乙醇醛光解离机理的理论研究

采用多参考态方法, 在CASPT2//CASSCF/6-311+G(2df, 2p) 水平上计算了乙醇醛(HOCH2CHO)分子在三个最低电子态(S0、S1和T1)上驻点的电子结构和解离势能面。结合势能面交叉点,探讨了HOCH2CHO与波长有关的光解离机理,分析了可能的光解离产物。结果表明, 在实验光解波长240 – 400 nm的激发下,HOCH2CHO分子主要发生S1态上的解离反应或通过S0和S1态之间的振动相互作用驰豫到基态,随之发生基态解离反应。C-C键断裂生成基态光解产物HOCH2 (2A′)+ HCO (2A′)是最主要的反应途径;而在一定波长下,生成CH3OH + CO的基态协同反应、脱醛基氢及脱羟基通道都是能量上可行的反应途径。本文的计算结果和实验观察一致。     

HNCO→(HV)NH+CO势能面研究

用从头算方法, 在UHF/6-311G**和CIS/6-311G**水平上, 分别考察了异氰酸光解反应在基态(S0)、第一激发三态(T1)和第一激发单态(S1)势 能面上的解离曲线. 并找到了S1/T1, T1/S0, S1/S0势能面的交叉点, 给出了极小能量交叉点的特征, 在此基础上根据梯度最速下降原理确定了态-态跃迁后交叉点构型在低能态势能面的变化及归宿. 计算结果表明光解反应存在三条竞争反应途径, 从动力学观点看, 途径A对反应最为有利.     

N_2H体系的势能面及振动激发态的理论研究

本文采用含Davidson校正的多参考组态相互作用MRCI+Q方法以及aug-cc-pVQZ基组计算了N2H体系的基态与第一激发态近24000个从头算能量点,使用三次样条插值法构建了高精度的全域绝热势能面.基态势能面具有一个极小点以及高于极小点0.44 eV的鞍点.基于该势能面上发生的N(~4S)+NH(X~3∑~-)→H(~2S)+N_2(X~1∑_g~g)反应是一个无能垒的放热反应,放热量为6.172 eV,比过去的势能面更接近实验值(5.956 eV).基态与第一激发态势能面间存在明显的锥形交叉,说明两个态之间存在非绝热耦合.此外,采用Lanczos算法预测了基态N_2H分子的振动能级.     

环丁酮光化学反应中S1/T1/S0交叉点的重要作用

势能面间的交叉在光化学反应中起着重要的作用 ,是由激发态反应物到基态产物发生无辐射跃迁的机制 .在本文中 ,我们用 CASSCF和态平均 CASSCF方法分别对环丁酮光化学反应的势能剖面及 S1,T1和 S0三个势能面间交叉进行了研究 .结果发现 ,基态和三态产物的形成是通过 S1,T1和 S0三个势能面交叉于同一区域 (称为 S1/T1/S0交叉点 )这一有效途径完成的 .　　60年代末 ,实验 [1－ 9]发现环丁酮和其它烷基酮 ,如丙酮、环戊酮的光化学反应机理很不一致 .主要体现在 ,i)环丁酮 (n,π态 )的α解离发生在 S1态势能面上 ,而其它烷基酮 (n,π态 …     

环丙酮光解离和热异构机理的从头算研究

运用精确的量子化学计算方法CASSCF, B3LYP和MP2, 结合cc-pVDZ基组, 优化了环丙酮的基态和激发态势能面上的驻点结构, 计算了它们的相对能量. 在此基础上, 深入探讨了环丙酮光解离反应的机理. 在292～365 nm波长的光的激发下, 环丙酮被激发至S₁态, 最可能的初始过程是α C—C键断裂. 我们的理论研究发现, 在α C—C键断裂途径上, 存在基态和第一激发势能面的交叉点, 它在随后的反应过程中起着重要作用. 一方面可形成单态双自由基中间体, 然后发生另一个C—C键的断裂, 生成基态产物一氧化碳和乙烯. 另一方面, 经过S₁/S₀交叉点可以回到热的基态. 在这种情况下, 体系具有足够的能量, 克服基态途径上的势垒, 生成同样的基态产物乙烯和一氧化碳. 此外, 还对环丙酮基态异构化反应进行了理论研究.     

2-硝基萘激发态衰减动力学和光解通道的从头计算研究

采用密度泛函理论(DFT)和完全活化空间自洽场理论(CASSCF)计算方法,在CASSCF(10,10)/6-31G(d)//CASPT2(10,10)/Aug-cc-PVDZ计算水平下,研究了2-硝基萘(2NN)S1(ππ*)激发态的衰减动力学.获得了2NN及其亚硝酸酯异构体(ISO)的基态、低能激发态和势能面交叉点的优化结构和能量,以及异构化反应的过渡态结构和能垒;给出了S0,T1和S1态时ISO沿N—O键的势能曲线;描绘了激发态衰变路径图.结果表明,被激发至S1(ππ*)激发态后,2NN经S1T3,S1T2交叉点发生S1!T3,S1!T2系间窜越过程再经T2T1锥形交叉点发生T2!T1内转换过程,最后无辐射衰减到T1态.具体衰减路径可以描述为S1-FC-2NN!S1T3-MIN-2NN或S1T2-MIN-2NN!T3-MIN-2NN或T2-MIN-2NN!T2T1-MIN-2NN!T1-MIN-2NN.该条路径能垒较小,T1态瞬态物种形成效率较高,为2NN激发态衰变动力学中最重要的无辐射衰变通道.在S0,T1和S1态势能面上,2NN!ISO异构化反应的能垒高,跃迁概率十分低下,难以形成芳氧自由基(Ar O"),2NN紫外光解离效率很低.     

气相中Cu+活化N2O中N-O键反应机理的理论研究

采用密度泛函UB3LYP/6 311+G(2d)方法计算研究了Cu+在基态和激发态下与N2O的反应机理,全参数优化了反应势能面上各驻点的几何构型,用频率分析方法和内禀反应坐标(IRC)方法对过渡态进行了验证,并用UCCSD(T)/6 311G(2d,p)、单点垂直激发、Harvey等人的方法分别进行各驻点单点能校正,单重态和三重态反应势能面交叉点CP确定,最低能量交叉点(MECP)的优化及MECP处相应的自旋 轨道耦合常数(SOC)计算。计算结果表明,该反应为一步反应,SOC值为84.2 cm-1,比较大的SOC值说明了在势能面上CP点处的翻转能够有效的降低反应的活化能,降低活化能值为27.6kJ.mol,增加反应放热126.7kJ.mol,这在动力学和热力学上对反应是非常有利。     

硫代乙酰胺光化学反应机理的理论研究

用B3LYP, MP2和CASSCF方法, 采用cc-pVDZ和6-31++G**基组, 研究了硫代乙酰胺在基态和最低三态上消除硫化氢以及其它光解离反应, 并考虑了单个溶剂分子参与反应对质子迁移反应的影响, 得到了消除硫化氢反应的反应机理, 计算结果可以很好地解释实验结果. 进而用CASSCF方法计算了第一激发单态上的各驻点, 以及各交叉点. 计算结果表明, 在S1和T1态上发生除分子内转动以外的化学反应的可能性比较小, 当分子被激发到S2态上时, 将通过S2/S1交叉点到S1态, 在S1态上的分子有两条途径去活化, 通过S1/S0交叉点到热基态, 通过S1/T1交叉点系间窜越到T1态. 因而得出CH3CSNH2发生光解离反应的可能性不大. 基于此, 可将硫代酰胺结构引入蛋白或多肽中, 有望在不破坏分子整体结构的情况下对其进行光化学研究.     

ArF2体系势能面和振动光谱的理论研究

利用MP4方法和一个含键函数的大基组研究了ArF2 体系的精确从头算势能面 .该势能面上存在两个局部极小值 :1个T型极小值和 1个L型 (线性 )极小值 .第 1极小值是L型几何结构 ,所对应的Rm 和Vm 分别为 0 .3883nm和 - 1 1 9.6 2cm-1;第 2极小值即T型极小值所对应的Rm 和Vm 分别为 0 .34 86nm和 - 85 .93cm-1;在势能面上θ =6 1°处有一个鞍点存在 ,所对应的R和V分别为 0 .3843nm和- 6 1 .5 3cm-1.用自洽场_组态相互作用方法预测了ArF2 体系的振动束缚态能级 .结果表明 ,该势能面支持 2 7个振动束缚态 ,其振动基态是两个简并的L型振动态 .     

Au+(1S, 3D)与N2O(1Σ+)反应机理的理论研究

采用密度泛函B3LYP方法, O和N用6-311+G*基组, Au+用赝势基组(8s7p6d)/[6s5p3d], 研究了Au+(1S, 3D)离子和N2O(1Σ+)分子的反应机理. 报道了在基态单重态和激发三重态势能面上各反应物、中间体和过渡态的构型特征及能量. 结果表明, 两个主反应通道Au+(1S)+ N2O(1Σ+)→1NA-Complex-1→1NA-TS1→1NA-Complex-2→1NA-Crossing→[3OAuNN]+和Au+(1S)+ N2O(1Σ+)→1NB-Complex→1NB-Crossing→[AuNN(1Σ+)]++O(3P)都需经过反应交叉势能面, 出现“系间窜越”. 用内禀坐标单点计算垂直激发态的方法确定了势能面交叉点, 并用含时密度泛函TD-B3LYP方法进一步探讨了自旋翻转机理.     

Ab initio study on the oxidation of NCN by O (3P): prediction of the total rate constant and product branching ratios

The reaction of NCN with O is relevant to the formation of prompt NO according to the new mechanism, CH+N2-->cyclic-C(H)NN- -->HNCN-->H+NCN. The reaction has been investigated by ab initio molecular orbital and transition state theory calculations. The mechanisms for formation of possible product channels involved in the singlet and triplet potential energy surfaces have been predicted at the highest level of the modified GAUSSIAN-2 (G2M) method, G2M (CC1). The barrierless association/dissociation processes on the singlet surface were also examined with the third-order Rayleigh-Schr?dinger perturbation (CASPT3) and the multireference configuration interaction methods including Davidson's correction for higher excitations (MRCI+Q) at the CASPT3(6,6)/6-311+G(3df)//UB3LYP/6-311G(d) and MRCI+Q(6,6)/6-311+G(3df)//UB3LYP/6-311G(d) levels. The rate constants for the low-energy channels producing CO+N2, CN+NO, and N(4S)+NCO have been calculated in the temperature range of 200-3000 K. The results show that the formation of CN+NO is dominant and its branching ratio is over 99% in the whole temperature range; no pressure dependence was noted at pressures below 100 atm. The total rate constant can be expressed by: kt=4.23x10(-11) T0.15 exp(17/T) cm3 molecule(-1) s(-1).     

Channels to Singlet and Triplet Phenylcarbenes in Phenyldiazomethane: A CASSCF and MRCI Study

Diazo compounds such as phenyldiazomethane (C₆H₅C(H)N₂) exhibit intriguing phenomena including the ultrafast formation of singlet carbene and the excited‐state rearrangement reaction (RIES). In this work, we have used multi‐reference configuration interaction with single and double excitations (MRCI‐SD) and complete active space self‐consistent field (CASSCF) methods to study the photodissociation dynamics of C₆H₅C(H)N₂. The equilibrium structures, transition states in the lowest three electronic states (S₁, T₁, and S₀), and S₁/S₀ and T₁/S₀ minimum‐energy crossing points both in the Franck–Condon region and on the pathway of the CN bond dissociation have been optimized. On the basis of the calculated S₁, T₁, and S₀ potential energy surfaces, we have uncovered the most efficient pathways to the lowest singlet and triplet phenylcarbenes (C₆H₅CH) in irradiated C₆H₅C(H)N₂.     

Quantum chemical calculations of electronically excited states: formamide, its protonated form and alkali cation complexes as case studies

The properties of formamide, its protonated form and interaction complexes with lithium and sodium cations were studied in electronically excited singlet states by means of high-level multireference ab initio methods. The vertical excitation energies show a marked influence on protonation with particular large effects found for the O-protonated form as compared to neutral formamide. Complexation with Li⁺ and Na⁺ leads to a pronounced shift of the n_O–π^* state to higher energies while the π–π^* state moves in opposite direction. Geometry optimizations in the lowest excited singlet show strong geometrical effects leading to pyramidalization at the N and C atoms. The photodynamical simulations performed for formamide in the first excited singlet state show that the main primary deactivation path is CN dissociation with a lifetime of about 420?fs.     

Singlet and Triplet Excited States and Intersystem Crossing in Free‐Base Porphyrin: TDDFT and DFT/MRCI Study

Extensive time-dependent DFT (TDDFT) and DFT/multireference configuration interaction (MRCI) calculations are performed on the singlet and triplet excited states of free-base porphyrin, with emphasis on intersystem crossing processes. The equilibrium geometries, as well as the vertical and adiabatic excitation energies of the lowest singlet and triplet excited states are determined. Single and double proton-transfer reactions in the first excited singlet state are explored. Harmonic vibrational frequencies are calculated at the equilibrium geometries of the ground state and of the lowest singlet and triplet excited states. Furthermore, spin–orbit coupling matrix elements of the lowest singlet and triplet states and their numerical derivatives with respect to nuclear displacements are computed. It is shown that opening of an unprotonated pyrrole ring as well as excited-state single and double proton transfer inside the porphyrin cavity lead to crossings of the potential energy curves of the lowest singlet and triplet excited states. It is also found that displacements along out-of-plane normal modes of the first excited singlet state cause a significant increase of the 2|H_so|S₁>, 1|H_so|S₁>, and 1|H_so|S₀> spin–orbit coupling matrix elements. These phenomena lead to efficient radiationless deactivation of the lowest excited states of free-base porphyrin via intercombination conversion. In particular, the S₁→T₁ population transfer is found to proceed at a rate of ≈10⁷ s⁻¹ in the isolated molecule.     

Ultraviolet Photodissociation Dynamics of m-Bromofluorobenzene at around 240 nm?

The photodissociation dynamics of m-bromofluorobenzene has been experimentally investigated at around 240 nm using the DC-slice velocity map imaging technique. The kinetic energy release spectra and the recoiling angular distributions of fragmented Br(²P_3/2) and Br(²P_1/2) atoms from photodissociation of m-bromofluorobenzene have been measured at different photolysis wavelengths around 240 nm. The experimental results indicate that two dissociation pathways via (pre-)dissociation of the two low-lying ¹ππ^* excited states dominate the production process of the ground state Br(²P_3/2) atoms. Because of the weak spin-orbit coupling effect among the low-lying triplet and singlet states, the spin-orbit excited Br(²P_1/2) atoms are mainly produced via singlet-triplet state coupling in the dissociation step. The similarity between the present results and that recently reported for o-bromofluorobenzene indicates that the substitution position of the fluorine atom does not significantly affect the UV photodissociation dynamics of bromofluorobenzenes.     

Potential energy surfaces for the photodissociation H2O→O(1Dg + H2(X1Σ+g)

The minimum energy pathways for symmetrical dissociation of water into O(¹D_g + H₂(X¹Σ⁺_g) are calculated by the MRD Cl technique for various excited states of H₂O and possible mechanism for the photodissociation are discussed.     

Non-adiabatic unimolecular dissociation of polyatomic molecules. II. The thermal dissociation of nitrous oxide

The theory presented in part I of this series is applied to the non-adiabatic spin-forbidden thermal dissociation N₂O(¹Σ⁺)→N₂(¹Σ⁺_g)+O(³P) as a test case. The molecular model is multidimensional and includes all vibrational modes of the molecule. Specifically considered is the fact that the initial singlet state of N₂O is linear and the final triplet state is bent. The best available data are used for describing the intersection of singlet and triplet potential energy surfaces. Calculated microcanonical rate constants are averaged over Boltzmann distribution of energies and compared with k_co, the high-pressure rate constant deduced from experiment. The agreement between theory and experiment is satisfactory. Analysis of the calculations shows that the driving force for the N₂O dissociation is the flow of energy into the bending vibrations. This is because the bendings have very different equilibrium angles in the initial and final states.     

The photodissociation of H2S: A study of indirect photodissociation

We have performed calculations for the photodissociation of H₂S using surfaces constructed to test a model proposed by van Veen et al., in which the dissociation occurs via predissociated levels of the bound ¹B₁ excited state. Total Franck-Condon factors for the photodissociation and partial Franck-Condon factors for the product vibrational distributions are presented.     

Effect of histidine on the sensitized generation of singlet oxygen in the complexes with chlorophyll

The energies of the intermolecular interactions of an O₂ molecule in the ground and excited states with the electron-excited and non-excited model complexes of chlorophyll were calculated using the DFT, CASSCF, SA-CASSCF, MCQDPT2, and XMCQDPT2 methods. The activation energies of formation and dissociation of the oxygen complexes were estimated. The radiative electric dipole moments of (0 → 0) spin-allowed S → S, T → T, and spin-forbidden S → T transitions were calculated taking into account the spin-orbit coupling, and rate constants of nonradiative transitions that determine the generation and deactivation of the O₂ molecule (¹Δ_g) were evaluated. The effect of histidine on the probability of singlet oxygen generation sensitized by the model chlorophyll complex was considered in detail.     

Singlet Sigma: The "Other" Singlet Oxygen in Solution

The lowest excited electronic state of molecular oxygen, O₂(a^1-DL_g), is often called simply singlet oxygen. This singlet delta state is an acknowledged and well-studied intermediate in many solution-phase photosystems. However, the second excited electronic state of oxygen, O₂(b¹δ_g+), is also a singlet. It has recently become possible to monitor this singlet sigma state in solution, which, in combination with studies of the singlet delta state, contributes to a better understanding of a variety of general problems in chemistry.