Ca(~1S_0)+CH_2Cl_2、CHCl_3和Ca(~3P)+CH_2Cl_2、CHCl_3反应的动态学研究

在微观反应动力学领域,研究反应物内能和平动能对产物能态布居的影响具有重大的意义。前人已较详细地考察了反应物平动能、振动能及转动能对产物内能态布居的影响,但有关反应物电子态能量对基电子态产物内能态布居影响的文献报道甚少。因为在大多数电子激发态金属原子与卤化物或氧化物的反应中,既可以形成电子激发态产物,又可形成基电子态产物,而来自电子激发态产物的辐射不仅给基电子态产物的激光诱导荧光(LIF)探测造成困难,而且影响初生基电子态产物的能态布居。据此,本文选择了小化学发光截面的反应(Ca~3P)+CH_2Cl_2、CHCl_3,利用直流放电选态和LIF技术,研究电子态能量对基电子态产物内能态布居的影响。     

分子动力学模拟飞秒激光对C60光裂解反应机理的影响

基于半经典分子动力学方法模拟超快激光诱导的C60光裂解反应.选择能量为2.0eV,半峰全宽(FWHM)分别为40和500fs的激光作用于C60分子,调节脉冲强度使其发生裂解反应,比较长短脉宽飞秒激光对C60裂解反应机理的影响.通过分析产物分布、原子平衡指数、温度以及吸收能量(包括动能、势能和电子能量),证实飞秒激光脉冲下C60的光裂解主要由电子激发态控制,非热力学效应在该反应中起重要作用.激光场的作用时间和强度均影响光裂解反应过程,而电子吸收能量饱和后光场强度的作用则变得不明显.     

三氟化氯和水反应的密度泛函理论研究

应用量子化学密度泛函理论(DFT), 对ClF3和H2O在不同比例下的反应进行了研究. 在B3pw91/6-31++G(d,p)水平上优化了反应物、中间体、过渡态和产物的几何构型, 计算出它们的振动频率和零点振动能(ZPVE), 并对能量进行了校正. 计算结果表明, ClF3和H2O的反应能垒很低, 反应极易进行;水足量有利于生成HClO2, 少量有利于生成其它卤氧化合物.     

C~6F~6^+→C~6F~5^++F气相单分子分解及碰撞诱导分解 反应研究

用质量分析离子动能谱(MIKES)研究了C~6F~6^+→C~6F~5^++F的气相单分子分解及其与Ar和He的碰撞诱导分解(CID)反应。实验结果表明,C~6F~6^+在电离室中获得足够能量而被激发到某一长效激发态,而CID是诱导其分解的必要步骤,且该分解过程有两条能量不同的反应途径。当碰撞气体为Ar时,两条途径所对应的能量变化分别为0eV和+9.8eV(将多余能量转化为动能),而当碰撞气体为He时,则分别为0eV和-17eV(将多余能量转化为内能)。CID/Ar诱导该长效激发态在C-F键断裂之前将多余能量转化为动能,而在CID/He中则将多余能量转化为内能。     

碘苯在266 nm激光作用下的碎片平动能谱

利用通用型交叉分子束装置测量了 2 6 6nm激光作用下碘苯的光致碎片化过程中产物碘原子的角分布及平动能分布 ,结果表明在所研究的激光强度 ( 10 8W/cm2 )下单光子反应与多光子过程之间存在着竞争 ,单光子解离反应为一慢速的预解离过程 ,各向异性参数被确定为 0 4左右 ,质心系的平均平动能只有 4 35 1kJ/mol,只有约 2 5 %的反应可资用能配置为产物的平动能 .而多光子碎片化过程为快速过程 ,其中的二光子过程的平均平动能为 2 16 .0 6 2kJ/mol,占反应可资用能的 35 %左右 ,另外一个主要的电离解离通道是一个更快的多光子过程 ,其最大峰值对应着四或五光子过程 ,三个通道的分支比约为 3∶3∶4     

单次碰撞反应的Ba+o,m,p-C_6H_4Cl_2的实验研究及统计理论解释

本文利用激光诱导荧光(LIF)和分子束技术,在单次碰撞条件下,研究了反应Ba+o,m,p-C_6H_4Cl_2.实验得到了反应产物BaCl的LIF光谱和这些反应的相对反应截面。对LIF光谱进行计算机模拟,得到了产物BaCl的振动布居。文中利用“入口”和“出口”过渡态“松”和“紧”的概念,并假设“出口”过渡态的弯曲振动转化为产物转动后,剩余能量全部转化为产物BaCl的振动能,对反应Ba+o,m,p-C_6H_4Cl_2进行了解释。     

Ba(~1S)+N_2O反应的化学发光偏振研究

当原子或分子之间经历反应碰撞后,产物分子的转动角动量矢量相对于初始相对速度矢量具有较明显的取向。达一信息可从测量反应生成的激发态产物分子的化学发光偏振度P得到。Zare等首先研究了反应Ba(~1S)+N_2O产物的化学发光偏振;近期Jalink等利用六极电场将反应物分子取向,研究了反应Ba(~1S)+N_2O的化学发光偏振与不同碰撞方位的关系。本文研究了化学发光偏振度随Ba(~1S)束速度(即反应物相对平动能)的变化,根据产物转动角动量分布对反应物相对平动能的依赖关系得到了Ba(~1S)+N_2O反应的反应机理及势能面的新图象。     

离子速度成像方法研究溴代环己烷的紫外光解动力学

利用二维离子速度成像方法对C6H11Br分子在234 nm附近的光解动力学行为进行了研究. 通过(2+1)共振增强多光子电离探测了光解产物Br*(2P1/2)和Br(2P3/2), 得到它们的相对量子产率. 从光解产物Br*(2P1/2)和Br(2P3/2)的速度图像得到了能量和角度分布. 结果表明, Br*原子主要来自于S1态的直接解离, 而Br则绝大部分是从S2态向T3态的系间交叉跃迁得到, 并导致了两种解离通道能量分布的差别. 实验发现C6H11Br分子解离过程中大部分能量都转化为内能, 但与其它长链溴代烷烃分子相比, 可资用能更多地被分配到平动能中, 结合软反冲模型分析了这种能量分配跟环烷基的构象和稳定性的关系.     

三维H+H~2(v, j)→H~2(v', j')+H反应中复合态生成及产物转动 态分布的量子 散射理论研究

用排列通道线性组合-散射波函数(LCAC-SW,linearcombinationofarrangementchannels-scatteringwavefunction)量子反应散射方法计算了H+H~2(v,j)→H~2(v',j')+H三维态-态反应几率,分析了反应体系的复合态生成(或能量共振结构),并由产物的转动态分布解释了能量共振的起源来自于平动态-内态之间的干涉效应。     

三维H+H~2(v,j)→H~2(v', j')+H反应中复合态生成及产物转动 态分布的量子 散射理论研究

用排列通道线性组合-散射波函数(LCAC-SW,linearcombinationofarrangementchannels-scatteringwavefunction)量子反应散射方法计算了H+H~2(v,j)→H~2(v',j')+H三维态-态反应几率,分析了反应体系的复合态生成(或能量共振结构),并由产物的转动态分布解释了能量共振的起源来自于平动态-内态之间的干涉效应。     

Barrier Height Effect on Cl+H2(D2) Reaction

Three-dimensional time-dependent quantum wave packet calculation was performed to study the reaction dynamics of Cl+H2(D2) on two potential energy surfaces (CW PESs). The first CW PES is with spin-orbit correction; the second is without spin-orbit correction. The integral cross-section and reaction probability as a function of collision energy are calculated in the collision energy range of 0.1 eV to 1.4 eV. For reaction of Cl with D2, the reaction section with spin-orbit correction has a shift toward the high energy because the barrier height increases. As for the reaction of Cl with H2 at low collision energy, it is more reactive on the PES with spin-orbit correction than on the low barrier height PES without spin-orbit correction, due to the tunnel effect for the reaction of the Cl with H2. When the collision energy is higher than 0.7 eV, the reactivity on the low barrier height PES is larger than that on the high barrier height PES. It is believed that the barrier height plays a very important role in the reactivity of Cl with (H2, D2). For the Cl+H2 reaction the barrier width is also very important because of the tunneling effect.     

Use of a single trajectory to study product energy partitioning in unimolecular dissociation: mass effects for halogenated alkanes

A single trajectory (ST) direct dynamics approach is compared with quasiclassical trajectory (QCT) direct dynamics calculations for determining product energy partitioning in unimolecular dissociation. Three comparisons are made by simulating C(2)H(5)F-->HF + C(2)H(4) product energy partitioning for the MP26-31G(*) and MP26-311 + + G(**) potential energy surfaces (PESs) and using the MP26-31G(*) PES for C(2)H(5)F dissociation as a model to simulate CHCl(2)CCl(3)-->HCl + C(2)Cl(4) dissociation and its product energy partitioning. The trajectories are initiated at the transition state with fixed energy in reaction-coordinate translation E(t) (double dagger). The QCT simulations have zero-point energy (ZPE) in the vibrational modes orthogonal to the reaction coordinate, while there is no ZPE for the STs. A semiquantitative agreement is obtained between the ST and QCT average percent product energy partitionings. The ST approach is used to study mass effects for product energy partitioning in HX(X = F or Cl) elimination from halogenated alkanes by using the MP26-31G(*) PES for C(2)H(5)F dissociation and varying the masses of the C, H, and F atoms. There is, at most, only a small mass effect for partitioning of energy to HX vibration and rotation. In contrast, there are substantial mass effects for partitioning to relative translation and the polyatomic product's vibration and rotation. If the center of mass of the polyatomic product is located away from the C atom from which HX recoils, the polyatomic has substantial rotation energy. Polyatomic products, with heavy atoms such as Cl atoms replacing the H atoms, receive substantial vibration energy that is primarily transferred to the wag-bend motions. For E(t) (double dagger) of 1.0 kcalmol, the ST calculations give average percent partitionings to relative translation, polyatomic vibration, polyatomic rotation, HX vibration, and HX rotation of 74.9%, 6.8%, 1.5%, 14.4%, and 2.4% for C(2)H(5)F dissociation and 39.7%, 38.1%, 0.2%, 16.1%, and 5.9% for a model of CHCl(2)CCl(3) dissociation.     

Nucleophilic substitution at silicon (SN2@Si) via a central reaction barrier

It is textbook knowledge that nucleophilic substitution at carbon (SN2@C) proceeds via a central reaction barrier which disappears in the corresponding nucleophilic substitution reaction at silicon (SN2@Si). Here, we address the question why the central barrier disappears from SN2@C to SN2@Si despite the fact that these processes are isostructural and isoelectronic. To this end, we have explored and analyzed the potential energy surfaces (PES) of various Cl-+CR3Cl (R=H, CH3) and Cl-+SiR3Cl model reactions (R=H, CH3, C2H5, and OCH3). Our results show that the nature of the SN2 reaction barrier is in essence steric, but that it can be modulated by electronic factors. Thus, simply by increasing the steric demand of the substituents R around the silicon atom, the SN2@Si mechanism changes from its regular single-well PES (with a stable intermediate transition complex, TC), via a triple-well PES (with a pre- and a post-TS before and after the central TC), to a double-well PES (with a TS; R=OCH3), which is normally encountered for SN2@C reactions.     

Accurate ab initio potential energy surface, thermochemistry, and dynamics of the Cl(2P, 2P(3/2) + CH4 → HCl + CH3 and H + CH3Cl reactions

We report a high-quality, ab initio, full-dimensional global potential energy surface (PES) for the Cl((2)P, (2)P(3/2)) + CH(4) reaction, which describes both the abstraction (HCl + CH(3)) and substitution (H + CH(3)Cl) channels. The analytical PES is a least-squares fit, using a basis of permutationally invariant polynomials, to roughly 16,000 ab initio energy points, obtained by an efficient composite method, including counterpoise and spin-orbit corrections for the entrance channel. This composite method is shown to provide accuracy almost equal to all-electron CCSD(T)/aug-cc-pCVQZ results, but at much lower computational cost. Details of the PES, as well as additional high-level benchmark characterization of structures and energetics are reported. The PES has classical barrier heights of 2650 and 15,060 cm(-1) (relative to Cl((2)P(3/2)) + CH(4)(eq)), respectively, for the abstraction and substitution reactions, in good agreement with the corresponding new computed benchmark values, 2670 and 14,720 cm(-1). The PES also accurately describes the potential wells in the entrance and exit channels for the abstraction reaction. Quasiclassical trajectory calculations using the PES show that (a) the inclusion of the spin-orbit corrections in the PES decreases the cross sections by a factor of 1.5-2.5 at low collision energies (E(coll)); (b) at E(coll) ≈ 13,000 cm(-1) the substitution channel opens and the H/HCl ratio increases rapidly with E(coll); (c) the maximum impact parameter (b(max)) for the abstraction reaction is ~6 bohr; whereas b(max) is only ~2 bohr for the substitution; (d) the HCl and CH(3) products are mainly in the vibrational ground state even at very high E(coll); and (e) the HCl rotational distributions are cold, in excellent agreement with experiment at E(coll) = 1280 cm(-1).     

A significant role of the totally symmetric valley-ridge inflection point in the bifurcating reaction pathway

Appearance of the valley-ridge inflection (VRI) point on the intrinsic reaction path (IRP) introduces geometrical instability of the reaction coordinate and sometimes leads to two different product minima on the potential energy surface (PES). A significant role of the totally symmetric VRI point on the IRP is discussed from the viewpoint of branching of the reaction pathway. As illustrative examples, ab initio calculations were performed to determine the IRP for XCHO^??+?CH₃Cl (X?=?H, CH₃) at the M?ller?CPlesset second-order perturbation theory (MP2) level with 6-31+G(d) basis sets and geometric features of the PES around the IRP have been analyzed.     

New mechanistic insights to the O(3P) + propene reaction from multiplexed photoionization mass spectrometry

The reaction of O((3)P) with propene (C(3)H(6)) has been examined using tunable vacuum ultraviolet radiation and time-resolved multiplexed photoionization mass spectrometry at 4 Torr and 298 K. The temporal and isomeric resolution of these experiments allow the separation of primary from secondary reaction products and determination of branching ratios of 1.00, 0.91 ± 0.30, and 0.05 ± 0.04 for the primary product channels CH(3) + CH(2)CHO, C(2)H(5) + HCO, and H(2) + CH(3)CHCO, respectively. The H + CH(3)CHCHO product channel was not observable for technical reasons in these experiments, so literature values for the branching fraction of this channel were used to convert the measured product branching ratios to branching fractions. The results of the present study, in combination with past experimental and theoretical studies of O((3)P) + C(3)H(6), identify important pathways leading to products on the C(3)H(6)O potential energy surface (PES). The present results suggest that up to 40% of the total product yield may require intersystem crossing from the initial triplet C(3)H(6)O PES to the lower-lying singlet PES.     

Quasiclassical trajectory study of the Cl+CH4 reaction dynamics on a quadratic configuration interaction with single and double excitation interpolated potential energy surface

An ab initio interpolated potential energy surface (PES) for the Cl+CH(4) reactive system has been constructed using the interpolation method of Collins and co-workers [J. Chem. Phys. 102, 5647 (1995); 108, 8302 (1998); 111, 816 (1999); Theor. Chem. Acc. 108, 313 (2002)]. The ab initio calculations have been performed using quadratic configuration interaction with single and double excitation theory to build the PES. A simple scaling all correlation technique has been used to obtain a PES which yields a barrier height and reaction energy in good agreement with high level ab initio calculations and experimental measurements. Using these interpolated PESs, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations, and internal energy distributions has been carried out for the Cl+CH(4) and Cl+CD(4) reactions, and the theoretical results have been compared with the available experimental data. It has been shown that the calculated total reaction cross sections versus collision energy for the Cl+CH(4) and Cl+CD(4) reactions is very sensitive to the barrier height. Besides, due to the zero-point energy (ZPE) leakage of the CH(4) molecule to the reaction coordinate in the quasiclassical trajectory (QCT) calculations, the reaction threshold falls below the barrier height of the PES. The ZPE leakage leads to CH(3) and HCl coproducts with internal energy below its corresponding ZPEs. We have shown that a Gaussian binning (GB) analysis of the trajectories yields excitation functions in somehow better agreement with the experimental determinations. The HCl(v'=0) and DCl(v'=0) rotational distributions are as well very sensitive to the ZPE problem. The GB correction narrows and shifts the rotational distributions to lower values of the rotational quantum numbers. However, the present QCT rotational distributions are still hotter than the experimental distributions. In both reactions the angular distributions shift from backward peaked to sideways peaked as collision energy increases, as seen in the experiments and other theoretical calculations.     

Photofragment imaging study of the CH2CCH2OH radical intermediate of the OH+allene reaction

These velocity map imaging experiments characterize the photolytic generation of one of the two radical intermediates formed when OH reacts via an addition mechanism with allene. The CH2CCH2OH radical intermediate is generated photolytically from the photodissociation of 2-chloro-2-propen-1-ol at 193 nm. Detecting the Cl atoms using [2+1] resonance-enhanced multiphoton ionization evidences an isotropic angular distribution for the Cl+CH2CCH2OH photofragments, a spin-orbit branching ratio for Cl(2P1/2):Cl(2P3/2) of 0.28, and a bimodal recoil kinetic energy distribution. Conservation of momentum and energy allows us to determine from this data the internal energy distribution of the nascent CH2CCH2OH radical cofragment. To assess the possible subsequent decomposition pathways of this highly vibrationally excited radical intermediate, we include electronic structure calculations at the G3//B3LYP level of theory. They predict the isomerization and dissociation transition states en route from the initial CH2CCH2OH radical intermediate to the three most important product channels for the OH+allene reaction expected from this radical intermediate: formaldehyde+C2H3, H+acrolein, and ethene+CHO. We also calculate the intermediates and transition states en route from the other radical adduct, formed by addition of the OH to the center carbon of allene, to the ketene+CH3 product channel. We compare our results to a previous theoretical study of the O+allyl reaction conducted at the CBS-QB3 level of theory, as the two reactions include several common intermediates.     

Understanding the complex dissociation dynamics of energy selected dichloroethylene ions: neutral isomerization energies and heats of formation by imaging photoelectron-photoion coincidence

The dissociative photoionization of 1,1-C(2)H(2)Cl(2), (E)-1,2-C(2)H(2)Cl(2), and (Z)-1,2-C(2)H(2)Cl(2) has been investigated at high energy and mass resolution using the imaging photoelectron photoion coincidence instrument at the Swiss Light Source. The asymmetric Cl-atom loss ion time-of-flight distributions were fitted to obtain the dissociation rates in the 10(3) s(-1) < k < 10(7) s(-1) range as a function of the ion internal energy. The results, supported by ab initio calculations, show that all three ions dissociate to the same C(2v) symmetry ClC═CH(2)(+) product ion. The 0 K onset energies thus establish the relative heats of formation of the neutral isomers, that is, the isomerization energies. The experimental rate constants, k(E), as well as ab initio calculations indicate an early isomerization transition state and no overall reverse barrier to dissociation. The major high energy channels are the parallel HCl loss and the sequential ClC═CH(2)(+) → HCCH(+) + Cl process, the latter in competition with a ClC═CH(2)(+) → ClCCH(+) + H reaction. A parallel C(2)H(2)Cl(2)(+) → C(2)HCl(2)(+) + H channel also weakly asserts itself. The 0 K onset energy for the sequential Cl loss reaction suggests no barrier to the production of the most stable acetylene ion product; thus the sequential Cl-atom loss is preceded by a ClC═CH(2)(+) → HC(Cl)CH(+) reorganization step with a barrier lower than that of the second Cl-atom loss. The breakdown diagram corresponding to this sequential dissociation reveals the internal energy distribution of the first C(2)H(2)Cl(+) daughter ion, which is determined by the kinetic energy release in the first, Cl loss reaction at high excess energies. At low kinetic energy release, this distribution corresponds to the predicted two translational degrees of freedom, whereas at higher energies, the excess energy partitioning is characteristic of only one translational degree of freedom. New Δ(f)H(o)(298K) of 3.7, 2.5, and 0.2 ± 1.75 kJ mol(-1) are proposed for 1,1-C(2)H(2)Cl(2), (E)-1,2-C(2)H(2)Cl(2), and (Z)-1,2-C(2)H(2)Cl(2), respectively, and the proton affinity of ClCCH is found to be 708.6 ± 2.5 kJ mol(-1).     

An ab initio based full-dimensional global potential energy surface for FH(2)O(X(2)A(')) and dynamics for the F + H(2)O → HF + HO reaction

A global potential energy surface (PES) for the ground electronic state of FH(2)O is constructed based on more than 30 000 ab initio points at the multi-reference configuration interaction level. The PES features a pre-reaction van der Waals well and two post-reaction hydrogen-bonded complexes, as well as a "reactant-like" transition state with a classical barrier of 3.8 kcal∕mol. The adiabatic F + H(2)O → HF + OH reaction dynamics on this PES was investigated using a standard quasi-classical trajectory method. In agreement with experiment, the HF product contains significant vibrational excitation with limited rotational excitation, while the OH product is internally cold, reflecting its spectator role in the reaction. The products are primarily scattered in the backward direction, consistent with a direct abstraction mechanism.