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      Gas-Phase Reactions of Cationic Vanadium-Phosphorus Oxide Clusters with C 2H x ( x=4, 6): A DFT-Based Analysis of Reactivity Patterns

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          The reactivities of the adamantane-like heteronuclear vanadium-phosphorus oxygen cluster ions [V x P 4− x O 10] .+ ( x=0, 2–4) towards hydrocarbons strongly depend on the V/P ratio of the clusters. Possible mechanisms for the gas-phase reactions of these heteronuclear cations with ethene and ethane have been elucidated by means of DFT-based calculations; homolytic C–H bond activation constitutes the initial step, and for all systems the P–O . unit of the clusters serves as the reactive site. More complex oxidation processes, such as oxygen-atom transfer to, or oxidative dehydrogenation of the hydrocarbons require the presence of a vanadium atom to provide the electronic prerequisites which are necessary to bring about the 2e reduction of the cationic clusters.

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          Optimization of an AMBER Force Field for the Artificial Nucleic Acid, LNA, and Benchmarking with NMR of L(CAAU)

          Locked Nucleic Acids (LNAs) are RNA analogues with an O2′-C4′ methylene bridge which locks the sugar into a C3′-endo conformation. This enhances hybridization to DNA and RNA, making LNAs useful in microarrays and potential therapeutics. Here, the LNA, L(CAAU), provides a simplified benchmark for testing the ability of molecular dynamics (MD) to approximate nucleic acid properties. LNA χ torsions and partial charges were parametrized to create AMBER parm99_LNA. The revisions were tested by comparing MD predictions with AMBER parm99 and parm99_LNA against a 200 ms NOESY NMR spectrum of L(CAAU). NMR indicates an A-Form equilibrium ensemble. In 3000 ns simulations starting with an A-form structure, parm99_LNA and parm99 provide 66% and 35% agreement, respectively, with NMR NOE volumes and 3 J-couplings. In simulations of L(CAAU) starting with all χ torsions in a syn conformation, only parm99_LNA is able to repair the structure. This implies methods for parametrizing force fields for nucleic acid mimics can reasonably approximate key interactions and that parm99_LNA will improve reliability of MD studies for systems with LNA. A method for approximating χ population distribution on the basis of base to sugar NOEs is also introduced.
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            Detailed Characterization of a Nanosecond-Lived Excited State: X-ray and Theoretical Investigation of the Quintet State in Photoexcited [Fe(terpy)2]2+

            Theoretical predictions show that depending on the populations of the Fe 3d xy , 3d xz , and 3d yz orbitals two possible quintet states can exist for the high-spin state of the photoswitchable model system [Fe(terpy)2]2+. The differences in the structure and molecular properties of these 5B2 and 5E quintets are very small and pose a substantial challenge for experiments to resolve them. Yet for a better understanding of the physics of this system, which can lead to the design of novel molecules with enhanced photoswitching performance, it is vital to determine which high-spin state is reached in the transitions that follow the light excitation. The quintet state can be prepared with a short laser pulse and can be studied with cutting-edge time-resolved X-ray techniques. Here we report on the application of an extended set of X-ray spectroscopy and scattering techniques applied to investigate the quintet state of [Fe(terpy)2]2+ 80 ps after light excitation. High-quality X-ray absorption, nonresonant emission, and resonant emission spectra as well as X-ray diffuse scattering data clearly reflect the formation of the high-spin state of the [Fe(terpy)2]2+ molecule; moreover, extended X-ray absorption fine structure spectroscopy resolves the Fe–ligand bond-length variations with unprecedented bond-length accuracy in time-resolved experiments. With ab initio calculations we determine why, in contrast to most related systems, one configurational mode is insufficient for the description of the low-spin (LS)–high-spin (HS) transition. We identify the electronic structure origin of the differences between the two possible quintet modes, and finally, we unambiguously identify the formed quintet state as 5E, in agreement with our theoretical expectations.
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              C-H bond activations by metal oxo compounds.


                Author and article information

                Chemistry (Weinheim an Der Bergstrasse, Germany)
                WILEY-VCH Verlag (Weinheim )
                25 February 2013
                15 January 2013
                : 19
                : 9
                : 3017-3028
                [[a] ]Institut für Chemie, Technische Universität Berlin Straße des 17. Juni 135, 10623 Berlin (Germany), Fax: (+49) 30-314-21102 E-mail: Maria.Schlangen@ Helmut.Schwarz@
                [[b] ]Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel Olshausenstraße 40, 24098 Kiel (Germany)
                [[c] ]Lab of Computational Chemistry and Drug Design, Laboratory of Chemical Genomics, Peking University Shenzhen Graduate School Shenzhen 518055 (P. R. China)
                [[d] ]Chemistry Department, Faculty of Science, King Abdulaziz University Jeddah 21589 (Saudi Arabia) E-mail: HSchwarz@
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                Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

                Re-use of this article is permitted in accordance with the Creative Commons Deed, Attribution 2.5, which does not permit commercial exploitation.

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