Generation/Reaction of Vinyl Groups on Cu(100)
J. Phys. Chem., Vol. 100, No. 30, 1996 12439
vinyl on Pt(111)48 and the facile (230 K) â-C-H bond breaking
that is observed for vinyl on Ni(100).5-7 Alkyl groups readily
undergo â-hydride elimination on Cu(100).22 Vinyl groups,
however, couple above 250 K on Cu(100) with no â elimination.
As a result, we can conclude that â elimination from vinyl is
slower than â-hydride elimination from alkyls. This difference
may reflect, in part, the orientational rigidity of surface vinyl
groups compared with surface alkyls. Recent studies have
shown that conformation plays a large role in determining the
rate of â elimination from alkyl groups,22 and since one might
expect a significant energy barrier for reorienting vinyl, achiev-
ing the adsorbate/surface geometry that minimizes the energy
required for the â-elimination pathway may be much higher
for vinyl than for alkyl groups. A similar barrier for reorienta-
tion may also account for the stability of the â-C-H bonds in
phenyl groups to temperatures above 300 K on Cu(100).49
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5. Summary
Our results show that monolayers of vinyl bromide and vinyl
iodide dissociate on Cu(100) to produce adsorbed vinyl groups.
Chemical displacement studies, NEXAFS experiments, and real
time work function change measurements indicate that the CsBr
bond cleavage occurs at 160 K in the vinyl bromide monolayer.
Vinyl groups are thermally stable up to 250 K where coupling
occurs to produce 1,3-butadiene, which also desorbs from the
surface at this temperature. NEXAFS studies indicate that
monolayer vinyl bromide molecules are oriented with their
molecular planes within ∼30° of parallel to the surface. By
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Acknowledgment. Financial support from the Dow Chemi-
cal Corporation, the National Science Foundation (Grant No.
CHE-93-18625), and the Camille and Henry Dreyfus Foundation
is gratefully acknowledged.
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