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Casalecchio di Reno, Italy

Mignone A.,University of Turin | Flock M.,SAP | Flock M.,Max Planck Institute for Astronomy | Stute M.,University of Tubingen | And 2 more authors.
Astronomy and Astrophysics | Year: 2012

Context. Explicit numerical computations of hypersonic or super-fast differentially rotating disks are subject to the time-step constraint imposed by the Courant condition, according to which waves cannot travel more than a fraction of a cell during a single time-step update. When the bulk orbital velocity largely exceeds any other wave speed (e.g., sound or Alfvén), as computed in the rest frame, the time step is considerably reduced and an unusually large number of steps may be necessary to complete the computation. Aims. We present a robust numerical scheme to overcome the Courant limitation by improving and extending the algorithm previously known as FARGO (fast advection in rotating gaseous objects) to the equations of magnetohydrodynamics (MHD) using a more general formalism. The proposed scheme conserves total angular momentum and energy to machine precision and works in cartesian, cylindrical, or spherical coordinates. The algorithm has been implemented in the next release of the PLUTO code for astrophysical gasdynamics and is suitable for local or global simulations of accretion or proto-planetary disk models. Methods. By decomposing the total velocity into an average azimuthal contribution and a residual term, the algorithm approaches the solution of the MHD equations through two separate steps corresponding to a linear transport operator in the direction of orbital motion and a standard nonlinear solver applied to the MHD equations written in terms of the residual velocity. Since the former step is not subject to any stability restriction, the Courant condition is computed only in terms of the residual velocity, leading to substantially larger time steps. The magnetic field is advanced in time using the constrained transport method in order to fulfill the divergence-free condition. Furthermore, conservation of total energy and angular momentum is enforced at the discrete level by properly expressing the source terms in terms of upwind Godunov fluxes available during the standard solver. Results. Our results show that applications of the proposed orbital-advection scheme to problems of astrophysical relevance provides, at reduced numerical cost, equally accurate and less dissipative results than standard time-marching schemes. © 2012 ESO. Source

Cavallo G.,Polytechnic of Milan | Metrangolo P.,Polytechnic of Milan | Metrangolo P.,VTT Technical Research Center of Finland | Pilati T.,Polytechnic of Milan | And 4 more authors.
RSC Advances | Year: 2016

Novel D-π-A push-pull chromophores were synthesized in good yields by CuAAc coupling of 4-X-2,3,5,6-tetrafluorophenyl-1-azides (X = H, Br, I) with 4-ethynyl-dimethylaniline. Thanks to the self-complementary binding sites at the molecular ends, the iodo derivative self-organizes in the solid state forming head-to-tail halogen-bonded one-dimensional unlimited chains. The second-order NLO properties of the iodo compound have been investigated by the solution-phase electric field induced second-harmonic generation method (EFISH). © The Royal Society of Chemistry 2016. Source

Bottoni A.,University of Bologna | Lombardo M.,University of Bologna | Miscione G.P.,University of Bologna | Montroni E.,University of Bologna | And 3 more authors.
ChemCatChem | Year: 2013

We have recently proposed the empirical concept of electrosteric activation to explain the improved catalytic performances observed for a series of ion-tagged catalysts compared to the parent tag-free structures. Here, the results of a combined experimental and computational investigation on the asymmetric aldol reaction between cyclohexanone and benzaldehyde, catalyzed by a family of tag-free and ionic-tagged prolines, are presented. Whereas diastereo- and enantioselectivities remain very high in all cases examined, the ion-tagged catalyst cis-4-(2-(3-methyl-imidazol-3-ium-1-yl)acetoxy)-proline bistriflimide, cis-7, displays a remarkably high activity compared to its tagged trans analogue and to the tag-free catalysts cis and trans-4-(2-phenylacetoxy)-proline 8. A computational investigation of ion-tagged and tag-free model systems shows that the transition state involving cis-7 is stabilized by a complex interplay of hydrogen bonds (in particular, those involving the counter ion oxygen atoms and the hydrogen atoms of the ionic tag), π-stacking interactions involving the aldehyde phenyl ring, and similar π interactions between the proline carboxyl group and the imidazole ring. The overall effect of these interactions accounts for the observed enhanced activity. Stack attack! The asymmetric aldol reaction between cyclohexanone and benzaldehyde in the presence of tagged 4-hydroxy-prolines is modeled to elucidate the empirical concept of electrosteric activation, introduced to account for the much higher activity of cis-ion-tagged prolines. A complex interplay of hydrogen bonds and π-stacking interactions is found to give rise to the observed enhanced activity. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Quintavalla A.,University of Bologna | Lanza F.,University of Bologna | Montroni E.,University of Bologna | Montroni E.,Consorzio Interuniversitario | And 2 more authors.
Journal of Organic Chemistry | Year: 2013

An efficient and highly enantioselective Michael addition of nitroalkanes to 3-ylidene oxindoles is described, mediated by thiourea-based bifunctional organocatalysts. The stereochemistry at Cα and C β centers is perfectly controlled, and the intermediate C-3 enolate is trapped with a second Michael acceptor. The developed one-pot three-component consecutive reactions generate up to four contiguous stereocenters, including the C-3 all-carbon quaternary center, in a perfectly defined configuration. The conversion of the β-nitro oxindole into the corresponding β-amino derivative discloses synthetically useful transformations, exploitable to generate pharmaceutically attractive molecular targets. © 2013 American Chemical Society. Source

Beghetto V.,University of Venice | Scrivanti A.,University of Venice | Bertoldini M.,University of Venice | Aversa M.,Consorzio Interuniversitario | And 2 more authors.
Synthesis (Germany) | Year: 2015

The fragrances (S)-(+)- and (R)-(-)-canthoxal [(S)-(+)- and (R)-(-)-3-(4-methoxyphenyl)-2-methylpropanal] and (+)- and (-)-Silvial® [(+)- and (-)-3-(4-isobutylphenyl)-2-methylpropanal] have been synthesized in high enantiopurity via a simple four-step strategy starting from the commercially available 4-substituted benzaldehydes. The key synthetic step is the catalytic asymmetric hydrogenation of the appropriate 3-aryl-2-methylacrylic acid which has been carried out employing an in situ prepared ruthenium/axially chiral phosphine catalyst (up to 98% ee). The olfactory activity of the single enantiomers has been evaluated. Source

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