Local Softness Research Articles

Local reactivity descriptors to predict the strength of Lewis acid sites in alkali cation-exchanged zeolites

Lewis acidity of alkali cation-exchanged zeolite is studied using local reactivity descriptors based on hard-soft acid–base (HSAB) concept. The local softness for nucleophilic attack ( s x +), local softness for electrophilic attack ( s x −) and their ratio, which is called `relative electrophilicity' ( s x +/ s x −), are calculated for the exchanged cations and Lewis acidity of the cations is found to decrease in the order: Li + > Na + > K + > Rb + > Cs +. Calculated blue shift of CO vibrational frequency (Δ ν) and interaction energy of CO molecule with alkali cation-exchanged zeolite clusters vary linearly with s x +/ s x − values.

Calculation of the Dipole Moment for Polypeptides Using the Generalized Born-Electronegativity Equalization Method: Results in Vacuum and Continuum-Dielectric Solvent

The electronegativity equalization methodology, EEM, is frequently used to calculate the charge distributionand reactivity index (e.g., local softness and hardness, condensed Fukui function) of molecules. However,recent work (Chelli, R. et al., J. Chem. Phys. 1999, 111, 8569) has shown a serious shortcoming of EEM inthe prediction of the polarizability for large molecules. In this paper, our goal is to show that we can obtaina reliable dipole moment for polypeptides in vacuum and continuum-dielectric solvent using the constrainedcharge approximation and the generalized Born-electronegativity equalization method. Different EEMparameterizations were tested and compared to the expected values of the dipole moment vector operator ascalculated at the ab initio B3LYP/6-311G(d,p) level. One EEM parameterization (Bakowies, D., Thiel, W.,J. Comput. Chem. 1996, 17, 87) when used with the constrained charge approximation and the generalizedBorn-electronegativity equalization method was comparable to the CM1 charge model (Storer et al.,J. Comput.-Aided Mol. Des. 1995, 9, 87) in the prediction of the dipole moment vector in vacuum and continuum-dielectric solvent, but was calculated with a much greater computational efficiency.

3D-QSAR analysis of metallocene-based catalysts used in ethylene polymerisation

Single site olefin polymerisation catalysts are suitable candidates for modelling purposes. Their well-defined structure and the almost complete elucidation of their polymerisation mechanisms, make these organometallic complexes ideal for studies based on quantitative structure-activity relationships (QSAR). Although the QSAR technique is extensively used in drug design, there are very few reports on its application to metallocene-based polymerisation catalysis. This probably has something to do with the difficulties inherent in controlling experimental conditions during the polymerisation process. In the present study, we obtained ethylene polymerisation data using a number of zirconocene catalysts under carefully controlled experimental conditions, i.e. keeping all polymerisation variables constant except catalyst structure. The catalytic activity and molecular weight of the resulting polyethylenes were experimentally determined. We then applied 3D-QSAR methodology to explain the experimental data in terms of three-dimensional (3D) field descriptors related to the structure of the metallocene catalyst. Our results provide useful correlations between experimental ethylene polymerisation activities and the steric, LUMO and local softness fields of the catalysts. The molecular weights calculated from comparative molecular field analysis (CoMFA) models including LUMO and local softness fields correlate well with the experimental ones. The predictive capacity of the models was also tested. Based on the proposed models, steric and electronic factors affecting polymerisation performance are discussed.

Use of DFT-Based Reactivity Descriptors for Rationalizing Radical Reactions: A Critical Analysis

Hydrogen abstraction by C2H, OH, CH3, CF3, C2H3, and C2H5 radicals from methane and propene and addition reactions of these radicals with substituted propenes have been investigated by using BHandHLYP/6-311G(d,p) level of theory. Transition states for all these reactions have been located. The reactivity of different radicals and substrates toward hydrogen abstraction and radical addition reactions has been critically analyzed by using density functional theory based reactivity descriptors, namely, local softness and electronegativity. The regiochemistry of the radical addition reaction has also been explained from the local softness values of the potential addition sites.

Adsorption of Small Molecules in Zeolites: A Local Hard—Soft Acid—Base Approach.

The interaction of small probe molecules with clusters chosen to represent Brφnsted acid sites in zeolites has been investigated using a local hard−soft acid−base approach. Condensed Fukui functions and local softness values of all the atoms of the probe molecules, namely, CH4, CO, and NH3, have been calculated, and these values are compared with that of the acidic hydrogen atoms of zeolite clusters to find out the most favorable adsorption sites. We calculated the interaction energy of the molecules using local HSAB principle, and the interaction energy values are in good agreement with the experimental results. Density functional and Hartree−Fock methods are used for the calculation of reactivity descriptors. We observed that the intermolecular interaction takes place via the atoms comparable softness values rather than via atoms of equal Fukui functions.

Global and Local Reactivity and Activation Patterns of HOOX (X = H, NO2, CO2-, SO3-) Peroxides with Solvent Effects

A detailed analysis of the global and local reactivity patterns of neutral and charged peroxides, including hydrogen peroxide (HOOH), peroxynitric acid (HOONO2), the peroxymonocarbonic ion (HOOCO2-), and the peroxymonosulfate ion (HOOSO3-) in the presence of a polar solvent (water) is presented. The polar solvent effects are included using an isodensity surface polarized continuum model (IPCM). The (1,2) hydrogen-shift reaction for these peroxides is studied. The transition states involved in the (1,2) hydrogen shift have been located and characterized at the B3LYP/6-311G** level of theory. The global analysis shows that, although the replacement of one hydrogen atom in HOOH by a neutral NO2 group enhances both the global softness and global electrophilicity, the substitution of one hydrogen atom by charged CO2- and SO3- groups results in a significant electrophilic deactivation of HOOCO2- and a moderate electrophilic activation of HOOSO3-. This result is observed in both the ground state and in some activated forms involved in the (1,2) hydrogen rearrangement. The local analysis, however, shows that the transition-state structures for the same rearrangement of HOOH and the HOOCO2- ion display an electrophilicity-enhanced pattern at the oxygen atom that is being deprotonated, thereby suggesting that this activated form is more likely to be involved in the oxygen-transfer reactions. The site-activation analysis performed on the basis of the variations in local softness from the ground to the transition states is also consistent with this local reactivity picture.

Importance of charge-transfer effects in [4+2] cycloaddition reaction

The self-consistent charge and configuration method for subsystems is used to study the [4+2] cycloaddition of 1- and 2-monosubstituted butadienes (C 4H 5X, X=CN, Cl, F, CH 3, OH, NH 2) with monosubstituted ethylenes (C 2H 3Y, Y=Cl, F, CH 3, OH). All calculations are performed for reactive systems (diene/dienophile) in the transition-state geometries. The system ‘electronic’ activation energies are decomposed into deformation, Heitler-London (the sum of electrostatic and exchange-repulsion), charge-transfer (CT) and polarisation components. It is shown that the CT energy, which is often considered as the starting point for constructing reactivity indicators such as: the frontier orbitals, local softnesses, and Fukui functions, is not responsible for regioselectivity. Such behaviour of CT energy suggests that these criteria have not energetic origin. Nevertheless, the CT energy is the only term, which is very sensitive to the type of substituent in diene and dienophile and depends on donor/acceptor character of the attached substituents. The results obtained confirm the ortho rule of regioselectivity for the normal, inverse, and neutral electron demand Diels-Alder reactions (1-substituted butadienes/ethylenes). The para rule is valid only for normal electron demand Diels-Alder reactions (2-substituted butadienes/ethylenes).

Inductive Electronegativity Scale. Iterative Calculation of Inductive Partial Charges

A number of novel QSAR descriptors have been introduced on the basis of the previously elaborated models for steric and inductive effects. The developed "inductive" parameters include absolute and effective electronegativity, atomic partial charges, and local and global chemical hardness and softness. Being based on traditional inductive and steric substituent constants these 3D descriptors provide a valuable insight into intramolecular steric and electronic interactions and can find broad application in structure-activity studies. Possible interpretation of physical meaning of the inductive descriptors has been suggested by considering a neutral molecule as an electrical capacitor formed by charged atomic spheres. This approximation relates inductive chemical softness and hardness of bound atom(s) with the total area of the facings of electrical capacitor formed by the atom(s) and the rest of the molecule. The derived full electronegativity equalization scheme allows iterative calculation of inductive partial charges on the basis of atomic electronegativities, covalent radii, and intramolecular distances. A range of inductive descriptors has been computed for a variety of organic compounds. The calculated inductive charges in the studied molecules have been validated by experimental C-1s Electron Core Binding Energies and molecular dipole moments. Several semiempirical chemical rules, such as equalized electronegativity's arithmetic mean, principle of maximum hardness, and principle of hardness borrowing could be explicitly illustrated in the framework of the developed approach.

Density Functional Study of the Complexation Reaction of Sn(CH3)3X (X = F, Cl, Br and I) with Halide Anions

AbstractThe Lewis acid‐base reaction between Sn(CH3)3X and Y− (with X, Y = F, Cl, Br and I) has been studied using quantum chemical calculations. Complexation energies were calculated at the Density Functional Theory (DFT) level and rationalized on the basis of a local application of the hard and soft acids and bases principle. It was observed that smaller differences in the local softness of the interacting sites in the Lewis acid and base correspond to stronger interactions. Moreover, the calculated sequences in complexation energies can be reproduced using equations containing chemical concepts introduced within the framework of conceptual density functional theory and rooted in the hard and soft acids and bases principle and referring only to the reactants. A method of treating the electronegativity and softness of the halide anions is presented based on a Taylor expansion of the electronegativity of the neutral halogens and the softness‐polarizability proportionality. Experimental evidence for the calculated sequences was gathered from measured 117Sn chemical shifts and 1J (13C‐119/117Sn) coupling constant changes upon complexation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

A Computational and Conceptual DFT Study of the Reactivity of Anionic Compounds: Implications for Enzymatic Catalysis

We present a DFT study on the reactivity of an alkoxide and a thiolate toward the neutral and all ionic forms of arsenate and phosphate. The isolated neutral, mono-, di-, and trianionic arsenate/phosphate (electrophiles) and thiolate/methanolate (nucleophiles) species are used as model systems of reacting partners in the enzymatic reactions catalyzed by arsenate reductase and phosphatase. The onset of the displacement reaction at the electrophilic center by the attacking nucleophile is described by the DFT descriptor local softness, applied in a HSAB context. The instability of multiply charged anions in gas phase necessitates the use of a solvent model with an appropriate dielectric constant mimicking the enzymatic environment. Gas-phase and solvent-reactivity studies of isolated compounds indicate that the nucleophilic attack of a thiolate during the first catalysis step of arsenate reductase (ArsC) and low molecular weight phosphatase (LMWPTPase) preferably occurs via the dianionic arsenate and phospha...

Local Softness versus Local Density of States as Reactivity Index

The definition of softness and local softness using the density of states (DOS) and local density of states (LDOS) given by Yang and Parr for metals has been casted into working equations and applied to sequences of finite cylindrical fullerenes modeling the (5,5) and (10,10) single-walled capped carbon nanotubes. The behavior of DOS, LDOS, and global and local softness as functions of length and tube type is investigated in the simplest Huckel approximation. DOS-based calculation of global softness gives results that depend less on the details of the electronic structure than those based on the finite-difference approximation. Armchair single-walled nanotubes with larger diameters are softer than those with smaller diameters at the same tube length. Local softness, after fluctuations in the end-cap region, settles down within the first 20 layers of hexagons of the tube body to a constant (diameter-dependent) value.

A theoretical approach to the regioselectivity in 1,3‐dipolar cycloadditions of diazoalkanes, hydrazoic acid and nitrous oxide to acetylenes, phosphaalkynes and cyanides

AbstractThe 1,3‐dipolar cycloadditions of diazoalkanes (R2CNN), hydrazoic acid (HNNN) and nitrous oxide (N2O) to some simple dipolarophiles such as acetylenes, phosphaalkynes and cyanides were studied using quantum chemical calculations at the B3LYP/6–311++G(d,p) level and, in the simplest case, using also the CCSD(T) method. Along with frontier molecular orbital coefficients, the local softness being a DFT‐based reactivity criterion is established as a useful tool to predict the regioselectivity in these cycloaddition reactions. Copyright © 2003 John Wiley & Sons, Ltd.

The interplay of electrostatic and covalent effects in 1-butene oxidation over vanadyl pyrophosphate

Abstract Quantum mechanical reactivity indices were computed from B3LYP calculations on 1-butene and active site clusters representing stoichiometric and phosphorous-enriched (VO) 2 P 2 O 7 (1 0 0) surfaces. Results indicate that 1-butene may chemisorb via formation of a π–cation complex at the stoichiometric (Open) surface, with comparison of local softnesses in 1-butene and the Open active site indicating a strong interaction. Coulombic repulsion at the phosphorous-enriched (Cleft) surface blocks vanadium sites from the CC region of 1-butene, precluding π–cation complex formation in the Cleft active site. The phosphorous-enriched surface naturally features a greater concentration of nucleophilic terminal PO oxygen species, which may promote CH, rather than CC, bond rupture in subsequent oxidation of adsorbed 1-butene. Hence, a rationale may be offered for both the ease of adsorption, and the low selectivity to maleic anhydride, observed experimentally when 1-butene undergoes oxyfunctionalisation over vanadyl pyrophosphate. Results also indicate that oxidative transformation of n -butane over (VO) 2 P 2 O 7 is unlikely to proceed via an olefin intermediate.

Adsorption of Small Molecules in Zeolites: A Local Hard−Soft Acid−Base Approach

The interaction of small probe molecules with clusters chosen to represent Brφnsted acid sites in zeolites has been investigated using a local hard−soft acid−base approach. Condensed Fukui functions and local softness values of all the atoms of the probe molecules, namely, CH4, CO, and NH3, have been calculated, and these values are compared with that of the acidic hydrogen atoms of zeolite clusters to find out the most favorable adsorption sites. We calculated the interaction energy of the molecules using local HSAB principle, and the interaction energy values are in good agreement with the experimental results. Density functional and Hartree−Fock methods are used for the calculation of reactivity descriptors. We observed that the intermolecular interaction takes place via the atoms comparable softness values rather than via atoms of equal Fukui functions.

Atom-Bond Electronegativity Equalization Method and its Applications Based on Density Functional Theory

The atom-bond electronegativity equalization method (ABEEM) and its applications for predicting intrinsic properties of large molecules, such as the charge distribution, the molecular energy, the local softness and the Fukui function, the regio- and stereo-selectivity of Diels–Alder reactions, the linear response function, and the charge polarization normal modes have been formulated. The examples show that there is a very good agreement of the ABEEM results with those of the corresponding ab initio quantum chemical calculations, demonstrating the reasonable and possible ABEEM's applications to the large molecular systems.

Philicity: A Unified Treatment of Chemical Reactivity and Selectivity

A generalized concept of philicity is introduced through a resolution of identity, encompassing electrophilic, nucleophilic, and radial reactions. Locally, a particular molecular site may be more prone to electrophilic attack or another may be more prone to nucleophilic attack, but the overall philicity of the whole molecule remains conserved. Local philicity is by far the most powerful concept of reactivity and selectivity when compared to the global electrophilicity index, Fukui function, local softness, or global softness because it contains information about almost all of the known global and local descriptors of chemical reactivity and selectivity.

Intermolecular reactivity study to scale adsorption property of para- and meta-substituted nitrobenzene over 2:1 dioctahedral smectite

Substituted nitrobenzene, a toxic series of molecules with its wide application in pesticides needs to be removed by a suitable sorbent, whose characteristic is to be determined using the reactivity index theory within the helm of the density-functional theory and as well inside the broad subject range of the hard–soft acid-base (HSAB) principle. We studied a series of meta- and para-substituted nitrobenzene molecules (NBMs) starting from –NH2, –OH, –OCH3, –CHO, –OCOCH3, –COCH3, –CF3, –CN, and –COOCH3 to monitor the effect of electrophilicity and nucleophilicity in their adsorption properties. From the values of the local softness and the charge on the hydrogen atom of the bridging/structural (occurring on the surface) hydroxyl attached to the octahedral/tetrahedral metal site present in smectite, it is concluded that the local acidities of the inorganic material systems are dependent on several characteristics which are of importance within the framework of HSAB principle. We used “group softness” for the interacting molecules, whose values were generated by adding the local softness values of constituent atoms of the respective group and found it can be a better descriptor for intermolecular interactions occurring in these NBMs. A semiquantitave scale is proposed from the group reactivity index present in NBM and the clay cluster.

Silaaromaticity in Polycyclic Systems: A Computational Study.

Density functional theory (B3LYP) calculations were performed to examine the effect of Si substitution on the aromaticity of some polycyclic hydrocarbons using geometric criterion (HOMA), isodesmic isomerization reactions, homodesmotic equations, NICS values, chemical hardness, and out-of-plane distortive tendencies. The HOMA values are lower and the NICS values are higher in the Si-substituted rings compared to those in the hydrocarbon counterpart, whereas the homodesmotic equations predict little loss of aromaticity upon Si replacement in polycylic systems. The chemical hardness values decrease and the out-of-plane distortive tendency increases upon silicon substitution. The relative energies of the positional isomers and the causative factors are analyzed. The high reactivity of some silaaromatics toward dimerization is explained based on local softness indices.

2,3,7,8-Tetrachloro dibenzo- p-dioxin can be successfully decomposed over 2:1 dioctahedral smectite—a reactivity index study

2:1 Dioctahedral smectite family has shown its capability to decompose 2,3,7,8-tetrachloro dibenzo- p-dioxin (TeCDD) using the active hydroxyl hydrogen attached with the central octahedral aluminum, as monitored using density functional theory (DFT). From the values of the local softness and the charge on the hydrogen atom of the bridging/structural (occurring on the surface) hydroxyl attached to octahedral/tetrahedral metal site present in smectite used as a first approximation to the local hardness, it is concluded that the local acidities of the inorganic material systems are dependent on several characteristics which are of importance within the framework of hard–soft acid–base (HSAB) principle. The first step in this process of decomposition is the abstraction of chlorine bound to TeCDD using surface hydrogen of smectites. This results in non-chlorinated dibenzo- p-dioxin (NCDD), which is less toxic than TeCDD. The second step is the formation of a dative bond between oxygen of NCDD and hydroxyl proton of smectite, with the breaking of CO bond of NCDD. The reaction mechanism is postulated within the helm of DFT using Fukui functions for all possible chlorinated and non-chlorinated dioxin varieties along with clay clusters. The material is identified to act for the decomposition of dioxin.

A DFT Study of Tin‐ and Crown‐Ether‐Based Host Molecules Capable of Binding Anions and Cations Simultaneously

AbstractDFT calculations are reported for a series of tin‐ and crown‐ether‐based host compounds {i.e., the trimethyl derivatives [18]‐crown‐6‐C6H3COOSn(CH3)3and [15]‐crown‐5‐C6H3COOSn(CH3)3} capable of binding cations and anions simultaneously. The B3LYP functional together with the 6‐31G* basis set was used for the atoms C, H, N, O, S, Na and K and the 3‐21G* basis set for Sn in order to obtain insights into the factors determining the nature of the interactions of these compounds with the neutral molecules acetone and H2O, the SCN−anion, and the Na+and K+cations. The interaction strength pattern with these molecules was explained by the use of a series of reactivity descriptors such as the Fukui function, hardness, local softness, and the MEP (molecular electrostatic potential). In all cases studied in this paper, the complexes with Na+were more stable than those with K+, correlating with the size of the cation and the volume available in the crown ether. Moreover, this finding is also in accordance with the greater hardness of Na+relative to K+, in combination with the hard environment of the crown ether moiety. This region was also analysed by computation of electrostatic potentials, which showed that highly negative values are associated with the inside region of the cavity of the crown, this region thus being amenable to electrophilic attack. The HSAB principle, characterising the reactive sites on the basis of local softness and the Fukui function, provided a firm explanation of the reactivity of the tin atom of the crown ether benzocarboxylate towards SCN−, acetone and water. The HSAB concept was also successfully used to explain the preference of the tin atoms in both crown ethers to bind with the nitrogen atom rather than the sulfur atom of SCN−. This result is a confirmation that the tin atoms in the compounds under consideration behave as hard atoms. Overall, these results fit remarkably well with previous experimentally measured NMR spectroscopy data and demonstrate that the interactions of this kind of molecules can be predicted and interpreted by the use of DFT calculations and DFT‐based reactivity descriptors, as well as MEP calculations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)