Crystals configurational disorder

Indeed, in the world of tomorrow we can expect new aspects of polymer solids to extend the conventional and successful structure ideas of this century. These, of course, were the recognition as molecular identities of the chains of repeating chemical monomers. The circumstances of those entities have resulted in interesting concepts of solubilities, viscosity, and other mechanics, and especially thermodynamic limitations m mutual solubility or comparability of polymer mixtures. But we have known for decades that even homogeneous regular chain polymers such as Carothers polyesters and polyamides formed solids with manifold imperfections and irregularities, such as order-disorder crystal configurations.(22,23)... 

The pattern of disorder and the relationship between racemic and enantiomeric crystal structures for phthalamide (11) is similar to that of the phthalate (10). The A -ray diffraction data were measured at —170 °C. The racemate crystallizes in space group PT (Z = 4) and the enantiomer in FI (Z = 4) (Table 2). Thus there are two independent molecules in the racemate and four in the chiral crystal. The s-butyl moeities in the racemate adopt the trans conformation and exhibit configurational disorder of different measures (44 56 and 28 72) at the two independent sites, as shown in Figure 11. The chiral crystal structure is shown in Figure 12. 

Thus both configurational disorder of the s-butyl group and the isomorphism between enantiomeric crystals and their racemic counterparts occur by virtue of the following molecular property a s-butyl group is able to occupy almost the same space occupied by its enantiomer by a change in conformation and position. We may define this property as conformational isomorphism. ... 

Increases in configurational disorder on melting are particularly prominent with flexible molecules, such as the homologues of long-chain aliphatic compounds with the general formula C H2 +xX, where X is an end-group such as H, I, COOH, etc. The crystal lattice normally selects only one out of a number of configurations, usually the one which is fully stretched, and packs... 

The crystal structures of four compounds of the type Me2XCHR CF2-XMe2 M(C0)4 (M = Cr or Mo X = P or As R = H or Cl) have been redetermined, and two previously reported crystal structures of this type re-interpreted. In all of these cases, the crystals contain disordered arrangements of molecules with normal geometries and dimensions, but each crystal contains two molecular configurations the ratio of which varies from 1 1 to ca. 6 1. Thus, the previous interpretation of such X-ray data, involving a single... 

As with the first and second laws, the Third Law is based on experimental measurements, not deduction. It is easy, however, to rationalize such a law. In a perfectly ordered3 crystal, every atom is in its proper place in the crystal lattice. At T— 0 Kelvin, all molecules are in their lowest energy state. Such a configuration would have perfect order and since entropy is a measure of the disorder in a system, perfect order would result in an entropy of zero.b Thus, the Third Law gives us an absolute reference point and enables us to assign values to S and not just to AS as we have been restricted to do with U, H, A, and G. 

In general it is not necessary to measure all possible configurations in order to identify all the phonons. However, it should be pointed out that frequently violations of the selection rules appear or that in a particular orientation forbidden phonons appear. These are believed to be caused by disorders in the single crystal. 

Structural disorder resulting from the statistical co-crystallization of different configurational repeating units (see Fig. 2). 

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