Lattice packing, reactive

A basic requirement of the subject is to obtain an understanding of the solid-state reactivity of dlacetylene monomers. This can, in principle, be used to develop a predictive capability and enable reactive monomer molecules to be engineered. Progress towards this goal has been achieved by the correlation of lattice packing and reactivity. Some recent studies, providing further information on this topic, are reviewed in the following section. 

Solid-state photoreactions are featured by their chemo-, regio-, and stereoselectivities, which are often quite different from those in solution (1). These features originate from the crystal structure of the parent molecule that is ordered with respect to packing, distance, mutual orientation, space symmetry, and molecular conformation. Reactions in crystals normally proceed with a minimum of atomic and molecular movement as a result of physical restraints by the crystal lattice (topochemi-cal principle) (2). To predict and control the crystal structure and reactivity by designing a chemical structure (crystal engineering) is one of the most attractive challenges in modern solid-state photochemistry (3). 

The photodegradation pathway of a drug in the solid state does not necessarily follow the degradation in solution. The difference arises mainly from a lack of molecular mobility, which restricts diffusion-controlled reactions and interactions between molecules, thereby influencing the reactant conformation. The photochemical properties of molecules in the solid state or embedded in a solid matrix depend greatly on the organization (if any) and nature of the solid lattice. In a number of cases, it has been possible to correlate the reactivity with packing of the reactant molecule in the crystal (Hadjoudis et al., 1986). 

Stability of, for example, (10,10)-nanotxibes, but kinetic reasons contribute as well to the effect. In achiral nanotubes, and especially in those of the armchair type, the replacement of metal atoms by carbon is much easier due to the orientation of the lattice structure (see below). What is more, several SWNTs at a time wiU emerge from very reactive sites. With the prevaiUng temperature being constant for all tubes nucleating in these zones, their respective diameters wiU also be more or less the same. Consequently, they may form a symmetric packing, which is why the most stable bundles are observed in these cases. 

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