Stacking axis

Figure 4-11. INDQ/SCI-caleulalcd evolution of the transition energies (upper pan) and related intensities (bottom pan) of the lowest two optical transitions of a cofacial dimer formed by two stilbenc molecules separated by 4 A as a function of the dihedral angle between the long molecular axes, when rotating one molecule around the stacking axis and keeping the molecular planes parallel (case IV of Figure 4-10). Open squares (dosed circles) correspond to the S(J - S2 (S0 — S, > transition.

Figure 5.18 (a) Drawing of the [Au(S2CN (C5Hii)2)]2 molecules in DMSO with two repeating units of chains in the structure viewed perpendicular to the stacking axis. Thermal ellipsoids are drawn at the 50% probability level. The pentyl moieties have been omitted forclarity. 

Figure 8.20 demonstrates the generation of ft (x) by displacement of the ID electron density77, Ap (y), with respect to its ghost, Ap (y -x), along the stack axis y. The direction x in the sketch is identical to the direction of the stack normal, r3, in Fig. 8.12 on p. 126. The sketch depicts a displacement x that is still so small that each domain is only correlated to itself (shaded areas). At such a position x we are still in... 

The General Series Expansion for Stacks. In practice, ID scattering intensities can always be modeled by programming the obvious series expansion of such a structural entity every correlated distance along the stack axis is producing an attenuated oscillation according to Hk (5) that is weighted by the probability of its occurrence under consideration of the zero-sum rule and the related correlation-construction principle (p. 158, Fig. 8.30)... 

The most reliable information on the structure of the polymer has been derived from X-ray diffraction and Raman-spectroscopic studies. In a few cases it has been possible to determine in detail the structures of both the monomer and the corresponding polymer. From such measurements and other optical studies the process is considered to be 132 — 133 for a symmetric diacetylene. In polymerizable structures the diacetylene rods are inclined at about 45° to the translation (stack) axis, with the ends of each diacetylene moiety approaching the adjacent triple-bond systems to a distance of s4 A. The polymer is a planar system in extended conformation and having alternate R groups trans to one another. 

Sanidic mesophase in which the molecular stacks are packed regularly side-by-side with long-range order along a stack normal as well as along the long stack-axis. 

Figure 1.15. Crystal structure of TTF-TCNQ perspective view along the stacking -axis. P2ilc, a = 1.230 nm, b = 0.382 nm, c = 1.847 nm, fi = 104.46°. C, S and N atoms are represented by black, medium grey and light grey balls, respectively. H atoms are not represented for clarity. Crystallographic data from Kistenmacher et al, 1974.

Some semiconducting organic CT complexes of mixed-stack architecture exhibit the rather unusual neutral-to-ionic (N-I) phase transition upon variation of an external variable of parameter hyperspace, such as P or r. The transition manifests itself by a change of q and a dimerization distortion with the formation of donor-acceptor dimers along the stacking axis in the I-phase. 

Fig. 16 Crystal structure of the BA/PFBA cocrystal viewed along the stacking axis. Interstack hydrogen bonding involving the carboxylic acid groups is shown as dashed lines. Hydrogen atoms are not shown, and the fluorine atoms of the PFBA molecules are shown in green...

Figure 6.42 Crystal structure of HMTTF-TCNQ. (a) Projection on the plane perpendicular to the stacking axis and (b) projection on a plane containing the stacking axis. (After Greene et ai, 1976.)...

The second structural motif is exemplified by Ni(bqd)2 (bqd= 1,2-benzoquinonedioximato) (Figure 23). The planar molecules (Table 54) are in a slipped-stacked (SS) arrangement which makes the Ni—Ni directions inclined with respect to the stacking axis. In this type of complex the Ni—Ni distances are longer, the packing is also more efficient and no channels are present. The complexes with SS structure are generally insulators. 

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