Download citation
Acta Cryst. (2014). A70, C188
Download citation

link to html
Charge-flipping has become a popular approach to ab-initio structure solution from X-ray powder diffraction data in particular due to its speed and need for minimal input other than lattice parameters. Given the appetite of charge-flipping for low d-spacing reflections, time-of-flight (TOF) neutron data should be a good match from a resolution standpoint, with easy access to high Q and lack of form-factor drop-off. One obvious issue with neutron data is the presence of elements with negative scattering lengths, where the inherent assumption of atoms always having positive `density' in the algorithm breaks down. This means that portions of the structure can be effectively invisible. Given that some of these elements (e.g. H and Mn) are commonly found in samples of interest the issue is more than simple academic curiosity. Of course such atoms can be found by difference maps, but the issue has also been addressed within the charge-flipping algorithm with the `band-flipping' modification [1]. Although Oszlányi & Sütö demonstrated the approach was viable with simulated neutron single crystal data [1], to the authors' knowledge it hasn't been used previously with experimental single crystal or powder neutron diffraction data. Powder diffraction data from POWGEN and wavelength-resolved TOF Laue single crystal data from TOPAZ at the Spallation Neutron Source have been used to probe the relative ease of charge-flipping with different TOF data using the TOPAS software package [2]. In addition the effectiveness of different customized band-flipping approaches has been tested to extract positions for positive and negative scattering elements simultaneously.

Download citation
Acta Cryst. (2014). A70, C922
Download citation

link to html
Synthetic biologically inspired complexes exhibiting reactivity similar to hydrogenase enzymes have provided evidence of hydride transfer to the metal and proton transfer to an amine, but key structural information about the intermediate is not readily discernible with X-rays. The greater sensitivity of neutron to hydrogen makes it ideal for studying the structure and dynamics of catalytic materials. The newly commissioned TOPAZ neutron single crystal diffractometer at the SNS is capable of continuous 3D diffraction space mapping from a small stationary crystal, permitting detailed structural study at atomic resolution. The structure measured on TOPAZ for an Fe-based mononuclear electrocatalyst confirms that reaction of [CpFeN-L)](BARF) (1) with H2 under mild conditions leads to heterolytic cleavage of the H-H bond into a proton and hydride[1]. The precise location of H atoms in [Fe-H···H-N]+ reveals an unconventional H-bonding interaction, where the ferrous hydridic site {Fe(II)-H-} acts as the H-bond acceptor and the nitrogen of the protic pendant amine {L-N-H+} as the H-bond donor. The neutron structure provides clear evidence of a crucial intermediate involving an Fe-H···H-N interaction in the oxidation of H2. The result clarifies the key role of the pendant amine in the iron complex and provides insights into the design of synthetic electrocatalysts sought as cost-effective alternatives to platinum in fuel cells. The reaction is also a critical step in homogeneous catalysts for hydrogenation of C=O and C=N bonds. A preliminary result from TOPAZ measurement shows that 1 undergoes further single-crystal to single-crystal chemical reaction with moisture in the air, leading to a Fe(H2O)+ complex. Abbreviations: Cp = pentafluoropyridylcyclopentadienide; N-L= 1, 5-di(tert-butyl)-3,7-di(benzyl)-1,5-diaza-3,7-diphospha-cyclooctane; BARF = [B[3,5-(CF3)2C6H3]4]-

Download citation
Acta Cryst. (2014). A70, C1552
Download citation

link to html
In recent years, semiconducting organic materials have attracted a considerable amount of interest to develop all-organic or hybrid organic-inorganic electronic devices such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), or photovoltaic cells. Rubrene (5,6,11,12-tetraphenyltetracene, RUB) is one of the most explored compound in this area as it has nearly 100% fluorescence quantum efficiency in solution. Additionally, the OFET fabricated by vacuum-deposited using orthorhombic rubrene single crystals show p-type characteristics with high mobility up to 20cm2/Vs (Podzorov et al., 2004). The large charge-carrier mobilities measured have been attributed to the packing motif (Fig a) which provides enough spatial overlap of the π-conjugated tetracene backbone. In the same time, RUB undergoes an oxidation in the presence of light to form rubrene endoperoxide (RUB-OX) (Fumagalli et al., 2011). RUB-OX molecules show electronic and structural properties strikingly different from those of RUB, mainly due to the disruption in the conjugate stacking of tetracene moieties. The significant semiconducting property of RUB is not clear yet. In this context, high resolution single crystal X-ray data of RUB (Fig b) and RUB-OX have been collected at 100K. Owing to the presence of weak aromatic stacking and quadrupolar interactions, the neutron single crystal data is also collected at 100K. The C-H bond distances and scaled anisotropic displacement parameters (ADP) of hydrogens from the neutron experiment are used in the multipolar refinements of electron density. The chemical bonding features (Fig c), the topology of electron density and strength of weak interaction are calculated by the Atoms in Molecules (AIM) theory (Bader, 1990). It is further supported by the source function description and mapping of non-covalent interactions based on the electron density. The detailed comparison of two organic semiconductors, RUB and RUB-OX will be discussed.
Follow Acta Cryst. A
Sign up for e-alerts
Follow Acta Cryst. on Twitter
Follow us on facebook
Sign up for RSS feeds