电负性研究(Ⅱ)—氢的电负性

氢的电负性值是氢元素性质的重要参数，１９３２年Pauling犤１～３犦定量确定氢的相对电负性值等于２．１，１９６１年Allred犤４，５犦用更准确的实验数据对Paul－ing电负性标度进行了修正，氢的电负性值被确定为２．２，目前这两个数值都在采用。元素的电负性值是与元素的性质紧密相关的，一个合适的电负性标度应该至少反映所有重要元素的电负性值，氢的化合物比任何其它元素都多，理应有一个基本的准确电负性值，然而一些电负性标度中却缺乏这样的数据。在Murphy等四人犤６犦最近发表的论文中，对Pauling电负性标度又进行了深入考查与…     

Molecular electronegativity in density functional theory (VI)

Based on the density functional theory and partitioning the molecular electron density ρ (r) into atomic electronic densities and bond electronic densities, the expressions of the total molecular energy and the “effective electronegativity” of an atom or a bond in a molecule are obtained. The atom-bond electronegativity equalization model is then proposed for the direct calculation of the total molecular energy and the charge distribution of large molecules. Practical calculations show that the atom-bond electronegativity equalization model can reproduce the correspondingab initio values of the total molecular energies and charge distributions for a series of large molecules with a very satisfactory accuracy.     

Estimation of electronegativity values of elements in different valence states

The electronegativities of 82 elements in different valence states and with the most common coordination numbers have been quantitatively calculated on the basis of an effective ionic potential defined by the ionization energy and ionic radius. It is found that for a given cation, the electronegativity increases with increasing oxidation state and decreases with increasing coordination number. For the transition-metal cations, the electronegativity of the low-spin state is higher than that of the high-spin state. The ligand field stabilization, the first filling of p orbitals, the transition-metal contraction, and especially the lanthanide contraction are well-reflected by the relative values of our proposed electronegativity. This new scale is useful for us to estimate some quantities (e.g., the Lewis acid strength for the main group elements and the hydration free energy for the first transition series) and predict the structure and property of materials.     

电负性均衡

电负性是分子中一个原子把电子拉向它自身的能力,是化学理论的基本概念之一。继Pauling建立第一个电负性标度后,提出了众多的电负性标度。只是在密度泛函理论的基础上,电负性概念和电负性均衡原理,才被精密地论证。近二十多年来,电负性理论的重要发展是:应用电负性均衡模型或方法,可以快速地计算分子体系的电荷分布,从而确定分子的其他性质,甚至包括分子的结构和反应性指标。通常的电负性均衡方法只把分子划分到原子区域,虽然简单直观,但其精度和应用范围受到限制。原子与键电负性均衡方法,把分子划分到包括原子区域、化学键区域和孤对电子区域,能够较快速精密地计算分子的电荷分布和其他性质,并被应用到构建新一代可极化或浮动电荷力场的探索中,有广阔的应用前景。     

Calculation of bond energies in diatomic molecules

Theoretical Chemistry Accounts - An extrapolation method is proposed for an approximative evaluation of covalent bonding powers of some elements from their electronegativity values. Using these...     

单硫醚气相色谱保留指数拓扑化学研究

在分子拓扑化学理论的基础上,根据分子中原子的特性,用分子中原子的平衡电负性对分子图进行着色,在距离矩阵的基础上结合分子中各原子的支化度构建一组新的拓扑指数NPm(m=1,2,3),利用多元线性回归技术将单硫醚在4种极性固定相的气相色谱保留指数与NPm(m=1,2,3)建立相应的定量结构-保留相关关系模型(QSRR),并用这种模型对单硫醚的气相色谱保留指数进行预测,结果表明,预测结果和实验值吻合较好。     

The importance of the external potential on group electronegativity

The electronegativity of groups placed in a molecular environment is obtained using CCSD calculations of the electron affinity and ionization energy. A point charge model is used as an approximation of the molecular environment. The electronegativity values obtained in the presence of a point charge model are compared to the isolated group property to estimate the importance of the external potential on the group's electronegativity. The validity of the "group in molecule" electronegativities is verified by comparing EEM (electronegativity equalization method) charge transfer values to the explicitly calculated natural population analysis (NPA) ones, as well as by comparing the variation in electronegativity between the isolated functional group and the functional group in the presence of a modeled environment with the variation based on a perturbation expansion of the chemical potential.     

Molecular electronegativity in density functional theory (II) --Direct calculation of group electronegativity and the atomic charges in a group

On the basis of a more precise expression of the atomic effective electronegativity deduced from the density functional theory and electronegativity equalization principle, a new scheme for calculating the group electronegativity and the atomic charges in a group is proposed and programed, and various parameters of electronegativity and hardness are given for some common atoms. Through calculation, analysis and comparison of more than one hundred groups, it is shown that the results from this scheme are reasonable and may be extended.     

再论电负性标度的发展

电负性是化学中的一个重要概念,在基础化学课程中起着关键作用。本文将电负性研究过程分为三个阶段,简述了关于电负性标度的认识不断深入、逐步发展的过程,在讨论不同电负性标度的基础上,着重讨论了绝对电负性标度和Pauling类型电负性标度的差别性、Allen电负性标度及其修正的Rahm标度等,对深刻理解电负性标度这一基本概念及促进无机化学教学内容的改革均有重要意义。     

电子亲合力与诱导效应的定量关系

本文提出了电子亲合力与诱导效应之间的定量关系。通过计算说明了电子亲合力与诱导效应的定量相关比电负性与诱导效应的定量相关好得多；并应用其定量关系式预测尚未实测到的取代基的电子亲合力值；根据计算结果, 可从不同实验方法测得的电子亲合力值中选择正确的实验值。该定量关系式能更加准确和定量地表征出诱导效应的作用。     

金属态原子电负性的计算及应用(I)

提出了一种表示金属态原子电负性的计论方法，即X＝Pexp（-KsR），其中P为Pauling电负性，Ks为Thomas-Fermi定义的表示金属晶体内某一金属原子核外电子受邻近配位原子屏蔽长度的倒数，R为该金属的一价离子半径．通过将由此获得的金属态原子电负性值及Pauling电负性值同金属表面的电子脱出功，CO、O2分子在某些过渡金属表面上的解离吸附热数值关联结果比较，发现提出的金属态原子电负性概念能更准确地反映过渡金属在催化过程中所表现出的各种热化学性质．     

Electronegativities of elements in covalent crystals

A new electronegativity table of elements in covalent crystals with different bonding electrons and the most common coordination numbers is suggested on the basis of covalent potentials of atoms in crystals. For a given element, the electronegativity increases with increasing number of bonding electrons and decreases with increasing coordination number. Particularly, the ionicity of a covalent bond in different environments can be well-reflected by current electronegativity values; that is, the ionicity of chemical bonds increases as the coordination number of the bonded atoms increases. We show that this electronegativity scale can be successfully applied to predict the hardness of covalent and polar covalent crystals, which will be very useful for studying various chemical and physical properties of covalent materials.     

预测环加成反应区域选择性的一个简易方法

长期以来,我们一直在研究化合物的性能与结构关系的定量化问题,其目的是为了寻求一个定量确定反应物反应活性的简易方法。本文研究了一种确定环加成反应区域选择性的新方法。早在七十年代初期,对于环加成反应的区域选择性问题就有报导,可以通过分子轨道法等加以确定。但是,应用分子轨道法的计算程序过于繁琐,所以应用并不广泛。1989年,S.K.Pal提出了处理这个问题的一个方程,即可以通过活性中心原有的电负性进行推算。这种原有的电负性可以用Huheey方法计算,但是这种计算方法在应用中仍然是相当复杂     

Ab initio calculations, electronegativity equalisation and group electronegativity

A correspondence betweenab initio calculations, the principle of electronegativity equalisation and group electronegativity has been established within the framework of Mulliken population analysis. Using this we have calculated electronegativities of some 37 groups/atoms. These electronegativities show excellent linear correlation with¹ J _CC coupling constants in monosubstituted benzenes and Inamoto’si scale and a satisfactory one with Wells’ group electronegativity data. The correspondence however required a scaling of charge (obtained byab initio calculations) and a proportionality between the electronegativity of the neutral group and its hardness. It is shown that using these electronegativity values it is possible to calculate group charges in molecules where groups under consideration interact with each other through σ bond only.     

A molecular electrostatic potential bond critical point model for atomic and group electronegativities

A consistent set of atomic electronegativities of main block and d-block transition elements has been obtained from the position and value of the molecular electrostatic potential bond critical point of the C-E bond of a methyl-element-hydride system, H(3)C-EH(n) (E is an element and n = 0, 1, 2, 3, 4, and 5 depending on the position of E in the periodic table). The new scale shows very good agreement with the popular electronegativity scales such as Pauling, Allen, Allred-Rochow, Mulliken, and Sanderson scales of electronegativity, especially for the main block elements. The present scale of electronegativity for transition elements is expected to be more accurate than the previously derived values because of a more consistent approach. Further, the same approach has led to the evaluation of group electronegativities when the hydrogens of E are replaced by other substituent groups. These group electronegativity values are found to correlate well with Inamoto and Mullay scales.     

A new scale of electronegativity based on electrophilicity index

By calculating the energies of neutral and different ionic forms (M2+, M+, M, M-, and M2-) of 32 elements (using B3LYP/6-311++G** level of theory) and taking energy (E) to be a Morse-like function of the number of electrons (N), the electrophilicity values (omega) are calculated for these atoms. The obtained electrophilicities show a good linearity with some commonly used electronegativity scales such as Pauling and Allred-Rochow. Using these electrophilicities, the ionicities of some diatomic molecules are calculated, which are in good agreement with the experimental data. Therefore, these electrophilicities are introduced as a new scale for atomic electronegativity, chi(omega)0. The same procedure is also performed for some simple polyatomic molecules. It is shown that the new scale successfully obeys Sanderson's electronegativity equalization principle and for those molecules which have the same number of atoms, the ratio of the change in electronegativity during the formation of a molecule from its elements to the molecular electronegativity (Delta chi/chi omega) is the same.     

电负性研究Ⅰ.建立元素电负性标度的有效核电荷数方法

通过价层电离能、价键轨道能量用有效核电荷数法建立了周期表中 90种元素的电负性新标度。χ=0 .41 2 3 -EV,该式表明电负性值与价键轨道能量的绝对值的平方根成正比 ,所得数值是一套无量纲的相对参数。元素电负性值随价态的升高与元素非金属性的增强相对应 ,元素电负性的大小不仅与单个成键电子有关 ,而且也与参加价键作用的多个电子甚至整个价层都有紧密的联系。氢的元素电负性值不同于Pauling值、等于 1 .52。用 1 6种氢化物中键的额外离子能Δ′对 (χA-χB) 2 作图 ,两者之间确实具有良好的线性关系。本方法充分体现了目前公认的三大电负性标度的优点 ,该标度同时也是价层电子在价键状态下的一种能量标度 ,是对元素周期律的定量描述和反映。     

Studies on monomer reactivity ratios. I. An electronegativity model

A model based on an electronegativity scheme is proposed for treatment of monomer reactivity ratios in free-radical bulk copolymerization. Values for each monomer are assigned to three parameters: a relative localization (or resonance stabilization) energy, a radical electronegativity, and a monomer electronegativity. Parameters for 17 monomers are given and calculated reactivity ratios are tabulated for a large number of copolymerizations. Agreement with experiment is usually obtained to within experimental error except for systems involving acrylonitrile. Computed parameters are rationalized on the basis of molecular orbital theory.     

A simple method for calculating reliable atomic charges in large molecules

Modifications are made to a previously developed scheme for calculating atomic charge which uses orbital electronegativity and which requires minimal calculational effort. The introduced changes are a result of deficiencies noted in the earlier method which were due to an inadequate accounting of effects from neighboring atom charges. Results obtained using the modified scheme for both model compounds as well as larger molecules of interest to biochemistry are compared to previous results and also to several levels of ab initio calculations. It is shown that a definite improvement is obtained and that the present method gives very good correlations with each calculational level. Comparisons are also made with other methods that use electronegativity theory. It is shown that the present scheme represents a definite improvement over alternate orbital electronegativity methods and is roughly comparable to a higher level scheme that utilizes atomic electronegativity values. A discussion comparing the latter method with the present one is included. Because of the small amount of calculational effort involved, the results indicate that the present method could be quite useful in providing reliable atomic charges for large molecular systems.     

Electronic properties of oxides: Chemical and theoretical approaches

An original analysis of the electronic and chemical properties of oxides is proposed based on the electronegativity χ and the chemical hardness η. This model which has been applied to various oxide based metals, degenerate semiconductors and optical properties of transition metal oxides allows explaining their electronic behaviors: Strong electronegativity and weak chemical hardness characterize oxides of transition elements with high oxidation state. Strong electronegativity and strong chemical hardness feature insulators with a large optical gap. Weak electronegativity and moderate chemical hardness describe alkali and alkaline earth oxides and weak electronegativity and strong chemical hardness are for ionic oxides with a relatively large optical gap. For a few illustrative case studies, ab intio electronic band structure calculations within the density functional theory framework are used.