理論化學之研究：1. 由氨基丙二?生成甘胺酸之反應機制　2. 1,3-丁二烯與1,4-二氮-1,3-丁二烯進行共軛雙烯[四加二]環加成反應(Diels-Alder)時，位能曲面與分子軌域作用之關係

朱鴻舜

指導教授 ： 何嘉仁

國立臺灣師範大學/理學院/化學系/博士(2005年)

摘要

無資料

關鍵字

甘胺酸 ； 1,3-丁二烯

並列摘要

There are two major themes in this thesis. I. The Formation of Glycine from Amino-Malononitrile. The scientific study of the origins of life was born in the 1920’s when Oparin and Haldane put forth the idea that the origin of life could be understood in terms of plausible chemical and physical process occurring on the primitive Earth. Glycine is the smallest amino acid molecule in the protein structure. It could be synthesized by the simple inorganic compound in the chemical evolution. We investigated the mechanism from amino malononitrile to glycine and the molecular orbital interaction in some reaction. There are two sections regarding to the subject studied and rendered below.

Section 1
Ab initio theoretical calculation was carried out to study the hydrolysis of amino malononitrile. The proposed scheme was considered as one of the possible reaction paths that the simplest amino acid, glycine, may be synthesized by the nature. Several other probable schemes based on the potential reaction sites of amino malononitrile were also examined. The optimized structures of the species on the reaction potential energy surfaces in addition to the activation energies were calculated at both HF and MP2 levels. The basis set superposition error (BSSE) for the correction of calculated energy was also performed. It came out that one of the proposed reactions had the lower potential energy profile in the sequential processes to form the amino acetonitrile. Most of the calculated barriers in this scheme were below 60 kcal/mol. The first added H2O in the hydrolysis of amino malononitrile was calculated to be at lower barrier (49.00 kcal/mol) on attacking one of the nitrile group of amino malononitrile and successively forming an amide, rather than attacking on the amino group of amino malononitrile (82.24 kcal/mol). Further frontier orbital analysis also proved the same fact. The second H2O molecule was added to hydrolyze the forming amide and produced carboxylic acid, which then underwent decarboxylation to form amino acetonitrile. Direct decarboxylation needs around 61 kcal/mol to cross the barrier, the highest one in all the processes derived in Scheme 1. Of course, it may be assisted by the third molecule such as H2O to lower the barrier (around 20 kcal/mol). From the calculated low barriers the proposed processes in Scheme 1 may be considered as one of the acceptable mechanisms in prebiotic chemical evolution on the primitive earth.

Section 2
Ab initio theoretical calculations were carried out to study the hydrolysis of amino acetonitrile (NH2CH2CN) and amino-cyano-acetic acid (NH2(CN)CHCOOH). Each of the proposed schemes was considered to be one of the possible reaction paths by which the simplest amino acid, glycine, may be synthesized by nature. The optimized structures of the species on the potential energy surfaces were calculated at both the HF and MP2 levels. We found that the direct hydrolysis of the nitrile group of amino acetonitrile required a higher energy barrier (52.38 kcal/mol) compared to the barrier for the hydrolysis of amino-cyano-acetic acid (46.11 kcal/ mol). Our calculated potential energy profiles revealed that this glycine evolution would not occur as easily in an anhydrous atmosphere as in moist surroundings. (The difference in the barriers may be more than 30 kcal/mol.) Molecular orbital interaction between H2O and the amino acetonitrile was also studied, and we found that the crucial part of this hydrolysis process was the transfer of the hydrogen atom of H2O to the N atom of the nitrile group rather than the formation of the C-O bond between the O atom of H2O and the C atom of the nitrile group. The schematic processes with calculated lower energy barriers in the proposed schemes might be considered to be possible mechanisms in the prebiotic chemical evolution on the primitive earth.

II. MO Interaction Correlating to the Potential Energy Surface in the Diels-Alder Reaction of 1,3-Butadiene and 1,4-Diaza-1,3-Butadiene

B3LYP/6-311G** levels have been used to assess the relative energies of 24 different transition structures of 1,3-butadiene and 1,4-diazabuta-1,3-diene. Two competitive pathways leading to cycloaddition will be rationalized. In our study, most reactions prefer 1,3-butadiene acting the role of diene. The relative activation energy can be affected by the HOMO-LUMO secondary orbital interaction and steric repulsion of the transition state. Because the stabilization of most intermolecular MOs will counteract the destabilization caused by the structure distortion, the barriers will be mainly affected by the destabilization of some π-MOs of reactants. For this reason, a well overlap MOs of reactants would lead to a slightly higher barrier if these MOs were too close to each other and causing repulsion. However, if the gap between the MOs were so large that needs to distort the structure to increase the overlap of MO, this distortion would raise the barrier.

並列關鍵字

ab initio ； DFT ； Diels-Alder ； HOMO-LUMO secondary interaction

參考文獻

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