduced by using benzene as a solvent (K = 0.2 dm3 mol-1) while, in cyclohexane, K = 12. The observed variations confirm the importance of solvents in influencing solute/solute interactions149. 

---