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RISS 인기검색어
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- 저자 : McCusker, James K. (검색결과 3 건)
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Complex zeolite structure solved by combining powder diffraction and electron microscopy
Gramm, Fabian,Baerlocher, Christian,McCusker, Lynne B.,Warrender, Stewart J.,Wright, Paul A.,Han, Bada,Hong, Suk Bong,Liu, Zheng,Ohsuna, Tetsu,Terasaki, Osamu Nature Publishing Group 2006 Nature Vol.444 No.7115
Many industrially important materials, ranging from ceramics to catalysts to pharmaceuticals, are polycrystalline and cannot be grown as single crystals. This means that non-conventional methods of structure analysis must be applied to obtain the structural information that is fundamental to the understanding of the properties of these materials. Electron microscopy might appear to be a natural approach, but only relatively simple structures have been solved by this route. Powder diffraction is another obvious option, but the overlap of reflections with similar diffraction angles causes an ambiguity in the relative intensities of those reflections. Various ways of overcoming or circumventing this problem have been developed, and several of these involve incorporating chemical information into the structure determination process. For complex zeolite structures, the FOCUS algorithm has proved to be effective. Because it operates in both real and reciprocal space, phase information obtained from high-resolution transmission electron microscopy images can be incorporated directly into this algorithm in a simple way. Here we show that by doing so, the complexity limit can be extended much further. The power of this approach has been demonstrated with the solution of the structure of the zeolite TNU-9 (|H<SUB>9.3</SUB>|[Al<SUB>9.3</SUB>Si<SUB>182.7</SUB>O<SUB>384</SUB>]; ref. 10) with 24 topologically distinct (Si,Al) atoms and 52 such O atoms. For comparison, ITQ-22 (ref. 11), the most complex zeolite known to date, has 16 topologically distinct (Si,Ge) atoms.
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Huse, Nils,Kim, Tae Kyu,Jamula, Lindsey,McCusker, James K.,de Groot, Frank M. F.,Schoenlein, Robert W. American Chemical Society 2010 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.132 No.19
<P>Solution-phase photoinduced low-spin to high-spin conversion in the Fe<SUP>II</SUP> polypyridyl complex [Fe(tren(py)<SUB>3</SUB>)]<SUP>2+</SUP> (where tren(py)<SUB>3</SUB> is tris(2-pyridylmethyliminoethyl)amine) has been studied via picosecond soft X-ray spectroscopy. Following <SUP>1</SUP>A<SUB>1</SUB> → <SUP>1</SUP>MLCT (metal-to-ligand charge transfer) excitation at 560 nm, changes in the iron L<SUB>2</SUB>- and L<SUB>3</SUB>-edges were observed concomitant with formation of the transient high-spin <SUP>5</SUP>T<SUB>2</SUB> state. Charge-transfer multiplet calculations coupled with data acquired on low-spin and high-spin model complexes revealed a reduction in ligand field splitting of ∼1 eV in the high-spin state relative to the singlet ground state. A significant reduction in orbital overlap between the central Fe-3d and the ligand N-2p orbitals was directly observed, consistent with the expected ca. 0.2 Å increase in Fe−N bond length upon formation of the high-spin state. The overall occupancy of the Fe-3d orbitals remains constant upon spin crossover, suggesting that the reduction in σ-donation is compensated by significant attenuation of π-back-bonding in the metal−ligand interactions. These results demonstrate the feasibility and unique potential of time-resolved soft X-ray absorption spectroscopy to study ultrafast reactions in the liquid phase by directly probing the valence orbitals of first-row metals as well as lighter elements during the course of photochemical transformations.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2010/jacsat.2010.132.issue-19/ja101381a/production/images/medium/ja-2010-01381a_0007.gif'></P>
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Cho, Hana,Strader, Matthew L.,Hong, Kiryong,Jamula, Lindsey,Gullikson, Eric M.,Kim, Tae Kyu,de Groot, Frank M. F.,McCusker, James K.,Schoenlein, Robert W.,Huse, Nils The Royal Society of Chemistry 2012 Faraday discussions Vol.157 No.-
<P>Ultrafast excited-state evolution in polypyridyl Fe(<SMALL>II</SMALL>) complexes is of fundamental interest for understanding the origins of the sub-ps spin-state changes that occur upon photoexcitation of this class of compounds as well as for the potential impact such ultrafast dynamics have on incorporation of these compounds in solar energy conversion schemes or switchable optical storage technologies. We have demonstrated that ground-state and, more importantly, ultrafast time-resolved X-ray absorption methods can offer unique insights into the interplay between electronic and geometric structure that underpins the photo-induced dynamics of this class of compounds. The present contribution examines in greater detail how the symmetry of the ligand field surrounding the metal ion can be probed using these X-ray techniques. In particular, we show that steady-state K-edge spectroscopy of the nearest-neighbour nitrogen atoms reveals the characteristic chemical environment of the respective ligands and suggests an interesting target for future charge-transfer femtosecond and attosecond spectroscopy in the X-ray water window.</P>
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