Approximate Self-Consistent Molecular Orbital Theory II. CNDO-MO Calculation of Methane Chlorine Derivatives

The CNDO-MO method with s-p separation model modification described in our former work¹⁾ is used here. Methane and it's chlorine derivatives are selected as the model molecules for such calculation. The nuclear coordinates are chosen from microwave spectroscopic data²⁾. Both the self consistent behavior and the invariance of orthogonal transformation²⁾ are quite successfully by preformed through the computation procedure. The ionization potentials predicted by Koopmans' theorem are reasonably good. The charge distribution and dipole moments of each case are calculated, and the results obtained by variation of bond parameter β are compared with the available experimental data satisfactorily.     

Approximate Self-Consistent Molecular Orbital Method and Repulsive Bonding of Monocarbocyclic Π System

Zero differential overlap (ZDO) approximation^1,2) and the consideration of long range (meta and para bonds etc) bonding are applied in this semiempirical LCAO-SCF MO method. Effective Huckel type formulation and diagonalization procedure of symmetry adapted analysis simplify the computational procedure as well as give a clear and intuitive localized bonding behavior of the carbocyclic molecules. The long range repulsive bonds are defined and discussed between aromatic and non-aromatic cases. Such bonds are closely correlated to the rigidity and distortion of ring systems. Benzene is the most stable species among the 6π electron systems, and there will be some new explanation of Dewar resonance structures due to the consideration of long range para repulsive bonds in this work.     

A Self-Consistent Model for Sorption and Transport in Polyimide-Derived Carbon Molecular Sieve Gas Separation Membranes

Demand for energy-efficient gas separations exists across many industrial processes, and membranes can aid in meeting this demand. Carbon molecular sieve (CMS) membranes show exceptional separation performance and scalable processing attributes attractive for important, similar-sized gas pairs. Herein, we outline a mathematical and physical framework to understand these attributes. This framework shares features with dual-mode transport theory for glassy polymers; however, physical connections to CMS model parameters differ from glassy polymer cases. We present evidence in CMS membranes for a large volume fraction of microporous domains characterized by Langmuir sorption in local equilibrium with a minority continuous phase described by Henry's law sorption. Using this framework, expressions are provided to relate measurable parameters for sorption and transport in CMS materials. We also outline a mechanism for formation of these environments and suggest future model refinements.     

Molecular states of atoms. I. Orbital energy parameters

Atomic valence state energies are analyzed to obtain values of orbital energy parameters that may be used in semiempirical molecular orbital calculations. Difficulty in defining the interaction between orbitals with non-integer electron populations is systematically avoided by distinguishing between a valence state and a molecular state of an atom, only the latter state having non-integer spin paired orbital occupancy. Application of the virial theorem to the molecular state enables a value for the orbital kinetic energy to be obtained from the valence state orbital energy parameters once an arbitrary configuration is defined as reference. The orbitals then are eigenfunctions of the atomic Fock operator for that reference molecular state and, with their energy parameters, may be employed as a fixed basis set for molecular orbital calculations.     

铌团簇和配合物的多面体分子轨道理论研究

应用多面体理论方法研究铌纯金属团簇和配合物的电子结构成键性质,并将理论预测结果用密度泛函方法验证.证实多面体理论是简便预测过渡金属簇,尤其是簇骨架电子结构的有效方法.铌的小团簇总是以低自旋密堆积结构为稳定构型,当体系的价电子数满足轨道成键数时,用该方法可较准确地推断成键性质(如Nb₄和[Nb₆Cl₁₂]⁺⁴);而对于不满足成键数的体系(如Nb₅和Nb₆),则可利用该理论分析畸变趋势.     

计算化学在分子轨道教学中的应用

分子轨道理论是理解分子电子结构与微观性质的重要理论之一,也是本科生与研究生结构化学教学中的重点与难点。学生对原子轨道组合形成分子轨道、分子轨道能级交叉混合等知识的理解缺乏形象直观、定量的认识。本文通过基于量子化学或密度泛函理论的Gaussian 03计算软件,计算、绘制并分析了F_2、O_2、N_2、HF、CO等的分子轨道能级图,将学生较难理解的内容定量、直观地呈现出来,形象地解释了分子轨道成键原则与电子填充原则等分子轨道理论中的重难点,加深了学生对分子轨道理论的理解,特别是sp轨道混杂导致的σ_(2p_z)与π_(2p)轨道能级交叉这一难点,激发了学生学习的主动性和积极性,提高了教学质量。在此基础上,利用分子轨道理论分析了CO_2的电子结构,使学生学会应用分子轨道理论解决实际问题,巩固了相关课堂理论知识。     

Self-Consistent APW–k · p method. I. Theory

A first principles method based on an APW – k · p band structure calculation is derived in order to obtain the self-consistent eigenenergies, crystal potential, and electronic density. The simplicity and extensions of the method are discussed, including comparison with the results obtained by the use of the Chadi and Cohen special points technique.     

Halogen-Bridged Mixed-Valence Linear-Chain Complexes: Molecular Orbital Model of Electron Localization

A simple tight-binding model is used to study electronic distributions and Peierls distortions in a series of mixed-valence linear-chain complexes [M(II)(LL)(2)][M(IV)(LL)(2)X(2)] (M = Ni, Pd, Pt; X = Cl, Br, I; LL = 1,2-diaminoethane, 1,3-diaminopropane). The calculated values of Peierls distortions for complexes with known structure are in good agreement with the experimental data. For the whole series, the model is employed as a qualitative tool to analyze structural and electronic properties. It is shown that charge disproportionation occurs in the related dimers. Thus, our quantum chemical calculations as well as previous phenomenological approaches indicate that, in metal-halogen chains, one deals with the interplay of local and cooperative effects.     

分子轨道理论的奠基人--弗里德里希?洪特

量子化学目前已发展成为一门成熟的学科。德国理论物理学家弗里德里希·洪特(1896—1997)对原子和分子电子结构的研究为量子化学的建立和发展奠定了基础,被称为分子轨道理论的奠基人之一。洪特因发现洪特规则和建立分子轨道理论被化学界熟知,本文介绍了弗里德里希·洪特的生平,并对洪特发现洪特规则和建立分子轨道理论的过程进行了详细论述。     

分子轨道成分的计算

量子化学文献中经常涉及到“分子轨道成分”概念, 然而相应的计算方法却没有普遍得到重视, 甚至存在严重误区. 专门对分子轨道中基函数的成分、原子轨道的成分以及原子的成分这些基本概念和计算方法进行了详细讨论, 通过实例分析比较了不同方法间的差异, 同时指出了计算和分析时需要注意的问题, 给出了在方法选择上的建议.     

分子轨道理论奠基人之一:约翰·爱德华·兰纳-琼斯

约翰·爱德华·兰纳-琼斯(1894—1954)是英国杰出的理论化学家。兰纳-琼斯因其在分子结构、原子价和分子间力等方面的研究而闻名,其中最重要的是提出了表达中性原子或分子之间的相互作用的一个简单的数学模型,这个模型被称为兰纳-琼斯势函数(也称L-J势函数或6～12势函数);他是第一个以目前普遍使用的方式使用原子轨道的线性组合来定量描述分子轨道(LCAO MO理论)的人,被称为分子轨道理论的奠基人之一。本文介绍了约翰·爱德华·兰纳-琼斯的生平,并对兰纳-琼斯提出兰纳-琼斯势函数和建立分子轨道的原子轨道线性组合法的过程进行了详细论述。     

θ溶剂中树枝形均聚物的自洽场理论计算

树枝形大分子因其高度支化的特殊结构，在传感、载药、催化等众多领域有着广泛的应用潜力.运用高斯链模型推导了溶液体系中树枝形均聚物的自洽场方程组.并在不考虑体积排除作用的条件下，计算了θ溶液中分子的中心链节官能度f₀、支化点官能度f、间隔链段的聚合度P、以及总代数G对链节浓度分布及均方回转半径的影响.计算结果显示，链节浓度分布总是符合"dense-core"的模型，即在所有计算参数组合下，链节浓度总是沿径向单调下降，并随f₀、f和G的增加而单调增加.自洽场计算得到的树枝形均聚物的均方回转半径R与Rouse动力学模型的计算结果基本一致，在f₀、f和G的数值较大时，都能够得到<R²>≈GPa²的标度律关系.     

柔性树枝形大分子溶液的自洽场理论计算

树枝形大分子是一种新型高分子材料,其浓度分布、末端官能团分布和分子尺寸决定了材料的性质.在柔性支化臂条件下,推导了均聚物树枝形大分子在溶剂中的自洽场理论,计算了不同代数G的树枝形大分子的链节浓度分布.计算结果表明：（1）在良溶剂（或绝热溶剂）中,柔性支化臂的树枝形大分子符合"dense-core"模型,链节浓度沿径向缓慢下降;（2）由于柔性链的折回构象,末端链没有向球形分子的外缘球壳层聚集的倾向;（3）不同代数的支化臂所受的拉伸强度不同,近中心处的第一代支化臂所受的拉伸程度最大;（4）自洽场计算与Flory平均场理论标度律估算结果一致,得到树枝形大分子在良溶剂中的分子尺寸具有标度律R~（GP）^1/5N^2/5,其中P是单个支化臂的聚合度,N是分子的总的链段数;（5）在G固定的情况下,该标度律简化为R~P^3/5.     

用分子轨道理论研究NO气体在TiO2表面吸附

根据一氧化氮(NO)气体在二氧化钛(TiO)表面吸附和脱附的实验结果,揭示了气体脱附量的变化规律．利用MOPAC和GAUSSIAN分子轨道理论计算了在TiO2(110)表面上吸附NO分子的原子簇模型,电荷分布以及原子簇的能级,推断了NO在TiO(110)表面吸附的稳定性．     

Composite Correlated Molecular Orbital Theory Calculations of Ring Strain for Use in Predicting Polymerization Reactions

Strained ring systems play an important role in synthesis and can be characterized by the ring strain energy (RSE). The RSE of 3, 4, 5, and 6 membered saturated and unsaturated ring systems containing N, O, P, and S heteroatoms and H, F, SiMe₃, and SO₂Me substituents were calculated at the G3(MP2) composite correlated molecular orbital theory level using up to 5 models to predict the RSE. Generally, the RSE decreased as ring size increased with a substantial decrease from 4 to 5 membered rings. Replacement of a ring CH₂ with P or S reduced the RSE, consistent with less angle strain. The RSE for unsaturated systems were generally greater than for saturated systems due to increased angle strain. No general trends were found with respect to substituent effects. The RSE values suggest that 3-pyrroline and 2-pyrroline and their derivatives may be able to support ring opening metathesis polymerization and warrant further study.     

EHMO方法的分子轨道定域化

用Foster-Boys的定域化准则讨论了EHMO方法的分子轨道定域化问题,提出用双中心重叠积分近似计算双中心轨道偶极矩积分方法,得到的EHMO定域分子轨道与严格定域化结果接近,与从头计算方法的定域化结果定性一致。     

Reactivity Study of Elementary Steps in the Polymerization Mechanism of Acrylfuranic Compounds by Frontier Molecular Orbital Theory

A reactivity study of the most important elementary steps (propagation, intermolecular degradative transfer, and re‐initiation) in free‐radical polymerization of acrylfuranic systems, furfuryl acrylate (FA), and furfuryl methacrylate (FM), using the frontier molecular orbital theory is described. A qualitative explanation of reactivity trends of these steps for both systems is given based on absolute values of the SOMO/HOMO gap. The small difference between values of k_p for FA and FM compared to that found for MA and MMA ( ) is justified semi‐quantitatively by applying a formulation for the change of energy in the transition state using second‐order perturbation theory.

     

简单Hückel分子轨道理论判断H4分子及离子的几何构型

简单Hückel分子轨道理论是结构化学课程内容的主要知识点之一,主要用于计算π电子成平面分布的离域体系的电子结构和轨道能量。本文把该理论推广到H_4非离域体系,定性地计算H_4非离域体系的轨道能量,帮助学生理解Walsh规则应用举例中难以理解的H_4构型为直线型,但H_4~+为四面体构型的原因。     

Molecular Orbital Studies of the Protonation of Diazomethane

CNDO/Z, MINDO/3 and ab initio molecular orbital calculations indicate that C-protonated diazomethane is more stable than N(end)-protonated diazomethane. Extrapolation of these results to solution chemistry as well as the kinetic references of protonation of diazomethane are discussed.     

An Ab Initio Molecular Orbital Study of Isophosphinines

Ab initio molecular orbital calculation at HF/6-31G*, HF/6-31G**, HF/6-311G**, HF/6-311++G**, RMP2-FC/6-31G*, and B3LYP/6-31G* levels of theory for geometry optimization and MP4(SDQ)/6-31G* for a single-point total energy calculation are reported for phosphinine and 13 isophosphinines 7-19 . Isomers 7-11 with an allenic system are calculated to be 8-18 kcal mol m 1 more stable than structures 12-17 with an acetylenic moiety. The calculated energy difference (66.19 kcal mol m 1 ) between phosphinine and the most stable isophosphinine (1-phospha-1,2,4-cyclohexatriene, 10 ) is smaller than the difference (78.96 kcal mol m 1 ) between benzene and the most stable isobenzene (cyclohexa-1,2,4-triene, 2 ). The isophosphinines 18 and 19 , with a butatriene moiety, are calculated to be the least stable isomers.