Frontier orbital and density functional study of the variation of the hard–soft behavior of monoborane (BH3) and boron trifluoride (BF3) as a function of angles of reorganization from planar (D3h) to pyramidal (C3v) shape

In chemical response the BH₃ and BF₃ molecules undergo the physical process of planar (D_3h) to pyramidal (C_3v) reorganization in shape as the condition precedent to the event of chemical reaction under the requirement of symmetry. A frontier orbital and density functional study of the variation of the stability of electronic structures and chemical reactivity of associated with the physical process of D_3h to C_3v geometry reorganization has been performed. The theoretical parameters viz. eigenvalues of HOMO and LUMO, the HOMO and LUMO energy gap, the global hardness and global softness, the chemical potential, the condensed Fukui function, and local softness of B atom, the reaction site, have been computed over a wide range of ∠XBX angles. The nature of variation in the intrinsic chemical reactivity, global and local, of the molecules associated with their geometry reorganization during the chemical event of charge transfer interaction involving their frontier molecular orbitals has been quantitatively explored. The hardness profiles as a function of reaction coordinates are consistent with the principle of maximum hardness (PMH). Results demonstrate that the hardness and softness are not a static and invariable property of molecules but a dynamic and variable function of molecular structure. The hardness parameters and the HOMO–LUMO gap of the molecules are so modified with the distortion of molecular geometry that, after a certain stage of molecular deformation, the profiles of such parameters of the molecules intersect and cross each other, signifying that the relative order of the intrinsic hardness of their equilibrium geometry is reversed. The intrinsically hard molecule BF₃ becomes softer than the intrinsically soft molecule BH₃ as a consequence of structural distortion. The increase in chemical reactivity computed in terms of density functional parameters are transparent and justified in terms of the profiles of the eigenvalues of the frontier orbitals. The profiles of chemical potential reveal the inherent difference in the tendency of backdonation from two molecules. The computed values of Fukui functions and local softness parameters of the B atom site demonstrate that the concept of local softness can be exploited for a theoretical analysis and understanding of the characteristic chemical events of the molecules under consideration. The profiles of the Fukui functions and local softness parameters of the two molecules seem to reflect and reveal their intrinsic difference in the tendency of receiving donation in the LUMO (electrophilicity) and that of backdonation from the HOMO (nucleophilicity) and the inherent difference of overall reactivity of the two molecules by a simultaneous operation of two opposing processes of charge transfer. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Quantifying Electronic Effects in QM and QM/MM Biomolecular Modeling with the Fukui Function

Multi-scale quantum-mechanical/molecular-mechanical(QM/MM) and large-scale QM simulation provide valuable insight into enzyme mechanism and structure-property relationships. Analysis of the electron density afforded through these methods can enhance our understanding of how the enzyme environment modulates reactivity at the enzyme active site. From this perspective, tools from conceptual density functional theory to interrogate electron densities can provide added insight into enzyme function. We recently introduced the highly parallelizable Fukui shift analysis(FSA) method, which identifies how frontier states of an active site are altered by the presence of an additional QM residue to identify when QM treatment of a residue is essential as a result of quantum-mechanically affecting the behavior of the active site. We now demonstrate and analyze distance and residue dependence of Fukui function shifts in pairs of residues representing different non-covalent interactions. We also show how the interpretation of the Fukui function as a measure of relative nucleophilicity provides insight into enzymes that carry out S_N2 methyl transfer. The FSA method represents a promising approach for the systematic, unbiased determination of quantum mechanical effects in enzymes and for other complex systems that necessitate multi-scale modeling.     

Regiocontrolled Nitration of 4-Quinolones at Ambient Conditions

Regiocontrolled nitration of 4-quinolone, the highly privileged scaffold, has been developed at ambient conditions. The nitro group can selectively be introduced at diverse positions simply by tuning the reactivity of the moiety. Discrimination is being achieved through the selective functionalization of the free N-H group. The functional group has been screened theoretically with the help of Fukui function and local softness calculation. Theoretical predictions are synchronized well with the experimental findings. Finally, this nitration technique allows quick access to the structurally diverse 4-quinolones.     

Fukui函数和局域软度应用于亲电加成反应的区位选择性的研究

Fukui函数、局域软度、广义Fukui函数以及广义软度通常被称为反应描述符。使用它们研究和探讨了HCl与不对称烯烃以及溴苯硒与不对称苯乙烯的亲电加成反应的区位选择性。在MP2/6-311++G(d, p)理论水平下,采用有限差分方法计算这些反应描述符,同时也使用ABEEMσπ方法进行了计算。ABEEMσπ模型下的局域软度和广义局域软度,分别结合局域硬-软酸碱(HSAB)原理,得出亲电试剂氯化氢与溴苯硒,更容易进攻不对称乙烯和苯乙烯中的马氏碳原子,符合马氏规则。而有限差分方法不能完全地解释该系列反应的区位选择性。此外,主要产物所对应的马氏碳原子的广义局域软度值,就能够预测出此类反应的活性序列,所得结果与速率常数有很好的关联。     

Quantum chemical studies on some thiadiazolines as corrosion inhibitors for mild steel in acidic medium

Density functional theory at the B3LYP/6-31G(d,p) basis set level was performed on three thiadiazolines, namely 4-chloro-N-(5-phenyl-1,3,4-thiadiazol-2(3H)-ylidene)aniline (TD01), 4-chloro-N-(5-(4-methoxyphenyl)-1,3,4-thiadiazol-2(3H)-ylidene)aniline (TD02), and 2-(5-(4-chlorophenylimino)-4,5-dihydro-1,3,4-thiadiazol-2-yl) phenol (TD03), and the inhibitive effect of these thiadiazolines against the corrosion of mild steel in acidic medium is elucidated. The calculated quantum chemical parameters correlated to the inhibition efficiency are E_HOMO (highest occupied molecular orbital energy), E_LUMO (lowest unoccupied molecular orbital energy), the energy gap (ΔE) hardness (η), softness (S), dipole moment (μ), electron affinity (EA) ionization potential (IE), the absolute electro negativity (χ), and the fraction of electron transferred (ΔN). The decreasing order of %IE of the thiadiazolines studied was found to be in agreement with experimental corrosion inhibition efficiencies. The local reactivity has been analyzed through the condensed Fukui function and local softness indices using population analysis.     

An example where orbital relaxation is an important contribution to the Fukui function

Density-functional electronic structure calculations are performed on the molecules Cr2(hpp)4, Mo2(hpp)4, and W2(hpp)4, where the bridging ligand, hpp, is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine. The calculated electronic densities are used to determine the Fukui functions. These molecules are unique not only in their ability as electron donors but also because orbital relaxation plays a decisive role in their reactivity. Unlike other examples in the literature, the reactivity of these compounds cannot be expressed solely in terms of the highest occupied and lowest unoccupied Kohn-Sham orbitals but only using the Fukui function, which includes the effects of orbital relaxation.     

Local and nonlocal density functional calculations of the molecular structure of isomeric thiadiazoles

The chemistry of thiadiazoles and their derivatives is of considerable interest in chemistry owing to their pharmacological and potential industrial applications. In this context, a detailed study of isomeric thiadiazole molecules has been done using local (SVWN; Slater, and Vosko, Wilk and Nusair) and nonlocal (BLYP; Becke, and Lee, Yang and Parr) density functionals and optimizing the molecular geometries by means of the gradient technique. A charge sensitivity analysis of the studied molecule has been performed by resorting to density functional theory, obtaining several sensitivity coefficients such as the molecular energy, net atomic charges, global and local hardness, global and local softness and Fukui functions. With these results and the analysis of the dipole moments, the molecular electrostatic potentials and the total electron density maps, several conclusions have been inferred about the preferred sites of chemical reaction of the studied compounds. The condensed Fukui functions are shown to be one of the best criteria for predicting chemical reactivity.     

亲电取代反应中活性位点预测方法的比较

预测发生亲电取代反应的活性位点具有重要的理论和实际意义.目前已提出了许多基于反应物自身电子结构的预测方法.本文选择14个单取代苯和8个双取代苯作为测试集,对14种预测方法的可靠性进行了详细的比较分析.结果表明,福井函数、平均局部离子化能等体现局部电子软度的方法特别适合含有邻对位定位基的单取代苯和双取代苯体系,但对于含有单个间位定位基的体系,这类方法往往预测失败.基于静电效应的预测方法整体表现明显不如体现局部软度的方法,但更适合含有单个间位定位基的体系.对所有体系预测能力最稳健的是双描述符,因此可以作为普适性的预测方法.     

Normal (decoupled) representation of electronegativity equalization equations in a molecule

The decoupled (normal) representation of the electronegativity (chemical potential) equalization equations, in which the hardness tensor {η_ij}={?μ_i/?N_j} becomes diagonal, is examined in the atoms-in-a-molecule (AIM ) approximation; μ_i=?E/?N_i is the chemical potential of the i-th AIM , N_i is its electron population, and E is the system energy. All relevant chemical potential, hardness, softness, and Fukui function quantities corresponding to the normal electron redistribution channels, Q_y, are discussed and expressed in terms of the canonical AIM parameters. The normal chemical potentials, μ _γ=?E/?Q_γ, provide a natural classification of the normal modes into three groups: (a) acceptor normal modes (μ _a < 0, positive mode Fukui function, hardness, and softness), (b) donor normal modes (μ _d > 0, negative mode Fukui function, hardness, and softness), and (c) polarization normal modes (μ _p=0, zero mode Fukui function, hardness, and softness). The implications of the normal mode analysis for the theory of chemical reactivity are briefly summarized.     

Comparison among four different ways to condense the Fukui function

Four different ways to condense the Fukui function are compared. Three of them perform a numerical integration over different basins to define the condensed Fukui function, and the other one is the most traditional Fukui function using Mulliken population analysis. The basins are chosen to be the basins of the electron density (AIM), the basins of the electron localization function (ELF), and the basins of the Fukui function itself. The use of the last two basins is new and presented for the first time here. It is found that the last three methods yield results which are stable against a change in the basis set. The condensed Fukui function using the basins of the ELF is not able to give information on the reactivity of an acceptor molecule. In general, the condensed Fukui function using the basins of the density or the basins of the Fukui function describe the reactivity trends well. The latter is preferred, because it only contains information about the Fukui function itself and it gives the right information for donor as well as acceptor centers.     

二元氯化物熔体中锂和钠离子析出电位的测定及去极化作用的研 究

本文利用I-V曲线法测定了锂和钠离子在二元混合氯化物体系(LiCl-KCl;NaCl-KCl)中的析出电位,为利用熔盐电解法制备低钠高纯铝-锂合金提供了理论依据。根据锂离子在铝阴极上产生的去极化值求出铝和锂发生合金化反应的热力学意义,提出二元合金类型与极化类型之间的经验关系。首次通过研究锂离子在铝合金电极上的去极化行为,发现某些第三合金元素的存在可以加强锂离子的去极化作用,为进一步制取低钠高纯三元铝-锂-M合金系列提供了参考。还研究了在氯化钾-氯化锂体系中,氯化锂浓度对锂离子在铝阴极上析出电位的影响,求出了氯化锂的平均活度系数。     

亲电取代反应中活性位点预测方法的比较

预测发生亲电取代反应的活性位点具有重要的理论和实际意义. 目前已提出了许多基于反应物自身电子结构的预测方法. 本文选择14 个单取代苯和8 个双取代苯作为测试集,对14 种预测方法的可靠性进行了详细的比较分析. 结果表明,福井函数、平均局部离子化能等体现局部电子软度的方法特别适合含有邻对位定位基的单取代苯和双取代苯体系,但对于含有单个间位定位基的体系,这类方法往往预测失败. 基于静电效应的预测方法整体表现明显不如体现局部软度的方法,但更适合含有单个间位定位基的体系. 对所有体系预测能力最稳健的是双描述符,因此可以作为普适性的预测方法.     

Phenomenological chemical reactivity theory for mobile electrons

We present herein a model to deal with the chemical reactivity, selectivity and site activation concepts of π electron systems derived by merging the classical Coulson–Longuet-Higgins response function theory based on the Hückel molecular orbital (HMO) theory and the conceptual density functional theory. HMO-like expressions for the electronic chemical potential, chemical hardness and softness, including their local counterparts, atomic and bond Fukui functions and non-local response functions are derived. It is shown that sophisticated non-local concepts as site activation may be cast into deeper physical grounds by introducing a simplified version of static response functions. In this way, useful quantities such as self and mutual polarizabilities originally defined through the HMO parameters can be redefined as self and mutual softnesses. The model is illustrated by discussing the classical Hammett free energy relationship describing inductive substituent effects on the reactivity of benzoic acids.     

First Principles Study of Al-Li Intermetallic Compounds

The structural properties, heats of formation, elastic properties, and electronic structures of four compositions of binary Al-Li intermetallics, Al₃Li, AlLi, Al₂Li₃, and Al₄Li₉, are ana-lyzed in detail by using density functional theory. The calculated formation heats indicate a strong chemical interaction between Al and Li for all the Al-Li intermetallics. In partic-ular, in the Li-rich Al-Li compounds, the thermodynamic stability of intermetallics linearly decreases with increasing concentration of Li. According to the computational single crystal elastic constants, all the four Al-Li intermetallic compounds considered here are mechani-cally stable. The polycrystalline elastic modulus and Poisson's ratio have been deduced by using Voigt, Reuss, and Hill approximations, and the calculated ratios of bulk modulus to shear modulus indicate that the four compositions of binary Al-Li intermetallics are brittle materials. With the increase of Li concentration, the bulk modulus of Al-Li intermetallics decreases in a linear manner.