Dimer molecules, packing

The unit-cell packing is illustrated in Figure 2. Packing of the dimeric molecules appears to be determined by intermolecular van der Waals and steric interactions among the alkyl hydrogen atoms. The closest intermolecular H H contact is 2.31 A between atom H(2) on C(25) and atom H(l) on C(37). 

The dominant supramolecular synthon present in these compounds is the pi-halogen dimer (PHD) interaction [22], This efficient packing motif results from two inversion-related host molecules packing such that one bromine atom from each is situated in the cleft of its V-shaped partner (Figure 3), in addition to aromatic endo, endo-facial interaction. 

In both polymorphs, the nitrofurantoin molecules pack in layers that are held together by van der Waals forces. The molecules associate to form centrosymmetric dimers through two identical intermolecular N—H... 0 hydrogen bonds. These dimers are themselves linked by a weaker C—... 

The packing of each molecule in the crystal is shown in Fig. 3. The 3-CyD dimers are packed in the manner of the brickwork pattern which is one of the cage type packing structures and has been frequently seen in the 3-CyD inclusion complex structure of the triclinic space group PI. 

Intetmolecular potentials between aromatic molecules, which involve n electron interactions, are different than those in aliphatic compounds. Thus, when calculating heats of sublimation and crystal packing values, special parameters for aromatics are needed. Nonbonded parameters, which include van der Waals s and R) and a Csp -H bond moment, should be included in a FF to reproduce crystal packing. In addition, accounting for electronic energy in a n system is needed to accurately calculate the heats of sublimation. For example, in the benzene dimer, molecules prefer to interact edge-to-face rather than parallel to each other or n stacked. With the appropriate nonbonded parameters in place and treatment of the electronic energy (see Section 2.2), MM3 can fit benzene and biphenyl crystal data. 

Figure 3.8 Schematic diagram of the dimeric Rop molecule. Each subunit comprises two a helices arranged in a coiled-coil structure with side chains packed into the hydrophobic core according to the "knobs in holes" model. The two subunits are arranged in such a way that a bundle of four a helices is formed.

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