Acidity and Basicity Concepts: The Density Functional Viewpoint Based on an Atoms-in-Molecules Sensitivity Analysis - a Tutorial

Baekelandt, BG; Janssens, Goa; Toufar, Helge; Mortier, Wilfried; Schoonheydt, Robert A.

doi:10.1007/978-94-011-0986-4_5

Cited by 7 publications

(10 citation statements)

References 24 publications

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“…The fact that the atomic charges and the correlation with the DFT-AIM charges do not undergo important changes does not necessarily mean that derived quantities would not undergo changes. Fukui functions derived from EEM for example, depend on the characteristics of the hardness matrix, especially the degree of diagonal dominance. − , This will naturally differ between the both types of hardness matrix.…”

Section: Resultsmentioning

confidence: 99%

“…N of these equations are obtained by equilibrating the individual atomic electronegativities to the molecular electronegativity (χ eq = χ 1 = χ 2 = ··· = χ N ), whereas one supplementary equation is obtained by constraining the sum of the atomic charges to equal the total molecular charge ( Q = q α ). In matrix form, this may be written as Finally, in addition to the molecular electronegativity and atomic charges, the EEM framework also allows a straightforward and transparent calculation of other fundamental properties such as the total electronic energy, hardness, and reactivity indices, such as Fukui functions and local softness. − A recent overview of the status and theory of EEM within the broad context of conceptual DFT can be found in Geerlings et al Chattaraj et al. recently published a review of reactivity descriptors from DFT, including those related to or obtainable from EEM…”

Section: Introductionmentioning

confidence: 99%

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High-Speed Calculation of AIM Charges through the Electronegativity Equalization Method

Bultinck¹,

Vanholme²,

Popelier³

et al. 2004

J. Phys. Chem. A

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The applicability of the electronegativity equalization method (EEM) is investigated for the calculation of atoms-in-molecules charges in prototypical organic molecules. A large training set of molecules was composed, comprising H, C, N, O, and F. Geometries and atomic charges are calculated at the B3LYP/6-31G* level, and from the calculated charges, effective electronegativity and hardness values are calibrated in a least-squares fashion. The quality of the EEM charges is assessed by comparison with B3LYP/6-31G* charges calculated for a test set of amino acids and the neuroleptic fluanisone, not contained in the training set. The EEM approach is found to be a very powerful way to obtain AIM charges without the computational cost of the ab initio approach.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

High-Speed Calculation of AIM Charges through the Electronegativity Equalization Method

Bultinck¹,

Vanholme²,

Popelier³

et al. 2004

J. Phys. Chem. A

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“…Trends in average oxygen charges can therefore be used as indicators of framework or structural basicity of zeolites. Aluminium being harder than Si not only increases the global hardness of the faujasite framework, but also softens the oxygens . This softening is expressed here by the increase of the averaged negative charge and thus of the basicity.…”

Section: Discussionmentioning

confidence: 98%

“…The EEM method − is a semi-empirical density functional theory method. It assumes an atom-in-molecule resolution, where each atom carries an effective charge q α .…”

Section: Description Of the Modelmentioning

confidence: 99%

Charge Sensitivity Analysis of Intrinsic Basicity of Faujasite-Type Zeolites Using the Electronegativity Equalization Method (EEM)

et al. 1996

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The Electronegativity Equalization Method (EEM) is used to study the dependence of the basicity of faujasitetype zeolites on the Si:Al ratio, the cation type, and the type of oxygen. The model structures were obtained by distance least-squares optimization within the F222 space group, and cations were located with the Monte Carlo technique, starting from experimental XRD positions. The main effect determining basicity, as indicated by the average negative charge on the oxygens, is the chemical composition of the framework. The average negative oxygen charge increases with decreasing Si:Al ratio, in agreement with the experimentally found increase of basicity. The secondary structure effect identifies the supercage oxygens O1 and O4 as the most basic. The details of the cation distribution further controls the basicity of these oxygens, which increases when Na + is replaced by Cs + on sites II and, especially, site III, also in agreement with experiment. Besides charges on oxygen, Fukui indices and regional sensitivities are also good indicators of basicity.Brønsted acid sites in zeolites, the so-called bridging hydroxyls tSisOHsAlt, have been extensively studied, both experimentally and theoretically. 1-8 Its acidity is chemical composition and structure dependent, the former being the most important and increases with increasing Si:Al ratio. As soon as the Si:Al ratio has reached the level that, statistically, no next nearest neighboring Al atoms (NNN) are present, the maximum acidity is reached. 4-7 This was theoretically addressed recently using the electronegativity equalization method (EEM). 8 Following a classical acid-base pair concept the surface oxygen is the conjugated base of the acidic bridging hydroxyl. The basicity of these oxygens is called the structural or intrinsic framework basicity. 9-11 As for the bridging hydroxyl we expect the properties of the basic sites to depend on the chemical composition (Si:Al ratio and the exchangeable cations) as well as on the structure type.Up to now, the intrinsic basicity of zeolites has been studied experimentally, either directly, by looking at the properties of the framework atoms, [9][10][11][12] or indirectly, by adsorption of probe molecules. [13][14][15] The O(1s) binding energy, deduced from XPS spectra, reflects the electron density of the oxygen atom in the framework. It decreases with increasing Al content, but at a given Si:Al ratio it increases with increasing electronegativity of the cation. 16,17 A decrease of the O(1s) binding energy is equivalent to an increase of the electron density around the O nucleus, or an increase of the negative charge on the oxygen, thus reflecting an increased basicity. XPS, however, gives only general trends and no distinction can be made between structurally different atoms. Similar results are obtained from calculations of averaged charges on framework oxygens with Sanderson's model of electronegativity: 18 the average negative charge on structural oxygens increases with the Al content of the framework and with dec...

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“…In the present study, the electronegativity equalization method ͑EEM͒, as introduced by Mortier et al, 11,12 is used to calculate atom condensed Fukui functions. In EEM, conceptual DFT quantities, like atom condensed Fukui functions, may be calculated employing separate systems of matrix equations as introduced by Baekelandt et al 13 and by Bultinck et al 14 An alternative method exists in using algebraic relations between these quantities and the molecular hardness matrix. 15 A complete discussion of EEM is well outside the scope of the present article, and the reader is referred to the work by Mortier et al 11,12 and subsequent work by Bultinck et al 14 -18 In short, EEM allows the calculation of atomic charges from the hardness matrix.…”

Section: ͑2͒mentioning

confidence: 99%

Negative Fukui functions: New insights based on electronegativity equalization

Bultinck

Carbó‐Dorca

Langenaeker³

2003

The Journal of Chemical Physics

120

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Fukui functions have been calculated for large numbers of organic molecules, and were found to always be positive. Numeric and algebraic considerations allowed the identification of several boundary conditions for negative values for Fukui functions. Negative Fukui functions are found to be very unlikely, except when very short interatomic distances are present. Recent hypotheses concerning the occurrence of negative Fukui functions are strongly supported by the present approach.

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Acidity and Basicity Concepts: The Density Functional Viewpoint Based on an Atoms-in-Molecules Sensitivity Analysis - a Tutorial

Cited by 7 publications

References 24 publications

High-Speed Calculation of AIM Charges through the Electronegativity Equalization Method

High-Speed Calculation of AIM Charges through the Electronegativity Equalization Method

Charge Sensitivity Analysis of Intrinsic Basicity of Faujasite-Type Zeolites Using the Electronegativity Equalization Method (EEM)

Negative Fukui functions: New insights based on electronegativity equalization

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