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Coupling Pathways (coupling + pathway)
Selected AbstractsEncapsulation of Cyanometalates by a Tris-macrocyclic Ligand Tricopper(II) Complex: Syntheses, Structural Variation, and Magnetic Exchange Coupling PathwaysCHEMISTRY - A EUROPEAN JOURNAL, Issue 3 2006Mihail Atanasov Prof. Dr. Abstract The reaction of [M(CN)6]3, (M=Cr3+, Mn3+, Fe3+, Co3+) and [M(CN)8]4,/3, (M=Mo4+/5+, W4+/5+) with the trinuclear copper(II) complex of 1,3,5-triazine-2,4,6-triyltris[3-(1,3,5,8,12-pentaazacyclotetradecane)] ([Cu3(L)]6+) leads to partially encapsulated cyanometalates. With hexacyanometalate(III) complexes, [Cu3(L)]6+ forms the isostructural host,guest complexes [{[Cu3(L)(OH2)2][M(CN)6]2}{M(CN)6}],[M(CN)6],30,H2O with one bridging, two partially encapsulated, and one isolated [M(CN)6]3, unit. The octacyanometalates of Mo4+/5+ and W4+/5+ are encapsulated by two tris-macrocyclic host units. Due to the stability of the +IV oxidation state of Mo and W, only assemblies with [M(CN)8]4, were obtained. The Mo4+ and W4+ complexes were crystallized in two different structural forms: [{Cu3(L)(OH2)}2{Mo(CN)8}](NO3)8,15,H2O with a structural motif that involves isolated spherical [{Cu3(L)(OH2)}2{M(CN)8}]8+ ions and a "string-of-pearls" type of structure [{[Cu3(L)]2[M(CN)8]}{M(CN)8}](NO3)4, 20,H2O, with [M(CN)8]4, ions that bridge the encapsulated octacyanometalates in a two-dimensional network. The magnetic exchange coupling between the various paramagnetic centers is characterized by temperature-dependent magnetic susceptibility and field-dependent magnetization data. Exchange between the Cu,,,Cu pairs in the [Cu3(L)]6+ "ligand" is weakly antiferromagnetic. Ferromagnetic interactions are observed in the cyanometalate assemblies with Cr3+, exchange coupling of Mn3+ and Fe3+ is very small, and the octacoordinate Mo4+ and W4+ systems have a closed-shell ground state. [source] NMR spin-spin couplings involving nuclei in the neighborhood of a carbonyl group.MAGNETIC RESONANCE IN CHEMISTRY, Issue 3 2008-substituted acetamides, 3JCH couplings in Abstract In this work 3JCH spin-spin coupling constants (SSCCs) for the cis- and trans- conformers for ,-X-acetamides (X = F, Cl, Br and CN) (1,4) were studied in detail since they were found to be notably different for both conformers. These differences are rationalized as originating in the changes of the strong negative hyperconjugative interactions that take place within the carbonyl group. Such changes are found to depend not only on conformation, but also on solvent. For the cis -conformers there is a close proximity between the X-substituent and the in-plane oxygen lone pair of pure p character, which affects notably their respective negative hyperconjugative interactions. Both the efficiency for transmitting the Fermi contact (FC) term through the coupling pathway of 3JCH SSCCs and its potential as a probe to study the stereochemical properties of the XH2C group are discussed. Copyright © 2008 John Wiley & Sons, Ltd. [source] The effect of carbonyl group in the asymmetry of 3, 4JCH coupling constants in norbornanonesMAGNETIC RESONANCE IN CHEMISTRY, Issue 2 2008Francisco P. dos Santos Abstract A rationalization of the known difference between the 3, 4J and 3, 4J couplings transmitted mainly through the 7-bridge in norbornanone is presented in terms of the effects of hyperconjugative interactions involving the carbonyl group. Theoretical and experimental studies of 3, 4JCH couplings were carried out in 3- endo - and 3- exo -X-2-norbornanone derivatives (X = Cl, Br) and in exo - and endo -2-noborneol compounds. Hyperconjugative interactions were studied with the natural bond orbital (NBO) method. Hyperconjugative interactions involving the carbonyl ,* and ,* antibonding orbitals produce a decrease of three-bond contribution to both 3, 4J and 3, 4J couplings. However, the latter antibonding orbital also undergoes a strong , , ,* interaction, which defines an additional coupling pathway for 3, 4J but not for 3, 4J. This pathway is similar to that known for homoallylic couplings, the only difference being the nature of the intermediate antibonding orbital; i.e. for 3, 4J it is of ,*-type, while in homoallylic couplings it is of ,*-type. Copyright © 2007 John Wiley & Sons, Ltd. [source] Multistep correlations via covariance processing of COSY/GCOSY spectra: opportunities and artifactsMAGNETIC RESONANCE IN CHEMISTRY, Issue 11 2008Gary E. Martin Abstract Long-range homonuclear coupling pathways can be observed in COSY or GCOSY spectra by the acquisition of spectra with larger numbers of increments of the evolution period, t1, than would normally be used. Alternatively, covariance processing of COSY-type spectra acquired with modest numbers of t1 increments, allows the observation of multistage correlations. In this work results obtained from covariance-processed GCOSY spectra are fully analyzed and compared to normally processed COSY and 80 ms TOCSY spectra. Multistage or ,RCOSY-type' correlations are observed when remote protons both exhibit correlations to the same coupling partner e.g. A , B and B , C gives rise to an A , C correlation. In the strict sense, RCOSY-type responses are artifacts albeit providing useful information. Nonbeneficial artifact correlations are observed when protons couple to other protons that overlap or partially overlap. The origin of artifact responses is also analyzed. Copyright © 2008 John Wiley & Sons, Ltd. [source] Isoquinoline alkaloids: a 15N NMR and x-ray study.MAGNETIC RESONANCE IN CHEMISTRY, Issue 11 2002Part Abstract Continuing our systematic 15N NMR study of isoquinoline alkaloids, we report a contribution extending our previous paper. The 15N NMR chemical shifts and 15N,1H long-range coupling pathways of tertiary and quaternary isoquinoline alkaloids of several constitutional types are presented. The selected compounds belong to the protoberberine, proaporphine, pavinane, rhoeadine and phtalideisoquinoline classes of alkaloids and were investigated by gradient-selected inverse-detected multiple bond correlation experiments (GHMBC and GSQMBC). In addition, x-ray data and the principal geometric parameters of stylopine, mecambridine, norchelerythrine, isothebaine and mecambrine are reported and discussed. Copyright © 2002 John Wiley & Sons, Ltd. [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] Exciton Migration in Conjugated Dendrimers: A Joint Experimental and Theoretical StudyCHEMPHYSCHEM, Issue 18 2009Muhammet E. Köse Prof. Abstract We report a joint experimental and theoretical investigation of exciton diffusion in phenyl-cored thiophene dendrimers. Experimental exciton diffusion lengths of the dendrimers vary between 8 and 17 nm, increasing with the size of the dendrimer. A theoretical methodology is developed to estimate exciton diffusion lengths for conjugated small molecules in a simulated amorphous film. The theoretical approach exploits Fermi's Golden Rule to estimate the energy transfer rates for a large ensemble of bimolecular complexes in random relative orientations. Utilization of Poisson's equation in the evaluation of the Coulomb integral leads to very efficient calculation of excitonic couplings between the donor and the acceptor chromophores. Electronic coupling calculations with delocalized transition densities revealed efficient coupling pathways in the bulk of the material, but do not result in strong couplings between the chromophores which are calculated for more localized transition densities. The molecular structures of dendrimers seem to be playing a significant role in the magnitude of electronic coupling between chromophores. Simulated diffusion lengths correlate well with the experimental data. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors in determining the size of the exciton diffusion length in amorphous films of conjugated materials. [source] | ||||||||||