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 (<375 K).
Data
from reflection−absorption infrared (RAIRS) and
temperature-programmed reaction spectroscopies show that
the pentalenyl intermediate is remarkably stable, persisting on the
surface up to at least 450 K. At temperatures
above 450 K, this intermediate fragments, liberating hydrogen, and
leaving a carbonaceous overlayer. The
formation of cyclooctatetraene, and thus the pentalenyl intermediate,
proceeds with the highest efficiency for
the two dienes and lowest for cyclooctane and
cis-cyclooctene. RAIRS data further serve to identify
several
intermediates formed along the reaction pathway leading to the
platinum-bound C8H6 species.