Structures and reactivities of cycloheptane, cycloheptene, 1,3-cycloheptadiene, and cycloheptatriene on Pt(111)
- Univ. of Illinois, Urbana, IL (United States)
The structures and reactivities of various cyclic C{sub 7} hydrocarbons (cycloheptane, cycloheptene, 1,3-cycloheptadiene, and cycloheptatriene) adsorbed on Pt(111) have been examined by means of temperature-programmed reaction, reflection-absorption infrared, and Auger electron spectroscopies. At temperatures below 200 K, cycloheptane binds weakly to the surface and exhibits vibrational mode softening consistent with the presence of agostic C-H{hor_ellipsis}M interactions. In contrast, the cyclic alkenes cycloheptene, 1,3-cycloheptadiene, and cycloheptatriene all bind strongly (and largely irreversibly) to the surface principally by means of interactions with their {pi}-systems. The chemistry of these adsorbates at higher temperatures has been mapped out in considerable detail. For cycloheptane, most simply desorbs but about 30% is dehydrogenated; interestingly, cycloheptene is not an intermediate in this process. Cycloheptene first loses two hydrogen atoms to give what the authors propose is an {eta}{sup 3},{eta}{sup 1}-C{sub 7}H{sub 10} product. 1,3-Cycloheptadiene first loses two hydrogen atoms to afford an {eta}{sup 5}-cycloheptadienyl product, which dehydrogenates further to give surface-bound cycloheptatriene in (apparently) high yield. Between 325 and 375 K, all of the cyclic C{sub 7} hydrocarbons dehydrogenate further and eventually yield a planar {eta}{sup 7}-cycloheptatrienyl (C{sub 7}H{sub 7}) species on the surface, although for cycloheptene this conversion is inefficient. The thermolytic decomposition of the {eta}{sup 7}-C{sub 7}H{sub 7} species begins at about 425 K and affords surface-bound hydrocarbons that are largely undetectable by RAIR spectroscopy. At higher temperatures (> 800 K), these hydrocarbons are completely dehydrogenated to form a carbonaceous overlayer. Ring contraction products, such as surface-bound benzene, are not observed as intermediates in any of the thermolytic reactions. The RAIR spectra of the {eta}{sup 5}-cycloheptadienyl and cycloheptatriene adsorbates both contain an unusual low-frequency vibrational band near 2,770 cm{sup {minus}1}. This band is not indicative of an agostic C-H{hor_ellipsis}M interaction, but is instead the signature of a methylene C-H bond that has been perturbed by hyperconjugative drain of electron density into the adjacent {pi}-system.
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- FG02-91ER45439
- OSTI ID:
- 691310
- Journal Information:
- Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical, Vol. 103, Issue 32; Other Information: PBD: 12 Aug 1999
- Country of Publication:
- United States
- Language:
- English
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