Description: This article discusses spin-flip probability in the inelastic scattering of 7.48-MeV neutrons from the 4.43-MeV state of 12C. An (n, n'y) coincidence method was used to determine the neutron spin-flip probability in inelastic scattering. The experimental method consists of neutron time-of-flight and neutron-y coincidence technique with the deexcitation y ray detected perpendicular to the neutron scattering plane. The angular distribution of the spin-flip probability for the first Jπ =2+ excitation in carbon has been determined at an energy of 7.48 MeV. The neutron spin-flip results were found to be similar to the proton spin-flip results in this energy region. The experimental results were compared to the predictions of an antisymmetrized distorted-wave calculation which did not provide good fits to the spin-flip data. The spin-flip predictions were sensitive to the optical-model parameters and were dominated by spin-orbit distortion in the elastic channels. The mechanism of core polarization dominated the inelastic amplitudes, masking the effects of the effective interaction on the extracore nucleons.

Description: This articles discusses an analysis of multiple ignition sites, primary circuits, and burst phase delay distributions. All higher order central nervous systems exhibit spontaneous neural activity, though the purpose and mechanistic origin of such activity remains poorly understood. The authors quantitatively analyzed the ignition and spread of collective spontaneous electrophysiological activity in networks of cultured cortical neurons growing in microelectrode arrays. Leader neurons, which form a mono-synaptically connected primary circuit, and initiate a majority of network bursts were found to be a small subset of recorded neurons. Leader/follower firing delay times formed temporally stable positively skewed distributions. Blocking inhibitory synapses usually resulted in shorter delay times with reduced variance. These distributions are characterizations of general aspects of internal network dynamics and provide estimates of pair-wise synaptic distances. The resulting analysis produced specific quantitative constraints and insights into the activation patterns of collective neuronal activity in self-organized cortical networks, which may prove useful for models emulating spontaneously active systems.

Description: This article discusses stability studies of transition-metal isomers. Several hypotheses to elucidate the linkage isomer preference of the thiocyanate (SCN¯) ion have been offered. For complexes with small coordination numbers (i.e. 1 and 2) and groups 11 (Cu-triad) and 12 (Zn-triad) metals, different levels of theory and a variety of basis sets have been employed to study linkage isomerism. Similar results are obtained for all density functionals tested, pure and hybrid. Overall, good agreement, vis-á-vis experimentally identified linkage isomers, is achieved for ab initio techniques, whereas semiempirical quantum mechanical methods show a bias toward S-ligated isomers. Despite the seeming ease for the a priori prediction of the most stable thiocyanate isomers using acid/base principles, this research highlights the sensitivity of quantitative calculations of transition-metal linkage isomerism to the choice of basis set and electron correlation, particularly with post-Hartree-Fock treatments.

Description: Front cover of the spring 2011 issue of the Hexagon, which features a statue James Joule wearing a coat and sitting in a chair. The inside cover includes a table of contents, a list of staff, an editorial, and a description of the cover.

Description: This article discusses stepwise reduction of dinitrogen bond order by a low-coordinate iron complex. Conversion of atmospheric N₂ into NH₃ is one of the most important chemical processes, because ammonia is the industrial and biological precursor to many nitrogen-containing compounds. Large-scale transformation of N₂ and H₂ into ammonia is performed in industry by the Haber-Bosch process, using "potassium-promoted" porous iron. A view of the N₂-reducing active site of iron-molybdenum nitrogenase, which contains unusual iron atoms with only three sulfur donors, is shown in Chart 1. The presence of iron in the active sites of this and other nitrogenases suggests that iron is again important for activating dinitrogen. Thus iron plays a major role in both natural and industrial N₂ reduction catalysis.