Intermolecular complex formation

Figure 21, Proposed model of adsorbed chiral selector (A-alkylproline)- Cu(U)-[free amino acid] mixed chelate complex, The lipophilized proline selector is held in position via intercalation of the alkyl chain. Case A the alkyl part of the mixed chelate complex is fixed by hydrophobic interactions with stationary phase (RP-J). Case B the complex formation is stabilized by other types of hydrophobic attraction. Chiral recognition and elution order is therefore not only dependent on the simple and isolatedly viewed chelate complex stability. In general, retention and chiral recognition in chiral LC is based on mixed-mode adsorption/dcsorption processes which act synergisticallv and also antagonistically with respect to the observed chiral resolution and intermolecular complex formation.

The ability of CDs to form intermolecular complexes with other molecules was already known in the early twentieth century. Another important property of CDs is that each glucose molecule in this macrocycle contains live chiral carbon atoms, which results in a chiral recognition ability in complex formation. This property of CDs was first evidenced by Cramer [1]. The relative easy availability from regenerable natural sources, the existence in various sizes, the stable structure, the localized hydrophobic area, the solubility in the hydrophilic solvents, the ability of intermolecular complex formation and the chiral recognition ability together with their nontoxicity, ultraviolet (UV) transparency, feasibility of their modification, and so forth contributed greatly to the establishment of CDs as a... 

The main focus of this study is to utilize excimer formation between pyrene groups attached to PEG chain ends as a molecular probe of intermolecular complex formation. The change upon complexation was monitored by UV-vlsible absorption, excitation, and fluorescence spectroscopies as well as by fluorescence lifetime measurements. 

Figure 4.21 Temperature dependence of the fluorescence peak wavelength during heating of thermotropic liquid-crystalline polyimide. Excitation at 320 nm. (Reprinted from Polymer, Volume 40, H.W. Huang, T.I. Kaneko, K. Horie and J. Watanabe, Fluorescence study on intermolecular complex formation between mesogenic terphenyldiimide moieties of a thermotropic liquid-crystalline polyimide, 3826, copyright 1999, with permission from Elsevier Science)...

The phenomenology of formation of intermolecular complexes leading to phase separation depends on the physical environment of the system. Thus the pH, polymer charge density, ionic strength, temperature and mixing ratio, all play a vital role in the formation of the complexes. The intermolecular complex formation is an associative interaction involving the attractive forces and entropy of the system. Thus, it is imperative to begin such studies with the electrophoretic characterization of the samples. 

We have provided several examples of intermolecular complex formation that has led to coacervation. In the past coacervate samples were probed by an array of techniques in order to determine the details of their micro-structure. The experimental results taken together reveals that the coacervate phase is a heterogeneous viscous material. The polymer-rich phase comprises physically crosslinked networks of constituent biomolecules. The presence of inter-penetrating networks of... 

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. 

The need of the acylurea site participating in intermolecular hydrogen bonding (cf. Figs. 11 and 12) for the complex formation is exemplified by the fact that a 1 1 mixture of JV-(p-dimethylaminophenyl)phenylacetamide (21) and JV-isobutyl-p-nitro-benzamide (22) gives no crystalline complexes under the same conditions as with 19 and 20. The trend of the complex formation often changes, when the combinations of R7 and R8 are reversed 35). 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

Mulliken [3] presented a classification of electron donor-acceptor complexes based on the extent of intermolecular charge transfer that accompanies complex formation. An outer complex is one in which the intermolecular interaction B- XY is weak and there is little intra- or intermolecular electric charge redistribution, while an inner complex is one in which there is extensive electric charge (electrons or nuclei) redistribution to give [BX] + - -Y . Inner complexes are presumably more strongly bound in general than outer complexes. 

In contrast to the dihalogens, there are only a few spectral studies of complex formation of halocarbon acceptors in solution. Indeed, the appearance of new absorption bands is observed in the tetrabromomethane solutions with diazabicyclooctene [49,50] and with halide anions [5]. The formation of tetrachloromethane complexes with aromatic donors has been suggested without definitive spectral characterization [51,52]. Moreover, recent spectral measurements of the intermolecular interactions of CBr4 or CHBr3 with alkyl-, amino- and methoxy-substituted benzenes and polycyclic aromatic donors reveal the appearance of new absorption bands only in the case of the strongest donors, viz. Act = 380 nm with tetramethyl-p-phenylendiamine (TMPD) and Act = 300 nm with 9,10-dimethoxy-l,4 5,8-... 

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. 

As an example of Mulliken CT complexation, let us consider the benzene bromine complex, whose optimized structure and leading intermolecular NBO interaction are displayed in Fig. 5.41. NBO charge analysis of the C6H6- -B complex reveals the presence of slight CT (0.(X)21 e) from C6H6 to Br2, which is consistent with the Mulliken description. Both monomers are altered by complex formation, with the Br—Br bond NBO becoming noticeably polarized away from the benzene ring,... 

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