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Quantum Chemical Methods (quantum + chemical_methods)
Selected AbstractsAzide,Tetrazole Ring-Chain Isomerism in Polyazido-1,3,5-triazines, Triazido- s -heptazine, and DiazidotetrazinesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2006Anton Hammerl Abstract The azide,tetrazole isomerism in several polyazido-1,3,5-triazines, triazido- sym -heptazine, and some diazido-1,2,4,5-tetrazines was investigated by ab initio quantum chemical methods in order to determine whether the polyazides are suitable starting materials for the synthesis of the isomeric tetrazoles. The effects of solvation in CCl4, DMSO and water on this isomerism were included using the self consistent reaction field (SCRF) method. The effect of amino- and nitrosubstituents on the azide,tetrazole isomerism was also examined. In the gas phase all investigated polyazidoheterocycles do not cyclize to form tetrazoles. An electron-donating amino group favors the ring closure to tetrazoles, whereas an electron-withdrawing nitro group favors the azides. Solvation in polar solvents favors the formation of a tetrazole ring system due to higher charge separation in the tetrazole ring system, but for all polyazido-1,3,5-triazines, including triazido- s -heptazine, the effects of solvation are not strong enough to shift the equilibrium to the tetrazole side, which explains why several attempts to detect these compounds have failed. The monotetetrazoles of diazidotetrazine and bis(azido)azo-1,2,4,5-tetrazine and the ditetrazole of bis(azido)hydrazo-1,2,4,5-tetrazine are the minimum energy species in DMSO and water. Thus we predict that the diazidoazo- and hydrazotetrazines will readily cyclize to the tetrazoles in polar solvents. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] The chemiionization reactions Ce + O and Ce + O2: Assignment of the observed chemielectron bandsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 10 2009Tanya K. Todorova Abstract Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemiionization reactions Ce + O , CeO+ + e, and Ce + O2 , CeO + e,. Selected spectroscopic constants for CeOn and CeO (n = 1, 2), as well as reaction enthalpies of the chemiionization reactions of interest, have been computed and compared with experimental values. In contrast to the lanthanum case, for both Ce + O2(X3,) and Ce + O2( a1,g), the Ce + O2 , CeO + e, reaction is shown to be exothermic, and thus, contributes to the experimental chemielectron spectra. The apparent discrepancy between the computed reaction enthalpies and the high kinetic energy offset values measured in the chemielectron spectra is rationalized by arguing that chemielectrons are produced mainly via two sequential reactions (Ce + O2 , CeO + O, followed by Ce + O , CeO+ + e,) as in the case of lanthanum. For Ce + O2 (a1,g), a chemielectron band with higher kinetic energy than that recorded for Ce + O2( X3,) is obtained. This is attributed to production of O( 1D) from the reaction Ce + O2( a1,g) , CeO + O( 1D), followed by chemiionization via the reaction Ce + O( 1D) , CeO+ + e,. Accurate potential energy curves for the ground and a number of excited states of CeO and CeO+ have been computed, and a mechanism for the chemiionization reactions investigated experimentally was proposed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] From quantum chemistry and the classical theory of polar liquids to continuum approximations in molecular mechanics calculations,INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2005Sergio A. Hassan Abstract Biological macromolecules and other polymers belong to the class of mesoscopic systems, with characteristic length scale of the order of a nanometer. Although microscopic models would be the preferred choice in theoretical calculations, their use in computer simulations becomes prohibitive for large systems or long simulation times. On the other hand, the use of purely macroscopic models in the mesoscopic domain may introduce artifacts, with effects that are difficult to assess and that may compromise the reliability of the calculations. Here is proposed an approach with the aim of minimizing the empirical nature of continuum approximations of solvent effects within the scope of molecular mechanics (MM) approximations in mesoscopic systems. Using quantum chemical methods, the potential generated by the molecular electron density is first decomposed in a multicenter-multipole expansion around predetermined centers. The monopole and dipole terms of the expansion at each site create electric fields that polarize the surrounding aqueous medium whose dielectric properties can be described by the classical theory of polar liquids. Debye's theory allows a derivation of the dielectric profiles created around isolated point charges and dipoles that can incorporate Onsager reaction field corrections. A superposition of screened Coulomb potentials obtained from this theory makes possible a simple derivation of a formal expression for the total electrostatic energy and the polar component of the solvation energy of the system. A discussion is presented on the physical meaning of the model parameters, their transferability, and their convergence to calculable quantities in the limit of simple systems. The performance of this continuum approximation in computer calculations of amino acids in the context of an atomistic force field is discussed. Applications of a continuum model based on screened Coulomb potentials in multinanosecond simulations of peptides and proteins are briefly reviewed. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Hydrodenitrification with PdCu Catalysts: Catalyst Optimization by Experimental and Quantum Chemical ApproachesISRAEL JOURNAL OF CHEMISTRY, Issue 1 2006Irena Efremenko A continuous process for nitrate and nitrite abatement from drinking water by catalytic hydrogenation has been developed in our lab. We describe the experimental process development procedure, and support it with semiempirical quantum chemical methods. Comparisons of activated carbon (ACC) and silica glass fiber (GFC) cloths as supports for mono- and bimetallic Pd-Cu catalysts show the former to be 45-fold and 15-fold more active for nitrite and nitrate hydrogenation, respectively, than the latter. Catalysts prepared by selective deposition of Cu on Pd/ACC led to better activity for nitrate hydrogenation than catalysts prepared by co-impregnation or ion exchange methods. The optimal Cu:Pd atomic ratio was found to be 1:2. The computational results show the following: (i) The dispersion of Pd catalysts supported on ACC is much higher than that on GFC due to the larger surface area and higher density of adsorption sites, and that accounts for the higher activity of PdCu/ACC; (ii) Nanosized Pd particles supported on ACC have a semispherical shape and possess preferentially close-packed triangular surfaces, while Pd/GFC particles are extended in the direction parallel to the support surface and show both fcc (100) and (111) planes; (iii) The interaction of Cu atoms with both supports is stronger than that of Pd; adsorbed Cu atoms show a greater ability to form monometallic than bimetallic bonds and that should result in poor mixing of the metal upon co-impregnation, as was observed experimentally; (iv) Cu atoms in bimetallic PdCu particles admit a significant positive charge; the experimentally measured solubility of metal atoms correlates with their calculated charges. The best catalyst (2 wt%Pd-0.6 wt%Cu/ACC) was employed in a novel continuous flow reactor for nitrate hydrogenation in distilled and tap water. The advantages of the reactor investigated over a conventional packed bed reactor are discussed, suggesting a potential for further process intensification. [source] Substituent electronic descriptors for fast QSAR/QSPRJOURNAL OF CHEMOMETRICS, Issue 3-4 2007Bahram Hemmateenejad Abstract Substituent electronic descriptors (SED), calculated by ab initio quantum chemical methods for radical substituents, were proposed as an efficient and simple to use descriptors for use in Quantitative structure-activity/property relationships (QSPR/QSAR) studies. Twenty five SED parameters were calculated for a set of simple substituents using orbital energies, local charges, and dipole moments. Calculation of these parameters for a substituent takes much lower time comparing with that for parent molecule. Different chemical and biological data were analyzed by the SED parameters and it was found that in addition to the simplicity and speed of calculations, models obtained by SED parameters have better or comparable efficiency in relative to existing models. Copyright © 2007 John Wiley & Sons, Ltd. [source] Electronic structure and reactivity of guanylthiourea: A quantum chemical studyJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2010Ahmed Mehdi Abstract Electronic structure analysis of guanylthiourea (GTU) and its isomers has been carried out using quantum chemical methods. Two major tautomeric classes (thione and thiol) have been identified on the potential energy (PE) surface. In both the cases conjugation of pi-electrons and intramolecular H-bonds have been found to play a stabilizing role. Various isomers of GTU on its PE surface have been analyzed in two different groups (thione and thiol). The interconversion from the most stable thione conformer (GTU-1) to the most stable thiol conformer (GTU-t1) was found to take place via bimolecular process which involves protonation at sulfur atom of GTU-1 followed by subsequent CN bond rotation and deprotonation. The detailed analysis of the protonation has been carried out in gas phase and aqueous phase (using CPMC model). Sulfur atom (S1) was found to be the preferred protonation site (over N4) in GTU-1 in gas phase whereas N4 was found to be the preferred site of protonation in aqueous medium. The mechanism of S-alkylation reaction in GTU has also been studied. The formation of alkylated analogs of thiol isomers (alkylated guanylthiourea) is believed to take place via bimolecular process which involves alkyl cation attack at S atom followed by CN bond rotation and deprotonation. The reactive intermediate RS(NH2)CNC(NH2)2+ belongs to the newly identified ,N(,L)2 class of species and provides the necessary dynamism for easy conversion of thione to thiol. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] MOLCAS 7: The Next GenerationJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2010Francesco Aquilante Abstract Some of the new unique features of the MOLCAS quantum chemistry package version 7 are presented in this report. In particular, the Cholesky decomposition method applied to some quantum chemical methods is described. This approach is used both in the context of a straight forward approximation of the two-electron integrals and in the generation of so-called auxiliary basis sets. The article describes how the method is implemented for most known wave functions models: self-consistent field, density functional theory, 2nd order perturbation theory, complete-active space self-consistent field multiconfigurational reference 2nd order perturbation theory, and coupled-cluster methods. The report further elaborates on the implementation of a restricted-active space self-consistent field reference function in conjunction with 2nd order perturbation theory. The average atomic natural orbital basis for relativistic calculations, covering the whole periodic table, are described and associated unique properties are demonstrated. Furthermore, the use of the arbitrary order Douglas-Kroll-Hess transformation for one-component relativistic calculations and its implementation are discussed. This section especially focuses on the implementation of the so-called picture-change-free atomic orbital property integrals. Moreover, the ElectroStatic Potential Fitted scheme, a version of a quantum mechanics/molecular mechanics hybrid method implemented in MOLCAS, is described and discussed. Finally, the report discusses the use of the MOLCAS package for advanced studies of photo chemical phenomena and the usefulness of the algorithms for constrained geometry optimization in MOLCAS in association with such studies. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010 [source] Systematic quantum chemical study of DNA-base tautomersJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2004M. Piacenza Abstract The relative energies of the energetically low-lying tautomers of pyridone, cytosine, uracil, thymine, guanine, and iso-cytosine are studied by a variety of different quantum chemical methods. In particular, we employ density functional theory (DFT) using the six functionals HCTH407, PBE, BP86, B-LYP, B3-LYP, and BH-LYP, and the ab initio methods Hartree-Fock (HF), standard second-order Møller-Plesset perturbation theory (MP2), an improved version of it (SCS-MP2), and quadratic configuration interaction including single and double excitations (QCISD) and perturbative triple corrections [QCISD(T)]. A detailed basis set study is performed for the formamide/formamidic acid tautomeric pair. In general, large AO basis sets of at least valence triple-, quality including f-functions (TZV) are employed, which are found to be necessary for an accurate energetic description of the various structures. The performance of the more approximate methods is evaluated with QCISD(T)/TZV(2df,2dp) data taken as reference. In general it is found that DFT is not an appropriate method for the problem. For the tautomers of pyridone and cytosine, most density functionals, including the popular B3-LYP hybrid, predict a wrong energetic order, and only for guanine, the correct sequence of tautomers is obtained with all functionals. Out of the density functionals tested, BH-LYP, which includes a rather large fraction of HF exchange, performs best. A consistent description of the nonaromatic versus aromatic tautomers seems to be a general problem especially for pure, nonhybrid functionals. Tentatively, this could be assigned to the exchange potentials used while the functional itself, including the correlation part, seems to be appropriate. Out of the ab initio methods tested, the new SCS-MP2 approach seems to perform best because it effectively reduces some outliers obtained with standard MP2. It outperforms the much more costly QCISD method and seems to be a very good compromise between computational effort and accuracy. © 2003 Wiley Periodicals, Inc. J Comput Chem 1: 83,98, 2004 [source] Complexation of the Vulcanization Accelerator Tetramethylthiuram Disulfide and Related Molecules with Zinc Compounds Including Zinc Oxide Clusters (Zn4O4)CHEMISTRY - A EUROPEAN JOURNAL, Issue 3 2008Ralf Steudel Prof. Abstract Zinc chemicals are used as activators in the vulcanization of organic polymers with sulfur to produce elastic rubbers. In this work, the reactions of Zn2+, ZnMe2, Zn(OMe)2, Zn(OOCMe)2, and the heterocubane cluster Zn4O4 with the vulcanization accelerator tetramethylthiuram disulfide (TMTD) and with the related radicals and anions Me2NCS2., Me2NCS3., Me2NCS2,, and Me2NCS3, have been studied by quantum chemical methods at the MP2/6-31+G(2df,p)//B3LYP/6-31+G* level of theory. More than 35 zinc complexes have been structurally characterized and the energies of formation from their components calculated for the first time. The binding energy of TMTD as a bidendate ligand increases in the order ZnMe2 Reaction of para -Hydroxy-Substituted Diphenylmethanes with tert -Butoxy RadicalCHEMPHYSCHEM, Issue 8 2004Catarina F. Correia What is the outcome of this reaction? In acetonitrile solution, the methylenic CH bond is approximately 25 kJ,mol,1 weaker than the OH bond in the same molecule (see picture), as demonstrated by time-resolved photoacoustic calorimetry and quantum chemical methods. However, as shown by electron spin resonance spectroscopy, the tert -butoxy radical selectively abstracts the hydrogen atom from the OH group. [source]
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