Descripción: An interesting case of 'halogen-bonding-promoted' crystal structure architecture is presented. The two title compounds, C8H8Br2O2 and C8H8I2O2, have almost indistinguishable mol-ecular structures but very different spatial organization, and this is mainly due to differences in the halogen-bonding inter-actions in which the different species present, i.e. Br and I, take part. The dibromo structure exhibits a π-bonded columnar array involving all four independent mol-ecules in the asymmetric unit, with inter-columnar inter-actions governed by C - Br⋯Br - C links and with no C - Br⋯O/N inter-actions present. In the diiodo structure, instead, the C - I⋯O synthon prevails, defining linear chains, in turn inter-linked by C - I⋯I - C inter-actions. © 2008 International Union of Crystallography.

Descripción: knowledge of the initial shape of strain markers allows calculation of the strain ellipsoid (axial relationships and orientation) when they show distorted geometries after deformation. This information is usually combined with the analysis of other micro- and mesostructures that constitute the dynamic framework in which the whole assemblage of structures developed. If only one deformational episode was responsible for all of them, the result is a highly coherent picture. In this paper, the quantification of strain and orientation of strain axes, is attempted on the basis of distorted hexagonal columns cropping out close to Puesto El Malabar (Pigüé, Sierras Australes de Buenos Aires). This columnar strucutre occurs in rhyolitic flows and tuffs of the Precambrian (?) basement in the area. It is apparent from this study that deformation in the central part of the north-western arc of Sierras Australes is markedly heterogeneous at all scales, with axial relationships giving values that record a variationin between almost no deformation and belts of high strain in which cleavage erases all evidence of columnar jointing. Micro- and mesostructural analysis shows dextral simple shear in plan view, oriented Az.145°-150° , and E-W stripes of sinistral shear that appear to be later and fewer than the earlier, one. Evidence of shear to the NE in vertical section has been found that is also heterogeneously developed. Secondary microstructures also show geometric relationships that are coherent with the general picture in the area. The regional significance of these data is analyzed in the regional picture, and it is concluded that it fits the previous model of an arcuate belt resulting from local development of conjugate megashears, with E-W (sinistral) and SW-NE (dextral) bands shaping the arc. © 1998 Asociación Geológica Argentina.

Descripción: Rapidly rotating turbulent flow is characterized by the emergence of columnar structures that are representative of quasi-two-dimensional behavior of the flow. It is known that when energy is injected into the fluid at an intermediate scale L f, it cascades towards smaller as well as larger scales. In this paper we analyze the flow in the inverse cascade range at a small but fixed Rossby number, Ro f≈0.05. Several numerical simulations with helical and nonhelical forcing functions are considered in periodic boxes with unit aspect ratio. In order to resolve the inverse cascade range with reasonably large Reynolds number, the analysis is based on large eddy simulations which include the effect of helicity on eddy viscosity and eddy noise. Thus, we model the small scales and resolve explicitly the large scales. We show that the large-scale energy spectrum has at least two solutions: one that is consistent with Kolmogorov-Kraichnan-Batchelor-Leith phenomenology for the inverse cascade of energy in two-dimensional (2D) turbulence with a ∼k⊥-5/3 scaling, and the other that corresponds to a steeper ∼k⊥-3 spectrum in which the three-dimensional (3D) modes release a substantial fraction of their energy per unit time to the 2D modes. The spectrum that emerges depends on the anisotropy of the forcing function, the former solution prevailing for forcings in which more energy is injected into the 2D modes while the latter prevails for isotropic forcing. In the case of anisotropic forcing, whence the energy goes from the 2D to the 3D modes at low wave numbers, large-scale shear is created, resulting in a time scale τ sh, associated with shear, thereby producing a ∼k -1 spectrum for the total energy with the horizontal energy of the 2D modes still following a ∼k⊥-5/3 scaling. © 2012 American Physical Society.