单态卡宾与臭氧反应机理的量子化学研究

为了研究单态卡宾与臭氧反应机理,本文采用密度泛函理论Gaussian-3方法(G3B3)优化了反应物、中间体、过渡态和产物的几何构型。探讨了单态卡宾与臭氧反应可能途径,并通过频率分析对过渡态和中间体进行了验证,研究结果表明：单态卡宾与臭氧反应有两条反应通道,分别具有亲核反应和亲电反应特征,相对而言亲核反应通道较易发生,且为强放热反应。     

4-硝基咪唑A-带激发态结构动力学

通过共振拉曼光谱实验和量子化学计算的方法研究了4-硝基咪唑(4NI)A-带激发态衰变动力学.对4NI的振动光谱、紫外电子吸收光谱、荧光光谱和共振拉曼光谱进行了指认.在全活化空间自洽场法(CASSCF)/6-31G(d)计算水平下获得了单重激发态S1(nOπ*)和S2(ππ*)和势能面交叉点S1(nOπ*)/S2(ππ*)的优化几何结构和能量,分析了A-带共振拉曼光谱的强度模式特征,获得了短时结构动力学,并结合全活化空间自洽场法(CASSCF)理论计算结果确定了4NI在S2(ππ*)态衰变通道主要是S2,FC→S2,min(ππ*)→S0辐射弛豫.     

·C2H3+O2→HC·O+H2CO 的密度泛函理论研究

应用密度泛函理论研究了@C2H3+O2→HC@O+H2CO的反应机理.在DFT(B3LYP/6-31G*)水平上对反应过程中所有反应物、中间体、过渡态和产物的几何构型进行优化,通过频率振动分析确认中间体和过渡态.计算IRC反应路径的能量,分析了中间体的异构化过程和各主要原子的自旋密度.     

单重态二溴卡宾和甲醛环加成反应的量化研究

采用量子化学密度泛函理论,研究了单重态二溴卡宾和甲醛环加成反应的机理.在B3LYP/6-31G*基组水平上,优化得到了反应途径上反应物、过渡态、中间体和产物的几何构型;计算并考察了四种可能反应途径势能面上各驻点的构型参数、振动频率和能量;通过振动分析对过渡态和中间体构型进行了确认.计算结果表明,二溴卡宾和甲醛反应有四条反应通道,其中c反应通道(即0°-0°型)控制步骤的活化能仅为13.7 kJ·mol－1,反应容易进行.     

O~3＋NH→HNO＋O~2反应机理的量子化学研究

用密度泛函(DFT)的B3LYP方法(6－31++G^*^*)研究了臭氧与NH自由基反应的微观机理,优化得到反应途径上的反应物,过渡态,中间体和产物的构型,通过振动分析对过渡态和中间体进行了确认。对单点用QCISD(T)/6－31++G^*^*方法计算能量,同时进行零点能校正。研究结果表明：NH与O~3反应有两条不同的反应通道,且均表现为亲电反应特征,两条不同的反应均为强放热反应。     

O~3＋NH→HNO＋O~2反应机理的量子化学研究

用密度泛函(DFT)的B3LYP方法(6－31++G^*^*)研究了臭氧与NH自由基反应的微观机理,优化得到反应途径上的反应物,过渡态,中间体和产物的构型,通过振动分析对过渡态和中间体进行了确认。对单点用QCISD(T)/6－31++G^*^*方法计算能量,同时进行零点能校正。研究结果表明：NH与O~3反应有两条不同的反应通道,且均表现为亲电反应特征,两条不同的反应均为强放热反应。     

异氰酸(HNCO)自旋禁阻脱羰反应中的旋-轨偶合效应

用自洽场从头算方法确定了异氰酸自旋禁阻脱羰反应单、三态势能面的零级近似能量极小交叉点(在MRSDCI/6-31G的水平上)。首次将分子中原子近似与有效单电子旋-轨偶合算符结合,在自洽场从头算CI的基础上得到单、三态的旋-轨偶合矩阵元,并用微扰方法据此确定单、三态波函数的混合系数,从而证明在零级近似能量极小交叉点邻域内,必伴随有单、三态的混合,异氰酸脱羰反应过程为:单态(反应物)→混合态(能量极小交叉点邻域)→三态(产物)。将零极近似能量极小交叉点视为反应过渡态,由此算得反应的活化势垒为354.163kJ·mol~(-1),这与实验结果极好的一致。     

CH_3自由基和O（~3P）反应机理的量子化学研究

用分子轨道从头计算MP2（full）方法和密度泛函理论（DFT）中的B3LYP方法 研究了CH_3自由基和三线态O原子反应的微观机理，优化得到了反应途径上的反应 物、过渡态、中间体和产物的几何构型，通过振动分析对过渡态和中间体构型进行 了确认，在G3不平上计算了能量，同时用经典过渡态理论对该反应的绝对速率常数 进行了理论计算。研究结果表明：CH_3自由基与O（~3P）反应有四条不同的放热反 应通道，主反应通道为IM1→TS1→CH_2O + H，同时反应可彻底裂解生成CO, H_2 及H。     

β-羟基丙醛基态与激发态氢迁移反应的从头算研究

本文用从头算RHF和UHF方法在3-21G基组上研究了β-羟基丙醛基态和激发态分解为甲醛和乙烯醇的反应机理。优化得到了各反应途径的过渡态和中间体, 其结果为: 基态β-羟基丙醛经过一个六元环过渡态和一个氢键中间体形成产物, 反应属于氢迁移和断键的协同过程; 激发三态β-羟基丙醛的分解途径首先经过一个氢迁移六元环过渡态形成双自由基中间体, 然后该中间体的分解包括两条相互竞争的途径, 它们各自经过一个断碳碳键的过渡态和一个氢键激-基态配合物中间体而形成两类产物, 一类为甲醛的基态和乙烯醇的激发态, 另一类为甲醛的激发态和乙烯醇的基态。激发态反应的两条通道均属于先氢迁移后断键分解的分步过程, 且反应的第二步为速控步骤。计算结果表明, 激发态反应活化位垒都比基态的低。     

OH+C_2H_2N←C_2H_3+NO→OH+C_2H_2N反应机理的密度泛函理论研究

应用密度泛函理论研究了反应通道(a)C2H3+NO→CH3+NCO和(b)C2H3+NO→OH+C2H2N的反应机理.在B3LYP/6-31G(d)水平上优化了反应物、中间体、过滤态、产物的几何构型,通过频率分析确定了11个中间体和10个过渡态.所有的反应物、中间体、过渡态、产物都在CCSD/6-311++G(d,p)水平上进行了单点能较正.并讨论了反应的异构化过程.计算结果表明10是能量最低的中间体,比反应物的能量低308 479kJ/mol;过渡态1/3,2/5,3/4,4/8比反应物的能量高,其中3/4是能量最高的过渡态,比反应物的能量高91 894kJ/mol.通道(a)和(b)的理论放热值分别为111 059和96 619kJ/mol.     

ANION AND SOLVENT EFFECTS ON THE RATE OF REDUCTION OF TRIPLET EXCITED THIAZINE DYES BY FERROUS IONS

Abstract— The lifetimes of the triplet excited states of thionine and methylene blue were measured in aqueous and 50 v/v% aqueous acetonitrile solutions acidified with 0.01 N sulfuric or trifluoromethyl-sulfonic acid. The rate constants for reaction of the triplet excited dyes with ferrous ions were measured in the same solutions. The triplet lifetimes in the absence of added quenchers were insensitive to a change in acid from trifluoromethylsulfonic to sulfuric or to a change in solvent from water to 50v/v% aqueous acetonitrile (τ for triplet thionine ˜7.5 μs, τ for triplet methylene blue ˜4.5 μs). In contrast, the rate constant for reaction of the triplet dyes with ferrous ions increased by nearly a factor of 10 with a change in acid from trifluoromethylsulfonic to sulfuric. In solutions containing sulfate ions this reaction rate constant increased with increasing sulfate concentration and with a change in solvent from water to 50 v/v% aqueous acetonitrile. The results are discussed in terms of the possibility of association of the positively charged reactive ions with sulfate anions. Quenching of the triplet excited dyes by ferric ions or by ground state dye molecules was shown to be negligible at the concentration used for the ferrous ion quenching study.     

Toward an understanding of the furoxan-dinitrosoethylene equilibrium

The tautomerism of furoxan (1,2,5-oxadiazole-2-oxide) has been investigated by different computational methods comprising modern density functionals as well as single-reference and multi-reference ab initio methods. The ring-opening process to 1,2-dinitrosoethylene is the most critical step of the reaction and cannot be treated reliably by low-level computations. The existence of cis-cis-trans-1,2-dinitrosoethylene as a stable intermediate is advocated by perturbational methods, but high-level coupled-cluster calculations identify this as an artifact. In contrast to the analogous reaction in benzofuroxans, cis-cis-cis-1,2-dinitrosoethylene was found to be a transition state rather than a local minimum. Model potentials were used to explain the occurrence and the disappearing of transition states and local minima relative to the reaction of benzofuroxan. Low-lying triplet states that can be accessed due to spin-orbit coupling were investigated as taking part in alternative routes to a proposed singlet pathway. Barriers for rotations of the nitroso groups on the S(0) and T(1) surfaces are reported.     

THE FLAVIN-SENSITIZED PHOTOOXIDATION OF NUCLEOTIDES—II. THE PARTICIPATION OF THE FLAVIN TRIPLET STATE

Abstract— –A study has been made of the effects of a series of nucleotides upon the electronic excited states of lumiflavin in order to determine the mechanism of their flavin-sensitized oxidation. A hydrogen-abstraction mechanism is ruled out, because if the nucleotide acts as a reducing agent for the excited dye molecules, it should increase the rate of reduction of the dye when the irradiation is carried out in the absence of oxygen. However, each of the nucleotides studied was found to reduce the rate of anaerobic photoreduction. While oxidation by an intermediate species such as the dye 'moloxide' or singlet oxygen is not entirely ruled out, our evidence suggests that the initial reaction is between the nucleotide and the flavin triplet. This results in a loss of the triplet excitation energy and is a very efficient reaction, guanosine monophosphate shewing 36 per cent of the triplet quenching efficiency of potassium iodide. The relative rates of reaction of the nucleotides with the flavin triplet exactly parallels their quantum yields of sensitized photo-oxidation. The formation of ground-state complexes between flavin and nucleotide and the participation of the singlet excited state of the flavin are not considered to be important.     

Theoretical Studies on Reaction Mechanism of CH3SO with NO

The potential energy surface （PES） of CH3SO radical with NO reaction has been studied at MP2/6-311G（2df, p） and QCISD/6-311G（2df, p） levels. Geometries of the reactants, transition states （TS） and products were optimized at B3LYP/6-311G （d,p） level. The geometries of the transition states were found for the first time. The calculated results show that the reaction can proceed via singlet-state or triplet-state PES. Because of the high energy barrier of triplet surface, the singlet surface reactions are dominant. The topological analysis of electron density shows that there are two kinds of structaral transition states （the bifurcation-type ring structure transition state and the T-shaped conflict structure transition state） in the titled reaction. The total electronic density of the reactants, TS and products and the spin electronic density on the triplet surface were also discussed in this paper.     

THE FLAVIN SENSITISED PHOTOOXIDATION OF ASCORBIC ACID: A CONTINUOUS AND FLASH PHOTOLYSIS STUDY

Abstract —The mechanism of the photooxidation of ascorbic acid by flavin mononucleotide has been studied by steady state and flash photolysis techniques. The primary reaction consists of an electron abstraction from ascorbic acid by the flavin singlet and triplet states by a diffusional process and in the latter case also by the formation of a triplet flavin-ascorbic acid complex. Under anaerobic conditions, the secondary reactions consist primarily of a radical recombination and a 'dark'chemical back reaction, leading to the reformation of unchanged flavin and ascorbic acid. In the presence of oxygen, the 'back reactions'are prevented, resulting in the efficient photooxidation of ascorbic acid.     

THE REDUCTIVE PHOTOALKYLATION OF FLAVIN BY N-ALLYLTHIOUREA

Illumination of flavin in the presence of N-allythiourea (ATU) inhibits catalytic turnover of flavin between its reduced and oxidized redox states by adduct formation, the adduct being no longer reoxidisable by oxygen. The first step in the mechanism of adduct formation is an electron transfer from ATU to flavin in the photoexcited triplet state. In further steps, the ATU radical cation deprotonates, electronic rearrangement occurs and radical combination with the flavosemiquinone follows, yielding a cyclic product in which ATU is added in the 4a- and 5-position to the flavin chromophore.
Reaction rates and yields were determined by flash photolysis and continuous illumination. A photochemical study by variation of the molecular structure of ATU was undertaken to prove the proposed mechanism and to determine the structural requirements of flavin inactivators.     

TRANSIENT SPECIES IN THE PHOTOCHEMISTRY OF EOSIN*

Abstract— Photochemical reactions of eosin in aqueous solution were studied using the flash photolysis technique. In deaerated solution the dye was converted quantitatively to the triplet state during flashing. The triplet dye decayed by first and second order reactions which partly regenerated the dye in the ground state and partly produced semioxidized and semireduced eosin. These radical species were formed in an electron dismutation reaction between two triplet molecules and also in a reaction between one triplet and one unexcited molecule. The radicals recombine rapidly to give the dye in the ground state.
An efficient reversible photooxidation reaction was observed in eosin solutions containing potassium ferricyanide. Semioxidized eosin was formed in high yield by reaction between the triplet dye and the oxidant. The dye was regenerated rapidly in a reverse reaction between the products of the oxidation reaction.
An analogous type of reaction was found to occur in eosin solutions containing p -pheny-lene diamine. This reagent reduced the triplet dye to semireduced eosin; the dye was regenerated in the ground state in a very efficient reverse reaction. The protolytic behaviour of semireduced eosin was studied by varying the pH.
Absorption spectra of the transient products were determined and rate constants for the observed reactions were measured. The results are compared with results from previous studies of fluorescein.     

一类Narcissistic反应过渡态的优化

我们对一类narcissistic 反应XCH_2-CH_2Y→[过渡态TS]→YCH_2—CH_2X (1)进行了研究,结果表明其过渡态能在对称性限制下由能量极小化方法较容易地给出. 采用IMSPAK分子轨道从头算程序,在IBM VM/370计算机系统上用STO—3G极小基组以平衡几何构型的能量梯度优化方法,在对称性限制下对反应(1)的几种反应体系的过渡态构型进行了优化,其结果列于表1.     

THE INFLUENCE OF THE NITRO GROUP ON THE DISSOCIATIONS OF SOME AROMATIC ACIDS AND BASES IN THEIR LOWEST EXCITED TRIPLET STATES*

Abstract— –Estimation of lowest excited triplet and singlet state dissociation constants of some nitro-aromatic acids and bases, from shifts in their phosphorescence and absorption spectra, respectively, indicate that intramolecular charge transfer to the nitro group is much more important in the lowest excited singlet state than in the ground or lowest excited triplet states. As a result, the effect of a nitro group on the acidity of the lowest excited singlet state of an acid or base is more exaggerated than that on the ground or lowest excited triplet state of the same compound. Furthermore, the basicity of the nitro group is greatly enhanced in the lowest excited singlet state. On this basis the increased rate of photoreduction of nitrobenzene in acidic solutions is found to be thermodynamically unfeasible in the lowest excited triplet state. Although the reaction is thermodynamically feasible in the lowest excited singlet state, the short lifetime of that state may make the reaction kinetically unfeasible. Rate-Hammett acidity profiles are therefore inadequate to alone establish the mechanism of photoreduction of nitrobenzene.     

基态氧(3^O2)氧化硅烯的密度泛函计算研究

采用密度泛函[B3LYP(full)/6-311+G^*]方法研究了基态氧(3^O2)氧化硅烯(Si2H4)的机理。计算了三重态初始中间体(IM(T1)到单重态中间体IM2(S0)反应交叉势能面,报道了各反应中间体、产物和过渡态的构型和能量,用频率分析方法对各过渡态进行了验证,进一步用IRC方法对主要的基元反应进行了考察,确定了历经生成1,2-二氧环氧硅烷中间体的氧化过程的主要反应通道。