Intermolecular association complexes

Smith et al. (104) discovered that when aqueous solutions of poly(ethylene oxide) (POE) and poly(aciylic acid) (PAA) were mixed in approximately equal proportions, a precipitate immediately formed. The precipitate could be molded to form clear, water-insoluble films and was found to be an intermolecular association complex of the two polymers. This complex is affected by hydrogen-ion concentration. Below a pH of 3.8, the complex precipitates. Precipitation also occurs above a pH of 12.3, but it is thought that only POE precipitates in this high-pH region (118). If a water-soluble consolute such as... 

In this Section, possible factors influencing the selectivity other than shape similarity and shape-specific weak interactions (Sect. 2.4) are discussed. These mainly include intermolecular association, exchange reactions, and hydrophobic interaction. In connection with intermolecular association and crystalline 1 1 complex formation (Sect. 2.3), tetrameric intermediates are also discussed. 

It is important to recognize that the intermolecular long-distance bonding with the participation of halogen derivatives represents a specific example of the broad general area of donor/acceptor interactions. Moreover, the complexes of molecular iodine, bromine and chlorine with aromatic donors represent classic examples of charge-transfer compounds [26-28] that are vital for the development of Mulliken theory of intermolecular association [29-31]. The latter thus provides the convenient framework for the... 

In spite of the numerous spectral observations of complex formation between aromatic and olefinic donors with the dihalogens, the preparations of the corresponding crystalline complexes have been hindered by their enhanced reactivity (as well as the relatively weak bonding). As such, only few examples of the X-ray structural characterization of the corresponding intermolecular associates are reported, the most notable exception being the dibromine complex with benzene. 

The stability of metal ion-alkane adducts such as shown in Figure 11 remains an interesting question. The bonding in such systems can be regarded as intermolecular "agostic" interactions (46). Similar adducts between metal atoms and alkanes have been identified in low-temperature matrices (47). In addition, weakly associated complexes of methane and ethane with Pd and Pt atoms are calculated to be bound by approximately 4 kcal/mol (43). The interaction of an alkane with an ionic metal center may be characterized by a deeper well than in the case of a neutral species, in part due to the ion-polarization interaction. 

Likewise, cationic acceptors afford mixed (positively) charged complexes with electron-rich donors,11 i.e., [D, A+] and anionic donors associate with electron-poor acceptors to form mixed (negatively) charged complexes,12 i.e., [D-, A]. In each case, the intermolecular (ionic) complexation or association represents the highly oriented organization of the donor/acceptor pair (independently of whether they bear positive, negative or no charge) that is often sufficient to afford crystalline complexes amenable to direct X-ray structure elucidation.13... 

The second important solvent effect on Lewis acid-Lewis base equilibria concerns the interactions with the Lewis base. Since water is also a good electron-pair acceptor129, Lewis-type interactions are competitive. This often seriously hampers the efficiency of Lewis acid catalysis in water. Thirdly, the intermolecular association of a solvent affects the Lewis acid-base equilibrium242. Upon complexation, one or more solvent molecules that were initially coordinated to the Lewis acid or the Lewis base are liberated into the bulk liquid phase, which is an entropically favourable process. This effect is more pronounced in aprotic than in protic solvents which usually have higher cohesive energy densities. The unfavourable entropy changes in protic solvents are somewhat counterbalanced by the formation of new hydrogen bonds in the bulk liquid. 

Analyses of the reeonstituted eomplexes by quantitative agarose gel electrophoresis [404,405] and analytical ultracentrifugation [266,406] in the presence of MgCl2 showed that the arrays were able to fold in a way that is almost indistinguishable from complexes reconstituted with major histones (see Fig. 14A-B). Despite this, it was found that histone H2A ubiquitination affects the MgCl2 solubility of the reconstituted complexes (see Fig. 14C) suggesting that this modification may play a role in enhancing the intermolecular associations between chromatin fibers [221]. 

One of the best-established functions of anthocyanins is in the production of flower color and the provision of colors attractive to plant pollinators. Considerable effort has been made to give explanations for the color variations expressed by anthocyanins in plants. Various factors including concentration and nature of the anthocyanidin, anthocyanidin equilibrium forms, the extent of anthocyanin glycosidation and acylation, the nature and concentration of copigmentation, metal complexes, intra- and intermolecular association mechanisms, and influence of external factors like pH, salts, etc. have been found to have impact on anthocyanin colors. ... 

Guanidines are basic molecules (pA of guanidine = 12.5) with a capacity to form intermolecular contacts mediated by H-bonding interactions. Consequently, they are potentially useful pharmacophores in medicinal chemistry, 1 have proven applications as artificial sweeteners,2,3 and are useful as probes in academic studies of intermolecular associations, including su-pramolecular complexes. Expedited access to these molecules via solid-phase synthesis is therefore a worthy goal. This chapter outlines various... 

Pyrroles having electron-withdrawing substituents in the /3-position are usually less soluble in nonpolar solvents than are the corresponding a-substituted compounds. As the importance of the zwitterionic canonical form to the resonance hybrid is less for the /3-isomers than for the a-substituted compounds, the lower solubility of the -substituted pyrroles most probably results from a difference in the intermolecular association of the two compounds. Pyrroles that have carbonyl substituents in the a-position readily form dimeric complexes via NH-0=C hydrogen bonds (25) in the liquid phase and in concentrated solution. The dimeric species may also exist in the solid phase and only in very dilute solutions (< ca. 10-4 M) do the com-... 

Porphyrin and nonporphyrin metals associated with asphaltenes have not been easy to identify in terms of molecular structure. This is partly due to the fact that the characteristics (i.e., spectra) of all possible model nonporphyrin compounds have not been studied. Nonporphyrin metals are probably small polar molecules that precipitate as asphaltenes (Filby, 1975) or complex at defect sites in large aromatic sheet structures of the type shown in Fig. 10. Porphyrins with increased aromaticity and systems with low aromaticity due to discontinued ring conjugation are both characterized as nonporphyrin species. These compounds do not have the characteristic visible absorption spectra and hence are not readily identified. It is also possible that some of the porphyrin in the residuum may not be extracted and identified due to intermolecular association with the asphaltene-generating molecules. 

The method of calculating of complex mixtures using the above equation would theoretically be correct only if the mixture behaves like an ideal solution. Since, most solvent mixtures may exhibit a high degree of intermolecular association af such system would lead to a deviation from the experimental data. The simpliLed Onsager-Kirkwood equation provides only a good approximate dielectric constant for mixed solvent systems. 

With the larger Ms(calix) crown (3.127),65 two dynamic processes are observed. The intermolecular association/disassociation equilibrium in which the complex equilibrates with the uncomplexed ligand and free metal cation is slow on the NMR time scale. The complex also exhibits a faster intramolecular vibration of the metal cation from one binding site to another (Scheme 3.26). Compound 3.127 is also interesting because the mechanism of this fast, intramolecular cation shuttling process may have important implications on the analogous movement of metal cations through transmembrane ion channels (Section 2.2). 

The monolayer also provides an environment of variable dielectric so that intermolecular association between photoactive molecules can readily occur. For example, molecular association of pyrene within a Langmuir-Blodgett film is clearly seen through time-resolved fluorescence measurements on the picosecond timescale [92], Attenuated total reflectance studies of dyes in cast films can similarly reveal their positions and photophysical interactions [93], Photochromism in a monolayer assembly has been attributed to excitation of ion-pair charge transfer complexes formed within the array [94]. 

If intermolecular associations are formed, hydration water molecules are removed from the interacting parts of the surface of a protein or a nucleic acid. This occurs, if the macromolecules form complexes with other macromolecules or with small substrates as, for instance, in enzymatic reactions (see Fig. 23.11). Because the contact between these interacting molecules is direct, water of hydration molecules have to be displaced from both partners. The question arises whether the water molecules located at the surface jump into the bulk water or whether they glide along the surface of the macromolecule and give way to the incoming, interacting molecules. 

Organotin halides and pseudohahdes are Lewis acids. As has already been indicated, they exhibit a strong tendency to expand their coordination number beyond four. This can be achieved by intermolecular associations or by forming complexes either with external ligands or via intramolecular complexation. 

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