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DFT Study (dft + study)
Selected AbstractsTetranuclear Manganese Complexes with [MnII4] and [MnII2MnIII2] Units: Syntheses, Structures, Magnetic Properties, and DFT Study,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 28 2010Lucjan B. Jerzykiewicz Abstract Two tetranuclear manganese compounds, [Mn4(,3,,2 -L)4Br4(LH)4] (1) and [Mn4(,3,,2 -L)2(,,,2 -L)4L2Br2] (2), with cubane and defect dicubane-like cores were synthesized and characterized by single-crystal X-ray diffraction, magnetic measurements, and DFT calculations (LH = 2-methoxyethanol). The magnetic properties of the resulting [Mn4] building blocks are presented and discussed in detail. In particular, in 2 the MnIII,O,MnIII angle 103.12(8)° is the largest observed to date for such a system. The conjunction of antiferromagnetic and ferromagnetic interactions within the tetranuclear mixed-valent MnII2MnIII2 complexes results in an unusual ST = 1 ground state. [source] Electronic Structure of Linearly Coordinated EQ Complexes of the Type [(N3N)W(EQ)] [N3N = N(CH2CH2NSiMe3)3; E = P, As, Sb, Bi; Q = O, S, Se, Te]: A DFT StudyEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 17 2007Gábor Balázs Abstract Density functional theory (DFT) calculations were carried out on the terminal EQ complexes [(N3N)W(EQ)] {N3N = N(CH2CH2NSiMe3)3; E = P, As, Sb, Bi; Q = O, S, Se Te} to clarify the bonding situation within the linear Nax,W,E,Q core. This unusual structural motif gives rise to a bonding arrangement in which the ,-electron density is delocalised over the three atoms of the W,E,Q unit. Fragment calculations and natural bond order (NBO) data indicated that the ,-bonding component of the Nax,W,E,Q unit comprises two occupied , orbitals, while the , component of bonding comprises two sets of degenerate , orbitals. In general, the , orbitals of the Nax,W,E,Q core are higher in energy compared to the , orbitals. The phosphorus monoxide (EQ = PO) complexes provide an exception to this rule, with the 1, orbitals of the W,P,O core lower in energy than the , orbitals. Generally, as the atomic number of either the pnicogen (E) or chalcogen (Q) atom increases the extent of ,-orbital delocalisation decreases, whereas the ,-orbital delocalisation increases. Fractional bond orders and Wiberg bond indices were used to establish whether localisation of the ,-electron density gives rise to a W,E or an E,Q double or triple bond. Both methods indicate a W,E as well as an E,Q double bond. The ionic nature of the complexes were analysed by inspection of the Hirschfeld charge distribution which shows only a moderate ionic character. Exceptions are the pnicogen monoxide complexes, which are more ionic. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] (4-Acyl-5-pyrazolonato)titanium Derivatives: Oligomerization, Hydrolysis, Voltammetry, and DFT StudyEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 17 2003Francesco Caruso Abstract Twenty 4-acyl-5-pyrazolonato (Q) titanium derivatives of varied nuclearity have been synthesized from Ti(OR)4 or TiCl4 and characterized with spectroscopic methods (IR, NMR, ESI-MS). While Ti,(,-diketonato) cleavage is not seen in isolated solids, Ti,O(alkoxy) (or Ti,Cl) bonds cleave upon hydrolysis, leading to several structural forms, including oligomers. Ionic Q species with no Ti, i.e., obtained after Ti,Q cleavage, are seen for some Ti,Q derivatives by ESI-MS, which also indicates a varied nuclearity for a given species, e.g., the isolated polynuclear [Q2Ti-,-O]n has several "n" values. Mononuclear Ti complexes are obtained under rigorous anhydrous conditions. The cis structures of the mononuclear species (QT)2Ti(OCH3)2, QT = 3-methyl-4-(neopentylcarbonyl)-1-phenylpyrazol-5-onato have been analyzed with DFT methods. A trans influence is a major driving force that accounts for several sets of Ti,O bonds. One of the cis stereoisomers is 56 kcal/mol higher in energy than the other two. In contrast, all (QT)2TiCl2cis isomers show similar energies. Voltammetry of the mononuclear species (QT)2Ti(OnPr)2 and the antitumor tetranuclear compound [(QB)2Ti-,-O]4, (QB = 4-benzoyl-3-methyl-1-phenylpyrazol-5-onato) indicate that the TiIV is less prone to reduction to TiIII in the latter (Epc for the TiIV/TiIII couple is ,1.71 V and ,1.46 V versus Fc+/Fc, respectively). Potential antitumor compounds having a Ti/Q ratio of 1:1 do not disproportionate, unlike the equivalent acetylacetonato derivatives, and are water-soluble. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] The Mechanism of the Stetter Reaction , A DFT StudyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 33 2008Kirsty J. Hawkes Abstract On the basis of Breslow's mechanism for benzoin condensation, a model asymmetric Stetter reaction has been investigated using DFT methods. In contrast to the concerted benzoin condensation, after formation of the Breslow intermediate the Stetter reaction is found to be a two-step process in which the rate-determining C,C coupling of the Breslow intermediate and the Michael acceptor precedes final proton transfer. In addition, the enolamine is found to play a significant role in the stereochemistry of the product, with the energy difference between stereoisomers of this intermediate reflected throughout the remainder of the reaction sequence. Consequently, electronic and steric control of the stereochemistry of this intermediate should directly enhance the ee values of the product. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Reaction of Carboxylic Acids with Isocyanides: A Mechanistic DFT StudyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 28 2008Tommaso Marcelli Abstract We present a computational investigation of the reaction between isocyanides and carboxylic acids. Our results indicate that this reaction begins with a stereoselective concerted ,-addition of the acid to the isocyanide, leading exclusively to a Z -acyl imidate. Isomerization to the E isomer and successive rate-limiting 1,3 O,N acyl migration yields an N -formyl imide. The calculated barriers are in good agreement with the experimental reaction conditions. Our results might provide an explanation for the peculiar reactivity observed when this reaction is carried out in a self-assembled capsule. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] DFT Study of Brřnsted Acid Catalyzed Nitroso Aldol Reaction Between Achiral Enamines and Nitrosobenzene: The Reason for Regio- and EnantioselectivityEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 25 2008Matsujiro Akakura Abstract The regio- and enantioselectivity of the nitroso aldol reaction between achiral enamines and nitrosobenzene catalyzed by chiral Brřnsted acid catalysts (TADDOL or 1-naphthylglycolic acid) were investigated in experimental and theoretical studies. The use of a model involving a simple organic acid (MeOH or AcOH) revealed that the reaction was catalyzed by two or more molecules of the organic acid and that the transition-state structure encompassed several hydrogen bonds. The role of hydrogen bonding in the regioselectivity was also examined carefully. The enantioselectivity in the reaction catalyzed by chiral organic acids (S,S)-TADDOL and (S)-1-naphthylglycolic acid was also confirmed through computational study.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Ligand Influence on Metathesis Activity of Ruthenium Carbene Catalysts: A DFT StudyADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1-2 2007Bernd Abstract A survey of the concept of active and inactive ligand conformations in ruthenium alkene carbene complexes of the Grubbs catalyst type is presented. This concept is extended to a variety of anionic ligand atoms. Density functional theory calculations at the B3LYP/LACV3P**+//B3LYP/LACVP* level of theory were performed on the precatalyst, 14 valence-electron intermediate, alkene carbene conformers and ruthena(IV)cyclobutane model intermediates for several ligands, such as methoxide, methanethiolate, fluoride, mesylate, water, and ammonia. The rule of the superiority of metathesis catalysts with small and electron-withdrawing halogens does not apply to fluoride ligands. Alkoxides and thiolates also destabilize active carbene conformations, while mesylate ligands lead to a balanced energetic relation of active and inactive carbene orientations. Cationic ruthenium carbene species with aqua or ammine ligands are limited by unfavored ligand dissociation to 14 valence-electron intermediates. A guideline for the design of novel ligand systems for ruthenium carbene complexes as metathesis catalysts is proposed. [source] ChemInform Abstract: Understanding the Reactivity and Basicity of Zeolites: A Periodic DFT Study of the Disproportionation of N2O4 on Alkali-Cation-Exchanged Zeolite Y.CHEMINFORM, Issue 32 2008Pierre Mignon Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Structural and Electronic Properties of Hetero-Transition-Metal Keggin Anions: A DFT Study of ,/,-[XW12O40]n- (X: CrVI, VV, TiIV, FeIII, CoIII, NiIII, CoII, and ZnII) Relative Stability.CHEMINFORM, Issue 16 2007Fu-Qiang Zhang Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] A DFT Study on the Electronic and Redox Properties of [PW11O39(ReN)]n- (n = 3, 4, 5) and [PW11O39(OsN)]2- .CHEMINFORM, Issue 8 2007Li-Kai Yan Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] The Stability and Eventual Lone Pair Distortion of the Hexahalide Complexes and Molecules of the Fifth to Eighth Main-Group Elements with One Lone Pair, as Isolated Entities or in Oligomeric Clusters: A Vibronic Coupling and DFT Study.CHEMINFORM, Issue 40 2005Mihail Atanasov No abstract is available for this article. [source] Comprehensive DFT Study of Nitrous Oxide Decomposition over Fe-ZSM-5.CHEMINFORM, Issue 18 2005Andreas Heyden No abstract is available for this article. [source] Environmental Effects on Vibrational Proton Dynamics in H5O2+: DFT Study on Crystalline H5O2+ClO4 -CHEMINFORM, Issue 12 2005Mikhail V. Vener Abstract For Abstract see ChemInform Abstract in Full Text. [source] A DFT Study of the Acidity of Ultrastable Y Zeolite: Where Is the Broensted/Lewis Acid Synergism?CHEMINFORM, Issue 32 2004Claudio J. A. Mota No abstract is available for this article. [source] Ab initio and DFT Study of the Molecular Mechanisms of SO3 and SOCl2 Reactions with Water in the Gas Phase.CHEMINFORM, Issue 25 2004Stanislav K. Ignatov No abstract is available for this article. [source] ChemInform Abstract: A DFT Study of the Electronic Spectrum of the ,-Keggin Anion [CoIIW12O40]6- .CHEMINFORM, Issue 21 2002Joan Miquel Maestre Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Unexpected Deformations Induced by Surface Interaction and Chiral Self-Assembly of CoII -Tetraphenylporphyrin (Co-TPP) Adsorbed on Cu(110): A Combined STM and Periodic DFT StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 38 2010Philip Donovan Abstract In a combined scanning tunnelling microscopy (STM) and periodic density functional theory (DFT) study, we present the first comprehensive picture of the energy costs and gains that drive the adsorption and chiral self-assembly of highly distorted CoII -tetraphenylporphyrin (Co-TPP) conformers on the Cu(110) surface. Periodic, semi-local DFT calculations reveal a strong energetic preference for Co-TPP molecules to adsorb at the short-bridge site when organised within a domain. At this adsorption site, a substantial chemical interaction between the molecular core and the surface causes the porphyrin macrocycle to accommodate close to the surface and in a flat geometry, which induces considerable tilting distortions in the phenyl groups. Experimental STM images can be explained in terms of these conformational changes and adsorption-induced electronic effects. For the ordered structure we unambiguously show that the substantial energy gain from the molecule,surface interaction recuperates the high cost of the induced molecular and surface deformations as compared with gas phase molecules. Conversely, singly adsorbed molecules prefer a long-bridge adsorption site and adopt a non-planar, saddle-shape conformation. By using a van der Waals density functional correction scheme, we found that the intermolecular ,,, interactions make the distorted conformer more stable than the saddle conformer within the organic assembly. These interactions drive supramolecular assembly and also generate chiral expression in the system, pinning individual molecules in a propeller-like conformation and directing their assembly along non-symmetric directions that lead to the coexistence of mirror-image chiral domains. Our observations reveal that a strong macrocycle,surface interaction can trigger and stabilise highly unexpected deformations of the molecular structure and thus substantially extend the range of chemistries possible within these systems. [source] Formation of Superoxo Species by Interaction of O2 with Na Atoms Deposited on MgO Powders: A Combined Continuous-Wave EPR (CW-EPR), Hyperfine Sublevel Correlation (HYSCORE) and DFT StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 23 2010Francesco Napoli Dr. Abstract The formation of O2, radical anions by contact of O2 molecules with a Na pre-covered MgO surface is studied by a combined EPR and quantum chemical approach. Na atoms deposited on polycrystalline MgO samples are brought into contact with O2. The typical EPR signal of isolated Na atoms disappears when the reaction with O2 takes place and new paramagnetic species are observed, which are attributed to different surface-stabilised O2, radicals. Hyperfine sublevel correlation (HYSCORE) spectroscopy allows the superhyperfine interaction tensor of O2,Na+ species to be determined, demonstrating the direct coordination of the O2, adsorbate to surface Na+ cations. DFT calculations enable the structural details of the formed species to be determined. Matrix-isolated alkali superoxides are used as a standard to enable comparison of the formed species, revealing important and unexpected contributions of the MgO matrix in determining the electronic structure of the surface-stabilised Na+O2, complexes. [source] Mechanistic Exploration of the Intramolecular Hydroalkoxylation of Allenyl Alcohols Mediated by Organolanthanide Complexes: A DFT StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2010Sven Tobisch Dr. An efficient route toward functionalised cyclic ethers: Computational mechanistic analysis indicates an operative LnO ,-bond insertion mechanism with a turnover-limiting cyclisation for the intramolecular hydroalkoxylation of allenyl alcohols (see scheme). [source] Transmetalation Reactions from Fischer Carbene Complexes to Late Transition Metals: A DFT StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 35 2008Israel Fernández Dr. Abstract Transmetalation reactions from chromium(0) Fischer carbene complexes to late-transition-metal complexes (palladium(0), copper(I), and rhodium(I)) have been studied computationally by density functional theory. The computational data were compared with the available experimental data. In this study, the different reaction pathways involving the different metal atoms have been compared with each other in terms of their activation barriers and reaction energies. Although the reaction profiles for the transmetalation reactions to palladium and copper are quite similar, the computed energy values indicate that the process involving palladium as catalyst is more favorable than that involving copper. In contrast to these transformations, which occur via triangular heterobimetallic species, the transmetalation reaction to rhodium leads to a new heterobimetallic species in which a carbonyl ligand is also transferred from the Fischer carbene to the rhodium catalyst. Moreover, the structure and bonding situation of the so far elusive heterobimetallic complexes are briefly discussed. La reacción de transmetalación catalítica desde cromo(0) Fischer carbenos a metales de transición tardía (Pd0, CuIand RhI) se ha estudiado computacionalmente usando DFT. Los resultados computacionales se han comparado con los datos experimentales disponibles. Las barreras y las energías de reacción se han comparado en los caminos de reacción obtenidos para los metales considerados. Mientras que los perfiles de reacción para la transmetalación desde Cr a Pd o Cu son similares, los valores calculados indican que aquellos procesos que implican catalizadores de Pd son más favorables que los que implican Cu. En claro contraste con estas transformaciones, que ocurren a través de especies heterobimetálicas con geometría triangular, la reacción con Rh forma nuevas especies heterobimetálicas en las que se ha transferido un ligando carbonilo desde el complejo de Fischer al catalizador de Rh. Adicionalmente, se discute la estructura y la forma de enlace de éstos complejos heterobimetálicos no aislados hasta este momento. [source] Why Platinum Catalysts Involving Ligands with Large Bite Angle Are so Efficient in the Allylation of Amines: Design of a Highly Active Catalyst and Comprehensive Experimental and DFT StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2008Guilhem Mora Abstract The platinum-catalyzed allylation of amines with allyl alcohols was studied experimentally and theoretically. The complexes [Pt(,3 -allyl)(dppe)]OTf (2) and [Pt(,3 -allyl)(DPP-Xantphos)]PF6 (5) were synthesized and structurally characterized, and their reactivity toward amines was explored. The bicyclic aminopropyl complex [Pt(CH2CH2CH2NHBn- , - C,N)(dppe)]OTf (3) was obtained from the reaction of complex 2 with an excess of benzylamine, and this complex was shown to be a deactivated form of catalyst 2. On the other hand, reaction of complex 5 with benzylamine and allyl alcohol led to formation of the 16-VE platinum(0) complex [Pt(,2 -C3H5OH)(DPP-Xantphos)] (7), which was structurally characterized and appears to be a catalytic intermediate. A DFT study showed that the mechanism of the platinum-catalyzed allylation of amines with allyl alcohols differs from the palladium-catalyzed process, since it involves an associative ligand-exchange step involving formation of a tetracoordinate 18-VE complex. This DFT study also revealed that ligands with large bite angles disfavor the formation of platinum hydride complexes and therefore the formation of a bicyclic aminopropyl complex, which is a thermodynamic sink. Finally, a combination of 5 and a proton source was shown to efficiently catalyze the allylation of a broad variety of amines with allyl alcohols under mild conditions. [source] Understanding the Reactivity and Basicity of Zeolites: A Periodic DFT Study of the Disproportionation of N2O4 on Alkali-Cation-Exchanged Zeolite YCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2008Pierre Mignon Dr. Abstract The disproportionation of N2O4 into NO3, and NO+ on Y zeolites has been studied through periodic DFT calculations to unravel 1),the role of metal cations and the framework oxygen atoms and 2),the relationship between the NO+ stretching frequency and the basicity of zeolites. We have considered three situations: adsorption on site,II cations with and without a cation at site,III and adsorption on a site III cation. We observed that cations at sites II and III cooperate to stabilize N2O4 and that the presence of a cation at site,III is necessary to allow the disproportionation reaction. The strength of the stabilization is due to the number of stabilizing interactions increasing with the size of the cation and to the Lewis acidity of the alkali cations, which increases as the size of the cations decreases. In the product, NO3, interacts mainly with the cations and NO+ with the basic oxygen atoms of the tetrahedral aluminium through its nitrogen atom. As the cation size increases, the NO3,,,,cation interaction increases. As a result, the negative charge of the framework is less well screened by the larger cations and the interaction between NO+ and the basic oxygen atoms becomes stronger. NO+ appears to be a good probe of zeolite basicity, in agreement with experimental observations. [source] Ethylene Biosynthesis by 1-Aminocyclopropane-1-Carboxylic Acid Oxidase: A DFT StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 34 2006Arianna Bassan Dr. Abstract The reaction catalyzed by the plant enzyme 1-aminocyclopropane-1-carboxylic acid oxidase (ACCO) was investigated by using hybrid density functional theory. ACCO belongs to the non-heme iron(II) enzyme superfamily and carries out the bicarbonate-dependent two-electron oxidation of its substrate ACC (1-aminocyclopropane-1-carboxylic acid) concomitant with the reduction of dioxygen and oxidation of a reducing agent probably ascorbate. The reaction gives ethylene, CO2, cyanide and two water molecules. A model including the mononuclear iron complex with ACC in the first coordination sphere was used to study the details of OO bond cleavage and cyclopropane ring opening. Calculations imply that this unusual and complex reaction is triggered by a hydrogen atom abstraction step generating a radical on the amino nitrogen of ACC. Subsequently, cyclopropane ring opening followed by OO bond heterolysis leads to a very reactive iron(IV),oxo intermediate, which decomposes to ethylene and cyanoformate with very low energy barriers. The reaction is assisted by bicarbonate located in the second coordination sphere of the metal. [source] Ring-Closing Olefin Metathesis on Ruthenium Carbene Complexes: Model DFT Study of StereochemistryCHEMISTRY - A EUROPEAN JOURNAL, Issue 13 2005Sergei F. Vyboishchikov Dr. Abstract Ring-closing metathesis (RCM) is the key step in a recently reported synthesis of salicylihalamide and related model compounds. Experimentally, the stereochemistry of the resulting cycloolefin (cis/trans) depends strongly on the substituents that are present in the diene substrate. To gain insight into the factors that govern the observed stereochemistry, density functional theory (DFT) calculations have been carried out for a simplified dichloro(2-propylidene)(imidazole-2-ylidene)ruthenium catalyst I, as well as for the real catalyst II with two mesityl substituents on the imidazole ring. Four model substrates are considered, which are closely related to the systems studied experimentally, and in each case, two pathways A and B are possible since the RCM reaction can be initiated by coordination of either of the two diene double bonds to the metal center. The first metathesis yields a carbene intermediate, which can then undergo a second metathesis by ring closure, metallacycle formation, and metallacycle cleavage to give the final cycloolefin complex. According to the DFT calculations, the stereochemistry is always determined in the second metathesis reaction, but the rate-determining step may be different for different catalysts, substrates, and pathways. The ancillary N-heterocyclic carbene ligand lies in the Ru-Cl-Cl plane in the simplified catalyst I, but is perpendicular to it in the real catalyst II, and this affects the relative energies of the relevant intermediates and transition states. Likewise, the introduction of methyl substituents in the diene substrates influences these relative energies appreciably. Good agreement with the experimentally observed stereochemistry is only found when using the real catalyst II and the largest model substrates in the DFT calculations. [source] Mechanism of Reppe's Nickel-Catalyzed Ethyne Tetramerization to Cyclooctatetraene: A DFT StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 12 2004Bernd F. Straub Dr. Abstract In this B3,LYP model study, homoleptic nickel(0) ethyne complexes have been predicted as the catalyst resting state for the title reaction. Ethyne ligand coupling of Ni(C2H2)3 yields monoethyne nickelacyclopentadiene in the rate-determining step. Ethyne coordination is followed by insertion of an ethyne ligand into the NiC , bond. A highly strained monoethyne trans -nickelacycloheptatriene is formed. This trans intermediate is unable to reductively eliminate benzene without prior isomerization to a cis -structure. Instead, it rapidly collapses to a nickelacyclononatetraene. Ethyne coordination induces reductive elimination to the cyclooctatetraene complex Ni(,2 -C2H2)(,2 -C8H8), followed by facile ligand exchange. Other ethyne coupling pathways have been computed to be less favored. The cyclooctatetraene ligand binds significantly weaker to nickel(0) than ethyne, both for mononuclear, and for dinuclear species. For this reason, CC bond formation steps at Ni2(,-cot) fragments have been predicted to feature prohibitively high overall reaction barriers. In dieser B3,LYP-Studie werden homoleptische Nickel(0)-Ethinkomplexe als Katalysatorruhezustand von Reppes Cyclooctatetraensynthese vorhergesagt. Kupplung zweier Ethinliganden in Ni(C2H2)3 ergibt Monoethin-Nickelacyclopentadien im geschwindigkeitsbestimmenden Schritt. Der Koordination von Ethin folgt die Insertion eines Ethinliganden in die NiC ,-Bindung. Ein hoch gespanntes Monoethin- trans -Nickelacycloheptatrien wird gebildet. Dieses trans -Intermediat ist nicht in der Lage, Benzol reduktiv zu eliminieren, ohne vorher in eine cis -Struktur zu isomerisieren. Stattdessen kollabiert es schnell zu Nickelacyclononatetraen. Koordination von Ethin induziert eine barrierefreie reduktive Eliminierung zum Cyclooctatetraen Komplex Ni(,2 -C2H2)(,2 -C8H8), gefolgt von einem einfach verlaufenden Ligandenaustausch. Andere Ethinkupplungspfade wurden als weniger begünstigt berechnet. Der COT-Ligand bindet sowohl für mononukleare als auch für dinukleare Spezies deutlich schwächer an Nickel(0) als Ethin. Aus diesem Grund werden für CC-Bindungsbildungsschritte am Ni2(,-cot)-Fragment prohibitiv hohe freie Aktivierungsenthalpien vorhergesagt. [source] DFT Study of Effects of Potassium Doping on Band Structure of Crystalline Cuprous AzideCHINESE JOURNAL OF CHEMISTRY, Issue 12 2008Wei-Hua ZHU Abstract The structure and defect formation energies of the K-doped CuN3 were studied using density functional theory within the generalized gradient approximation. The results show that the K-doping breaks the azide symmetry and causes asymmetric atomic displacement. As the K-doping level increases, the band gap of the doped system gradually increases. The K impurity is easily incorporated into the crystal thermodynamically. The Cu vacancy is easily created thermodynamically and the K impurity can serve as nucleation centers for vacancy clustering. Finally the effects of K-doping concentrations on the sensitivity of CuN3 were understood based on electronic structures. [source] Mechanism of Oxidation of (olefin)RhI and -IrI Complexes by H2O2EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2004Peter H. M. Budzelaar Abstract A DFT study of the oxidation of (tpa)MI(C2H4)+ and (dpa-R)MI(cod)+ complexes (M = Rh, Ir) by H2O2 indicates that the reaction starts with heterolytic cleavage of the peroxide O,O bond, leading to MIII(olefin)(OH)2+ species. These can then undergo cyclisation, followed by deprotonation to oxetanes. In the oxidation of COD complexes, further cyclisation to internal ethers has a modest barrier, and the observed products (ethers for Rh, oxetanes for Ir) are suggested to be the thermodynamic ones. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] The Effects of Fluorine and Chlorine Substituents across the Fjords of Bifluorenylidenes: Overcrowding and StereochemistryEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 22 2006Sergey Pogodin Abstract The bistricyclic aromatic enes (BAEs) (E)- and (Z)-1,1,-difluorobifluorenylidene, 1,8,1,,8,-tetrafluorobifluorenylidene, (E)- and (Z)-3,3,-difluorobifluorenylidene, 3,6,3,,6,-tetrafluorobifluorenylidene, and their chlorinated analogues were subjected to a DFT study of overcrowding in their fjord regions. The B3LYP hybrid functional was employed to calculate energies and geometries of the twisted conformations of these BAEs. The diastereomers E11,F2 and Z11,F2 have identical twist angles (, = 37.1°) and similar degrees of overcrowding, but differ in the degree and mode of pyramidalization, ,. In E11,F2, ,(C9) = +,(C9,) = 7.0° (syn -pyramidalization), while in Z11,F2, ,(C9) = ,,(C9,) = 1.0° (anti -pyramidalization). By contrast, in E11,Cl2 and Z11,Cl2, , = 40.6° and 42.7°, respectively. Introducing four halogen substituents results in higher twist angles: , = 40.3° in 181,8,F4 and 52.6° in 181,8,Cl4. Surprisingly, Z11,F2 is more stable than E11,F2 (,H298 = ,1.9 kJ/mol), whereas Z11,Cl2 is less stable than E11,Cl2 (,H298 = 2.2 kJ/mol). Both results are consistent with the experimental relative stabilities of these diastereomers. The unexpected stability of Z11,F2 is explained by a combination of steric and electronic effects. Calculations of Coulomb energies for point charge systems of atoms C, F, and H in the fjord regions shows stabilization of the (Z) diastereomer by ,45.5 kJ/mol. The dipole,dipole interactions in the fjord region destabilize Z11,F2 by 6.4 kJ/mol relative to E11,F2. Careful examination of the NMR spectra of E11,F2 and Z11,F2 shows, in the latter, evidence of long-range fluorine,fluorine coupling over seven bonds (11.4 Hz) and carbon,fluorine coupling over six bonds (4.8 Hz).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] DFT study of polymorphism of the DNA double helix at the level of dinucleoside monophosphatesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 13 2010Valeri I. Poltev Abstract We apply DFT calculations to deoxydinucleoside monophosphates (dDMPs) which represent minimal fragments of the DNA chain to study the molecular basis of stability of the DNA duplex, the origin of its polymorphism and conformational heterogeneity. In this work, we continue our previous studies of dDMPs where we detected internal energy minima corresponding to the "classical" B conformation (BI-form), which is the dominant form in the crystals of oligonucleotide duplexes. We obtained BI local energy minima for all existing base sequences of dDMPs. In the present study, we extend our analysis to other families of DNA conformations, successfully identifying A, BI, and BII energy minima for all dDMP sequences. These conformations demonstrate distinct differences in sugar ring puckering, but similar sequence-dependent base arrangements. Internal energies of BI and BII conformers are close to each other for nearly all the base sequences. The dGpdG, dTpdG, and dCpdA dDMPs slightly favor the BII conformation, which agrees with these sequences being more frequently experimentally encountered in the BII form. We have found BII-like structures of dDMPs for the base sequences both existing in crystals in BII conformation and those not yet encountered in crystals till now. On the other hand, we failed to obtain dDMP energy minima corresponding to the Z family of DNA conformations, thus giving us the ground to conclude that these conformations are stabilized in both crystals and solutions by external factors, presumably by interactions with various components of the media. Overall the accumulated computational data demonstrate that the A, BI, and BII families of DNA conformations originate from the corresponding local energy minimum conformations of dDMPs, thus determining structural stability of a single DNA strand during the processes of unwinding and rewinding of DNA. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2548,2559, 2010 [source] Copper-catalyzed phosphinidene transfer to ethylene, acetylene, and carbon monoxide: A computational studyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 9 2010Matthew J. Amme Abstract A DFT study of phosphinidene transfer by copper model catalysts is reported. PR-transfer pathways are highly exergonic with respect to catalyst and phosphinidene transfer reagent. Calculated free energy barriers by which (dhpe)Cu(PMe) active species yields functionalized products are reasonable for modeled substrates,ethylene, acetylene, and carbon monoxide. Calculations suggest a (dhpe)CuI(-PMe·,) formulation as more appropriate than (dhpe)CuII(=PMe2,). The preferred pathway for production of phosphirane (phosphirene) is via direct [1 + 2] addition of ethylene (acetylene) to the PMe group of (dhpe)Cu(PMe), which contrasts the [2 + 2] mechanism for the reaction of ethylene with Ni0 -phosphinidenes. In light of simulations for neutral and cationic models, it is concluded that the extra electron in copper destabilizes [2 + 2] pathways. Calculated energetics for (dhpe)Cu(PMe) versus (en)Cu(PMe), dhpe = 1,2- bis(dihydrophosphino)ethane, en = ethylenediamine, indicate that the former is a more potent for PR-transfer. Thus, it is inferred that modifications that result in a more electron deficient metal center will yield better group transfer catalysts. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] | |||||||||||||||||||||||||