Carbon-Carbon Bond Forming Reactions of Metal-Bonded Hydrocarbon Groups on Ag(111): Steric, Electronic, and Carbon Hybridization Effects on the Coupling Rates

• Main Authors: Long-chen Lee, 李龍晟
• Format: Dissertation
• Language: English
• Published: 2006

碩士 國立中山大學 化學系研究所 94 The alkyl substitution effects and the hybridization effects on the rate of coupling of adsorbed hydrocarbon groups on Ag(111) have been investigated under ultrahigh vacuum by temperature programmed reaction/desorption (TPR/D). For these two different issues, two types of halide precursors were used. One is to form adsorbed fragments bearing Cα(sp3) and Cα-H, the other is to yield adsorbed fragments with different hybridized α-carbons without Cα-H. The desired hydrocarbon groups were generated on Ag(111) by the thermal dissociation of the C-X (X = I or Br) bond in the corresponding halogenated compounds. Substitution of alkyl for hydrogen in the adsorbed alkyl groups systematically raises the coupling temperature. For example, 3-pentyl groups homo-couple at temperatures ~ 70 K higher than the ethyl homo-coupling reaction. The concept of “geminal repulsion” can account for our experimental results while the size and the number of the alkyl substitution groups increase. Different hybridized Cα (metal-bonded carbon) species cause various angle strain energies in the cyclic transition state for the coupling reaction. The Cα(sp) species (CH3C≡C(ad) and (CH3)3SiC≡C(ad)) have rather high coupling temperatures (~ 460 K) due to the unidirectional sp orbital and the stronger Ag-C(sp) bond in the transition state. The relative rates for homo-coupling as a function of the hybridization of the metal-bound carbon follow the trend sp3 > sp2 > sp on the Ag(111) surface. Lastly, we found that the isobutyl groups undergo a β-hydride elimination instead of homo-coupling on the Ag(111) surface. It may be due to that isobutyl groups have a total of nine β-hydogens among all the hydrocarbon groups, which makes this rare reaction pathway possibly occur on Ag(111).