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.     

Modified Karplus equations and their stereochemical applications

The current status of research on the Karplus equation for the vicinal spin—spin coupling constants (SSCC)³ J _HH, of fundamental significance in applications of NMR spectroscopy for establishing the configuration and conformational features of organic compounds in solutions, is discussed in the review. The data on the analysis of the fundamental molecular factors that affect the³ J _HH SSCC were generalized: the dihedral angle between interacting protons, the electronegativities of - and -substituents, the H-C-C valence angles, the C-C bond length, and effects of "proximity of substituents." The stereochemical applications of different modifications of the Karplus equation proposed in recent years were examined and comparatively analyzed. Possible ways of improving the accuracy of predicting the³ J _HH SSCC by using modern modifications of the Karplus equation and the method of molecular mechanics were discussed.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 7, pp. 1483–1506, July, 1992.     

The reactivity of organoboranes in radical substitution reactions

The parameters characterizing substitution reactions of the types R^· + BR₃, RO^· + BR₃, ROO^· + BR₃, and RS^· + BR₃ were calculated from the experimental data using the parabolic model of bimolecular radical reaction. Along with the enthalpy of the reaction, the following factors affect the activation energy: triplet repulsion in the transition state, difference in electronegativities of the atoms forming the reaction center, -bonds in the -position to the reaction center, steric hindrances, and force constants of the reacting bonds. The change in the dissociation energy of the B--C bond in organoboranes, in which alkyl substituents were replaced by alkoxyl and thiyl substituents, was estimated from the kinetic data. The parameters obtained make it possible to calculate the activation energies of individual reactions of four types under study.     

B_nAH型分子中A─H键振动频率与基团B_nA的电负性

Ｂ_ｎＡＨ型分子中Ａ─Ｈ键振动频率与基团Ｂ_ｎＡ的电负性宋小平，韩长日（海南师范学院化学系海口５７１１５８）关键词共价氢化物分子，振动频率，基团电负性对于ＢｎＡＨ型分子中Ａ─Ｈ键的振动频率的规律性，国内外学者从不同角度进行了较多研究，其中运用元素电负性...     

异构现象与最大硬度原理

利用原子-键电负性均衡方法计算了700多个异构体的硬度, 通过与标准生成焓所确定的相对稳定性比较后发现, 多数异构体并不遵守最大硬度原理.     

分子电性距离矢量用于三嗪类化合物的构效关系研究

采用分子电性距离矢量(Molecular Electronegativity Distance Vector,MEDV)表征了三嗪类化合物的分子结构,并运用多元线性回归(Multiple Linear Regression,MLR)建立了该类化合物结构与其发光菌和大型蚤毒性的定量结构-毒性相关(Quanti-tative Structure-Toxicity Relationship,QSTR)模型,同时采用留一法交互检验对所建模型进行了分析和验证,建模计算值的相关系数R分别为0.970和0.952,留一法交互检验预测值的相关系数RLOO分别为0.917和0.921,并进一步阐述了结构与毒性之间的关系。结果表明,三嗪环上π电子离域程度减小有利于毒性增加,侧链N上取代基数目增加,化合物毒性减小。为进一步预测该类化合物的毒性,进行药物筛选提供了有效的理论依据。     

主族金属元素电子脱出功与拓扑指数~mP的相关性

利用Pauling电负性xpi、原子的价电子数mi和原子的价电子层数ni构建了点价iδ.由点价iδ构建了拓扑指数mP.利用其0阶指数0P与23种金属元素电子脱出功关联,拟合成3个线性回归方程.相关系数为0.9803,0.9870和0.9878,均优于文献值.预测取得了较好的结果.     

Application of the electronegativity indices of organic molecules to tasks of chemical informatics

An efficient structure filtration method for the operation with chemical databases containing information on the structures and properties of organic molecules was proposed. The technique involves the use of electronegativity indices for generation of identification keys and for isomorphism tests of the molecular graphs corresponding to the structural formulas. The test set for the method proposed included a total of 95,000,000 molecules containing up to sixty carbon atoms. Tests revealed a high discriminating capability of the electronegativity indices and high efficiency of the method for solving both general problems (recognition of chemical structures, chemical database management systems) and specific tasks (generation of molecular graphs, etc.) in chemical informatics. Dedicated to Academician N. S. Zefirov on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2166–2176, September, 2005.     

Electronegativity force of cations and thermal decomposition of complex fluorides

The thermal decompositions of anhydrous fluorosilicates M₂SiF₆ (M=Li, Na, K, Rb, Cs) and MSiF₆ (M=Ca, Sr, Ba) were investigated. The decompositions proceeded according to a simple acid-base mechanism with evolution of SiF₄. The influence of the cationic counter-ion acidity (expressed by means of the electronegativity force) on the course of thermal decomposition was estimated quantitatively.