A computational study of the cycloaddition of thiobenzophenone S-methylide to thiobenzophenone

The cycloaddition of thiobenzophenone S-methylide to thiobenzophenone, an experimentally well-known reaction, was studied, using (U)HF/3-21G* for finding stationary points and (U)B3LYP/6-31G*//(U)HF/3-21G* single-point calculations for energies. Some optimizations were performed by (U)B3LYP/ 6-31G* to check the reliability of the calculations. The comparison of the concerted pathways and stepwise reactions via C,C-biradicals and C,S-zwitterions showed that the formation of a tetraphenyl-substituted C,C-biradical and its ring closure to 4,4,5,5-tetraphenyl-1,3-dithiolane constitutes the energetically most probable pathway of product formation, despite the fact that the regioisomeric 2,2,4,4-tetraphenyl-substituted product is more favorable by 17 kcal mol(-1). Model calculations on bond dissociation energies showed that (U)B3LYP with various basis sets overestimates radical stabilization, whereas CBS-QB3 closely reproduced experimental values. Results with the BLYP functional are similar to those with B3LYP. The consequences of the overestimation of radical stability for the cycloaddition mechanism involving biradicals are discussed. Thiobenzophenone S-methylide, if not captured by a dipolarophile, dimerizes to 2,2,3,3-tetraphenyl-1,4-dithiane. Calculation disclosed likewise a tetraphenyl-substituted C,C-biradical as intermediate.     

Thioformaldehyde S-methylide and thioacetone S-methylide: an ab initio MO study of structure and cycloaddition reactivity

The mechanisms of cycloaddition of thioformaldehyde S-methylide and thioacetone S-methylide, as models for an alkyl-substituted ylide, to thioformaldehyde and thioacetone, as well as to ethene as a model for a C=C double bond have been studied by ab initio calculations. Restricted and unrestricted B3LYP/6-31G* calculations were performed for the geometries of ground states, transition structures, and intermediates. Although basis sets with more polarization functions were tested, the 6-31G* basis set was applied throughout. Single-point CASPT2 calculations are reported for analysis of the unsubstituted system. The stabilities of structures with high biradical character seem to be overestimated by DFT methods in comparison to CASPT2. The general trends of the results are independent of the level of theory. Thioformaldehyde adds to thioformaldehyde S-methylide without activation energy, and the activation energies for two-step biradical pathways to 1,3-dithiolane are low. C,S biradicals are more stable than C,C biradicals. The two-step cycloaddition is not competitive with the concerted cycloaddition. Methyl substitution in the 1,3-dipole and the dipolarophile does not change the mechanistic relationships. TSs for the concerted formation of the regioisomeric cycloadducts of thioacetone Smethylide and thioacetone were located. Concerted addition remains the preferred reaction. The reactivity of the C=S double bond is high relative to that of the C=C double bond.     

Aromaticity of the Bergman, Myers-Saito, Schmittel, and directly related cyclizations of enediynes

Aromaticity criteria (magnetic susceptibility exaltations, nucleus independent chemical shifts (NICS), and aromatic stabilization energy (ASE) evaluations) for enediyne and enyne-allene cyclizations evaluated at (UBS)-BLYP/6-31G* all agree that the degrees of cyclic electron delocalization of the benzenoid systems formed by the Bergman (3) and Myers-Saito reactions (5) are comparable to benzene. The reaction enthalpy differences between the parent cyclizations and their benzannelated analogues are not entirely due to disparities in gained ASE during the reactions. The alternative formation of fulvene biradicals is not accompanied by favorable aromatic effects.     

Theoretical study of some 1,3-substituted (1,3-diketonato)borondifluorides. Potential energy curve relevant to the barrier crossing reaction

Ab initio Hartree-Fock calculations utilising STO-3G, 3-21G^* and 6-31G^* basis sets have been performed on three neutral and highly polar molecules, (diformylmethine)borondifluoride, (acetylacetonato)borondifluoride and (dibenzylmethine) borondifluoride. The calculated and experimental structures are well correlated when using the HF/3-21G^* basis set, except for the structure parameters involving the boron atom. The HF/6-31G^* basis set does not improve the accuracy in structure calculations. The conformational analysis is in agreement with the experimentally observed C_2v symmetrical structures, where the boron atom is tetrahedrally coordinated. The calculations support a one-dimensional ground state barrier crossing reaction for (dibenzylmethine)borondifluoride, where the phenyl torsion is the most likely reaction coordinate. Both HF/6-31G^* calculations and the second-order Møller-Plesset correction with the 3-21G^* basis set suggest an activation energy of the ground state reaction of about 30 kJ mol⁻¹. The ground state barrier crossing reaction kinetics is evaluated by the Kramers theory. The calculated ground state parameters relevant to the barrier crossing reaction are compared with the experimentally observed excited state values.     

Photocyclization of iminyl radicals: theoretical study and photochemical aspects

The irradiation of acyloximes was studied by theoretical methods. CASPT2/6-31G*//CASSCF/6-31G* calculations, using an active space of 14 electrons in 11 orbitals, indicate that S2 should be the spectroscopic state, and its relaxation leads directly to N-O bond breakage due to coupling between the imine pi* and the sigma* N-O orbitals. Subsequent calculations at the B3PW91/6-31+G* level suggest that the resulting iminyl radicals are able to cyclize to the five- or six-membered ring, depending on the presence of a phenyl group as a spacer, a process that has been verified experimentally. The photochemical aspects of the more common five-membered ring formation, such as excited-state quenching, quantum yield, excited-state sensitizers, laser flash photolysis experiments, Stern-Volmer plot, and luminescence measurements, were investigated. These studies indicate that singlet and triplet excited states undergo the same reaction. Emission lifetimes of ca. tau = 10.6 micros for compound 11 are suggestive of triplet parentage, while no fluorescence was detected, in agreement with the computed MEP energy profile.     

Excited state proton transfer in guanine in the gas phase and in water solution: a theoretical study

Theoretical investigations were performed to study the phenomena of ground and electronic excited state proton transfer in the isolated and monohydrated forms of guanine. Ground and transition state geometries were optimized at both the B3LYP/6-311++G(d,p) and HF/6-311G(d,p) levels. The geometries of tautomers including those of transition states corresponding to the proton transfer from the keto to the enol form of guanine were also optimized in the lowest singlet pipi* excited state using the configuration interaction singles (CIS) method and the 6-311G(d,p) basis set. The time-dependent density function theory method augmented with the B3LYP functional (TD-B3LYP) and the 6-311++G(d,p) basis set was used to compute vertical transition energies using the B3LYP/6-311++G(d,p) geometries. The TD-B3LYP/6-311++G(d,p) calculations were also performed using the CIS/6-311G(d,p) geometries to predict the adiabatic transition energies of different tautomers and the excited state proton transfer barrier heights of guanine tautomerization. The effect of the bulk aqueous environment was considered using the polarizable continuum model (PCM). The harmonic vibrational frequency calculations were performed to ascertain the nature of potential energy surfaces. The excited state geometries including that of transition states were found to be largely nonplanar. The nonplanar fragment was mostly localized in the six-membered ring. Geometries of the hydrated transition states in the ground and lowest singlet pipi* excited states were found to be zwitterionic in which the water molecule is in the form of hydronium cation (H3O(+)) and guanine is in the anionic form, except for the N9H form in the excited state where water molecule is in the hydroxyl anionic form (OH(-)) and the guanine is in the cationic form. It was found that proton transfer is characterized by a high barrier height both in the gas phase and in the bulk water solution. The explicit inclusion of a water molecule in the proton transfer reaction path reduces the barrier height drastically. The excited state barrier height was generally found to be increased as compared to that in the ground state. On the basis of the current theoretical calculation it appears that the singlet electronic excitation of guanine may not facilitate the excited state proton transfer corresponding to the tautomerization of the keto to the enol form.     

Theoretical studies on the gas-phase pyrolysis of 2-phenoxycarboxylic acids: an ONIOM approach

Various ONIOM combinations-ONIOM(HF/6-31G*: PM3), ONIOM(B3LYP/6-31G*: PM3), ONIOM(MP2/6-31G*: PM3), and ONIOM(MP2/6-31G*: HF/3-21G)--were applied to investigate thermal decomposition mechanisms of four 2-phenoxycarboxylic acids (2-phenoxyacetic acid, 2-phenoxypropionic acid, 2-phenoxybutyric acid, and 2-phenoxyisobutyric acid) in the gas phase. All the transition states and intermediates of the reaction paths were optimized. The reaction pathway of four reactants yielding the phenol, CO, and the corresponding carbonyl compound was characterized on the potential energy surface and found to proceed stepwise. The first step corresponds to the elimination of phenol and the formation of alpha-lactone intermediate through a five-membered ring transition state, and the second step is the cycloreversion process of alpha-lactone intermediate to form CO and the corresponding carbonyl compound. The reaction pathway of latter three compounds to produce the carboxylic acid and phenol via a four-membered cyclic transition structure was also examined theoretically. Comparison with experiment indicates that the activation parameters for the fist reaction channel are accurately predicted at the ONIOM(MP2/6-31G*: HF/3-21G) level of theory.     

1. 5-环辛二烯-3, 7-二炔(C~8H~4)结构和光谱性质的理论研究

采用量子化学从头算方法研究了1,5-环辛二烯-3,7-二炔(C~8H~4)的结构和光谱性质,根据等键反应分析和自然键轨道方法研究了它的稳定性、成键情况和共轭性。结果表明1,5-环辛二烯-3,7-二炔(C~8H~4)分子为平面刚性结构,可能稳定存在。分子中C≡C键与C=C键存在一定程度的共轭,可能具有芳香性。     

2-甲基噻吩光异构化成3-甲基噻吩的理论研究

用CASSCF方法以6-31G基组研究了2-甲基噻吩光异构化为3-甲基噻吩的光化学反应和基态(S₀)及三重激发态(T₁)的相关势能面.反应主要发生在三重态(T₁)上,其间经历了两个双自由基,1个三元环中间体及4个过渡态.沿着反应路径找到了2个T₁/S₀势能面交叉点,其结构都类似于双自由基.在第二个T₁/S₀势能面交叉点附近由T₁向S₀的系间窜越(ISC)最为有利.     

异硫氰酸与甲亚胺环加成反应机理的理论研究

对异硫氰酸与甲亚胺形成四元或六元环产物的环加成反应进行了理论研究,结果表明,一分子异硫氰酸与一分子甲亚胺形成四元环(1:1)产物P1的反应(1)为经过一个两性离子中间体的分步反应,其中第二步为速控步骤,其活化热垒为107.86kJ/mol.此外,反应(1)的中间体还可与另一甲亚胺或异硫氰酸分子继续反应形成两个不同的六元环(1:2或2:1)产物P2或P3;这两个反应均为协同反应,其活化势垒分别为15.88和21.82kJ/mol.这些结果与当异硫氰酸酯与亚胺发生环加成反应时,只有类似于P2和P3的两种六元环产物生成的实验事实一致。     

二氧化钛分子电致发光激发特性研究

采用密度泛函B3P86方法优化得到了沿分子平面不同外电场作用下TiO₂分子的基态稳定构型, 在优化构型下利用杂化CIS (CI-Singles)-B3P86方法在6-311＋G*基组水平上, 研究了不同外电场下TiO₂分子前六个激发态的激发能和跃迁波长等激发特性. 研究结果表明, TiO₂分子多个激发态满足偶极跃迁定则, 跃迁光谱对应多个峰值, 在分子水平上, 可以增大利用太阳光的比例. 在外电场作用下, 能隙随电场的增大而减小, 电子易从最高占据轨道跃迁到最低空轨道形成空穴, 各个激发态跃迁波长均有随电场增大发生红移的趋势, 最长589 nm, 因而利用外电场可以控制材料的发光光谱范围在可见光区域扩展.     

Untangling the excited states of DR1 in solution: an experimental and theoretical study

We report the experimental observations and the theoretical predictions of the fully separated n-pi* and pi-pi* bands of Disperse Red One in acidified methanol solution. The analysis of the linear and two-photon absorption spectra is presented in four specific solvents. We demonstrate the possibility to establish the position of the first two excited states combining linear and nonlinear spectroscopy. Calculations using density functional theory at TD-B3LYP/6-31G*//HF/6-31G* level accurately predict the spectral region of the n-pi* and pi-pi* transitions of DR1 in all solvents.     

Kinetics of the multichannel reaction of methanethiyl radical (CH3S*) with 3O2

The CH3S* + O2 reaction system is considered an important process in atmospheric chemistry and in combustion as a pathway for the exothermic conversion of methane-thiyl radical, CH3S*. Several density functional and ab initio computational methods are used in this study to determine thermochemical parameters, reaction paths, and kinetic barriers in the CH3S* + O2 reaction system. The data are also used to evaluate feasibility of the DFT methods for higher molecular weight oxy-sulfur hydrocarbons, where sulfur presents added complexity from its many valence states. The methods include: B3LYP/6-311++G(d,p), B3LYP/6-311++G(3df,2p), CCSD(T)/6-311G(d,p)//MP2/6-31G(d,p), B3P86/6-311G(2d,2p)//B3P86/6-31G(d), B3PW91/6-311++G(3df,2p), G3MP2, and CBS-QB3. The well depth for the CH3S* + 3O2 reaction to the syn-CH3SOO* adduct is found to be 9.7 kcal/mol. Low barrier exit channels from the syn-CH3SOO* adduct include: CH2S + HO2, (TS6, E(a) is 12.5 kcal/mol), CH3 + SO2 via CH3SO2 (TS2', E(a) is 17.8) and CH3SO + O (TS17, E(a) is 24.7) where the activation energy is relative to the syn-CH3SOO* stabilized adduct. The transition state (TS5) for formation of the CH3SOO adduct from CH3S* + O2 and the reverse dissociation of CH3SOO to CH3S* + O2 is relatively tight compared to typical association and simple bond dissociation reactions; this is a result of the very weak interaction. Reverse reaction is the dominant dissociation path due to enthalpy and entropy considerations. The rate constants from the chemical activation reaction and from the stabilized adduct to these products are estimated as functions of temperature and pressure. Our forward rate constant and CH3S loss profile are in agreement with the experiments under similar conditions. Of the methods above, the G3MP2 and CBS-QB3 composite methods are recommended for thermochemical determinations on these carbon-sulfur-oxygen systems, when they are feasible.     

氰乙基对几种芳胺结构和光谱的影响

采用量子化学中密度泛函理论(DFT)的B3LYP方法分别用6-31G*和6-311+G*基组对苯胺、对氯苯胺和对甲苯胺及其氰乙基衍生物的几何构型进行全优化, 探讨了氨基上氰乙基的引入对分子电荷转移、前线轨道能量和电子光谱等性质的影响规律. 在此基础上采用含时密度泛函方法(TD-DFT)计算了分子第一激发态的电子跃迁能, 得到最大吸收波长λmax. 计算结果表明, 氨基上氰乙基的引入, 对前线分子轨道组成影响虽然小, 但使得最大吸收波长红移, 与实验值λmax有较好的一致性, 发现该类物质主要吸收光谱源于分子内的π→π*的电子跃迁.     

竹红菌甲素光敏活性的量子化学研究

利用密度泛函B3LYP/6-31G^*方法对茈醌类光敏剂竹红菌甲素hypocrellin A(HA)及其模型化合物的结构性质和分子内氢传递(IHT)过程进行了系统的计算研究,比较了各侧链对分子结构和IHT过程的影响.利用CIS/6-31G^*方法对HA的活性中心苝醌(PQ)及与其结构类似的一系列化合物激发态的IHT过程进行了研究.得到的主要结论包括:(1)常温下,处于基态的HA分子能够进行快速的分子内氢传递.(2)HA的几种模型化合物在基态时的IHT势垒、分子内电荷分布与HA差别很小,这说明侧链对IHT过程的影响不大.(3)HA模型化合物的IHT势垒与氧氢键键长变化和氢键键长变化呈良好的线性关系.(4)IHT反应中伴随着一个电荷分离过程,在氢传递过程中占主导地位的是静电相互作用,这说明此IHT过程本质上是质子转移过程.(5)共轭结构对该类分子的激发态IHT过程具有重要影响,总的趋势是对小共轭体系,单重激发态和三重激发态IHT反应的势能曲线分离,随着分子共轭结构的增大,两个激发态势能曲线逐渐接近,发生交叉的可能性以及交叉的程度也随之增大,即从单重态到三重态发生系间窜越的可能性也随之增大.由于分子内氢传递导致单重激发态和三重激发态势能曲线交叉,致使发生系间窜越,使体系三重态量子效率显著提高,从而提高了茈醌类光敏剂的光敏活性.这说明IHT过程对该类分子保持其光敏活性具有重要的意义.     

Photochemistry of heterocycles: VIII. photocycloaddition of acetylenic compounds to 1,3-dimethyl-6-azathymine

The photocycloadditions of 1,3-dimethyl-6-azathymine to 2-butyne-1,4-diol and 1,4-dimethoxyl-2-butyne have been studied. Three novel compounds, 7,8-bis(hydroxymethyl)-3,5-dioxo-2,4, 6-trimethyl-1,2,4-triazabicyclo [4,2,0]-7-octene (1), 7,8-bismethoxymethyl-3,5-dioxo-2,4,6-trimethyl- 1,2,4-triazabicyclo[4,2,0]-7-octene (2) and 8,9-bismethoxymethyl-4-oxo-1,3,5-trimethyl-7-oxa-2,3,5-triazaspiro[5,3]- 1,8-nonadiene (3) were obtained. The proposed reaction mechanism, which includes excited triplet complexes and biradicals as intermediates, was supported by kinetic and photophysical studies.     

The reaction of cyclopentyne with ethene: concerted vs stepwise mechanism?

The cycloaddition of cyclopentyne with ethene was examined using (U)B3LYP and CASSCF methods to discern the reaction mechanism. (U)B3LYP/6-31G* and (U)B3LYP/6-311+G* slightly favor the concerted pathway, whereas CASSCF(4,4)/6-31G* and CASCF(6,6)/6-31G* favor the diradical pathway. MRMP2 using the CASSCF(4,4) wave function also favors the diradical mechanism. In the context of a diradical pathway, the experimentally observed complete retention of stereochemistry for this reaction is understood in terms of stereochemical control resulting from dynamic effects.     

Ab initio and DFT calculations on the initial step in thiamin catalysis

The diphosphate ester (ThDP) of thiamin (vitamin B₁) is an important cofactor of enzymes within the carbohydrate metabolism. The initial reaction step shared by all ThDP-dependent enzymes is the deprotonation of the C2–H of the thiazolium ring. The replacement of the 4′-amino group by a hydroxyl one in the pyrimidine ring leads to the oxy-ThDP analogue which is known as an antagonist in thiamin catalysis.

Ab initio and DFT calculations on the MP2/6-31G* and B3LYP/6-31G* level were performed to study the proton relay function in thiamin and oxythiamin systems. Both MP2 and B3LYP calculations show significant differences of the reaction coordinate of the ylide formation in the systems. Tautomers, protonated and deprotonated species of both systems show different trends regarding their stability. The influence of correlation effects on the results is discussed by comparison with the HF-SCF/6-31G* data. Frequency calculations on the B3LYP/6-31G* level were performed to characterize the minima and transition state structures, respectively.     

可乐定分子构象异构和互变异构的理论研究

采用HF/6-31G*方法, 对氨基型和亚胺型可乐定中性分子以及质子化的可乐定分子进行了构象分析. 之后采用B3LYP/6-31G**方法对处于势能面上的能量极小构象异构体进行全自由度几何优化和频率分析. Onsager反应场溶剂模型用于水相的计算. 结果表明, 在气相和水相中可乐定中性分子主要以亚胺型存在, 这同已有实验结果一致. 进一步, 寻找构象异构化过渡态和氨基型亚胺型互变异构化过渡态, 探讨质子化和水溶剂化效应对异构化过程的几何结构和能量的影响. 为了考察氯苯的共轭效应对可乐定互变异构体稳定性的影响, 在B3LYP/6-31G**水平上, 研究了2-氨基-2-咪唑啉的互变异构化反应机理.     

DFT法研究3-羟基丙烯醛的双键旋转异构反应机理

利用密度泛函理论(DFT)分别在B3LYP/6-31G**和B3LYP/6-311＋＋G**的计算水平上优化了基态3-羟基丙烯醛分子在双键旋转异构反应过程中的平衡态以及过渡态的几何构型，分析了反应过程中键参数的变化，计算了该反应的内禀反应坐标(IRC)，发现在重排反应途径上存在一个四元环骨架的中间体.通过振动分析对平衡态和过渡态进行了确认，并得到了零点能.计算结果表明，基态3-羟基丙烯醛分子的双键旋转异构反应经过两步完成，第一步反应位垒稍高，第二步反应位垒较低，存在着发生重排反应的可能性.