5-氟胞嘧啶气相及水助质子转移异构化的理论研究

采用密度泛函B3LYP/6-311G**方法,对6种5-氟胞嘧啶异构体孤立分子的稳定性及质子转移引起的酮式-烯醇式、氨基式-亚胺式互变异构反应机理进行了计算研究,获得了零点能、吉布斯自由能及质子转移过程的反应焓、活化能、活化吉布斯自由能和速率常数等参数.计算结果表明,气相中烯醇-氨基式FC4是最稳定的异构体.分子内质子转移设计了FC1→FC2和FC1→FC6两条通道,分别标记为P(1)和P(2),各通道速控步骤的活化能和速率常数分别为155.9 kJ·mol-1,4.70×10-15 s-1和173.1 kJ·mol-1,1.41×10-18 s-1.水助催化时,相应通道P(3) 和P(4) 速控步骤的活化能和速率常数分别为51.0 kJ·mol-1,1.41×103 s-1和88.2 kJ·mol-1,4.53×10-3 s-1.可见,水分子的加入极大地降低了质子转移的活化能垒.另外发现,水分子参与形成协同的双质子转移机理比水助单质子转移机理更利于降低活化能垒.     

5-氯尿嘧啶质子转移异构化的密度泛函理论研究

采用密度泛函B3LYP/6-311+G**方法,对5-氯尿嘧啶分子内质子转移及水助催化质子转移引起的互变异构反应机理进行了计算研究,获得了互变异构过程的反应焓、活化能、活化吉布斯自由能和质子转移反应的速率常数等参数。计算结果表明,5-氯尿嘧啶无论是孤立分子还是一水合物,其双酮式CU1是最稳定异构体,由双酮式向烯醇式异构化找到3条通道(P1,P2,P3),各通道速控步骤的活化能分别为177.85、177.05和197.58kJ/mol。当水分子参与反应以双质子转移机理异构化时,活化能显著降低,各通道速控步骤的活化能依次降为66.24、69.36和77.85kJ/mol,有利于双酮式向烯醇式或酮醇式转变。计算结果还表明,氢键作用在增大5-氯尿嘧啶一水复合物稳定性、降低质子转移异构化反应活化能等方面起着重要作用。     

2,6-二巯基嘌呤质子转移异构化的密度泛函理论研究

采用密度泛函B3LYP方法, 在6-311＋G(d,p)基组水平上对2,6-二巯基嘌呤质子转移引起的硫醇式与硫酮式互变异构反应进行了计算研究, 获得了互变异构过程的反应焓、活化能、活化吉布斯自由能和质子转移反应的速率常数等性质. 计算结果表明, 2,6-二巯基嘌呤无论是孤立分子还是一水合物, 其二硫酮式R是最稳定异构体. 由二硫酮式通过分子内质子转移向二硫醇式异构化共有6条反应通道, 其主通道(1)速控步骤的活化能为139.1 kJ•mol^－1, 速率常数为2.16×10^－12 s^－1; 当水分子参与反应以双质子转移机理异构化时, 活化能显著降低, 有利于硫酮式向硫醇式转变, 其主通道(7)速控步骤的活化能为61.3 kJ•mol^－1, 速率常数为1.33×10 s^－1. 计算结果还表明, 氢键作用在增大2,6-二巯基嘌呤氢键一水合物稳定性、降低质子转移异构化反应活化能等方面起着重要的作用.     

黄嘌呤异构体质子转移异构化反应机理的理论研究

采用量子化学密度泛函B3LYP/6-31G(d,p)和M06-2X/6-311++G(d,p)方法对黄嘌呤两种酮式异构体X(1,3,7)与X(1,3,9)间质子转移引起的互变异构反应机理进行了计算研究,获得了异构化反应过程的反应焓﹑活化吉布斯自由能和质子转移反应的速率常数等参数。水相计算采用极化连续(PCM)模型。结果表明,由于可能的氢迁移顺序差异,分子内由X(1,3,7)向X(1,3,9)异构化可能共有16条反应通道,涉及11个中间体和20个过渡态,其主反应通道速控步骤的活化吉布斯自由能为183.10k J/mol,速率常数为5.17×10-20s-1,其余各通道速控步骤活化吉布斯自由能均较高,而且整体水溶剂效应不利于质子转移的发生。     

尿嘧啶水助质子转移反应机理的研究

用密度泛函理论,在B3LYP/6-311++G**计算水平下分别对尿嘧啶所有的气相、液相、过渡态和质子转移异构体的结构进行全优化,获得它们在气相和水相中的几何结构和电子结构,PGM反应场溶剂模型用于水相计算.结果显示:在气相和水相中,水参与反应降低了互变异构质子迁移的反应活化能,对互变异构质子迁移的反应起到催化作用,但...     

胞嘧啶…NO复合物结构与性质的理论研究

用密度泛函理论在BL3YP/6-311＋G*基组水平上对胞嘧啶…NO复合物体系进行了理论计算, 发现了6个能量极小的复合物. 其结合方式是NO的N或O原子与胞嘧啶的N—H键形成氢键, 最稳定的复合物的结合能为－9.65 kJ/mol. NO的N原子与胞嘧啶的结合具有更强的优势, N结合的复合物中NO的键长缩短, 而O结合的复合物中NO键长伸长. 同时, 对复合物的振动分析发现, 在胞嘧啶中所有的与NO结合的N—H键的伸缩频率下降, 而所有氨基的面内弯曲振动频率是增加的.     

5-甲基胞嘧啶-BH3复合物构型及异构化机理的理论研究

用B3LYP/6-311＋G(d)及MP2/6-311＋G(d)计算方法, 对5-甲基胞嘧啶与BH₃所形成复合物及异构化反应进行了研究, 获得了6种复合物及4个异构化过渡态. 在考虑基组重叠误差校正基础上, 得到了结合能及变形能等信息, 并用自然键轨道分析法讨论了其相互作用情况. 结果表明, BH₃与5-甲基胞嘧啶中N(3), O及N(4)相连形成稳定复合物a, b₁, b₂和c, 其结合能分别为104.58, 92.25, 63.49和43.41 kJ•mol^－1; 复合物a, b₁, b_2, c和d既可通过BH₃与5-甲基胞嘧啶不同部位结合直接生成, 也可通过BH₃整体迁移实现相互转化; 甲基化对复合物稳定性及相互异构化能垒无明显影响.     

三羟甲基氨基甲烷水杨醛类席夫碱质子转移异构化的理论研究

采用密度泛函(DFT)中的B3LYP方法,在6-311+G(d,p)基组水平上对三羟甲基氨基甲烷水杨醛席夫碱气相、水溶剂及甲醇溶剂中的分子内质子转移和衍生席夫碱的互变异构反应机理进行了计算研究,获得了反应焓、活化能、活化吉布斯自由能和质子转移反应的速率常数等参数.液相计算采用Onsager模型.结果表明,不论在气相、水溶剂还是甲醇溶剂中,三羟甲基氨基甲烷水杨醛席夫碱(L3=H,L5=H)烯醇亚胺式异构体R1和醌型的酮烯胺异构体P1可以共存,但以苯环型的烯醇亚胺式R1为主要形式.当由苯环型的烯醇亚胺R1向醌型的酮烯胺P1分子内质子转移时活化能较低,室温常压下反应容易进行.水和甲醇溶剂对异构化反应影响较小.当—NO2,—OMe取代生成衍生席夫碱时(L3=H,L5=NO2;L3=OMe,L5=H),结果表明,苯环型的烯醇亚胺式和醌型的酮烯胺式异构体都能共存,质子转移异构化反应的活化能垒也较低.     

二苯甲酰甲烷互变异构反应机理溶剂效应的理论研究

采用密度泛函B3LYP方法,在6-31+G(d,p)基组水平上对二苯甲酰甲烷质子转移引起的酮式-烯醇式互变异构反应机理进行了计算研究,获得了零点能、总能量、吉布斯自由能及质子转移过程的反应焓、活化能、活化吉布斯自由能和速率常数等参数.3种非质子溶剂中的优化和频率计算采用Onsager模型进行计算.计算结果表明,不论在气相还是3种溶剂中,二苯甲酰甲烷的烯醇式较酮式稳定,烯醇式向酮式气相转变需要较高的活化能垒,在不同极性的溶剂中,随着溶剂介电常数的增大,异构化反应活化能垒减小,反应速率常数增大.     

p-羟基苯丙酮酸互变异构反应机理溶剂效应的理论研究

采用密度泛函B3LYP方法,在6-31+G**基组水平上对p-羟基苯丙酮酸质子转移引起的烯醇式-酮式互变异构反应机理进行了计算研究,获得了互变异构过程的反应焓、活化能、活化吉布斯自由能和质子转移反应的速率常数等参数。Onsager反应场溶剂模型用于水相和甲醇相的计算,计算结果表明,p-羟基苯丙酮酸无论在气相还是水相、甲醇相中,其酮式异构体都是最稳定的存在形式,烯醇式异构体也会以一定量共存。气相中孤立分子内质子转移几何构型改变较显著,异构化反应需要较大的活化能,不利于发生质子转移,Onsager模型计算方法对质子转移反应的影响较小。     

Laser Flash Photolysis on Electron Transfer Reactions between 1,8-Dihydroxyanthraquinone with Adenine and Cytosine

Electron transfer （ET） reactions between 1,8-dihydroxyanthraquinone （DHAQ） and two DNA bases, adenine （A） and cytosine （C）, have been investigated in CH3CN/H20 solution with nanosecond time-resolved laser flash photolysis. After irradiation at 355 nm, the triplet DHAQ is produced via intersystem crossing and reacts with two nucleobases. ET processes for both reactions have been definitely identified, in which two bases play a significant role of electron donor. Based on the measured decay dynamics of various intermediates and the corresponding quenching rates, an initial ET process followed by a secondary proton-transfer reaction is suggested for both the overall reactions. By plotting the observed quenching rate against the concentration of two DNA bases, the bimolecular quenching rate constants are determined as 9.0-10s L/（mol.s） for the 3DHAQ＊＋C reaction and 3.3x10^8 L/（mol.s） for the 3DHAQ＊＋A reaction, respectively.     

Theoretical Studies on the Hydrogen Bond Transfer and Proton Transfer between Anamorphoses of the Dihydrated Glycine Complex

The conversion between anamorphoses of the dihydrated glycine complex was studied by means of B3LYP/6-31++G**. It was found that proton transfer was accompanied by hydrogen bond transfer in the process of conversion between different kinds of anamorphoses. With proton transfer, the electrostatic action was notably increased and the hydrogen-bonding action was evidently strengthened when the dihydrated neutral glycine complex converts into dihydrated zwitterionic glycine complex. The activation energy required for hydrogen bond transfer between dihydrated neutral glycine complexes is very low (6.32 kJ·mol-1); however, the hydrogen bond transfer between dihydrated zwitterionic glycine complexes is rather difficult with the required activation energy of 13.52 kJ·mol-1 due to the relatively strong electrostatic action. The activation energy required by proton transfer is at least 27.33 kJ·mol-1, higher than that needed for hydrogen bond transfer. The activation energy for either hydrogen bond transfer or proton transfer is in the bond-energy scope of medium-strong hydrogen bond, so the four kinds of anamorphoses of the dihydrated glycine complex could convert mutually.     

Theoretical Study of the Intramolecular Proton Transfer in the Tautomers of Cytosine Assisted by Water

Ab initio MP2 and DFT studies on the tautomers of cytosine and the related hydrated tautomers have been carried out. The ground‐state structures of four tautomers of cytosine and related transition states were fully optimized. The vibrational frequency analysis was performed on all the optimized structures. Detailed intrinsic reaction coordinate (IRC) calculations were carried out to guarantee the optimized transition‐state structures being connected to the related tautomers. We obtained the relative stability order for the tautomers of cytosine and the related hydrated tautomers. In the isolated and hydrated condition, the bond types of C(2) O(7) and C(4) N(8) greatly affect the stability of the cytosine tautomers. Moreover, we have explored the influence of the water molecules on the intramolecular proton transfer between the keto and enol forms of the cytosine tautomers. The first water molecule obviously decreases the isomerization activation energy for the monohydrated cytosine tautomers. It is shown that the isomerization energy barrier changes only a little when the second and third water molecules are added in the reaction loop. The solvent effects have an obvious influence on the proton‐transfer barrier of the isolated cytosine. However, the solvent effects seem to be insignificant for the isomerization energy barriers of the monohydrated, dihydrated and trihydrated cytosine. The water molecule in these complexes can be looked on as the explicit water. Therefore, the explicit water model may be more credible to explore the intramolecular proton transfer, in comparison with the PCM which is the implicit water model.     

HCO+和HOC+与C2H2气相质子转移反应的理论研究

在 HF,B3LYP和 QCISD(T)水平上研究了 HCO+和 HOC+与 C2 H2 的气相质子转移反应 .结果表明 :高级电子相关效应对研究质子转移过程极为重要 ;HCO+和 HOC+通过质子垂直进攻 C2 H2 的 π-键分别生成中间体 OC· HC2 H+2 和 CO· HC2 H+2 ,最终生成主要产物为π-质子化的乙炔 HC(H) CH+;对于质子转移反应 HOC+比 HCO+更为活泼 .计算结果与实验结果符合得较好 ,这将有助于理解星际化学以及燃烧化学中的质子转移过程     

吲唑、3-卤代吲唑互变异构反应机理的理论研究

The molecular structures of indazole and 3-halogeno-indazole tautomers were calculated by the B3LYP method at the 6-311G** level, both in the gaseous and aqueous phases, with full geometry optimization.The geometry and electronic structure of the tautomers of indazole, 3-halogeno-indazole and their transition states were obtained. The Onsager solvate theory model was employed for the aqueous solution calculations.The results of the calculation indicated that the N1-H form of the studied molecule is more stable than that of the N2-H form. The influences of the different 3-halogeno and solvent effects on the geometry, energy,charge and activation energy were discussed. The reaction mechanism of the tautomerization of indazole and 3-halogeno-indazole was also studied and a three-membered cyclic transition state of the tautomer reaction has been obtained.     

Theoretical Studies on the Proton Transfer Mechanism in Four-water Hydrated Glycine Complexes

The mp2/6-31++g**//b3lyp/6-31++g** method has been employed to study the proton transfer mechanism in 4H2O-glycine complexes(4W-G).Compared with bare glycine the four-water hydrated neutral glycine complexes(4W-GN) can turn into the corresponding zwitterionic glycine(4W-GZ) through proton transfer.The most stable conformation of 4W-GZ has a "double water-chain" structure and is more stable than its corresponding precursor 4W-GN by 0.97 KJ/mol though it is less stable than the most stable 4W-GN by 7.80 KJ/mol.It is a spontaneous reaction to form the most stable conformation of 4W-GZ,and the potential barrier is only 1.97 KJ/mol,so the probability of this reaction is very high and the most stable 4W-GZ may be detected in experiment or in the early stage of experiment.     

溶液中Zn2+与腺嘌呤异构体间相互作用的理论研究

用B3LYP/6-311++G**方法和PCM及Onsager模型研究了Zn2+与腺嘌呤异构体在溶液中的11种较为稳定配合物. 结果显示, 这些配合物在溶液中的稳定性顺序与气相中明显不同, 其结合位点表现出如下的规律性, 在亚氨基类配合物中, Zn2+与腺嘌呤的N7、N6位结合比与N1、N6位结合形成的配合物更稳定; 氨基类配合物中, Zn2+以"双齿"形式与腺嘌呤异构体上的氮结合时的优先顺序为(N3和N9)>(N7和N6)>(N1和N6). 研究表明, 不论气相还是溶液相, 孤立的腺嘌呤分子内的质子转移较困难, 结合Zn2+后也不能明显降低关键步骤的活化能; 结合Cu2+却能明显地降低气相中关键步骤的活化能, 但溶剂效应却不利于Cu2+引发腺嘌呤分子内的质子转移.     

Proton mobility in the solid inorganic hydrates of acids and acid salts

The method for the calculation of the proton-transfer frequency (n_t) and its activation energies (E_t) was suggested. Results of the calculations were presented. The experimental data on the activation energy of proton-containing group rotation and protonconductivity values for some hydrates of inorganic acids and acid salts were compared with the calculated ones.     

3-羟基对氧萘酮分子内激发态质子转移反应位能面和机理的研究

用MNDO和INDO-SCF-CI法研究了3-羟基对氧萘酮分子内激发态质子转移反应基态和激发态的位能面,求得相应能垒△E₁、△E₂和荧光移位,并对光化学反应机理进行了探讨.