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
• Other Authors: Chao-ming Chiang
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/42194234126866765801

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).