Periodic bond chains, definition

The very idea of an intermolecular chemical bond, in conjunction with crystal periodicity, leads to the concept of chain formation. One is never too careful, however, because crystal structures are by definition periodic in space and, vice versa, the concept of chain can surreptitiously sneak in without the need of a real chemical bond. Once again, there is a dichotomy between geometrical appearance -a ribbon of molecules close to each other - and objective physics, or the presence of an effective electronic interaction between the partners in the supposed periodic structure. The basic question is, how and when is one allowed to trace a chemical bond, and thus legitimately to draw a chain structure The answer - or at least what in this author s opinion is the only objective and physically sound answer - must come from quantitative evaluation of the involved contact energies, as discussed in the next paragraphs. 

The crystal structure of polyphosphazenes is of definite interest for understanding the genesis of the structure of the mesophases [42, 55-61]. The crystal unit cell parameters are reported in Table 4.5 for a series of polyphosphazenes forming thermotropic mesophases. All of the crystals are assigned to a monoclinic or orthorhombic system. The identity period is close to 4.9 A in all polyphosphazenes, which corresponds to an almost planar chain conformation in the crystals with cis-trans bond conformation [55, 61] (Fig. 4.18). The directivity of the bonds in the macromolecules of polyphosphazenes implies the possibility of different packing in the same crystd [55]. 

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