Anisometric molecules

Mislow [18] has proposed a classification of isomers based not on the bonding connectivity of atoms as above, but on the pairwise interactions of all atoms (bonded and nonbonded) in a molecule. The operation of comparison of all pairwise interactions is called isometry (for detailed explanations, see [19]). Isomers in which all corresponding pairwise interactions are identical are said to be isometric, and they are anisometric if this condition is not fulfilled. Isometric molecules may be superimposable, in which case they are identical (homomeric), or they may be nonsuperimposable, in which case they share an enantiomeric relationship. As regards anisometric molecules, they are categorized as diastereoisomers or constitutional isomers, depending on whether their constitution is identical or not. This discussion is schematically summarized in the lower half of Fig. 2. 

Triglycerides are large and highly anisometric molecules that can assume many conformations. This gives rise to polymorphism the main polymorphs are called a, p, and p, in order of increasing melting point and stability. 

Mesomorphic Phase A phase consisting of anisometric molecules or particles that are aligned in one or two directions but randomly arranged in other directions. Such a phase is also commonly referred to as a liquid-crystalline phase or simply a liquid crystal. See also Neat Soap. 

Mesomorphic Phase. A phase consisting of anisometric molecules or particles that are aligned in one or two directions but randomly arranged in other directions. Such a phase is also commonly referred to as a liquid-crystalline phase or simply a liquid crystal. The mesomorphic phase is in the nematic state if the molecules are oriented in one direction, and in the smectic state if oriented in two directions. Mesomorphic phases are also sometimes distinguished on the basis of whether their physical properties are mostly determined by interactions with surfactant and solvent (lyotropic liquid crystals) or by temperature (thermotropic Uquid crystals). See also Neat Soap. 

Consider first an anisometric molecule with the longitudinal p, and transversal p, permanent dipole moments in an isotropic phase. There are two relaxation modes mode 1, rotations of p, around the long axis, and mode 2, reorientation of p,. Figure 10-1. The mode 1 has a smaller relaxation time, Tj < Tj, because of the smaller moments of inertia involved. When this isotropic fluid is cooled down into the NEC phase, the dynamics is affected by the appearance of the nematic potential associated with the orientational order along the director n. The mode 1 remains almost the same as in the isotropic phase, and contributes to both the parallel and perpendicular components of dielectric polarization (determined with respect to n). Mode 2 is associated with small changes of the angle between p, and n it contributes to the parallel component of dielectric polarization. Mode 3 is associated with conical rotations of p, around the director (as the axis of the cone) it is effective when the applied electric field is perpendicular to n and contributes... 

The optical anisotropy (i.e. the birefringence) of a nematic liquid crystal is a very important property for the contrast ratio and viewing angle in liquid crystal displays. The optical anisotropy of the nematic is caused by the anisotropy of the molecular polarisability of the constituent anisometric molecules (rod-like molecules) and their long range orientational ord. ... 

Diffusion and sedimentation measurements on dilute solutions of flexible chain molecules could be used to determine the molecular extension or the expansion factor a. However, the same information may be obtained with greater precision and with far less labor from viscosity measurements alone. For anisometric particles such as are common among proteins, on the other hand, sedimentation velocity measurements used in conjunction with the intrinsic viscosity may yield important information on the effective particle size and shape. ... 

Birefringence induced by flow in liquids, solutions and dispersions of optically anisotropic, anisometric or deformable molecules or particles due to a non-random orientation of the molecules or particles. 

The nematic phase (N) is the least ordered, and hence the most fluid liquid crystal phase. The order in this type of LC phases is based on a rigid and anisometric (in most cases rod-shaped or disc-shaped) molecular architecture. Such molecules tend to minimize the excluded volume between them, and this leads to long range orientational order. For rod-like molecules the ratio between molecular length and its broadness determines the stability of the nematic phase with respect to the isotropic liquid state and the stability rises with increase of this ratio. In most cases the rigid cores are combined with flexible chains, typically alkyl chains, which hinder crystallization and in this way retain fluidity despite of the onset of order. 

Segregation of the incompatible molecular segments takes place with formation of distinct nano-compartments organized on a one-dimensional (ID), two-dimensional (2D), or three-dimensional (3D) periodic lattice, separated by interfaces. These interfaces tend to be minimal in order to reduce the interfacial energy stored in the system. For amphiphilic molecules without anisometric segments (flexible amphiphiles) the mesophase type is mainly determined by the relative volume of the two incompatible segments, as shown in Fig. 2. 

The rotational crystalline phase has conceptually the same meaning as that of the smectic liquid crystal [8]. Molecules of substances forming liquid crystals have, as a rule, an anisometrically elongated shape and low symmetry. The main structural feature of the liquid-crystalline state is a parallel array of molecules with very light contacts between them. 

Both theoretical approaches qualitatively describe the "thermotropic" and "lyotropic" liquid crystalline state of rod-like molecules ( see also D.B. DuPre, R. Parthasarathy, this book). Combination of both theories (Flory, Ronca)(7) slightly improves the predictions compared to the experimental findings. Anisotropic dispersion interactions and/or anisometric molecular shape can thus be the basis for explaining theoretically the appearance of "lyotropic" and "thermotropic" liquid crystalline phases. 

When we apply an electric field to a suspension of anisometric colloid particles (or a solution of polyelectrolyte molecules), the particles tend to orient into the field direction, and this results in a macroscopic optical anisotropy effect. This effect may be expressed by the equation... 

Another factor that presumably contributes to AG is the loss of orientational and conformational entropy of the molecules that become incorporated in an embryo. It is seen that the difference between theory and experiment of the preexponential factor increases when going from ice (five orders of magnitude) to sucrose (seven) to tristearate (twelve), and this is at least in qualitative agreement with the loss in entropy, which will be larger for more anisometric and more flexible molecules. Especially tristearate molecules will lose very much conformational freedom upon crystallization. 

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