Generic model, dimers

Figure 16. The linear and bent conformers of the generic model for liquid crystal dimers. The axis system associated with the bent conformer and reflecting its synunetry which is used to define the director orientation is also shown.

Given this brief outline of the generic model, we are now able to consider certain of its predictions. The first is the dependence of the entropy of transition on the composition of the isotropic phase. In the limit that this is unity, the system corresponds to a pure system of rods and so is analogous to a rodlike monomer. At the other limit, of x equal to zero, the system is again a single component but now of bent molecules indeed within our parametrization of the model the particles have the maximum biaxiality. In between these two extremes, the system corresponds to a liquid crystal dimer even dimers would have x[ of about 0.5 whereas odd dimers should have x P of less than 0.1. From... 

Although the generic model which we have discussed in the preceding section provides a valuable route to understanding the properties of liquid crystal dimers it cannot be used to predict their behavior quantitatively. To achieve this it is necessary to include in the model all of the conformational states which the spacer can adopt. The simplest way to describe these is with Flory rotamer-ic state model [60] which restricts the conformers to a discrete number. The confor-... 

Our aim here is not to make a detailed comparison of the various parametrizations which have been proposed for the potential of mean torque. Instead we wish to illustrate the nature of the results which can be obtained with models which include all of the conformations for the dimer, suitably weighted [78]. The calculations proceed in an analogous manner to the generic model for example to determine the N-I transition temperature it is necessary to determine when the molar Helmholtz free energy of the isotropic phase... 

Table 5. Binding Energies Ei, for the Methyl-Substituted Benzene Dimer Cations Based on Application of the Generic Standard Hydrocarbon Model to the Radiative Association Kinetic Data. I obs is the Efficiency per Collision of Radiative...

Molecular dynamics studies of diatomic model detonations were first carried out by Karo and Hardy in 1977 [14]. They were soon followed by other groups [15, 16]. These early studies employed predissociative potentials, in which the reactant dimer molecules are metastable and can dissociate exothermically. More realistic models, combining an endothermic dissociation of reactants with an exothermic formation of product molecules, were introduced by White and colleagues at the Naval Research Laboratory and U.S. Naval Academy, first using a LEPS (London-Eyring-Polanyi-Sato) three-body potential for nitric oxide [17], and later a Tersoff-type bond-order potential [18] for a generic AB model, loosely based on NO [19, 20]. 

---