Theoretical study on the structure and energetics of alkali halide clusters

The structures of alkali halide clusters Na_nF_n, Li_nF_n and Na_nCl_n, and their metal-excess clusters Na_nF_n−1⁺, Li_nF_n−1⁺ and Na_nCl_n−1⁺ were investigated by the ab initio molecular orbital method for cluster sizes from 1 to 14. The magic numbers for the neutral clusters Na_nF_n, Li_nF_n and Na_nCl_n are 4, 6, and 8. The most stable structure for these cluster sizes is a perfect crystallite for Na_nF_n and Na_nCl_n, and a double ring for Li_nF_n. The magic numbers for the metal-excess clusters are 5 and 8, which are near ideal cuboids with (100) facets.     

Theoretical investigation on hydroformylation reactions: II. Hydrocarbonyliron intermediates in reppe synthesis

Extended Hückel theory calculations have been carried out to investigate the possible formation of stable π and σ complexes between olefins and some hydrocarbonylirons postulated as active catalysts in Reppe synthesis. It was found that H₂Fe(CO)₄ and HFe(CO)₄^- do not coordinate ethylene, unlike the corresponding CO deficient species H₂Fe(CO)₃ and HFe(CO)₃^-, which interact with the olefin to give stable π complexes. Moreover ethylene—H₂Fe(CO)₃ is more stable than ethylene—HFe(CO)₃^-, in line with the conclusions based on the experimental results. Stable alkyl intermediates are predicted starting from the coordinatively saturated hydrocarbonyls.     

Electrophilic intermediates and their reactions in superacids

Acid-catalyzed reactions have played a major role in hydrocarbon chemistry involving electron-deficient intermediates such as carbocations, carbodications, onium ions, etc. The pioneering discovery of the use of superacids by George A. Olah, in the early 1960s, to characterize such intermediates under so-called long-lived stable ion conditions led to the understanding of their structures and reactivity patterns much more clearly. Continuing studies in this area in the past 30 years have resulted in a paradigm shift in comprehending the stability and reactivity of electrophilic intermediates in superacid media.     

Application of decarboxylation reactions in catalytic analysis determination of aluminium

Summary The activation of the catalytic decarboxylation of oxaloacetic acid with some pyridine derivatives was studied in order to select suitable activators for increasing the sensitivity of this type of analytical catalytic reactions. The catalytic activity of some bivalent metal ions (Cu²⁺, Zn^2s+, Cd²⁺, Pb²⁺) as well as that of Al³⁺ was also studied. On that ground a catalytic method for determination of aluminium was developed. The sensitivity of the method is 0.5g of aluminium in the sample.

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Zusammenfassung Die Aktivierung der katalytischen Decarboxylierung von Oxalessigsäure durch einige Pyridinderivate wurde untersucht, um geeignete Aktivatoren zur Steigerung der Empfindlichkeit dieser analytisch auswertbaren Reaktion zu finden. Die katalytische Aktivität einiger zweiwertiger Metallionen (Cu, Zn, Cd, Pb) sowie von Al wurde ebenfalls geprüft. Auf dieser Grundlage wurde eine Methode zur Aluminiumbestimmung entwickelt, deren Empfindlichkeit zur Bestimmung von 0,5g Al je Einwaage ausreicht.

     

Bonding character of intermediates in on-surface Ullmann reactions revealed with energy decomposition analysis

On-surface synthesis has become a thriving topic in surface science. The Ullmann coupling reaction is the most applied synthetic route today, but the nature of the organometallic intermediate is still under discussion. We investigate the bonding nature of prototypical intermediate species (phenyl, naphthyl, anthracenyl, phenanthryl, and triphenylenyl) on the Cu(111) surface with a combination of plane wave and atomic orbital basis set methods using density functional theory calculations with periodic boundary conditions. The surface bonding is shown to be of covalent nature with a polarized shared-electron bond supported by π-back donation effects using energy decomposition analysis for extended systems (pEDA). The bond angle of the intermediates is determined by balancing dispersion attraction and Pauli repulsion between adsorbate and surface. The latter can be significantly reduced by adatoms on the surface. We furthermore investigate how to choose computational parameters for pEDA of organic adsorbates on metal surfaces efficiently and show that bonding interpretation requires consistent choice of the density functional.     

Structure and interconversion of intermediates in pyridine-catalyzed mesylation reactions

The structure of intermediates of pyridine-catalyzed mesylation processes, i.e. sulfene and sulfonyl salts, and the ways of their interconversion have been studied by the joint use of semi-empirical MNDO approximation and methods of molecular mechanics. Calculation results agree well with the kinetic data.

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On the nature of factors determining reactivity of strongly bound surface species as intermediates in heterogeneous catalytic redox reactions

The rate-limiting stage of heterogeneous catalytic redox reactions with participation of strongly bound surface species, such as nitrates, carbonates and sulfites, is the interaction of these heteroatomic oxidants with the reducing agents. It appears to be mainly determined by their bonding strength with the surface, being independent of the nature of the central atom (carbon, sulfur, nitrogen) and the energy of its bond with oxygen.     

Structure, energetics, and reactions of alkali tetramers

Electronic structure calculations have been carried out for all possible alkali tetramers that can be formed from X(2) + X(2) → X(2)X(2), X(2) + Y(2) → X(2)Y(2), and XY + XY → X(2)Y(2) alkali dimer association reactions. Vibrationally stable rhombic (D(2h)) and planar (C(s)) structures are found for all possible tetramers formed from the alkali metals, Li to Cs. All tetramer formation reactions (from ground state singlet homonuclear or heteronuclear dimers) are found to be exothermic with binding energies ranging from 6282 cm(-1) for Li(2)Li(2) to 1985 cm(-1) for Cs(2)Cs(2). Extensive calculations, carried out at long-range for several reactant pairs, indicate that there are barrier-less pathways for the formation of tetramers from dimer association reactions. At low temperatures, direct formation of tetramers is unlikely, owing to the large exothermicity associated with these association reactions, but atom exchange reactions (X(2) + Y(2) ? XY + XY) are possible for some species.     

Identification of intermediates in the catalytic cycle of chloroperoxidase

Background: Chloroperoxidase (CPO) is the most versatile of the hemethiolate proteins, catalyzing the chlorination of activated CH bonds and reactions reminiscent of peroxidase, catalase, and cytochrome P450. Despite 30 years of continuous efforts, no intermediates of the enzyme's catalytic cycle have been identified except for compound I. Thus, in the absence of conclusive evidence it is generally believed that the halogenation of substrates proceeds by means of ‘free HOCl’ in solution.Results: The pH profile of chloroperoxidase from Caldariomyces fumago revealed a new active-site complex that can be detected only at pH 4.4. According to ultra-violet (UV) spectroscopy, and by comparison with suitable enzyme models, this intermediate is the HOCl adduct of the iron(III) protoporphyrin(IX). Inactivation of chloroperoxidase by diethyl pyrocarbonate, which interrupts the proton shuttle by modification of the distal histidine, led to the formation of the ⁻OCl adduct of the iron complex, which was identified by comparison with a corresponding active site analogue.Conclusions: The availability of enzyme models of heme-thiolate proteins allowed the identification by UV spectroscopy of both the ⁻OCl adduct and the HOCI adduct of the iron(III) protoporphyrin(IX) of chloroperoxidase. The existence of these previously elusive intermediates suggests that the chlorination catalyzed by CPO, and its corresponding active site analogue, proceeds by Cl⁺ transfer from the HOCl adduct to the substrate bound in the distal pocket of the enzyme.     

The structure and energetics of cryolite melts

A method of solvation energy computation is proposed for ions and molecules in the environment of an ionic melt, based on the approximation of the ionic melt as an ideal conductor. The method is used to compute equilibrium constants of some equilibria in cryolite melt. Theoretically obtained results predict that aluminium is bound in tetrafluorocomplexes AlF₄ ⁻. Received: 16 March 1998 / Accepted: 19 June 1998 / Published online: 7 October 1998     

Alkali hydrolysis of diethyl malonates: Theoretical conformation analysis for substrates and tetrahedral intermediates

Molecular-mechanics methods have been applied to the conformations of substituted diethyl malonates; models for reaction complexes formed with hydroxide anions have been constructed. The relative preferences for the diastereometric tetrahedral intermediates have been determined for both stages in the alkali hydrolysis. Potential-surface sections give a qualitative interpretation of the steric factors governing the reactivity, which can be evaluated precisely from the steric indices IS_V.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 6, pp. 675–681, November–December, 1990.     

Rate-limiting steps, rate-determining parameters, and apparent activation energy of stepwise reactions, 2. Simple stepwise consecutive catalytic reactions linear in respect to intermediates

The paper considers the application of a recently suggested mathematically correct approach for determining the rate-limiting steps, rate-determining parameters as well as the apparent activation energy for the steady occurrence of simple stepwise consecutive catalytic reactions which are linear in respect to catalytic intermediates. Under consideration are situations of both the small and large coverage of the surface with catalytic intermediates. This revised version was published online in June 2006 with corrections to the Cover Date.     

Synthetic and catalytic intermediates in a magnesium promoted Tishchenko reaction

Magnesium-aryloxide and -amide compounds supported by amidinate ligands are accessible in two-steps from a suitable Grignard reagent. Both species are active for the dimerization of benzaldehyde. The slower initiation by the aryloxide was exploited in the isolation of a bis-mesitylaldehyde adduct.