臺灣博碩士論文加值系統

藉著密度泛函理論的研究，可發現黃嘌呤二聚物(X2)具有六種可能的結構。透過自然鍵性軌域的計算，可獲得超共軛的未定域化能量，而這個能量大小可作為判斷此六個嘌呤二聚物(X2)的相對穩定性。這些二聚物(X2)的穩定性主要是靠穩定酮基上氧之孤對電子的能力來決定，也就是氧上的孤對電子到氮氫鍵的反鍵結軌域間之超共軛未定域化的現象。除此之外，氮原子的重新混成現象也可用來驗證這些二聚物的氫鍵，但是這個混成現象會被超共軛效應所超越。
在氟取代的甲烷系統中，隨著氟取代數目的增加導致碳氫鍵中碳的混成軌域的s-軌域特徵也會增加。然而，這裡的重新混成效應的影響反而大於負超共軛效應的影響，而成為決定碳氫鍵的強度的主要因素。超共軛效應的削弱是可以被理解的，因為氟原子上的孤對電子主要是由碳氟鍵的負超共軛現象來穩定，相對來說碳氫鍵的負超共軛現象會減少，同樣地在氯取代和溴取代的甲烷中可以觀察到上述的現象。而對於單取代的鹵化甲烷如氟化甲烷、氯化甲烷以及溴化甲烷，其碳氫鍵長是由超共軛現象來決定，此現象在去氧核醣核酸中的鹼基對之發現相同。

Six possible conformations of Xanthine dimers (X2) has been found by density functional theory (DFT) studies. The relative stabilities of the six X2 dimers are determined by the magnitudes of hyperconjugative delocalization energies, which have been obtained by the natural bond orbital (NBO) calculations. The dimers’s stabilities are dominated by the ability to stabilize the lone-pair electrons on the carbonyl oxygen, via hyperconjugative delocalization from the oxygen’s lone-pair to the NH antibonding orbital, n����*NH. The rehybridization of the N-atom can be verified in these H-bonding but its effects are outweighed by the hyperconjugation effects.

In the fluorinated methanes, increase of the degree of fluorine substitution leads to increasing s-character of the carbon’s hybrid orbital in forming the remained C-H bond(s). This rehybridization effect outweighs the negative hyperconjugation in determination the strengths of C-H bond(s). The weakened hyperconjugation effect is understood since the lone-paired electrons on the F-atom(s) are predominantly stabilized by the negative hyperconjugation with the C-F fragments, which in return reduces their tendency of negative hyperconjugation interaction with the C-H bond(s). The same observation has been found for the chlorine and bromine analogues. For the mono-halogenation methanes, CH3X (X=F, Cl, Br), the CH bond lengths are dominated by the hyperconjugation effects as found in DNA base pairs.

摘要 I
Abstract II
致謝 III
目錄 IV
表目錄 VI
圖目錄 VII
Chapter I. Introduction 1
Chapter II. Theoretical Background 4
2-1 H-bonding and the natural bond orbital (NBO) approach19 4
2-2 H-bonding present in the Watson-Crick base pairing6 4
2-3 Hyperconjugation and Negative Hyperconjugation8-17 --- Stabilization of localized electron pairs in Lewis Structures 5
2-4 Development of the Negative Hyperature2b 6
2-4-1 The Origine 6
2-4-2 The Anionic Hyperconjugation 7
2-4-3 Hyperconjugation-dependent conformational energy 8
2-4-4 The work by Schleyer8-10 9
2-5 Quantization of the hyperconjugation within the NBO approach 9
2-6 Two types of H-bonding26 10
2-7. Theoretical studies of H-bonding 11
Chapter III. Methods of Calculations 13
Chapter IV. Results and Discussion 14
4-1 Structural information obtained from Stabilization Energy (SE)1a 14
4-1-1 Values of SE for dimers calculated for XX、TT and XT dimers 14
4-1-2 Values of SE and some spectroscopic parameters 19
4-2 Values of stabilization energies (SE) and the second order perturbation energies-lowering the energy of lone pair electrons 20
4-3 Stabilization energy and rehybridization 22
4-4 Rationalization of relative H-bonding strengths 23
4-4-1 The role played by the energy gap between electron donor-acceptor bond orbitals: E(i,j)=E(j)-E(i) in Equation 1 24
4-4-2 The role played by the orbital interaction energy: F(i,j) in Equation 1. 26
4-4-3 Tables 29
4-5 Negative Hyperconjugation and C—H bond strength---- Relative ability among halogens in strengthening the C—H bonding 31
4-5-1 Tables 36
Chapter V. Conclusion 38
References 39

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