Packings, structured sheeting

In the case of [ Au(C = lBu) 2[p.-1,2-C6Mc4(C = C)2 ] [57] the molecules pack in sheets (Figure 5.31a), which, in turn, stack to give a three-dimensional supramolecular structure (Figure 5.31b) this packing seems to be attributable to intermolecular van der Waals —H- Au interactions. 

In an X-ray refinement procedure that additionally allowed to be a parameter, Arnott et al. (1967) were able to find an APPS structure with statistical packing of sheets that gave good agreement with the observed intensities. In this structure, Ab, although it could not be well refined, was felt to be probably between 0 and -0.65 A. However, Ab can be obtained relatively accurately from a TDC analysis of the amide I modes (Moore and Krimm, 1976b), and was found to be -0.27 A. 

A close-packed monolayer of long, planar molecules must necessarily contain at least "islands" of local order in which molecules are packed plane-to-plane. One notices that crystals of these dyes tend to pack in sheets, within which the molecules have their long axes parallel to one another and parallel to the plane of the sheet. The molecular short axes are nearly perpendicular to the plane of the sheet, and the intermolecular contacts within the sheet tend to occur at the graphitic distance, except for molecules which are bent or twisted out of planarity. Twisted molecules ("overcrowded" in Brooker s classification) generally make poor sensitizers, probably because they have a fast route of deexcitation directly from the excited singlet state to the ground state opened by the existence of the twisted, stressed structure. 

The structure has a hexagonal symmetry and unit cell. The a- and b-axes lie in the close-packed A sheet, and the hexagonal c-axis is perpendicular to the stacking and runs from one A sheet to the next above it, (Figure 7.3). There are two spheres (two atoms) in a unit cell, at positions 0,0,0 and A, 2A, A. If the spheres just touch, the relationship between the sphere radius, r, and the lattice parameter a, is ... 

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