尿酸质子转移反应的理论研究I——分子内质子转移研究

使用量子化学中的Hartree-Fock方法和密度泛函理论中的B3LYP方法，分别在3－21G^*和6－31G（d)水平上，计算了尿酸分子从三羰基异构体向三羟基异构体的转化。结果表明，转化过程经历了单羟基和双羟基异构体2种中间物和3种过渡态时的分子内质子转移（IPT），转移中的H原邻近的N，O和C原子形成了具有四元环结构的过渡态。随着IPT的进行，N－H键逐渐被削弱和断裂，O－H键则逐渐生成。3个反应的活化能分别为190.3kJ/mol,181.4kJ/mol和249.9kJ/mol(B3LYP/6-31G(d))。较高的活化能表明在室温下，无催化剂的IPT难以进行。     

2-[(4-氯苯基亚氨基)甲基]-8-羟基喹啉质子转移的理论研究

研究了2-[(4-氯苯基亚氨基)甲基]-8-羟基喹啉的三种质子转移途径: 分子内质子转移、水分子辅助质子转移和甲醇分子辅助质子转移. 以该席夫碱化合物的晶体结构作为模型, 在B3LYP/6-31＋G(d)水平上, 优化得到稳定态和过渡态的几何构型. 对三类质子转移前后的结构、能量、红外光谱、化学位移进行研究, 结果表明水分子辅助质子转移和甲醇分子辅助质子转移中, 水和甲醇分子利用氢键作用参与质子转移过程, 形成七元环状过渡态, 大大降低了反应的能垒, 有利于质子的转移, 氢键在降低活化能方面起着重要作用.     

N'-苯亚甲基-苯并酰肼气相高温分解反应机理的密度泛函研究

利用密度泛函理论研究了N'-苯亚甲基-苯并酰肼气相高温分解生成苯腈和苯甲酰胺的反应机理.首先用B3LYP/6-31G(d,p)方法优化反应中反应物、过渡态、中间体及产物的几何构型,然后通过振动分析确认了过渡态的结构,并通过内禀反应坐标方法(IRC)确认能量最低的反应途径.报道了3条可能的反应通道,包括1条直接协同高温分解反应和2条先成环后协同高温分解反应途径,其中直接协同高温分解反应由于能垒低.因此发生的几率较大.     

6-甲基-4-羟基嘧啶单体及二聚体质子转移过程的理论研究

应用密度泛函理论的B3LYP/6-311+G(d)方法研究了6-甲基-4-羟基嘧啶单体及二聚体质子转移的异构化反应.对反应势能面的研究发现,该化含物可能存在9种单体异构体,对其最稳定的单体构型进行分析.各单体间异构化反应的过渡态共有9种,反应的活化能最小为22.06 kJ/mol,最大为356.55 kJ/mol,最可能的反应路径在室温下即可进行. 研究了2种二聚体及其异构化反应的过渡态,发现二聚体均比其对应的单体稳定,而且质子转移所需要的活化能仅为20.13 kJ/mol,比单体低很多. 氢键在这种变化中起了主要作用,由单体和二聚体的总能量计算了氢键的键能.     

3,4,4-三甲基-1-乙酰基咪唑啉盐向邻苯二胺转移一 碳单元反应的理论研究

用PM3-MO半经验方法对叶酸辅酶模型化合物3,4,4-三甲基-1-乙酰基咪唑啉盐向邻苯二胺转移一碳单元的反应进行了理论研究。结果表明,咪唑环有两种开环方式,该反应可能通过两种途径实现,每一种途径都经历了六个反应步骤,包括两次质子转移步骤,其中第二次质子转移是限速步骤。优化计算了所有步骤的中间体和过渡态的结构,各个中间体和过渡态具有不同的构型,构象和能量。     

3,4,4-三甲基-1-乙酰基咪唑啉盐向邻苯二胺转移一 碳单元反应的理论研究

用PM3-MO半经验方法对叶酸辅酶模型化合物3,4,4-三甲基-1-乙酰基咪唑啉盐向邻苯二胺转移一碳单元的反应进行了理论研究。结果表明,咪唑环有两种开环方式,该反应可能通过两种途径实现,每一种途径都经历了六个反应步骤,包括两次质子转移步骤,其中第二次质子转移是限速步骤。优化计算了所有步骤的中间体和过渡态的结构,各个中间体和过渡态具有不同的构型,构象和能量。     

咪唑啉盐转移-碳单元反应的量子化学研究

用量子化学从头算与PM3相结合的方法对叶酸辅酶模型——2-甲基-1-乙酰基咪唑啉盐向邻苯二胺转移一碳单元的反应进行了理论研究.结果表明,由于咪唑啉环有两种开环方式,反应可以通过两条互相竞争的途径完成,得到相同的产物.每一条途径都经历了6个反应步骤,包括两种反应物的结合,质子转移,五元环的断裂,苯并咪唑衍生物的形成,再次质子转移和最终产物的生成.上述反应的每一条途径中,两次质子转移过渡态的能量较高,说明该反应存在一般酸碱催化作用,与酶催化甲酸氧化态一碳单元转移的实验结果一致.     

N-亚苯基氨基酰胺气相高温分解反应机理的密度泛函研究

利用密度泛函理论研究了N-亚苯基氨基酰胺气相高温分解生成苯腈和苯甲酰胺的反应机理。首先用B3LYP／6-31G(d,p)方法优化反应中反应物、过渡态、中间体及产物的几何构型,通过振动分析确认了过渡态的结构,并通过内稟反应坐标方法(IRC)确认能量最低的反应途径。本文报道了三条可能的反应通道,包括一条直接协同高温分解反应和两条先成环后协同高温分解反应途径,其中直接协同高温分解反应由于能垒低,因此发生的几率较大     

N-亚苯基氨基酰胺气相高温分解反应机理的密度泛函研究

利用密度泛函理论研究了N-亚苯基氨基酰胺气相高温分解生成苯腈和苯甲酰胺的反应机理。首先用B3LYP／6-31G(d,p)方法优化反应中反应物、过渡态、中间体及产物的几何构型,通过振动分析确认了过渡态的结构,并通过内稟反应坐标方法(IRC)确认能量最低的反应途径。本文报道了三条可能的反应通道,包括一条直接协同高温分解反应和两条先成环后协同高温分解反应途径,其中直接协同高温分解反应由于能垒低,因此发生的几率较大     

单嘧磺隆除草剂的晶体构象-活性构象转换的密度泛函理论研究

应用密度泛函理论在B3LYP/6-31G(d)水平上对新型除草剂单嘧磺隆绕脲桥部分两个C—N键的内旋转势能面进行计算, 然后对势能面上的驻点进行构型优化和过渡态搜索, 得到单嘧磺隆4种稳定构象和构象转换过程所涉及的8个过渡态结构. 研究结果表明, 单嘧磺隆晶体构象-活性构象转换过程中涉及4种稳定构象和8条转换途径, 脲桥部分NH基团与嘧啶环上N原子所形成的分子内氢键对于构象的稳定性及转换过程起着十分重要的作用. 应用极化连续介质溶剂模型(PCM)在B3LYP/6-31＋＋G(d, p)水平下进行溶剂化效应计算, 结果表明单嘧磺隆从晶体构象转换成活性构象主要是在水相中进行的.     

Computational analysis of some possible initial steps in the unimolecular thermal decompositions of 1,3-diazacyclobutane and its 1,3-dinitramine derivative

Summary We have investigated some possible initial steps in the unimolecular thermal decompositions of 1,3-diazacyclobutane and its 1,3-dinitramine derivative, the latter being selected as the simplest example of a symmetric cyclic nitramine. Vibrational analyses were used to identify normal modes that, in the extreme limits, would correspond to bond rupture and molecular decomposition. The energy requirements for ring fragmentation and N-N bond-breaking were computed at the MP4/6-31G level, using SCF 3-21G optimized structures. It was concluded that ring-fragmentation is a probable initiating step in the decomposition of the unsubstituted molecule, and that it is roughly competitive with N-N bond scission for the dinitramine. The nitronitrite rearrangement is predicted, on the basis of SCF calculations, to be less likely than either of the other two processes. It is proposed that N-N bond-breaking may be of primary importance for nitramine stability, but that energetic performance may be determined more by decomposition pathways having energy barriers.     

Experimental and theoretical evidence of basic site preference in polyfunctional superbasic amidinazine: N(1),N(1)-dimethyl-N(2)-beta-(2-pyridylethyl)formamidine

The gas-phase basicity (GB) of the flexible polyfunctional N(1),N(1)-dimethyl-N(2)-beta-(2-pyridylethyl)formamidine (1) containing two potential basic sites (the ring N-aza and the chain N-imino) is obtained from proton-transfer equilibrium constant measurements, using Fourier-transform ion-cyclotron resonance mass spectrometry. Comparison of the experimental GB obtained for 1 with those reported for model amidines and azines indicates that the chain N-imino in the amidine group is the favored site of protonation. Semiempirical (AM1) and ab initio calculations (HF, MP2, and DFT), performed for 1 and its protonated forms, confirm this interpretation. These results are in contrast to those found previously for N(1),N(1)-dimethyl-N(2)-azinylformamidines (containing the amidine function directly linked to the azinyl ring), in which the ring N-aza is the most basic site in the gas phase. The separation of the two potential basic sites in 1 by the ethylene chain interrupts the resonance conjugation between the two functions and changes their relative basicities and, thus, the preferable site of protonation. It also increases the chelation effect against the proton and the gas-phase basicity of 1 in such a magnitude that consequently 1 may be classified as a superbase (GB = 241.1 kcal mol(-)(1)). A transition state corresponding to the internal transfer of the proton (ITP) between the ring N-aza and the chain N-imino in 1 is investigated at the DFT(B3LYP)/6-31G level. The energy barrier calculated for the ITP between the two basic sites is small and vanishes when zero-point vibrational terms and thermal corrections are applied to obtain the enthalpy or Gibbs energy of activation for the proton transfer. Additional calculations at the DFT(MPW1K)/6-31G level confirm this behavior. This indicates that the quantum-chemical ITP in 1 has a single-well character. The proton is located on the N-imino site, and the H-bond is formed with the N-aza site.     

Theoretical investigation of unimolecular decomposition channels of furan4

Density functional theory and high-level ab initio calculations were carried out to investigate three unimolecular decomposition channels of furan. All equilibrium and transition state structures along the proposed decomposition channels are fully optimized by B3LYP/6-31G** and characterized at the same level of theory by vibrational and intrinsic reaction coordinate analyses. Relative energies of the optimized structures were evaluated at theoretical levels up to QCISD(T)/6-311++G**. The theoretical results suggest that the unimolecular decomposition channel of isoxazole, proposed in an experimental study and implied to be the main decomposition channel of furan, is responsible only for the formation of HC(TRIPLE BOND)CH and H₂O(DOUBLE BOND)C(DOUBLE BOND)O, minor products of furan thermal decomposition. A new decomposition mechanism, proposed in the present study, is shown to be more likely responsible for the formation of CH₃C(TRIPLE BOND)CH and CO, major products of furan thermal decomposition. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 240–249, 1998     

氧负离子自由基与苯的反应机理研究

采用密度泛函理论研究了氧负离子自由基与苯分子的反应. 针对可能存在的抽氢反应、置换氢反应、生成水反应及羟基化反应通道, 在B3LYP/6-31+G(d, p)的水平上得到了反应物、产物、中间体复合物和过渡态的优化构型、振动频率以及能量, 证实了这个反应的多产物通道性质, 较完整地描述了反应的机理和产物生成途径. 并且利用G2MP2能量计算结果, 从反应途径的能垒高度定性说明了实验观测结果的合理性, 同时也解释了以往实验的矛盾结果.     

2-羟基吡啶质子转移过程的理论研究

采用量子化学中的密度泛函理论,在B3LYP/6-31G(d)基组水平上,计算并考察了2-羟基吡啶分子醇式结构和酮式结构进行结构互变的质子转移过程中的4种可能途径:(a)分子内质子转移,(b)水助催化质子转移,(c)同种二聚体双质子转移和(d)异种二聚体间双质子转移.计算结果表明,途经c所需要的活化能最小(2.6 kJ•mol－1,逆反应则为27.1 kJ•mol－1),而过程a所需要的活化能最大(137.2 kJ•mol－1),途径b和d的活化能居中间(分别为38.7和17.3 kJ•mol－1).研究还表明,氢键在降低反应活化能方面起着重要的作用.     

Dihydrogen trioxide clusters, (HOOOH)n (n = 2-4), and the hydrogen-bonded complexes of HOOOH with acetone and dimethyl ether: implications for the decomposition of HOOOH

Hydrogen-bonded gas-phase molecular clusters of dihydrogen trioxide (HOOOH) have been investigated using DFT (B3LYP/6-311++G(3df,3pd)) and MP2/6-311++G(3df,3pd) methods. The binding energies, vibrational frequencies, and dipole moments for the various dimer, trimer, and tetramer structures, in which HOOOH acts as a proton donor as well as an acceptor, are reported. The stronger binding interaction in the HOOOH dimer, as compared to that in the analogous cyclic structure of the HOOH dimer, indicates that dihydrogen trioxide is a stronger acid than hydrogen peroxide. A new decomposition pathway for HOOOH was explored. Decomposition occurs via an eight-membered ring transition state for the intermolecular (slightly asynchronous) transfer of two protons between the HOOOH molecules, which form a cyclic dimer, to produce water and singlet oxygen (Delta (1)O 2). This autocatalytic decomposition appears to explain a relatively fast decomposition (Delta H a(298K) = 19.9 kcal/mol, B3LYP/6-311+G(d,p)) of HOOOH in nonpolar (inert) solvents, which might even compete with the water-assisted decomposition of this simplest of polyoxides (Delta H a(298K) = 18.8 kcal/mol for (H 2O) 2-assisted decomposition) in more polar solvents. The formation of relatively strongly hydrogen-bonded complexes between HOOOH and organic oxygen bases, HOOOH-B (B = acetone and dimethyl ether), strongly retards the decomposition in these bases as solvents, most likely by preventing such a proton transfer.     

Ab initio方法研究CH~3+OClO反应的可能通道

利用abinitio方法研究了CH~3+OClO反应的三个可能通道,首次应用UMP2(full)/6-31G(d,p)方法得到各反应物、产物、中间物及过渡态的优化构型和谐振频率;然后采用G2MP2理论计算各通道反应焓变和势垒高度。理论计算表明产物通道CH~2O+HOCl是最可能发生的途径,反应放热为443.80kJ·mol^-^1。可能的反应过程为：CH~3和OClO自由基先经无垒过程生成了一个富能中间物,继而通过较低的势垒解离成HOCl+H~2CO。     

ESIPT和TICT荧光发射的电子结构特征及发射能计算

采用含时密度泛函理论(TDDFT)与单激发组态相互作用(CIS)相结合的计算方案对八种结构相似的水杨酰苯胺衍生物及其类似物第一激发单重态(S1)进行考察, 证实它们的荧光发射分属分子内质子转移(ESIPT)和分子扭转-电荷转移(TICT)两种不同机制且结论与已知实验事实相符. ESIPT发光的化合物在电子跃迁前后无明显的电荷转移发生, 发射能计算的适用泛函是OLYP和BLYP等无Hartree-Fock(HF)交换成分的纯泛函; TICT发光的化合物在电子跃迁前后发生明显的电荷转移, 其适用泛函为含约37% HF交换成分的混合型泛函(例如mPW1B95和MPW1K). 按上述原则来选择适用泛函, 即可在TDDFT/6-31G(d)//CIS/3-21G(d)理论水平上正确预测水杨酰苯胺衍生物和类似物的发射能, 平均精度可达0.2 eV. 兼具质子转移与电荷转移双反应通道的化合物, 两者的竞争遵从能量最小原理, 结果使荧光发射仅选择其中一个通道进行. 泛函的选择只与实际发生的反应有关, 与并未实际发生的反应通道无关. 附加的八个算例进一步表明, 此成功的计算方案可望推广应用于其它类型的ESIPT和TICT荧光有机物.     

过氧仲丁基自由基内氢转移及解离机理的理论计算研究

采用量子化学从头算MP2方法在6-31G(d)基组水平上系统地研究了过氧仲丁基自由基内三个不同碳位上的氢原子向O—O基的端O转移的过渡态结构, 讨论了这些氢转移反应过程中相关化学键的变化, 并在此基础上进一步研究了氢转移产物解离的过渡态结构. 结果表明具有四元环、五元环、六元环结构的三个氢转移过渡态对应的氢转移反应位垒分别是195.86, 176.43, 122.49 kJ•mol－1; 相应的氢转移产物进一步解离的反应位垒分别是3.53, 126.23, 154.2 kJ•mol－1, 对应的最终产物分别为HO自由基和丁酮, HO2自由基和丁烯, HOOCHCH3和乙烯.