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論文名稱 Title |
銀(111)單晶表面碳氫基團之碳-碳鍵形成反應:立體能障、電子誘導效應與碳原子混成軌域對速率的影響 Carbon-Carbon Bond Forming Reactions of Metal-Bonded Hydrocarbon Groups on Ag(111): Steric, Electronic, and Carbon Hybridization Effects on the Coupling Rates |
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系所名稱 Department |
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畢業學年期 Year, semester |
語文別 Language |
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學位類別 Degree |
頁數 Number of pages |
59 |
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研究生 Author |
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指導教授 Advisor |
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召集委員 Convenor |
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口試委員 Advisory Committee |
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口試日期 Date of Exam |
2006-07-18 |
繳交日期 Date of Submission |
2006-08-06 |
關鍵字 Keywords |
程溫脫附/反應、β-脫氫反應、銀(111)、混成軌域、電子誘導效應、立體效應、烷基取代、超高真空系統、自偶合反應、1-3 立體排斥作用力 Ag(111), alkyl substitution, steric effects, β-H elimination, homo-coupling, UHV, inductive electronic effects, 1-3 repulsive steric interactions, TPD/R, orbital hybridization |
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統計 Statistics |
本論文已被瀏覽 5720 次,被下載 1711 次 The thesis/dissertation has been browsed 5720 times, has been downloaded 1711 times. |
中文摘要 |
由於銀與氫親和力很低以及銀-氫鍵較其它金屬-碳鍵為弱,導致碳氫自由基在銀表面傾向自偶合(homo-coupling)形成碳-碳鍵而不容易進行碳-氫鍵裂斷反應。本文旨在於超高真空條件下,使用程溫脫附/反應 (TPD/R) 來研究烷基取代以及α碳(和金屬表面或鹵素鍵結的碳)之混成軌域對於碳氫化合物自偶合反應速率的影響。基於這兩種不同的研究目標,所使用的鹵化前驅物也分為兩大部分:一部份為α碳(皆為sp3)具有氫的鹵化烷類分子;另一部份則是 α碳(有各種不同的混成軌域)上不具有氫的鹵化碳氫化合物,再進一步利用碳-溴或碳-碘鍵容易熱烈解的特性在Ag(111)上產生相對應的吸附基團。在烷基取代的研究中,我們有系統的改變α碳(sp3)上烷基取代的大小以及數目並觀察各個物種在Ag(111)上其自偶合產物脫附的溫度。結果顯示,3-戊烷基的產物脫附溫度比乙烷基高了約70 K,這表示在烷基取代效應中立體障礙的影響大於電子誘導效應,而由立體障礙所導致的反應活化能升高和以"geminal repulsion"的大小所預測的趨勢是符合的。另一方面,在針對α碳上不同混成軌域對自偶合反應速率影響的實驗中,α碳為sp 混成的物種(1-丙炔碘(CH3C≡CI)以及三甲基矽基乙炔碘((CH3)3SiC≡CI))被觀察到有特別高的產物脫附溫度約在460 K左右,在自偶合反應的過渡態中有牽涉到銀-碳鍵斷裂以及碳-碳鍵形成,我們認為由於Cα(sp)物種具有較強的銀-碳鍵以及較高的混成軌域方向性因而造成較高的活化能。同時從實驗的結果中也可以發現自偶合反應速率快慢和α碳的混成有著以下順序: sp3 > sp2 > sp。最後在異丁烷基(isobutyl)的實驗中,並沒有如預期的有自偶合反應發生,取而代之的是在銀表面上鮮少發生的β-hydide elimination,我們推測這是由於異丁烷基擁有在所有碳氫化合物中最多的九個β氫,而使得β-H elimination 反應發生的可能性提高許多。 |
Abstract |
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). |
目次 Table of Contents |
Chapter 1 Introduction ……………………………………………… 1 Chapter 2 Experimental ……………………………………………… 5 Chapter 3 Results 3.1 The Surface Chemistry of iodoethane, 1-iodopropane, and 2-iodopropane on Ag(111) ……………………………………………… 8 3.2 The Surface Chemistry of 2-iodobutane on Ag(111) ……………………………………………… 14 3.3 The Surface Chemistry of 3-bromopentane on Ag(111) ……………………………………………… 17 3.4 The Surface Chemistry of Propynyl Iodide ( Cα(sp) ) on Ag(111) ……………………………………………… 20 3.5 The Surface Chemistry of 1-iodo-2-(trimethylsilyl) acetylene ( Cα(sp) ) on Ag(111) ……………………………………………… 23 3.6 The Surface Chemistry of Phenyl Iodide ( Cα(sp2) ) on Ag(111) ……………………………………………… 25 3.7 The Surface Chemistry of 2-iodo-2-methylpropane ( Cα(sp3) ) on Ag(111) ……………………………………………… 28 Chapter 4 Discussion 4.1 Steric Effect on the Transition State for Alkyl Coupling Reactions ……………………………………………… 31 4.2 Hybridization Effect on the Coupling Rate of the Adsorbed Hydrocarbon Groups ……………………………………………… 37 Chapter 5 Conclusions ……………………………………………… 41 References ……………………………………………… 43 |
參考文獻 References |
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