中 文 摘 要
本論文藉由理論計算的方法，在HF/6-31G、B3LYP/6-311++G**的計算層級下，對各穩定點結構做全面性的幾何優選，探討2-戊烷氧自由基及其衍生物在大氣對流層中異構化反應的各項性質，並且將討論影響這一系列反應活化能的相關因素，全文共分為以下五個單元進行討論：
第一單元　討論2-戊烷氧自由基經由不同位置碳的分子內氫原子轉移。計算結果發現經由六圓環反應在B3LYP/6-311++G**層級所得的反應速率常數為1.25 105 (s-1)，與實驗值非常的接近；異構化反應經由六圓環過渡狀態結構的活化能為8.63kcal/mol，遠小於經由三圓環、四圓環及五圓環過渡狀態結構的反應，主要是由於過渡狀態幾何張力的影響；所得的產物有一級、二級及三級自由基，六圓環過渡狀態的產物為一級自由基，若再經由分子內氫轉移變為三級的自由基所需的活化能為18.61kcal/mol。
第二單元　將2-戊烷氧自由基的碳鏈改為不飽和的雙鍵，討論這個改變對於分子內氫原子轉移的影響，計算結果發現雙鍵系統的活化能並不如單鍵系統的結果般單純，單鍵系統的活化能高低和幾何環張力有很高的相關性，而雙鍵的部分存在著另外一個不可忽略的因素，那就是產物的相對能量高低，末端雙鍵的四圓環產物為類似丙烯自由基的穩定產物，因此活化能為20.89kcal/mol，比單鍵低；同理如果將雙鍵位置改為C3=C4，則六圓環的產物將會比較穩定，經由六圓環反應的活化能6.90kcal/mol也比單鍵低。
第三單元　討論2-戊烷氧自由基異構化反應的取代基效應，這一個部分將只討論經由六圓環過渡狀態的異構化反應，我們在接氧的碳上更換取代基，討論反應物對於活化能的影響；在離去氫的碳上更換取代基，討論產物對於活化能的影響。從計算的結果發現在接氧的碳上更換取代基活化能並沒有很顯著的改變，而在離去氫的碳上更換取代基活化能最高的為8.63kcal/mol最低的為3.77kcal/mol，有比較大的影響；另外從產物和活化能的相關度我們可以證明產物的穩定性對於活化能是有一定程度的影響的。
第四單元　將2-戊烷氧自由基(RO•)上的氧自由基換成氮自由基及硫自由基，討論R(NH)•和RS•經由四圓環、五圓環及六圓環轉移的情形，計算結果顯示R(NH)•的異構化反應活化能跟RO•的異構化反應活化能相近；而RS•的異構化反應活化能則和RO•的異構化反應活化能有比較多的不同，五圓環反應的活化能為17.05kcal/mol，比六圓環活化能17.07 kcal/mol低。
第五單元　這一個部分我們將綜合上面幾項不同因素的計算，討論影響2-戊烷氧自由基系列反應活化能高低的因素，我們認為在這一個系統中，影響異構化反應活化能的最大因素是過渡狀態幾何環張力，另外一個不可忽略的因素是產物間的相對穩定性，產物的相對能量越低，活化能也會相對降低。

Abstract
This thesis deals with the calculation of isomerization reactions of 2-pentoxy radical and its derivatives by density function theory. All of the local minimum structures are optimized with 6-31G and 6-311++G** basis set at the levels of HF and B3LYP. There are five sections rendered here.
Section 1: We study the intramolecular hydrogen transfer from five different positions of carbon of the 2-pentoxy. There are five paths of hydrogen transfer discussed, which include the transition structure of 3-, 4-, 5-, and 6-member ring. The results indicate that the rate constant of 6-member ring calculated at the B3LYP/6-311++G** level is very close to the experimental value. The rate constant is 1.25×105(s-1). The order of the energy barriers among five possible pathways are 3-, 4- >5- >6- member ring. The energy barrier of 6-member ring is 8.63 kcal/mol. The ring strain of the transition state surely dominates the energy barrier of the intramolecular hydrogen transfer.
Section 2: We study the double bond isomerization reactions at these series reactions. The ring strain of the transition state is not the only influence factor on the barrier of the double bond reaction. The second factor is the stability of each product. The pent-1-ene-4-oxy radical undergoes isomerization by 1,3-hydrogen shift via a lower energy barrier than that of 2-pentoxy radical. The energy barrier is 20.89 kcal/mol. The energy barrier of pent-2-ene-4-oxy radical isomerization via a six-member ring transition state, 6.9 kcal/mol, is also lower than the analogous process of 2-pentoxy radical.
Section 3: We study substitution effect of isomerization of 2-pentoxy radical. In this section, we only discuss isomerization proceeding through a 6-member ring. The lowest barrier of these reactions is 5-methyl-2-hexoxy radical. The energy barrier is 3.77 kcal/mol. We find that the relative energy of the product is an important factor relating to the barrier-height of these isomerization reactions.
Section 4: We study isomerization reactions of R(NH)•and RS•proceeding through 4-,5-, and 6-member ring. The reactions of R(NH)• are similar to the reactions of RO•, but the reactions of RS• are quite different. The barrier of the reactions of RS• via 5-member ring is lower than that via 6-member ring.
Section 5: We summarize two important factors on the barriers of isomerization reactions of alkoxy radicals and its derivatives. One is ring strain of the transition structure, and the other is the relative energy of the product.

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