Wilfried Mortier

The use of global and local molecular parameters for the analysis of the gas-phase basicity of amines.

It is demonstrated that the variation of the gas-phase basicities of amines can be analyzed by using two parameters: one global and one local (that is, site-dependent). Two global quantities (the average ”effective” electronegativity and the geometric average of the isolated-atom electronegativities) and two local quantities (the fukui function and the residual charges) are tested. A two-parameter linear model containing one global and one local quantity produces satisfactory correlations with the experimental gas-phase basicities. It is shown how to express the fukui function, which reflects the site reactivity in density functional theory f l i t ) = [ap(F)/aNl,, ,) , in terms of the variation of the Mulliken gross charges (4,) of an atom in a molecule, which is accomDanied with a change in the total number of electrons ( N ) in this molecule: f,’ = q, (N + 1) q,(N); f,= q,(N) qi(N .l) andf,’ = 1/2[qiTN + 1 ) qi (N I ) ] . The electron density distribution is fundamental for understanding chemical reactivity and naturally leads to explaining nucleophylic and electrophylic attack on the basis of electrostatic interactions. However, the electron density by itself does not explain everything, and it was recognized that the change in electron density under the influence of an approaching reagent is also of importance. The frontier-electron theory of chemical reactivity by Fukui’*2 recognizes the key role of the valence electrons in forming molecules and considers therefore the distribution of the highest energy orbital electron density as being most important for electrophilic attack and the lowest energy vacant orbitals in nucleophilic substitution reactions. In reactions with radicals both orbitals become important. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are in this way considered as the principal factors governing the easiness of chemical reactions and the stereoselective path. It was recently demonstrated by Parr et aL3q4 that most of the frontier-electron theory of chemical reactivity can be rationalized from the density functional theory of the electronic structure of molecules.5%6 For a system of N electrons with ground-state energy E[n,v] , where G‘ is the potential energy acting on an electron due to the presence of all nuclei, several quantities of fundamental importance can be defined. The chemical potential p of the electrons, Le., the negative of the electronegativity x, is given by p = (aE/am,( , and has the same value everywhere. The change of p with the number of electrons was defined by Parr and Pearson’ as a measure for the “absolute hardness” as 7 = (&.L/c~N),(, = (C3*E/aM),(,. The frontier function or fukui function for a University of North Carolina. *University of North Carolina and K. U. Leuven. molecule, reflecting the reactivity of a site, was defined by Parr and Yang3 as Le., the functional derivative of the chemical potential with respect to a change in the external potential or, because of the Maxwell relations, identical with the change in electron density accompanied with a change in the number of electrons. The hardness and the fukui function are local quantities and reflect the properties of the different sites within the molecule. Probing the reactivity of the different sites within a molecule, being of importance for understanding reaction mechanisms, only partially meets the chemists aspirations. A comparison of different molecules is also of importance. The object of the present study is to explore different ways to make this comparison by using easily obtainable parameters. Substituent group properties have been used extensively in this context. However, as the atomic properties change from molecule to molecule, so will substituent groups be subject to variations. Alkyl groups, e.g., may act either as electron donors or as electron acceptors, depending on the substrate. One of the properties of interest of substituent groups is their tendency to attract electrons, i.e., their electronegativity. Depending on the actual molecular configuration, this may qualify a group as electron donor or electron acceptor. In a study of the gas-phase (!) Fukui, K. Theory of Orientation and Stereoselection; Springer-Verlag: (2) Fukui, K. Science (Washington, DC) 1982, 218, 747-754. (3) Parr, R. G.; Yang, W. J. Am. Chem. SOC. 1984, 106, 4049-4050. (4) Yang, W.; Parr, R. G.; Pucci, R. J . Chem. Phys. 1984,81,2862-2863. ( 5 ) Hohenberg, P.; Kohn, W. Phys. Reu. 1964, 136, B864-8871. (6) Parr, R. G. Annu. Reu. Phys. Chem. 1983, 34, 631-656. (7) Parr, R. G.; Pearson, R. G. J . Am. Chem. So?. 1983,105, 7512-7516. Berlin, 1973; p 134. 0002-7863/86/1508-5708

Zeolite electronegativity related to physicochemical properties

01.50/0

An attempt to rationalize stretching frequencies of lattice hydroxyl groups in hydrogen-zeolites

Abstract The influence of the Al content and cation type on the physicochemical properties of aluminosilicates is explained using Sandersons electronegativity scale. Atomic charges can easily be derived from the compound formula and correlate with acidity, Tue5f8O vibrations and interactions of sorbed molecules with the cations. Complicated chemical systems can be compared on the basis of their composition and predictions of their overall properties can be made.

Intrinsic framework electronegativity: A novel concept in solid state chemistry

Hydrogen zeolites of different structure and chemical composition have been prepared and their lattice OH vibrations determined by high-resolution infrared spectroscopy. The frequency of all the lattice OH groups in these zeolites has been rationalized in terms of two concepts: (i) the frequency of the unperturbed OH groups — vibrating in structural elements larger than eight-membered rings — is linearly correlated with the Sanderson intermediate electronegativity of the zeolites, which in its turn varies with the chemical composition; (ii) hydroxyl groups vibrating either in 6- or 8-membered rings undergo a bathochromic shift with respect to their unperturbed positions. This shift is caused by electrostatic interactions with the nearest oxygen atoms and therefore inversely proportional to the average of the squared distances.

On the nature of the charged silver clusters in zeolites of type A, X and Y

The charge‐dependent ‘‘effective’’ electronegativity χα of an atom α in a molecule or crystal, i.e., χα=(χ0α+Δχα)+2(η0α +Δηα)qα +∑β≠αu2009qβ/Rαβ differs from the expression for the isolated atom (χ0α+2η0αqα) because of corrections to the electronegativity (Δχα) and hardness (Δηα), respectively, and due to the external potential generated by the surrounding charges qβ at distances Rαβ (new quantum‐mechanical proof given). After calibration of Δχα and Δηα for all atom types, ab initio charges can be reproduced to within a few hundredths of an electron by the electronegativity equilization method (using χav=χα=χβ⋅⋅⋅ and ∑βu2009qβ=constant). Application of this algorithm to the solid state (i.e., charges and external potential being generated in a self‐consistent way and using Ewald’s method for determining the Madelung potential) leads to a novel method for determining ‘‘ab initio quality’’ charges and the value of the average electronegativity. If applied to SiO2 polymorphs, a relation between the refractive index ...

Influence of the temperature and water adsorption on the cation location in NaY zeolites

The formation of charged silver clusters and the concomitant colour formation in zeolites of type A (NaAg-A) and faujasite-type zeolites was followed by X-ray powder structure determinations. Associations of three (or two) silver cations were located in faujasite-type zeolites with one silver cation in the centre of the hexagonal prism and two (or one) silver cation(s) in the adjacent sites after dehydration and oxygen treatment at 693 K. The number of such clusters increases with the Al content. The dimensions are comparable to analogous strings in A-type zeolites. The intensity of the colour (yellow) increases with the number of such clusters formed. The red colour in Ag-A type zeolites can be ascribed to two interaction clusters while in faujasite-type zeolites, no such interaction is possible.

Properties of zeolites in relation to their electronegativity: Acidity, carboniogenic activity and strength of interaction in transition metal complexes

Abstract The structures of hydrated and dehydrated Naue5f8Y zeolites were determined as a function of the temperature, using X-ray powder diffraction methods. No significant difference was observed between the dehydrated structures, determined at 323 K and at 723 K. In the presence of an equilibrium water pressure of 1870 Pa, an increased site I occupancy is observed, which at 723 K, is higher than in the absence of water molecules. Neither the site I, nor the site II populations reach the level of the occupancies of the dehydrated state. In neither case could all cations be located at the sites I, I, and II.

Influence of the structure type on the intrinsic framework electronegativity and the charge distribution in zeolites with SiO2 composition

Abstract The Sanderson electronegativity model was used on order to explain changes in overall properties of molecules in the gas phase, adsorbed on zeolites, and of functional groups in zeolites. Using the Sandersons concepts, the changes in gas phase basicity of primary, secondary and tertiary alkylamines could be related to the changes in their electronegativity. In zeolites with different structure, the changes in stretching frequencies and integrated absorption coefficients of hydroxyl groups vibrating in the main pores of the structure could be rationalized in terms of the residual charge on the proton as calculated by the theory. Changes in overall acidity of these zeolites as measured by the rate of dehydration of isopropanol per active centre could be related to compositional changes of these materials. Also the changes of the symmetric deformation frequency of ammonia adsorbed on Ag + ions in zeolites with different structure and chemical composition could be rationalized using the electronegativity model of Sanderson.

Theoretical investigations on the interaction of benzene with faujasite

The intrinsic electronic properties of 33 zeolite structure types with hypothetical SiO 2 composition were estimated by the Electronegativity Equalization Method. For each structure, the average electronegativity and the connectivity-dependent partial charges for silicon and oxygen are calculated. These constitute fundamental properties for systems described in density functional theory and should correlate with the physicochemical properties. The predominant factor determining the electronegativity is the framework density, the highest value coinciding with the most open framework type of faujasite. The charge distribution is site-dependent and largely confirms existing structural-chemical concepts. Framework electronegativity and framework ionicity are not related.

Oxidation and reduction of silver in zeolite Y: a structural study

The interaction of benzene with Faujasite (FAU) type structures with varying Si ∶ Al ratio and cation content has been analysed. Using the electronegativity equalization method (EEM), the charge perturbation of a benzene molecule located in the supercage (on the three-fold axis) and the interaction energies with the framework have been evaluated. On this basis two distinct adsorption sites have been identified in agreement with experiments. The relative strength of the benzene–framework interaction at either of these sites depends strongly on the aluminium content of the framework and, most importantly, on the nature of the charge-compensating cation. This dependence correlates with the electrostatic field gradient in the supercage.