Cycloaddition of cyclopentadiene and dicyclopentadiene on Si(100)-2x1: Comparison of monomer and dimer adsorption

The adsorption of cyclopentadiene and its dimer form, dicyclopentadiene, on the Si(100)-2x1 surface has been investigated using polarized multiple int ernal reflection Fourier transform infrared (MLR-FTIR) spectroscopy togethe r with ab initio quantum chemistry calculations. The results show that cycl opentadiene adsorbs at room temperature to form a [4 + 2] cycloaddition pro duct and at saturation covers less than two-thirds of the surface. Dicyclop entadiene adsorbs intact, most likely by a [2 + 2] cycloaddition reaction, and is estimated to adsorb onto 30-40% more Si=Si dimer sites than cyclopen tadiene despite its larger size. Dicyclopentadiene is not found to react vi a a surface-catalyzed retro-Diels-alder pathway whereby it splits into cycl opentadiene while chemisorbing, even though this pathway is theoretically p redicted to be much more thermodynamically favored.