键能的分子轨道理论研究2: 键能与Mulliken布居对键强度的 判断的比较

用键能E~A~B和Mulliken布居对化学键强度的判别进行了分析比较。结果表明,键能判据比Mulliken布居判据所得结论更符合实际情况。作为衡量原子间化学键强度的尺度,不仅应考虑原子轨道间的布居因素,还应考虑分子轨道(或原子轨道)的能量因素。     

键能的分子轨道理论研究Ⅱ.键能与Mulliken布居对键强度的判断的比较

用键能EAB和Mulliken布居对化学键强度的判别进行了分析比较.结果表明,键能判据比Mulliken布居判据所得结论更符合实际情况.作为衡量原子间化学键强度的尺度,不仅应考虑原子轨道间的布居因素,还应考虑分子轨道(或原子轨道)的能量因素.     

分子轨道成键性质及原子间化学键强度的成键能判据

本文定义了成键能Eb并用作分子轨道成键性质和分子中原子间化学键强度的判据。与Mulliken重叠布居Pb不同, 在成键能Eb中同时包含了原子轨道间的重叠因素和原子轨道的能量因素。对一些分子所作计算结果表明, 成键能判据较Mulliken重叠布居判据所得结论与实验更相符。     

原子-键电负性均衡方法参数的确定与探讨

原子-键电负性均衡方法(ABEEM)是以密度泛函理论(DFT)和电负性均衡原理为基础发展而来，它明确地考虑了化学键是不引入任何实验数据的带纯理论性和计算的方法．使用统一标准并具有代表性和全面性地选择了200多个模型分子，利用可得到较准确结构的MP2／6-31G*优化结构，心／STO-3G单点计算得到Mulliken重叠布居，再用最小二乘法拟合得到许多主族元素在分子体系中的诸原子(包括单、双和叁键等不同成键状态)和化学键的ABEEM参数．所得到的原子的价态电负性可与已提出的其他电负性标度相比拟，计算CO得到的电荷负端为C(与从头计算的结果相反)，结果与实验相符，且原子电荷的正负不完全由原子电负性决定．     

有关键能的几个问题

本文讨论了与键能有关的几个问题：即键能和键焓，键离解能和平均键能，原子轨道的成键能力和键的强弱等。     

基团电负性研究Ⅶ 基团电负性与R-X键的键裂能

化学键的键裂能(也称离解能)定义为在标准条件下:键均裂前后各个物种生成热的代数和,即:DH~0(R—X)=△tH_(208)~0(X·())+△fH_(298)~0(R·())—△fH_(298)~0,(RX())(1) 因此测量化学键的键裂能等价于测量自由基的生成热。Benson等经过近三十年,准确测     

键离子性与键长的计算

一、引言键长是分子结构的一个重要参数,对于化学键键长实验和计算,前人已做了许多工作。但键长和物质结构及性能的关系,各类无机物、有     

铁的单核羰基络合物键能和成键特性的研究

通过电荷自洽叠代的ＥＨＭＯ量子化学计算,求得Ｆｅ（ＣＯ）５,ＰＸ５（Ｘ＝Ｆ,Ｃｌ）及Ｆｅ（ＣＯ）４Ｈ２分子的键能ＥＡＢ并分析它与键强度的关系,解释了Ｆｅ（ＣＯ）５分子中键长长的Ｆｅ－Ｃ键其键强度反而比键长短的Ｆｅ－Ｃ键强,Ｆｅ（ＣＯ）５和Ｆｅ（ＣＯ）４Ｈ２中Ｃ－Ｏ键键长相等,但键强度又不一样的“反常”现象     

键长、价层轨道能与A─H键振动频率

键长、价层轨道能与Ａ─Ｈ键振动频率喻典，吕祖美（重庆师范学院重庆６３００４７）（重庆沙坪坝区教师进修学校重庆６３００００）关键词：氢化物，振动频率，价层轨道平均能键长振动频率的大小反映了化学键的强弱和分子的稳定性，由于测定的实验数据有限，建立简单而准...     

成键能力大的杂化轨道形成共价键的键能也大吗?

在我们的许多教科书中,当讲到原子轨道杂化的原因时,都是这样叙述的:杂化轨道具有更强的方向性,伸展得更远,可以与其他原子的原子轨道进行更大程度的重迭,从而提高成键能力,形成更牢固的化学键,使分子更加稳定。于是给学生形成了这样一个“基本概念”,“杂化轨道的成键能力越大,形成的共价键键能     

键能的分子轨道理论研究 1: 理论公式

从LCAO-MO出发, 给出了一个计算键能的近似方法, 即EAB(i)-∑∑CaiSabCbiεi为第i个占据分子轨道(MO)中的一对电子对A-B键键能的贡献。对所有分子轨道求和即为该键的键能: EAB=∑EAB(i)。按该方法, 不仅可以计算各种不同分子中每两个相键连原子间的键能, 还可以从MO及AO角度分析每一具体键, 如σ, π, δ键的键能以及各AO对键能的贡献。该方法虽有别于求键焓和平衡离解能De, 但计算结果和De的实验值甚相符合。通过对键能的分析研究, 能较好地揭示原子间的相互作用关系及化学键的强弱, 从而可进一步探讨化学反应活性, 反应速率等化学性质。     

Orbital overlap and chemical bonding

The chemical bonds in the diatomic molecules Li(2)-F(2) and Na(2)-Cl(2) at different bond lengths have been analyzed by the energy decomposition analysis (EDA) method using DFT calculations at the BP86/TZ2P level. The interatomic interactions are discussed in terms of quasiclassical electrostatic interactions DeltaE(elstat), Pauli repulsion DeltaE(Pauli) and attractive orbital interactions DeltaE(orb). The energy terms are compared with the orbital overlaps at different interatomic distances. The quasiclassical electrostatic interactions between two electrons occupying 1s, 2s, 2p(sigma), and 2p(pi) orbitals have been calculated and the results are analyzed and discussed. It is shown that the equilibrium distances of the covalent bonds are not determined by the maximum overlap of the sigma valence orbitals, which nearly always has its largest value at clearly shorter distances than the equilibrium bond length. The crucial interaction that prevents shorter bonds is not the loss of attractive interactions, but a sharp increase in the Pauli repulsion between electrons in valence orbitals. The attractive interactions of DeltaE(orb) and the repulsive interactions of DeltaE(Pauli) are both determined by the orbital overlap. The net effect of the two terms depends on the occupation of the valence orbitals, but the onset of attractive orbital interactions occurs at longer distances than Pauli repulsion, because overlap of occupied orbitals with vacant orbitals starts earlier than overlap between occupied orbitals. The contribution of DeltaE(elstat) in most nonpolar covalent bonds is strongly attractive. This comes from the deviation of quasiclassical electron-electron repulsion and nuclear-electron attraction from Coulomb's law for point charges. The actual strength of DeltaE(elstat) depends on the size and shape of the occupied valence orbitals. The attractive electrostatic contributions in the diatomic molecules Li(2)-F(2) come from the s and p(sigma) electrons, while the p(pi) electrons do not compensate for nuclear-nuclear repulsion. It is the interplay of the three terms DeltaE(orb), DeltaE(Pauli), and DeltaE(elstat) that determines the bond energies and equilibrium distances of covalently bonded molecules. Molecules like N(2) and O(2), which are usually considered as covalently bonded, would not be bonded without the quasiclassical attraction DeltaE(elstat).     

A variational biorthogonal valence bond method

The details of a simple and efficient scheme for performing variational biorthogonal valence bond calculations are presented. A variational bound on the energy functional is obtained through the use of a complete configuration expansion in a well-chosen subset of orbitals. The resultant wave functions are clearly dominated by the covalent (spin-coupled) structures, with a negligible contribution from ionic structures. The orbitals obtained compare favorably with overlap enhanced atomic orbitals obtained by other valence bond approaches. The method is illustrated by calculations on water and dioxygen difluoride. © 1994 by John Wiley & Sons, Inc.     

The nature of the chemical bond in UO2

The nature of the chemical bond in UO₂ was analyzed taking into account the X-ray photoelectron spectroscopy (XPS) structure parameters of the valence and core electrons, as well as the relativistic discrete variation electronic structure calculation results for this oxide. The ionic/covalent nature of the chemical bond was determined for the UO₈ (D_4h) cluster, reflecting uranium's close environment in UO₂, and the U₁₃O₅₆ and U₆₃O₂₁₆ clusters, reflecting the bulk of solid uranium dioxide. The bar graph of the theoretical valence band (from 0 to ~35 eV) of XPS spectrum was built such that it was in satisfactory agreement with the experimental spectrum of a UO₂ single crystalline thin film. It was shown that unlike the crystal field theory results, the covalence effects in UO₂ are significant due to the strong overlap of the U 6p and U 5f atomic orbitals with the ligand orbitals, in addition to the U 6d atomic orbital (AO). A quantitative molecular orbital (MO) scheme for UO₂ was built. The contribution of the MO electrons to the chemical bond covalence component was evaluated on the basis of the bond population values. It was found that the electrons of inner valence molecular orbitals (IVMO) weaken the chemical bond formed by the electrons of outer valence molecular orbitals (OVMO) by 32% in UO₈ and by 25% in U₆₃O₂₁₆.     

Maximum bond order hybrid orbitals

Summary Based on the simplified calculation scheme of the maximum bond order principle and the basic idea of the maximum overlap symmetry orbital method, a simple procedure is suggested for constructing systematically the bonding hybrid orbitals, called maximum bond order hybrid orbitals, for a given molecule from the first-order density matrix obtained from a molecular orbital calculation. As an example, the proposed procedure is performed for some typical small molecules by use of the density matrix obtained from CNDO/2 calculation. It is shown that the bonding hybrid orbitals constructed by using the procedure are extremely close to those by using the natural hybrid orbital procedure and in good agreement with chemical intuition, and that the proposed procedure can be performed more easily than the natural hybrid orbital procedure and can given simultaneously the values of the maximum bond order for all bonds in molecules.The project was supported by National Natural Science Foundation of China and also supported partly by Foundation of Hubei Education Commission     

Orbital Interaction and the Mulliken-Walsh Diagram for AH2 Systems

We define θ- (or R-) bonding (or antibonding) character of the molecular orbitals according to the slopes of the orbital energy curves when the internuclear angle (or internuclear distance) is varied. So far the slope of the orbital curve has only been accounted for by the qualitative argument based on two factors: the orbital overlap and the s-p mixing. We employ the bond orbitals instead of the usual atomic orbitals as the basis set to analysis the character of the molecular orbitals. The Fock matrix in the bond orbital basis can then quantitatively account for the effects of both the overlap and s-p mixing factors. Our analysis also show that a third factor, the orbital interaction, is essential to account for both the “typical” and “abnormal” behavior of the slopes.     

Unique bonding pattern and resulting bond stretch isomerism in

The bond‐stretch isomers are characterized by a principal change in the bond‐length with the rest of the molecule being unaltered. The electronic structure regulates the bond stretch isomerism phenomenon in which has been investigated with density functional theory, ab initio CASSCF, highly efficient n‐electron valence state perturbation theory and multireference configuration interaction calculations. Two isomers are distinguished on different potential energy surfaces and the corresponding avoided crossing is also studied in details. The bonding pattern in two isomers are analysed through adaptive natural density partitioning analysis and quantum theory of atoms in molecules analysis. The bonds in both the isomers primarily involve the 2p orbitals, which overlap face‐to‐face in long‐bond isomer. Whereas, in‐plane π‐bonding occurs at the short‐bond isomer leading to unusual bent bond. © 2015 Wiley Periodicals, Inc.     

Exploring the actinide-actinide bond: theoretical studies of the chemical bond in Ac2, Th2, Pa2, and U2

Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemical bond in the actinide diatoms Ac2, Th2, Pa2, and U2. Scalar relativistic effects and spin-orbit coupling have been included in the calculations. In the Ac2 and Th2 diatoms the atomic 6d, 7s, and 7p orbitals are the significant contributors to the bond, while for the two heavier diatoms, the 5f orbitals become increasingly important. Ac2 is characterized by a double bond with a 3Sigmag-(0g+) ground state, a bond distance of 3.64. A, and a bond energy of 1.19 eV. Th2 has quadruple bond character with a 3Dg(1g) ground state. The bond distance is 2.76 A and the bond energy (D0) 3.28 eV. Pa2 is characterized by a quintuple bond with a 3Sigmag-(0g+) ground state. The bond distance is 2.37 A and the bond energy 4.00 eV. The uranium diatom has also a quintuple bond with a 7Og (8g) ground state, a bond distance of 2.43 A, and a bond energy of 1.15 eV. It is concluded that the strongest bound actinide diatom is Pa2, characterized by a well-developed quintuple bond.     

Spectroscopic calculation of CH bond dissociation energy for aliphatic derivatives from the ethylene series

The bond-dissociation energy of CH bonds in molecules of the ethylene homological series has been determined by spectroscopic and quantum chemical methods. Spectroscopic values for the CH bond dissociation energy were calculated based on the fundamental absorption bands in the anharmonic approximation by the variation method using the Morse anharmonic basis. Quantum chemical computations were performed with 6-311G(3d, 3p)/B3LYP basis. There are discussed the obtained regularities of changes in the bond dissociation energy when the structure of a molecule is changed.