Intermolecular associations

The variation of Cp for crystalline thiazole between 145 and 175°K reveals a marked inflection that has been attributed to a gain in molecular freedom within the crystal lattice. The heat capacity of the liquid phase varies nearly linearly with temperature to 310°K, at which temperature it rises more rapidly. This thermal behavior, which is not uncommon for nitrogen compounds, has been attributed to weak intermolecular association. The remarkable agreement of the third-law ideal-gas entropy at... 

A large number of thermodynamic studies of binary systems were undertaken to find and determine eventual intermolecular associations for thiazole Meyer et al. (303, 304) discovered eutectic mixtures for the following systems -thiazole/cyclohexane at -38.4°C, Wt = 0.815 -thiazole/carbon tetrachloride at -60.8°C, Mt = 0.46 -thiazole/benzene at -48.5°C, nr = 0.70. 

In aqueous solution intermolecular association between carboxylic acid molecules IS replaced by hydrogen bonding to water The solubility properties of carboxylic acids are similar to those of alcohols Carboxylic acids of four carbon atoms or fewer are mis cible with water m all proportions... 

Since polar solvents would be expected to stabilize polar forms, a retreat towards the hydroxy tautomer (71) would be predicted in solvents less polar than water, and in the vapour phase. This is borne out in practice at equilibrium both 2- and 4-hydroxypyridine (as well as the 3-hydroxy compound, which even in water exists as an approximate 1 1 mixture of OH and NH forms) exist as such, rather than as the pyridinones. However, the 2- and 4-quinolinones remain in the NH (keto) forms, even in the vapour phase. Hydrocarbon or other solvents of very low polarity would be expected to give results similar to those in the vapour phase, but intermolecular association by hydrogen bonding often leads to a considerably greater proportion of polar tautomers being present than would otherwise have been predicted (77ACR186, 78JOC177). 

Isoxazole dissolves in approximately six volumes of water at ordinary temperature and gives an azeotropic mixture, b.p. 88.5 °C. From surface tension and density measurements of isoxazole and its methyl derivatives, isoxazoles with an unsubstituted 3-position behave differently from their isomers. The solubility curves in water for the same compounds also show characteristic differences in connection with the presence of a substituent in the 3-position (62HC(17)1, p. 178). These results have been interpreted in terms of an enhanced capacity for intermolecular association with 3-unsubstituted isoxazoles as represented by (9). Cryoscopic measurements in benzene support this hypothesis and establish the following order for the associative capacity of isoxazoles isoxazole, 5-Me, 4-Me, 4,5-(Me)2 3-Me> 3,4-(Me)2 3,5-(Me)2 and 3,4,5-(Me)3 isoxazole are practically devoid of associative capacity. 

As noted earlier in this section, branched alkanes have lower boiling points than theii unbr anched isomer s. Isomer s have, of cour se, the sane number of atoms and electrons, but a molecule of a br anched alkane has a smaller sur face ar ea than an unbr anched one. The extended shape of an unbranched alkane permits more points of contact for intermolecular- associations. Compare the boiling points of pentane and its isomer s ... 

Weak interactions may occur between molecules (intermolecular association) as well as within a molecule (intramolecular) for chalcogen-nitrogen ring systems. This behaviour is especially significant for odd electron species, e.g., [EsNa]" (4.14, E = S, Se) and [PhCNaEa] (4.15, E = S, Se), both of which are seven r-electron molecules. As mentioned in the previous section, it also occurs for the eight r-electron dithiatriazines 4.10... 

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 resulting derivatives, unlike most organotin carboxylates, are hydrolyzed relatively readily in air. An X-ray crystallographic study 399) showed that trimethyltin glycinate has an intermolecularly associated, polymeric structure, with bridging amino groups. 

Coacervation occurs in tropoelastin solutions and is a precursor event in the assembly of elastin nanofibrils [42]. This phenomenon is thought to be mainly due to the interaction between hydro-phobic domains of tropoelastin. In scanning electron microscopy (SEM) picmres, nanofibril stmc-tures are visible in coacervate solutions of elastin-based peptides [37,43]. Indeed, Wright et al. [44] describe the self-association characteristics of multidomain proteins containing near-identical peptide repeat motifs. They suggest that this form of self-assembly occurs via specific intermolecular association, based on the repetition of identical or near-identical amino acid sequences. This specificity is consistent with the principle that ordered molecular assembhes are usually more stable than disordered ones, and with the idea that native-like interactions may be generally more favorable than nonnative ones in protein aggregates. 

Nucleotides and nucleic acids are critical tools in the areas of gene expression, therapeutics, and diagnostics. However, there are certain challenges associated with their large-scale purification and subsequent characterization. While solid-state oligonucleotide synthesis is relatively simple and can be totally automated, intra- and intermolecular associations may occur involving shorter sequences that may hybridize with the desired full length... 

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. 

Figure 4.6 Intermolecular association by hydrogen bonding in the crystal lattice of linear trans quinacridone, 65a...

C. I. Pigment Red 183 (214), which range in shades from yellow to bluish-red and brown and exhibit excellent fastness properties. Their good stability to light and heat and their insolubility is attributed to extensive intermolecular association as a result of hydrogen bonding and dipolar forces in the crystal structure, as illustrated in Figure 9.3. 

Figure 9.3 Intermolecular association in the crystal structure of a benzimidazolone azo pigment...

The observed anomaly in the viscous properties of dilute organotin copolymer solutions seems to be attributable to the existence of both intra- and intermolecular associates,due to coordination interactions between SnR3 and C=0 fragments of side groups. 

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