Solid-state reactions geometric factors

The rate equations which have found most widespread application to solid state reactions are summarized in Table 3.3. Other functions can be found in the literature. The expressions are grouped according to the shape of the isothermal a-time curves as acceleratory, sigmoid or deceleratory. The deceleratory group is further subdivided according to the controlling factor assumed in the derivation, as geometrical, diffusion or reaction order. 

Novel vinyl liquid crystalline (l.c.) polymers were synthesized with the UV-sensitive p-methoxycinnamate chromophore incorporated into the side chain of the polymers. The objective of this synthesis was to determine if a molecularly organized environment could influence the yield of a chemical reaction in the solid state. The investigation into the photochemical and physical processes of these thin films revealed that the photodimerization of the p-methoxycinnamate moieties was very sensitive to their geometrical arrangement in the polymer matrix. The relative quantum yield of cyclobutane formation increased by a factor of approximately 8 for the l.c. p-methoxycinnamate film compared to its amorphous analog. This quantum yield approaches the theoretical limit for this system. 

Another factor which affects the reactivity of polymer molecules is that the reactants must be expected to achieve a particular geometrical relationship with respect to each other in order to form the excited complex from which reaction occurs. If mobility is severely restricted, this geometry may not be achievable within the lifetime of the excitation, and chemists not familiar with macromolecular reactions might assume that the restrictions of motion in the solid state would be so great that reaction would not occur at all. In fact, this is not the case, and there are many examples of photochemical reactions which are just as efficient in solid polymers as they are in dilute solutions of small-... 

Experimental evidence on the relationship between structure and reactivity in Norrish type II photochemistry of spirobenzoyladamantanes in the solid state has been produced. Geometric factors clearly determine partitioning between cleavage, cychzation, and hydrogen transfer reactions of the 1,4-biradical intermediate Cleavage becomes predominant when the singly occupied orbitals overlap poorly with one another but well with the central CC bond, which, as earlier sections indicate, has been presumed for years based on ketone structure. 

---