Comparison of α CH and CF activation in alkyl transition metal complexes: a DFT and CASSCF study

DFT (B3PW91) and CASSCF calculations have been carried out to study the relative α migratory abilities of H and F in alkyl transition metal complexes. It is shown that the activation energy is considerably lower to migrate H than F, whereas the energies of reaction are similar for the two reactions. A study of the electron configurations and the orbitals describing these configurations shows that the high activation energy for F is due to a 4-electron repulsion between an F lone pair and the occupied Ru=C π orbital.     

Cluster modelling of O(1s) core excitons at the (100) surface of alkaline-earth oxides

Calculations have been performed on the O(1s) X-ray absorption spectra of the O^2- ion in the series of alkaline-earth oxides MgO, CaO, SrO. The static exchange approach has been used to calculate the near-edge X-ray absorption fine structure (NEXAFS) spectra for a range of cluster models (up to 50 atoms), representing both the (100) surface and the bulk of the different lattices. The clusters have been embedded in a proper representation of both the surrounding crystal ions and the Madelung potential generated by the infinite crystal. In the surface clusters discrete levels lying around 1.5 eV below the calculated ionization potential were found and characterized. This surface state has been studied further by performing complete active space self-consistent field (CASSCF) calculations in which the polarization of the crystal ions has been taken into account by means of a core polarization potential (CPP) method as well as the Mott-Littleton approach. After inclusion of this potential, the calculated ionization potentials show good agreement with XPS measurements.     

AN INDICATOR FOR ECOSYSTEM EXTERNALITIES IN FISHING

Ecosystem externalities arise when one use of an ecosystem affects its other uses through the production functions of the ecosystem. We use simulations with a size‐spectrum ecosystem model to investigate the ecosystem externality created by fishing of multiple species. The model is based upon general ecological principles and is calibrated to the North Sea. Two fleets are considered: a “forage fish” fleet targeting species that mature at small sizes and a “large fish” fleet targeting large piscivorous species. Based on the marginal analysis of the present value of the rent, we develop a benefit indicator that explicitly divides the consequences of fishing into internal and external benefits. This analysis demonstrates that the forage fish fleet has a notable economic impact on the large fish fleet, but the reverse is not true. The impact can be either negative or positive, which entails that for optimal economic exploitation, the forage fishery has to be adjusted according to the large fish fishery. With the present large fish fishery in the North Sea, the two fisheries are well adjusted; however, the present combined exploitation level is too high to achieve optimal economic rents.     

A PRESSURE CORRECTION METHOD FOR UNSTRUCTURED MESHES WITH ARBITRARY CONTROL VOLUMES

A pressure correction procedure for general unstructured meshes is presented. It is a cell-centred, collocated finite volume method and the pressure–velocity coupling is treated using SIMPLEC. The cells can have an arbitrary number of grid points (cell vertices). In the present study the number of faces on the cells varies between three and six. The discretized equations are solved using either a symmetric Gauss–Seidel solver or a conjugate gradient solver with a preconditioner. The method is applied to three two-dimensional test cases in which the flow is incompressible and laminar. The extension to three dimensions as well as to turbulent flow using transport models is straightforward. It can also be extended to handle compressible flow.