International Journal of Molecular Sciences

63 KiB

Open AccessArticle

Nature of Copper Active Sites in CuZSM-5: Theory and Experiment

by Ewa Broclawik, Jerzy Datka, Barbara Gil and Pawel Kozyra

Int. J. Mol. Sci. 2002, 3(4), 435-444; https://doi.org/10.3390/i3040435 - 25 Apr 2002

Cited by 15 | Viewed by 9368

We report here a concise resume reporting the way of constructing the model of an active site composed of transition metal cation exchanged in zeolites. The main goal was to devise the model of CuZSM-5 capable of describing geometrical and electronic properties of [...] Read more.

We report here a concise resume reporting the way of constructing the model of an active site composed of transition metal cation exchanged in zeolites. The main goal was to devise the model of CuZSM-5 capable of describing geometrical and electronic properties of metal sites and adsorption complexes with small molecules. The models were built up starting from simple ring structures encountered in ZSM-5 framework to fused rings’ model selected as the representative of α position for hosting the exchanged cation. Geometrical and electronic properties of the basal model, composed of the extended framework cluster with Cu⁺ or Cu²⁺ cation, and adsorption complexes with diatomic molecules were extracted from DFT calculations. The stress was put here on direct confirmation of structural changes on copper reduction/oxidation and adsorption. Electron donor/acceptor properties of the sites combined with electronic properties of adsorbed molecules led to the proposal for the mechanism of NO activation by Cu⁺ZSM-5: transfer of electrons from copper d orbitals to antibonding states of NO should cause large weakening of the bond, which was evidenced also by IR measurements. Full article

(This article belongs to the Special Issue Application of Density Functional Theory)

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425 KiB

Open AccessArticle

Zeolite-Catalyzed Hydrocarbon Formation from Methanol: Density Functional Simulations

by Niranjan Govind, Jan Andzelm, Kurt Reindel and George Fitzgerald

Int. J. Mol. Sci. 2002, 3(4), 423-434; https://doi.org/10.3390/i3040423 - 25 Apr 2002

Cited by 49 | Viewed by 12635

Abstract

We report detailed density functional theory (DFT) calculations of important mechanisms in the methanol to gasoline (MTG) process in a zeolite catalyst. Various reaction paths and energy barriers involving C-O bond cleavage and the first C-C bond formation are investigated in detail using [...] Read more.

We report detailed density functional theory (DFT) calculations of important mechanisms in the methanol to gasoline (MTG) process in a zeolite catalyst. Various reaction paths and energy barriers involving C-O bond cleavage and the first C-C bond formation are investigated in detail using all-electron periodic supercell calculations and recently developed geometry optimization and transition state search algorithms. We have further investigated the formation of ethanol and have identified a different mechanism than previously reported [1], a reaction where water does not play any visible role. Contrary to recent cluster calculations, we were not able to find a stable surface ylide structure. However, a stable ylide structure built into the zeolite framework was found to be possible, albeit a very high reaction barrier. Full article

(This article belongs to the Special Issue Application of Density Functional Theory)

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83 KiB

Open AccessArticle

The O-H Bond Dissociation Energies of Substituted Phenols and Proton Affinities of Substituted Phenoxide Ions: A DFT Study

by Asit K. Chandra and Tadafumi Uchimaru

Int. J. Mol. Sci. 2002, 3(4), 407-422; https://doi.org/10.3390/i3040407 - 25 Apr 2002

Cited by 147 | Viewed by 15738

Abstract

The accurate O-H bond dissociation enthalpies for a series of meta and para substituted phenols (X-C₆H₄-OH, X=H, F, Cl, CH₃, OCH₃, OH, NH₂, CF₃, CN, and NO₂) have been [...] Read more.

The accurate O-H bond dissociation enthalpies for a series of meta and para substituted phenols (X-C₆H₄-OH, X=H, F, Cl, CH₃, OCH₃, OH, NH₂, CF₃, CN, and NO₂) have been calculated by using the (RO)B3LYP procedure with 6-311G(d,p) and 6-311++G(2df,2p) basis sets. The proton affinities of the corresponding phenoxide ions (XC₆H₄-O-) have also been computed at the same level of theory. The effect of change of substituent position on the energetics of substituted phenols has been analyzed. The correlations of Hammett’s substituent constants with the bond dissociation enthalpies of the O-H bonds of phenols and proton affinities of phenoxide ions have been explored. Full article

(This article belongs to the Special Issue Application of Density Functional Theory)

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373 KiB

Open AccessArticle

First Principles Molecular Dynamics Study of Catalysis for Polyolefins: the Ziegler-Natta Heterogeneous System.

by Mauro Boero, Kiyoyuki Terakura and Michele Parrinello

Int. J. Mol. Sci. 2002, 3(4), 395-406; https://doi.org/10.3390/i3040395 - 25 Apr 2002

Cited by 4 | Viewed by 10303

Abstract

We review part of our recent ab initio molecular dynamics study on the Ti-based Ziegler-Natta supported heterogeneous catalysis of α-olefins. The results for the insertion of ethylene in the metal-carbon bond are discussed as a fundamental textbook example of polymerization processes. Comparison with [...] Read more.

We review part of our recent ab initio molecular dynamics study on the Ti-based Ziegler-Natta supported heterogeneous catalysis of α-olefins. The results for the insertion of ethylene in the metal-carbon bond are discussed as a fundamental textbook example of polymerization processes. Comparison with the few experimental data available has shown that simulation can reproduce activation barriers and the overall energetics of the reaction with sufficient accuracy. This puts these quantum dynamical simulations in a new perspective as a virtual laboratory where the microscopic picture of the catalysis, which represents an important issue that still escapes experimental probes, can be observed and understood. These results are then discussed in comparison with a V-based catalyst in order to figure out analogies and differences with respect to the industrially more successful Tibased systems. Full article

(This article belongs to the Special Issue Application of Density Functional Theory)

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242 KiB

Open AccessArticle

Can Unrestricted Density-Functional Theory Describe Open Shell Singlet Biradicals?

by Jürgen Gräfenstein, Elfi Kraka, Michael Filatov and Dieter Cremer

Int. J. Mol. Sci. 2002, 3(4), 360-394; https://doi.org/10.3390/i3040360 - 25 Apr 2002

Cited by 187 | Viewed by 19580

Abstract

Unrestricted density functional theory (UDFT) can be used for the description of open-shell singlet (OSS) biradicals provided a number of precautions are considered. Biradicals that require a two-determinantal wave function (e.g. OSS state of carbenes) cannot be described by UDFT for principal reasons. [...] Read more.

Unrestricted density functional theory (UDFT) can be used for the description of open-shell singlet (OSS) biradicals provided a number of precautions are considered. Biradicals that require a two-determinantal wave function (e.g. OSS state of carbenes) cannot be described by UDFT for principal reasons. However, if the overlap between the open-shell orbitals is small (the single electrons are located at different atomic centers) errors become small and, then, the principal failure of UDFT in these cases is not apparent and may even be disguised by the fact that UDFT has the advantage of describing spin polarization better than any restricted open shell DFT method. In the case of OSS biradicals with two- or multiconfigurational character (but a onedeterminantal form of the leading configuration), reasonable results can be obtained by broken-symmetry (BS)-UDFT, however in each case this has to be checked. In no case is it reasonable to lower the symmetry of a molecule to get a suitable UDFT description. Hybrid functionals such as B3LYP perform better than pure DFT functionals in BS-UDFT calculations because the former reduce the self-interaction error of DFT exchange functionals, which mimics unspecified static electron correlation effects, so that the inclusion of specific static electron correlation effects via the form of the wavefunction becomes more effective. Full article

(This article belongs to the Special Issue Application of Density Functional Theory)

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293 KiB

Open AccessArticle

Chemical Reactivity Dynamics and Quantum Chaos in Highly Excited Hydrogen Atoms in an External Field: A Quantum Potential Approach

by P. K. Chattaraj and B. Maiti

Int. J. Mol. Sci. 2002, 3(4), 338-359; https://doi.org/10.3390/i3040338 - 25 Apr 2002

Cited by 16 | Viewed by 9355

Abstract

Dynamical behavior of chemical reactivity indices like electronegativity, hardness, polarizability, electrophilicity and nucleophilicity indices is studied within a quantum fluid density functional framework for the interactions of a hydrogen atom in its ground electronic state (n = 1) and an excited electronic state [...] Read more.

Dynamical behavior of chemical reactivity indices like electronegativity, hardness, polarizability, electrophilicity and nucleophilicity indices is studied within a quantum fluid density functional framework for the interactions of a hydrogen atom in its ground electronic state (n = 1) and an excited electronic state (n = 20) with monochromatic and bichromatic laser pulses. Time dependent analogues of various electronic structure principles like the principles of electronegativity equalization, maximum hardness, minimum polarizability and maximum entropy have been found to be operative. Insights into the variation of intensities of the generated higher order harmonics on the color of the external laser field are obtained. The quantum signature of chaos in hydrogen atom has been studied using a quantum theory of motion and quantum fluid dynamics. A hydrogen atom in the electronic ground state (n = 1) and in an excited electronic state ( n = 20) behaves differently when placed in external oscillating monochromatic and bichromatic electric fields. Temporal evolutions of Shannon entropy, quantum Lyapunov exponent and Kolmogorov – Sinai entropy defined in terms of the distance between two initially close Bohmian trajectories for these two cases show marked differences. It appears that a larger uncertainty product and a smaller hardness value signal a chaotic behavior. Full article

(This article belongs to the Special Issue Application of Density Functional Theory)

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78 KiB

Open AccessArticle

The Concept of Density Functional Theory Based Descriptors and its Relation with the Reactivity of Molecular Systems: A Semi-Quantitative Study

by K. R. S. Chandrakumar and Sourav Pal

Int. J. Mol. Sci. 2002, 3(4), 324-337; https://doi.org/10.3390/i3040324 - 25 Apr 2002

Cited by 73 | Viewed by 12078

Abstract

In this present paper, we have made an attempt to explain the theoretical basis for the empirical hardness/softness concepts to address the reactivity of molecular complexes in a semi-quantitative way within the framework of density functional theory. A model based on local hard-soft-acid-base [...] Read more.

In this present paper, we have made an attempt to explain the theoretical basis for the empirical hardness/softness concepts to address the reactivity of molecular complexes in a semi-quantitative way within the framework of density functional theory. A model based on local hard-soft-acid-base principle has been proposed. The results obtained using some prototype charge transfer complexes, Lewis acid-base complexes and hydrogen-bonded complexes as examples, are in good agreement with the standard ab initio values. Although the model contains ad hoc parameters, it may form the basis of semi-quantitative description of inter-molecular interactions using hardness/softness parameters. The limitation, weakness and other critical issues of the present model are also discussed. Full article

(This article belongs to the Special Issue Application of Density Functional Theory)

120 KiB

Open AccessArticle

Use of Local Softness for the Interpretation of Reaction Mechanisms

by Asit K. Chandra and Minh Tho Nguyen

Int. J. Mol. Sci. 2002, 3(4), 310-323; https://doi.org/10.3390/i3040310 - 25 Apr 2002

Cited by 99 | Viewed by 11155

Abstract

The application of reactivity parameters derived from density functional theory in a local sense, in particular the softness and Fukui function, to interpret and predict the mechanisms of various organic reactions has been discussed. Local softness is shown to be successful in determining [...] Read more.

The application of reactivity parameters derived from density functional theory in a local sense, in particular the softness and Fukui function, to interpret and predict the mechanisms of various organic reactions has been discussed. Local softness is shown to be successful in determining the site-selectivity and regiochemistry and can be used as an alternative to the traditional frontier orbital theory. Full article

(This article belongs to the Special Issue Application of Density Functional Theory)

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504 KiB

Open AccessArticle

Chemical Reactivity as Described by Quantum Chemical Methods

by P. Geerlings and F. De Proft

Int. J. Mol. Sci. 2002, 3(4), 276-309; https://doi.org/10.3390/i3040276 - 25 Apr 2002

Cited by 152 | Viewed by 15638

Abstract

Density Functional Theory is situated within the evolution of Quantum Chemistry as a facilitator of computations and a provider of new, chemical insights. The importance of the latter branch of DFT, conceptual DFT is highlighted following Parr's dictum "to calculate a molecule is [...] Read more.

Density Functional Theory is situated within the evolution of Quantum Chemistry as a facilitator of computations and a provider of new, chemical insights. The importance of the latter branch of DFT, conceptual DFT is highlighted following Parr's dictum "to calculate a molecule is not to understand it". An overview is given of the most important reactivity descriptors and the principles they are couched in. Examples are given on the evolution of the structure-property-wave function triangle which can be considered as the central paradigm of molecular quantum chemistry to (for many purposes) a structure-property-density triangle. Both kinetic as well as thermodynamic aspects can be included when further linking reactivity to the property vertex. In the field of organic chemistry, the ab initio calculation of functional group properties and their use in studies on acidity and basicity is discussed together with the use of DFT descriptors to study the kinetics of S_N2 reactions and the regioselectivity in Diels Alder reactions. Similarity in reactivity is illustrated via a study on peptide isosteres. In the field of inorganic chemistry non empirical studies of adsorption of small molecules in zeolite cages are discussed providing Henry constants and separation constants, the latter in remarkable good agreement with experiments. Possible refinements in a conceptual DFT context are presented. Finally an example from biochemistry is discussed : the influence of point mutations on the catalytic activity of subtilisin. Full article

(This article belongs to the Special Issue Application of Density Functional Theory)

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119 KiB

Open AccessArticle

Density Functional Theory and Materials Modeling at Atomistic Length Scales

by Swapan K. Ghosh

Int. J. Mol. Sci. 2002, 3(4), 260-275; https://doi.org/10.3390/i3040260 - 25 Apr 2002

Cited by 3 | Viewed by 8655

Abstract

We discuss the basic concepts of density functional theory (DFT) as applied to materials modeling in the microscopic, mesoscopic and macroscopic length scales. The picture that emerges is that of a single unified framework for the study of both quantum and classical systems. [...] Read more.

We discuss the basic concepts of density functional theory (DFT) as applied to materials modeling in the microscopic, mesoscopic and macroscopic length scales. The picture that emerges is that of a single unified framework for the study of both quantum and classical systems. While for quantum DFT, the central equation is a one-particle Schrodinger-like Kohn-Sham equation, the classical DFT consists of Boltzmann type distributions, both corresponding to a system of noninteracting particles in the field of a density-dependent effective potential, the exact functional form of which is unknown. One therefore approximates the exchange-correlation potential for quantum systems and the excess free energy density functional or the direct correlation functions for classical systems. Illustrative applications of quantum DFT to microscopic modeling of molecular interaction and that of classical DFT to a mesoscopic modeling of soft condensed matter systems are highlighted. Full article

283 KiB

Open AccessArticle

Applications of the Information Theory to Problems of Molecular Electronic Structure and Chemical Reactivity

by Roman F. Nalewajski

Int. J. Mol. Sci. 2002, 3(4), 237-259; https://doi.org/10.3390/i3040237 - 25 Apr 2002

Cited by 39 | Viewed by 10839

Abstract

Recent studies on applications of the information theoretic concepts to molecular systems are reviewed. This survey covers the information theory basis of the Hirshfeld partitioning of molecular electron densities, its generalization to many electron probabilities, the local information distance analysis of molecular charge [...] Read more.

Recent studies on applications of the information theoretic concepts to molecular systems are reviewed. This survey covers the information theory basis of the Hirshfeld partitioning of molecular electron densities, its generalization to many electron probabilities, the local information distance analysis of molecular charge distributions, the charge transfer descriptors of the donor-acceptor reactive systems, the elements of a “thermodynamic” description of molecular charge displacements, both “vertical” (between molecular fragments for the fixed overall density) and “horizontal” (involving different molecular densities), with the entropic representation description provided by the information theory. The average uncertainty measures of bond multiplicities in molecular “communication” systems are also briefly summarized. After an overview of alternative indicators of the information distance (entropy deficiency, missing information) between probability distributions the properties of the “stockholder” densities, which minimize the entropy deficiency relative to the promolecule reference, are summarized. In particular, the surprisal analysis of molecular densities is advocated as an attractive information-theoretic tool in the electronic structure theory, supplementary to the familiar density difference diagrams. The subsystem information density equalization rules satisfied by the Hirshfeld molecular fragments are emphasized: the local values of alternative information distance densities of subsystems are equal to the corresponding global value, characterizing the molecule as a whole. These local measures of the information content are semi-quantitatively related to the molecular density difference function. In the density functional theory the effective external potentials of molecular fragments are defined, for which the subsystem densities are the ground-state densities. The nature of the energetic and “entropic” equilibrium conditions is reexamined and the entropy representation forces driving the charge transfer in molecular systems are introduced. The latter combine the familiar Fukui functions of subsystems with the information densities, the entropy representation “intensive” conjugates of the subsystem electron densities, and are shown to exactly vanish for the “stockholder” charge distribution. The proportionality relations between charge response characteristics of reactants, e.g., the Fukui functions, are derived. They are shown to follow from the minimum entropy deficiency principles formulated in terms of both the subsystems electron densities and Fukui functions, respectively. Full article

(This article belongs to the Special Issue Application of Density Functional Theory)

25 KiB

Open AccessEditorial

Special Issue on Application of Density Functional Theory in Chemical Reactions

by Abhijit Chatterjee

Int. J. Mol. Sci. 2002, 3(4), 234-236; https://doi.org/10.3390/i3040234 - 25 Apr 2002

Cited by 6 | Viewed by 6706

Abstract

Rapid advances are taking place in the application of density functional theory (DFT) to describe complex chemical reactions.[...] Full article

(This article belongs to the Special Issue Application of Density Functional Theory)

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Int. J. Mol. Sci., Volume 3, Issue 4 (April 2002) – 12 articles , Pages 234-444

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