Spin Crossover under Extreme Conditions: A Structural Approach

Structural studies of several iron complexes that show spin crossover (SCO) properties are presented and intrinsic links between the structure and properties have been established for these systems. X-ray diffraction experiments of samples under extremes of temperature and pressure and after irradiation with laser light have been performed to characterise the structure of these various compounds. The incomplete thermal SCO observed in the dinuclear species [2 4,4’- bipy]•2MeOH has been investigated, revealing a complex series of crystallographic phase transitions that result in a material with a mixture of high spin (HS) and low spin (LS) states that displays long-range order at low temperature. The meta-stable high spin (HS*) state that arises as a result of the Light Induced Excited Spin State Trapping (LIESST) effect has also been structurally characterised at 30 K. Further single crystal X-ray diffraction experiments at high pressure have shown that it is possible to achieve a complete spin transition to a fully LS state, which is not thermally accessible. Another species that shows crystallographic ordering of mixed spin states is an FeIII complex with a hexadentate Schiff base ligand, which has been studied using both single crystal and powder diffraction techniques. The structure of the photo-induced HS* state of [Fe(bapbpy) NCS)2] has been determined at 30 K from diffraction data which comprise contributions from both the HS* state and the residual LS component of the crystal, which is non-merrohedrally twinned. The structure of [Fe(picen)(NCS)2] has been determined at 30 K after a range of cooling speeds, showing that cooling of the sample results in partial population of the HS* state. The photo-induced HS* state, and that induced by thermal trapping are shown to be structurally similar. A gradual spin transition has been identified purely from structural analysis in an FeII complex with a hexadentate N6-donating ligand. The gradual nature of the spin transition may be attributed to the poor cooperativiy observed between iron centres. This study shows how structural features can be used to gain insights into some of the more unusual aspects of the SCO phenomenon such as stepped spin transitions, ordered mixed spin intermediate phases, trapping of meta-stable states as a result of light irradiation or temperature and pressure-induced spin transitions

Aerospace medicine and biology: A continuing bibliography with indexes (supplement 341)

This bibliography lists 133 reports, articles and other documents introduced into the NASA Scientific and Technical Information System during September 1990. Subject coverage includes: aerospace medicine and psychology, life support systems and controlled environments, safety equipment, exobiology and extraterrestrial life, and flight crew behavior and performance

Special Libraries, February 1972

Volume 63, Issue 2https://scholarworks.sjsu.edu/sla_sl_1972/1001/thumbnail.jp

Phase Combination and its Application to the Solution of Macromolecular Structures: Developing ALIXE and SHREDDER

[eng] Phasing X-ray data within the frame of the ARCIMBOLDO programs requires very accurate models and a sophisticated evaluation of the possible hypotheses. ARCIMBOLDO uses small fragments, that are placed with the maximum likelihood molecular replacement program Phaser, and are subject to density modification and autotracing with the program SHELXE. The software receives its name from the Italian painter Giuseppe Arcimboldo, who used to compose portraits out of common objects such as vegetables or flowers. Out of most possible arrangements of such objects, only a still-life will result, and just a few ones will truly produce a portrait. In a similar way, from all possible placements with small protein fragments, only a few will be correct and will allow to get the full “protein’s portrait”. The work presented in this thesis has explored new ways to exploit partial information and increase the signal in the process of phasing with fragments. This has been achieved through two main pieces of software, ALIXE and SHREDDER. With the spherical mode in ARCIMBOLDO_SHREDDER, the aim is to derive compact fragments starting from a distant homolog to our unknown protein of interest. Then, locations for these fragments are searched with Phaser. These include strategies for refining the fragments against the experimental data and giving them more degrees of freedom. With ALIXE, the aim is to combine information in reciprocal space from partial solutions, such as the ones produced by SHREDDER, and use the coherence between them to guide their merging and to increase the information content, so that the step of density modification and autotracing starts from a more complete solution. Even if partial solutions contain both correct and incorrect information, the combination of solutions that share some similarity will allow to get a better approximation to the correct structure. Both ARCIMBOLDO_SHREDDER and ALIXE have been used on test data for development and optimisation but also on datasets from previously unknown structures, which have been solved thanks to these programs. These programs are distributed through the website of the group but also through software suites of general use in the crystallographic community such as CCP4 and SBGrid

Aerospace medicine and biology: A continuing bibliography with indexes (supplement 335)

This bibliography lists 143 reports, articles and other documents introduced into the NASA Scientific and Technical Information System during March, 1990. Subject coverage includes: aerospace medicine and psychology, life support systems and controlled environments, safety equipment, exobiology and extraterrestrial life, and flight crew behavior and performance

Systematic structural studies in metal complex chemistry

A procedure is presented for detecting geometrical preferences, deformations and interconversion pathways between different geometries for the transition metal coordination sphere ML(_n). A discrepancy index [R(_ang)(x)] was proposed initially to address the problems of dimensionality and permutation complexity in the systematic analysis of coordination sphere geometry with higher coordination numbers (n > 7). But it can also be used generally for the lower coordination numbers. A set of standard geometries for coordination numbers 2-9 are presented and the angles between the center point and each vertex for the polyhedra which are used to describe the coordination sphere geometries for coordination numbers 7-9 are idealised. These angles correspond to the metal-ligand valence angles in the coordination complex and are used as the standard values to measure the deviation of a real coordination sphere in the complex from these standard polyhedra. Geometry of each coordination sphere (ML(_7-9)) from the Cambridge Structural Database (CSD) is identified by the calculations of R(_ang)(x) values. Also the unique enumeration numbers of the ligands corresponding to each geometry can be derived over the n! ligand permutations. The different geometrical clusters and interconversion pathways from one to another are mapped in a designed two-dimensional plot. The symmetry coordinates and principal component analysis are initially applied in these higher coordination number systems. They not only map the clusters represented to those standard geometries in the different symmetric point groups but also provide and confirm the interconversion pathways between the different geometries. The other systematic study involves the analysis and correlation of the metal σ-π bond in the transition metal alkyne and alkene complexes from the CSD. Geometrical features of this specific bond are examined in the view of structure and some useful correlation between the key geometrical parameters are defined. Finally, X-ray crystal structure determinations are briefly described and the crystal structures of ten transition metal compounds in coordination numbers 4-6 are presented

Integrated polymer photonics : fabrication, design, characterization and applications

[no abstract

Chemical Information Bulletin

Periodic supplement for "the regular journals of the American Chemical Society," containing annotated bibliographies of chemical documentation literature as well as information about meetings, conferences, awards, scholarships, and other news from the American Chemical Society (ACS) Division of Chemical Literature