Free-radical reactions under diffusional constraints: orientation does matter in hydrogen transfer.
Pyrolysis of silica-immobilized 1,3-diphenylpropane at 375 degrees C has been examined in the presence of a series of isomeric (by point of attachment) co-attached hydroaromatic spacer molecules. Under the diffusional constraints, the pyrolysis rate is sensitive to the orientation of the spacer molecule, which must transfer hydrogen to intermediate benzylic radicals on the surface. Spacer molecules possessing a meta-orientation of the benzylic hydrogens with respect to the surface linkage are able to attain a better geometry for the hydrogen transfer on the surface resulting in faster reaction rates.