RADICAL CHEMISTRY UNDER DIFFUSIONAL CONSTRAINTS - IMPACT OF SPACER MOLECULES ON THE THERMOLYSIS OF SURFACE-IMMOBILIZED BIBENZYL

Silica nanoparticle surfaces have been chemically modified to contain a molecular probe, 1,2-diphenylethane (bibenzyl), as well as a second spacer molecule of variable structure. Thermolysis kinetics and produc ts at 400 degrees C have been determined for bibenzyl, and the impact of diffusional constraints and spacer molecular structure on the multi pathway radical chemistry has been analyzed relative to fluid phases. Unimolecular homolysis rate constants, k = (7-9) x 10(-6) s(-1), are f ound to be independent of spacer molecular structure (naphthalene, dip henylmethane, tetralin) and similar to values measured in fluid phases , indicating the lack of a cage effect on the silica surface. However, the total rate of bibenzyl thermolysis and the resulting product sele ctivities are profoundly affected by both surface immobilization and t he structure of the spacer molecule. Naphthalene spacers behave as ''m olecular walls'' serving as physical barriers to bimolecular hydrogen transfer steps on the surface and augmenting the effects of diffusiona l constraints. In contrast, diphenylmethane spacers are found to serve as hydrogen transfer, radical relay catalysts that translocate radica l sites across the surface and diminish the impact of diffusional cons traints. Tetralin spacers undergo significant reaction with free-radic al intermediates, selectively producing the isomeric methylindan via a chain pathway that is promoted by the restrictions on diffusion.