Home About us Contact
|
Home About us Contact | ||
|
|
||
Similar Energies (similar + energy)
Selected Abstracts(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] Microbiology and geochemistry of Little Hot Creek, a hot spring environment in the Long Valley CalderaGEOBIOLOGY, Issue 2 2010T. J. VICK A culture-independent community census was combined with chemical and thermodynamic analyses of three springs located within the Long Valley Caldera, Little Hot Creek (LHC) 1, 3, and 4. All three springs were approximately 80 °C, circumneutral, apparently anaerobic and had similar water chemistries. 16S rRNA gene libraries constructed from DNA isolated from spring sediment revealed moderately diverse but highly novel microbial communities. Over half of the phylotypes could not be grouped into known taxonomic classes. Bacterial libraries from LHC1 and LHC3 were predominantly species within the phyla Aquificae and Thermodesulfobacteria, while those from LHC4 were dominated by candidate phyla, including OP1 and OP9. Archaeal libraries from LHC3 contained large numbers of Archaeoglobales and Desulfurococcales, while LHC1 and LHC4 were dominated by Crenarchaeota unaffiliated with known orders. The heterogeneity in microbial populations could not easily be attributed to measurable differences in water chemistry, but may be determined by availability of trace amounts of oxygen to the spring sediments. Thermodynamic modeling predicted the most favorable reactions to be sulfur and nitrate respirations, yielding 40,70 kJ mol,1 e, transferred; however, levels of oxygen at or below our detection limit could result in aerobic respirations yielding up to 100 kJ mol,1 e, transferred. Important electron donors are predicted to be H2, H2S, S0, Fe2+ and CH4, all of which yield similar energies when coupled to a given electron acceptor. The results indicate that springs associated with the Long Valley Caldera contain microbial populations that show some similarities both to springs in Yellowstone and springs in the Great Basin. [source] Magnetic impurities in small metal clustersANNALEN DER PHYSIK, Issue 9-10 2005G.M. Pastor Abstract Magnetic impurities in small metallic clusters are investigated in the framework of the Anderson model by using exact diagonalization and geometry optimization methods. The singlet-triplet spin gap ,E shows a remarkable dependence as a function of band-filling, cluster structure, and impurity position that can be interpreted in terms of the environment-specific conduction-electron spectrum. The low-energy spin excitations involve similar energies as isomerizations. Interesting correlations between cluster structure and magnetic behavior are revealed. Finite-temperature properties such as specific heat, effective impurity moment, and magnetic susceptibility are calculated exactly in the canonical ensemble. A finite-size equivalent of the Kondo effect is identified and its structural dependence is discussed. [source] Regio- and Stereoselective Palladium-Pincer Complex Catalyzed Allylation of Sulfonylimines with Trifluoro(allyl)borates and Allylstannanes: A Combined Experimental and Theoretical StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 26 2006Olov A. Wallner Dr. Abstract Regio- and stereoselective palladium-pincer complex catalyzed allylation of sulfonylimines has been performed by using substituted trifluoro(allyl)borates and trimethylallylstannanes. The reactions provide the corresponding branched allylic products with excellent regioselectivity. The stereoselectivity of these processes is very high when trifluoro(cinnamyl)borate and trimethyl cinnamyl stannane are employed as allylic precursors; however, the reaction with trifluoro(crotyl)borate results in poor stereoselectivity. The major diastereomer formed in these reactions was the syn isomer, while the (previously reported) reactions with aldehyde electrophiles afforded the anti products, indicating that the mechanism of the stereoselection is dependent on the applied electrophile. Therefore, we have studied the mechanistic aspects of the allylation reactions by experimental studies and DFT modeling. The experimental mechanistic studies have clearly shown that potassium trifluoro(allyl)borate undergoes transmetallation with palladium-pincer complex 1,a affording an ,1 -allylpalladium-pincer complex (1,e). The mechanism of the transfer of the allyl moiety from palladium to the sulfonylimine substrate was studied by DFT calculations at the B3PW91/LANL2DZ+P level of theory. These calculations have shown that the electrophilic substitution of sulfonylimines proceeds in a one-step process with a relatively low activation energy. The topology of the potential energy surface in the vicinity of the transition-state structure proved to be rather complicated as nine different geometries with similar energies were located as first order saddle points. Our studies have also shown that the high stereoselectivity with cinnamyl metal reagents stems from steric interactions in the TS structure of the allylation reaction. In addition, these studies have revealed that the mechanism of the stereoselection in the allylation of aldehydes and sulfonylimines is fundamentally different. [source] Ab initio prediction of optical rotation: Comparison of density functional theory and Hartree-Fock methods for three 2,7,8-trioxabicyclo[3.2.1]octanesCHIRALITY, Issue 4 2002P.J. Stephens Abstract We report ab initio calculations of the frequency-dependent electric dipole-magnetic dipole polarizabilities, ,(,), at the sodium D line frequency and, thence, of the specific rotations, [,]D, of 2,7,8-trioxabicyclo[3.2.1]octane, 1, and its 1-methyl derivative, 2, using the Density Functional Theory (DFT) and Hartree-Fock/Self-Consistent Field (HF/SCF) methodologies. Gauge-invariant (including) atomic orbitals (GIAOs) are used to ensure origin-independent [,]D values. Using large basis sets which include diffuse functions DFT [,]D values are in good agreement with experimental values (175.8° and 139.2° for (1S,5R)- 1 and - 2, respectively); errors are in the range 25,35°. HF/SCF [,]D values, in contrast, are much less accurate; errors are in the range 75,95°. The use of small basis sets which do not include diffuse functions substantially lowers the accuracy of predicted [,]D values, as does the use of the static limit approximation: ,(,) , ,(o). The use of magnetic-field-independent atomic orbitals, FIAOs, instead of GIAOs, leads to origin-dependent, and therefore nonphysical, [,]D values. We also report DFT calculations of [,]D for the 1-phenyl derivative of 1, 3. DFT calculations find two stable conformations, differing in the orientation of the phenyl group, of very similar energy, and separated by low barriers. Values of [,]D predicted using two different algorithms for averaging over phenyl group orientations are in good agreement with experiment. In principle, the absolute configuration (AC) of a chiral molecule can be assigned by comparison of the optical rotation predicted ab initio to the experimental value. Our results demonstrate the critical importance of the choice of ab initio methodology in obtaining reliable optical rotations and, hence, ACs, and show that, at the present time, DFT constitutes the method of choice. Chirality 14:288,296, 2002. © 2002 Wiley-Liss, Inc. [source] | ||||||||||