Effects of Molecular Shape

HTP of some chiral azobenzene compounds in E44 are given in Table 10.1. As described in the preceding section, HTPs of the chiral compounds increased as the distance between azobenzene group and chiral group became short. Therefore, the 

TABLE 10.1. HTPs [( x 108m mol g-E44), Experimental], the Length of the Long Axis, L, and the Diameter, D [(A), Measured from Molecular Orbital Calculation with PM3 Method], Data of Chiral Azobenzene Compounds 

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An example of intramolecular hydrogen bonding is provided by the cis- and trans- forms of the acid HOOC—CH=CH—COOH. The trans- form, fumaric acid, has a higher melting point than the cis- form, maleic acid. In addition to the general effect of molecular shape (mentioned earlier in this chapter), another reason for... 

Using the formalism of statistical mechanics, Giddings et al. [135] investigated the effects of molecular shape and pore shape on the equilibrium distribution of solutes in pores. The equilibrium partition coefficient is defined as the ratio of the partition function in the pore... 

M.R. Robinson, S. Wang, A.J. Heeger, and G.C. Bazan, A tetrahedral oligo(phenylene vinylene) molecule of intermediate dimensions effect of molecular shape on the morphology and electroluminescence of organic glasses, Adv. Funct. Mater., 11 413 419, 2001. 

Table 4.3 Effect of Molecular Shape on Molecular Polarity... 

The spherically shaped cryptophanes are of much interest in particular for their ability to bind derivatives of methane, achieving for instance chiral discrimination of CHFClBr they allow the study of recognition between neutral receptors and substrates, namely the effect of molecular shape and volume complementarity on selectivity [4.39]. The efficient protection of included molecules by the carcerands [4.40] makes possible the generation of highly reactive species such as cyclobutadiene [4.41a] or orthoquinones [4.41b] inside the cavity. Numerous container molecules [A.38] capable of including a variety of guests have been described. A few representative examples of these various types of compounds are shown in structures 59 (cyclophane) 60 (cubic azacyclophane [4.34]), 61a, 61b ([4]- and [6]-calixa-renes), 62 (cavitand), 63 (cryptophane), 64 (carcerand). 

It should also be noted that the stability of the distinct mesophases can be quite different. It seems that there is a significant effect of molecular shape and topology, stabilizing SmA phases in the system 41/43 and Colhex phases in the system 35/37. In addition, the mesophase stability is often reduced close to the transition to another mesophase (see Fig. 15). Hence, the order-disorder temperatures can only be roughly estimated based on segmental solubility parameters [24, 25]. 

Y. Queneau, J. Gagnaire, J. J. West, G. Mackenzie, and J. W. Goodby, The effect of molecular shape on the liquid crystal properties of the mono-0-(2-hydroxydodecyl)sucroses, J. Mater. Chem., 11 (2001) 2839-2844. 

Figure 6. Effect of molecular shape on local concentration at 400 K. Sorption of n-...

Krukowski et al. [24] studied the effect of molecular shape in details by performing exact enumerations on lattice models of different molecular shapes. They calculated the entropic component of the chemical potential, i.e.,... 

Braga, D. and Grepioni, F. (1991). Effect of molecular shape on crystal building and dynamic behavior in the solid state from crystalline arenes to crystalline metal arene complexes. Organometallics, 10, 2563-9. [85]... 

The density functional method as applied by Tarazona to deal with classical fluids has been used to calculate the orientation of triatomic molecular fluids near the solid-liquid interface. The results give valuable suggestions about the effect of molecular shape on the orientation of real molecules such as liquid crystals near the solid-fluid interface. 

Normally in chromatography (except affinity chromatography) effects of molecular shape on retention are of minor importance in comparison to the effects of molecular size and molecular polarity. In the case of planar non-polar PAH isomers, retention was linearly related to a shape descriptor (a degree of elongation of the analyte molecule) [43,541. 

Two characteristics determine the shape of molecular aggregates. The first is the shape of the constituent molecules, which sets the curvature of the aggregate. The second is coupled to the chirality of the molecules, which also determines the curvature of the aggregate, via the geodesic torsion. The bulk of this chapter is devoted to an exploration of the effect of molecular shape on aggregation geometry. An account of the theory of self-assembly of chiral molecules is briefly discussed at the end of this chapter. 

Knowing the shape of a substance s molecules is a key to understanding its physical and chemical behavior. One of the most important and far-reaching effects of molecular shape is molecular polarity, which can influence melting and boiling points, solubility, chemical reactivity, and even biological function. 

Nonpolar molecules that deviate from spherical shape and small molecules of only moderate polarity are described quite well by the three-constant correlation that utilizes the acentric factor. The acentric factor, therefore, can be viewed as an empirical parameter that encompasses the effect of molecular shape, and to some extent polarity. Molecules in this category constitute the class of normal fluids, and include small molecules such as O2, N, as well many lower hydrocarbons. 

Effect of molecular shape. For a pair of nonpolar substances with the same molar mass, stronger attractions occur for a molecular shape that has more area over which the electrons can be distorted. For example, the two five-carbon alkanes, n-pentane and neopentane (2,2-dimethylpropane) are structural isomers— same molecular formula (C5H12) but different properties. n-Pentane is more cylindrical and neopentane more spherical (Figure 12.15). Thus, two -pentane molecules make more contact than do two neopentane molecules, so dispersion forces act at more points, and n-pentane has a higher boiling point. 

Jabbarzadeh, A., Atkinson, ID. and Tanner, R.I. (2003) Effect of molecular shape on rheological properties in molecular dynamics simulation of star, H, comb, and linear polymer melts. Macromolecules. 36(13) pp. 5020-5031. 

The Debye-Stokes-Einstein relation assumes a particle to be spherical. For a nonspherical particle an alternate equation must be chosen that takes into consideration the effect of molecular shape on the diffusion properties of the particle. 

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