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The mechanism and intermediates of a Diels–Alder-type cycloaddition reaction between dienes and the silicon dimers of a Si(100)-2×1 surface, which was theoretically predicted by Doren and Konecny, have been investigated. The reactions of 1,3-butadiene and 2,3-dimethyl-1,3-butadiene were studied using multiple internal reflection infrared spectroscopy, thermal desorption spectrometry, and near edge x-ray absorption fine structure (NEXAFS) measurements. The results show that the compounds physisorb on Si(100)-2×1 at cryogenic temperature. Infrared studies of the room temperature adsorption of both dienes indicate that reaction leads to the formation of stable, chemisorbed Diels–Alder adducts. By NEXAFS measurements on 2,3-dimethyl-1,3-butadiene, we determine that the angle between the π orbitals of the reaction product and the Si(100)-2×1 surface is near 40°. Upon heating, the chemisorbed butadienes primarily decompose to form adsorbed carbon and hydrogen at the surface. Hydrogenation of chemisorbed butadienes by atomic hydrogen was also investigated, and conversion from the Diels–Alder geometry to [2+2] bonding is observed. This effect is attributed to cleavage of the Si–Si dimer bond upon atomic hydrogen exposure. Temperature dependent studies suggest the presence of an activation barrier for chemisorption at low temperature. The parallels between our experimental results and the theoretical studies of these reactions are discussed.

The room-temperature chemisorption, thermal reactivity, and reaction with atomic hydrogen were compared for 1,3-cyclohexadiene, 1,4-cyclohexadiene, and cyclohexene on Si(100)-2×1 using multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), and temperature programmed reaction/desorption techniques. For both dienes, a CdC double bond remains in the chemisorbed product, indicating that surface bonding does not involve both alkene groups in the reactant. For 1,3-cyclohexadiene, multiple chemisorption configurations are implicated, consistent with [4+2] and [2+2] cycloaddition products. The average angle between the π bond in the 1,3-cyclohexadiene adsorbate and the Si(100)-2×1 surface is 35°according to NEXAFS measurements. Upon heating, both cyclohexadienes decompose, resulting in the evolution of benzene at temperatures ranging from 400 to 700 K and release of H 2 from the silicon surface at 780 K, although the behavior differs slightly for the two compounds. IR annealing studies for 1,3-cyclohexadiene confirm that decomposition begins between 400 and 500 K. Following atomic hydrogenation of the chemisorbed C 6 cyclic hydrocarbons, the same vibrational spectra are obtained for cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, and benzene; these spectra are also similar to that of chemisorbed cyclohexene. These similarities suggest a common hydrogenation product, whose structure is consistent with a hydrogenated [2+2] adduct, independent of whether before hydrogenation the chemisorbed hydrocarbon formed a [2+2] or [4+2] product.

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