J. Phys. Chem. B 1998, 102, 2391-2402
2391
Carbon-Hydrogen Bond Activation and Cyclodehydrogenation Reactions of Cyclic C8 Hydrocarbons on Pt(111) William L. Manner,† Lawrence H. Dubois,*,‡ Gregory S. Girolami,*,† and Ralph G. Nuzzo*,† School of Chemical Sciences and the Frederick Seitz Materials Research Laboratory, UniVersity of Illinois at UrbanasChampaign, Urbana, Illinois 61801, and Defense AdVanced Resarch Projects Agency, Arlington, Virginia 22203 ReceiVed: September 18, 1997; In Final Form: January 13, 1998
The mechanisms of the thermolytic decomposition of a series of unsaturated cyclic C8 hydrocarbons adsorbed on a platinum(111) single crystal surface are described. This study both confirms and extends the results reported by Frei and Campbell which corrected errors in an earlier report from our laboratory on the mechanisms of thermal decomposition seen in this adsorbate system. We find that the dehydrocyclization of cyclooctene, cyclooctadiene (1,3 and 1,5 isomers), and cyclooctatetraene on Pt(111) proceeds through a related set of intermediates and ultimately yields a novel surface-bound bicyclic ring system. As noted by Frei et al., our earlier finding that benzene is produced in high yield is incorrect. The spectroscopic evidence reported here demonstrates that all four of the cyclic C8 hydrocarbons form a bicyclo[3.3.0]octenyl (or, as more commonly named, a pentalenyl) intermediate of stoichiometry C8H6. This bicyclic ring system forms via a dehydrocyclization of cyclooctatetraene, which itself is formed in varying yields from each of the less-unsaturated c-C8 adsorbates. The formation of the pentalenyl species proceeds at low temperatures (
