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 products at 400 degrees C have been determined for bibenzyl, and the impact of diffusional constraints and spacer molecular structure on the multipathway radical chemistry has been analyzed relative to fluid phases. Unimolecular homolysis rate constants, k = (7-9) x 10(-6) s(-1), are found to be independent of spacer molecular structure (naphthalene, diphenylmethane, 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 selectivities are profoundly affected by both surface immobilization and the structure of the spacer molecule. Naphthalene spacers behave as "molecular walls" serving as physical barriers to bimolecular hydrogen transfer steps on the surface and augmenting the effects of diffusional constraints. In contrast, diphenylmethane spacers are found to serve as hydrogen transfer, radical relay catalysts that translocate radical sites across the surface and diminish the impact of diffusional constraints. Tetralin spacers undergo significant reaction with free-radical intermediates, selectively producing the isomeric methylindan via a chain pathway that is promoted by the restrictions on diffusion.