非过渡元素原子d轨道的σ成键作用

非过渡元素原子d轨道是否参加σ成键,在理论上存在两种截然不同的看法.本文介绍了这两种观点及其对共价化合物σ成键作用的处理,即共振模型、三中心键模型、原子参加化合时影响d轨道状态的四个因素,并对d轨道是否参加σ成键进行了讨论.     

共价键稳定性的影响因素

虽然小分子中的共价键强度可以很方便地通过高斯计算而相当准确地得到,一些手册和数据库中也可以直接查出部分键能/离解能数据,但共价键的强弱变化的影响因素分析在化学教学中仍然显得非常重要。共价键的强弱与成键原子及其环境密切相关,其中成键原子因素主要包括原子半径、成键类型、成键轨道类型、相对论效应、电负性、成键数量、反馈效应和孤电子对效应,而成键环境因素包括键间张力效应、离域效应、次级化学键效应、诱导效应和位阻效应。在教学中,我们可以通过对化学键影响因素的分析帮助学生理解共价键键能的变化规律。本文分析了影响共价键强弱的主要因素,并介绍了这种分析思路在化学教学中的应用。     

共价键中结合力的本质及其强度的定量表达方法: 键强参数的概念

建立 A—B 键中某价电子的键强参数的概念:f_(1,A-B)=(?)ε_(?)/(?)ι_(A-B)表达价分子轨道 i 上一个电子对 A—B 键中化学结合力的贡献.从理论上说明它是 i-电子云对 A 及 B 核的“拉力”以及对其它电子的“推力”的“净效应”大小.就 CO,N_2,O_2说明其所反映的“成键性质”与 UPS 的实验数据相一致.就几个均匀环状化合物证明其有很好的“加和性”.多中心键的 F_1与其中“双中心”键的 f_1之和相等.其所反映的“成键”性质与 MO 的对称性及电荷密度分布图的定性结论完全一致.f_(▲_B)=(?)所反映的键强度与键长及“键电荷”的大小顺序相适应.     

键长与价层轨道平均能的关系

关于键长的计算方法颇多。本文试从键长与原子价层轨道平均能的关系建立一个简单且更接近实验值的计算公式。我们知道,对于每个成键原子来说,一方面作为带电体使对方电子云发生变形;另一方面在对方的作用下,本身发生变形。如果成键原子吸引键合电子的能力相等,各自的电荷分布将很少变化,键长就等于成键     

γ-TiAl中掺杂原子取代位置的量子化学研究

用(EHT)紧束缚能带计算方法计算了g -TiAl及其掺杂合金的原子间成键强度,并依据键强度建立了判断掺杂原子取代位置的模型。掺杂原子的位置应根据取代前后的键强度相近的取代方式进行确定。模型预测的结果与场理论方法的结果是一致的,也与实验结果基本相符,据此能够定性判断掺杂原子的取代位置。     

共价键中结合力的本质及其强度的定量表达方法: 键强参数的概念

建立A-B键中某价电子的键强参数的概念, 表达价分子轨道i上一人电子对A-B键中化学结合力的贡献。从理论上说明它是i-电子云对A及B核的"拉力"以及对其它电子的"推力"的"净效应"大小, 就CO, N2, O2说明其所反映的"成键性质"与UPS的实验数据相一致。就几个均匀环状化合物证明其有很好的"加和性"。多中心键的F1与其中"双中心"键的f1之和相等。其所反映的"成键"性质MO的对称性及电荷密度分布图的定性结论完全一致。表达式所反映的键强度与键长及"键电荷"的大小顺序相适应。     

γ—TiAI中掺杂原子取代位置的量子化学研究

用（EHT）紧束缚能带计算方法计算了γ-TiAl及其掺杂合金的原子间成键强度，并依据键强度建立了判断掺杂原子取代位置的模型。掺杂原子的位置应根据取代前后的键强度相近的取代方式进行确定，模型预测的结果与场理论方法的结果是一致的，也与实验结果基本相符。据此能够定性判断掺杂原子的取代位置。     

双核V配合物中金属-金属键的INDO定域化方法研究

本文利用INDO和E_R定域化方法,对Ⅴ~2(μ-η^2-S~2)~2(SCCH~2)~4和[V~2O~2Cl~6]^2-双核配合物进行了量子化学计算研究,讨论了它们的电子结构和化学键性质,并讨论了利用mulliken键序判别金属原子间成键的可靠.进而基于过渡金属原子主要是以d亚层轨道参与成键,建议用金属原子d亚层的键序讨论键强度的新方法.     

关于原子轨道杂化及有关化学现象的讨论

本文根据多原子分子中键角、键的共价强度及键长等化学结构数据和事实,总结其间的变化规律及联系,说明了原子间的共价成键作用及成键过程中的电子相关作用是原子轨道杂化的原因和动力。据此讨论和解释了多原子分子中的杂化类型和一些分子的异常立体构型,对惰性电子对效应的概念和原因提出新的看法。     

哌嗪二酮的气相Hel光电子能谱及其量化计算

给出了哌嗪二酮的气相HeI紫外光电子能谱(UPS), 并进行了化合物分子的HAM/3, MNDO, MINDO/3, INDO, CNDO/2和EHMO等量子化学计算研究. UPS谱低电离能(<11.00 eV)区的四重峰被指认为分子体系中氧-氧, 氮-氮原子孤对轨道间的通过键相互作用导致的分裂峰. 表明HAM/3和MNDO计算法是预指该化合物实验电离能正确次序、轨道对称性类型以及通过键相互作用导致分裂大小的较好方法.     

环脲硝胺化合物的性能和电子结构之间的关系I. 羰基伸缩频率和键级、双原子作用能的近似线性相关

本文通过CNDO/2计算研究环脲硝胺同系化合C=O基伸缩频率与所得计算参量的关系。获得的结果是: 羰基伸缩频率实验值与C=O的Mulliken键级、双原子作用能等计算值之间, 存在近似的线性关系, 对现有的9个文献化合物和新合成的2,4,6,8-四硝基半甘脲的~γCO和计算参量作了线性回归处理, 相关系数为0.95。讨论了一些理论有机的传统观点与本文结果的关系。     

卤化物中化学键的稳定度与硬软酸的定量分类

本文提出卤化物化学键的稳定度的概念,用于衡量卤化物化学键的稳定性。给出了键稳定度公式S_(AB)=E_(AE)/(r_(AB)|x_A—x_B|),S_(AB)为键稳定度,E_(AB)为键能,r_(AB)为键长,x_A和x_B为两原子的电负性。卤化物建稳定度的物理意义是:在平衡状态下,卤化物单键上两原子的单位电负性差、单位键长的键能。用公式计算了52个金属离子卤化物的键稳定度,得到260个数据,用于硬软酸的定量分类。结果表明,用其中的(S_F—S_1)值能有效地把硬酸、交界酸、软酸分开,S_F—S_I>10为硬酸,-10Cl>Br>I,软酸的S值为F相似文献   

Effects of fluorine on the structures and energetics of the propynyl and propargyl radicals and their anions

[reaction: see text] The adiabatic electron affinity (EA(ad)) of the CH(3)-C[triple bond]C(*) radical [experiment = 2.718 +/- 0.008 eV] and the gas-phase basicity of the CH(3)-C[triple bond]C:(-) anion [experiment = 373.4 +/- 2 kcal/mol] have been compared with those of their fluorine derivatives. The latter are studied using theoretical methods. It is found that there are large effects on the electron affinities and gas-phase basicities as the H atoms of the alpha-CH(3) group in the propynyl system are substituted by F atoms. The predicted electron affinities are 3.31 eV (FCH(2)-C[triple bond]C(*)), 3.86 eV (F(2)CH-C[triple bond]C(*)), and 4.24 eV (F(3)C-C[triple bond]C(*)), and the predicted gas-phase basicities of the fluorocarbanion derivatives are 366.4 kcal/mol (FCH(2)-C[triple bond]C:(-)), 356.6 kcal/mol (F(2)CH-C[triple bond]C:(-)), and 349.8 kcal/mol (F(3)C-C[triple bond]C:(-)). It is concluded that the electron affinities of fluoropropynyl radicals increase and the gas-phase basicities decrease as F atoms sequentially replace H atoms of the alpha-CH(3) in the propynyl system. The propargyl radicals, lower in energy than the isomeric propynyl radicals, are also examined and their electron affinities are predicted to be 0.98 eV ((*)CH(2)-C[triple bond]CH), 1.18 eV ((*)CFH-C[triple bond]CH), 1.32 eV ((*)CF(2)-C[triple bond] CH), 1.71 eV ((*)CH(2)-C[triple bond]CF), 2.05 eV ((*)CFH-C[triple bond]CF), and 2.23 eV ((*)CF(2)-C[triple bond]CF).     

On the nature of metal-carbon bonding: AIM and ELF analyses of MCH(n) (n = 1-3) compounds containing early transition metals

Ab initio and DFT calculations have been performed on a series of organometallic compounds, according to the formula MCH(n), where M = K, Ca, Sc, Ti, V, Cr, or Mn and n = 1-3. Various theoretical methods are compared, the B3LYP level yielding the same agreement with the experimental geometries available as the correlated MP2 and CISD methods, with the 6-311++G(3df,2p) basis set for C and H and Wachter's (15s11p6d3f1g)/[10s7p4d3f1g] basis set for transition metals. The main geometric and electronic features of the molecules studied are described, analyzing the M-C bonding characteristics in terms of the atoms in molecules theory (AIM) and the electron localization function (ELF). Although multiple bonding is expected from the Lewis bonding scheme, the results indicate an almost pure ionic bond for all of the systems studied. The net charge transfer from the metal to the carbon atom ranges from 0.5 to 1 e(-), and the electronic structure of the CH(n)(-) moiety is unaltered after the interaction with the metal cation, showing little or no effect on the shape of the electron pairing. The bond paths corresponding to a possible alpha-agostic bond for these systems are not present.     

Magnitude and directionality of the interaction energy of the aliphatic CH/pi interaction: significant difference from hydrogen bond

The CCSD(T) level interaction energies of CH/pi complexes at the basis set limit were estimated. The estimated interaction energies of the benzene complexes with CH(4), CH(3)CH(3), CH(2)CH(2), CHCH, CH(3)NH(2), CH(3)OH, CH(3)OCH(3), CH(3)F, CH(3)Cl, CH(3)ClNH(2), CH(3)ClOH, CH(2)Cl(2), CH(2)FCl, CH(2)F(2), CHCl(3), and CH(3)F(3) are -1.45, -1.82, -2.06, -2.83, -1.94, -1.98, -2.06, -2.31, -2.99, -3.57, -3.71, -4.54, -3.88, -3.22, -5.64, and -4.18 kcal/mol, respectively. Dispersion is the major source of attraction, even if substituents are attached to the carbon atom of the C-H bond. The dispersion interaction between benzene and chlorine atoms, which is not the CH/pi interaction, is the cause of the very large interaction energy of the CHCl(3) complex. Activated CH/pi interaction (acetylene and substituted methanes with two or three electron-withdrawing groups) is not very weak. The nature of the activated CH/pi interaction may be similar to the hydrogen bond. On the other hand, the nature of other typical (nonactivated) CH/pi interactions is completely different from that of the hydrogen bond. The typical CH/pi interaction is significantly weaker than the hydrogen bond. Dispersion interaction is mainly responsible for the attraction in the CH/pi interaction, whereas electrostatic interaction is the major source of attraction in the hydrogen bond. The orientation dependence of the interaction energy of the typical CH/pi interaction energy is very small, whereas the hydrogen bond has strong directionality. The weak directionality suggests that the hydrogen atom of the interacting C-H bond is not essential for the attraction and that the typical CH/pi interaction does not play critical roles in determining the molecular orientation in molecular assemblies.     

The influence of CH…π interaction on hydrogen bonding ability of –CONH<Subscript>2</Subscript> functional group of benzamide

Interplay between CH…π and hydrogen bond interactions of benzamide has been investigated by quantum mechanical calculations. The effect of the substituents on geometrical parameters has also been studied at the B3LYP level with 6-311++G(d,p) basis set. The electron-withdrawing substituents enhance the total interaction energy of the complexes. The results indicated that the cooperativity of interactions leads to extra stability of the ternary complexes. The CH…π interaction and the hydrogen bond energies have been estimated using the electron densities calculated by the atoms in molecules (AIM) method at hydrogen bond critical points. The strength of hydrogen bonding increases in the presence of CH…π interaction in the ternary complexes. The effect of CH…π interaction on the hydrogen bond interaction has also been studied by the natural bond orbital, AIM and the molecular electrostatic potential analyses.     

CH3--MoF], [CH2=MoHF], and [CH[triple bond]MoH2F] formed by reaction of laser-ablated molybdenum atoms with methyl fluoride: persistent photoreversible interconversion through alpha-hydrogen migration and agostic interaction

Simple molybdenum methyl, carbene, and carbyne complexes, [CH3--MoF], [CH2=MoHF], and [CH[triple chemical bond]MoH(2)F], were formed by the reaction of laser-ablated molybdenum atoms with methyl fluoride and isolated in an argon matrix. These molecules provide a persistent photoreversible system through alpha-hydrogen migration between the carbon and metal atoms: The methyl and carbene complexes are produced by applying UV irradiation (240-380 nm) while the carbyne complex is depleted, and the process reverses on irradiation with visible light (lambda>420 nm). An absorption at 589.3 cm(-1) is attributed to the Mo--F stretching mode of [CH3--MoF], which is in fact the most stable of the plausible products. Density functional theory calculations show that one of the alpha-hydrogen atoms of the carbene complex is considerably bent toward the metal atom (angle-spherical HCMo=84.5 degrees ), which provides evidence of a strong agostic interaction in the triplet ground state. The calculated C[triple chemical bond]Mo bond length in the carbyne is in the range of triple-bond values in methylidyne complexes.     

N, P, and As ylides and aza- and arsa-Wittig reactions from topological analyses of electron density

The nature of bonding between N, P, and As constituent atoms in ylide systems with the R(3)XYR' formula (X = N, P, As; Y = N, P, As; R = F, H; R' = H, CH(3)) has been characterized by ab initio (MP2/6-311++G**) and density functional theory (B3LYP/6-311++G**) calculations. Its electronic structure has been analyzed through electron density with the quantum theory of atoms in molecules and the electron localization function (ELF). The characteristics of the central bond are inspected with the calculated rotational barriers. The results show that N has a behavior different from that of the remaining pnicogen atoms (P, As), where the bond is much stronger. Fluorine substituents strengthen the X-Y bond, reduce the bond distance, and increase the electron density in the central bond so that the substituent pulls charge from the bond in the pnicogen X atom. For the N-pnicogen ylides, the results showed different bonding characters between F and X atoms; depending on the position of the F atom, the difference of the bond character is sensed by the basin synaptic order, as it is deduced from the analysis of the ELF basins. The energy profiles of the rotational barriers have been calculated at the MP2/6-311++G** level, indicating that the electronegativity of the substituents is a relevant factor that has consequences in the characteristics of the X-Y bond.     

甲硫醇在Cu(111)表面的解离吸附: 密度泛函理论研究

采用基于密度泛函理论的第一性原理方法和平板模型研究了CH₃SH分子在Cu(111)表面的吸附反应.系统地计算了S原子在不同位置以不同方式吸附的一系列构型, 第一次得到未解离的CH₃SH分子在Cu(111)表面顶位上的稳定吸附构型,该构型吸附属于弱的化学吸附, 吸附能为0.39 eV. 计算同时发现在热力学上解离结构比未解离结构更加稳定. 解离的CH₃S吸附在桥位和中空位之间, 吸附能为0.75-0.77 eV. 计算分析了未解离吸附到解离吸附的两条反应路径, 最小能量路径的能垒为0.57 eV. 计算结果还表明S―H键断裂后的H原子并不是以H₂分子的形式从表面解吸附而是以与表面成键的形式存在. 通过比较S原子在独立的CH₃SH分子和吸附状态下的局域态密度, 发现S―H键断裂后S原子和表面的键合强于未断裂时S原子和表面的键合.