Molecular column

Columnar mesophase characterised by a hexagonal packing of the molecular columns. 

Two models have been developed to describe the superstructure found in these salts. In the Kobayashi et al. model, cation ordering in the channels adjacent to the [Pt(C204)2] chain is responsible for the development of the superstructure and the 3D modulation of the Pt atom chain.79 On the other hand, Bertinotti and coworkers have proposed that the chains are fragmented into micro-domains by periodic intrinsic defects associated with polarons.78 82 Three orthogonal deformation modes, one longitudinal and two transverse, are present in each chain and a fourth mode corresponds to a global sliding of the molecular column. 

Source Distance Density n(H2) (cm 3) Tem- perature Tk(K) Molecular column densities (cm 2) ... 

We will not mention effects on molecular formation due to shocks and shock fronts in dense molecular clouds, nor will we discuss the chemistry of the cir-cumstellar environment, where an abundance of molecular species has been detected during the past several years. In the warm, dense envelopes of stars the abundances can be matched by chemical-equilibrium calculations, in contrast to the chemical reactions which can take place in the cold interstellar molecular clouds. For example theoretical calculations based on chemical equilibrium have been performed for the expanding molecular envelope of the cool carbon star IRC H-10216 by McCabe et al. (1980), in agreement with the observed molecular column-densities. 

Fig. 5a, b a Packing of host molecules DGTD within the column, and a symbolic description. b Types of molecular column orientation of DGTD observed in its inclusion complexes I - parallel columns in space group C2, II - antiparallel columns in space group P2[ and P2i2i2i, III - columns rotated by 120° in space group P32. (Adopted from [33] with permission)... 

The common feature observed in each of the crystals is the presence of molecular columns formed by the host (Fig. 5a) molecules aggregate in the head-to-taiF mode, i.e. the disulfide bridge contact methyl groups from the opposite side of the DGTD, and they are repeated by translation along the column. 

We report here the macrocyclic conformation of the complexed and the uncomplexed host molecules, hostrguest interactions, the molecular columns formed by stacking of the macrocycles and the guest dependent polymorphism (crystal structure). 

In the solid state, the maxrocycle exists as R -conformer or S -conformer. A molecular column is chiral because a column consists of all R -conformers or all S"-conformers. Thus, "R -conformers make R -columns and "S"-conformers make "S"-columns. Chiral crystals of 3 CHCl3 consists of all "R"-columns or all "S"-columns, while racemic crystal of CH2Cl2 consists of both "R"-columns and "S"-columns. 

Figure 4. Molecular columns formed by stacking of the macrocycles along -axis.

It turns out, however, that the HATn have a very low intrinsic carrier concentration, owing to the large band-gap in these materials. They are therefore good insulators (Table 1) with high breakdown fields of 4 MV cm [9]. The mobility of an injected charge along the molecular columns (wires) is, however, fairly high [10-12]. 

The combination of these properties, together with other unique properties of columnar liquid crystals such as the selforganizing propensity of the molecular columns [13] which eliminates long lived grain boundaries which trap charge, and the ability readily to wet the surface of metals and... 

Figure 2. The structure of the chromonic N and M phases The basic structural unit of both phases is the untilted stack of molecules. The N phase is a nematic array in which these stacks lie in a more or less parallel pattern, but where there is no positional ordering. Tlie M phase is a hexagonal array of these columns. The six-fold symmetry is a result of orientational (but not positional) disorder. A schematic diagram of a localized region, as shown in (a) has only orthorhombic symmetry, but, averaged over the whole structure, each column actually lies in a site with sixfold symmetry (b). The restrictions to the possible orientations of the columns are shown in (c). Because of packing considerations, for any particular orientation of a column, as shown on the left, an adjacent column (right) can take up only two of the three possible orientations (i) and (ii). A representation of the orientationally disordered state of the M phase is given in Fig. 9. Note that the molecular columns are shown here in a highly stylized way. They are not necessarily such simple one-molecule-wide stacks.

Fig. 14.14. The best P2 c crystal structure for dicarbonyl-corannulene, 3. In spite of the sbghtly different viewing, the structure is similar to that of the chloroderivative in Fig. 14.11, with alternating up and down molecular columns. Inter-column contacts involve favorable coulombic interactions between oxygens and hydrogens (dark and white calottes, respectively).

Cassel [29,30] showed, using Gibbs adsorption isotherm, that the surface tension of the adsorbed film atP = Pq is negative, arising from the total disregard of the interaction forces. Since the BET model assumes the existence of localized adsoq>tion at all levels, the molecules being located on top of one another, and since the adsorption can take place in the nth layer before the (n-/>th layer is filled, the adsorbed phase is built up not as a series of continuous layers, but as a random system of vertic molecular columns. Halsey [31] pointed out that the combinational entropy term associated with these random molecular piles is responsible for the stability of the BET adsorbed layers at... 

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