Anisotropic intermolecular interaction

Milet, A., Korona, T., Moszynski, R., Kochanski, E., 1999, Anisotropic Intermolecular Interactions in Van der Waals and Hydrogen-Bonded Complexes What Can we Get from Density Functional Theory , J. Chem. Phys., Ill, 7727. 

Jerry and Monnerie l " have proposed a modified theory of rubber elasticity which includes anisotropic intermolecular interactions U12 (favoring the alignment of neighbouring chain segments) in the form U,2 = ZUL(r12) PL(0i) PL(02), where r,2 is the intermolecular distance, 0X and 02 are the angles between the molecular axes... 

If collisional systems involving one or more molecules are considered, the internal degrees of freedom of the molecule(s) (e.g., rotation, vibration) have to be taken into account. This often leads to cumbersome notations and other complications. Furthermore, we now have to deal with anisotropic intermolecular interactions which again calls for a significant modification of the formal theory. In that sense, this Chapter differs from the previous one but otherwise the reader will find here much the same material, techniques, etc., as discussed in Chapter 5. 

Milet A, Korona T, Moszynski R, Kochanski E (1999) Anisotropic intermolecular interactions in Van der Waals and hydrogen-bonded complexes What can we get from density functional calculations J Chem Phys 111 7727-7735... 

The liquid crystal state (LCS) shows order in one or two dimensions it lacks the three-dimensional long-range order of the crystalline state. LCS has characteristics intermediate between those of the crystalline and the disordered amorphous states. These phases are called liquid crystals because many of them can flow like ordinary liquids but they display-birefringence and other properties characteristic of crystalline soHds. In liquid crystal phases the molecules can move but the orientational order is conserved in at least ne direction. The LCS can be displayed by small molecules and by polymersj but in both cases a characteristic chemical structure is needed. The existence of the liquid crystal state is related to the molecular asymmetry and the presence of strong anisotropic intermolecular interactions (19-21). Thus, molecules with a rigid rod structure can form highly ordered... 

The computation of the structure of crystals with charge transfer is difficult, since here different, anisotropic intermolecular interactions contribute in the determination of the minimum energy. The crystal-growth conditions play a still stronger role than in crystals consisting of only one type of molecules, as do purity and the temperature. In general, the theoretical prediction of the structures of charge-transfer salts is hardly possible. 

A more sophisticated analysis is required for quantitative simulation of experimental data. In particular, we explicitly consider both chromophore shape (within a hard object approximation) and do not average out anisotropic intermolecular interactions. As discussed elsewhere (3), this sq>proach leads to... 

Anisotropic Intermolecular Interaction Potential and Solute Orientation... 

The starting point for a theory of the anisotropic intermolecular interaction in liquid crystals is the Maierand Saupe theory [114,115,116,118].This theory is based on the assumption that the intermolecular interaction potential in nematic liquid crystals is determined primarily by Lx)ndon dispersion forces. The effective anisotropic potential U of a molecule C in the anisotropic dispersion field generated by its oriented neighbors s is calculated by averaging the pair potential between two molecules C and s over all orientations of the solvent molecules s and over all... 

Collective motions are elastic deformations in liquid crystalline samples, but can also exist in their isotropic phases with a finite coherence length, giving rise to pretransitional phenomena [6.2]. These motions are perceived as hydrodynamic phenomena and are influenced by molecular properties such as elastic constants and viscosities of the liquid crystalline medium. At best, director fluctuations can only provide indirect information on the anisotropic intermolecular interactions. On the contrary, motion on a molecular level must reflect the shape of the instantaneous potential of mean torque on each molecule. Both both molecular rotation and translation are expected to be sensitive to the nature of anisotropic interactions, which determine the formation of various liquid crystalline structures. 

We start with the microscopic definitions and discussion of the nematic and smectic order parameters and then proceed with some elementary information about anisotropic intermolecular interactions in liquid crystals. Then we discuss in more detail the main molecular theories of the nematic-isotropic phase transition and conclude with a consideration of molecular models for smectic A and smectic C phases. 

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