42 research outputs found
Dark matter halos in the multicomponent model. II. Density profiles of galactic halos
The multicomponent dark matter model with self-scattering and
inter-conversions of species into one another is an alternative dark matter
paradigm that is capable of resolving the long-standing problems of
CDM cosmology at small scales. In this paper, we have studied in
detail the properties of dark matter halos with obtained in -body cosmological simulations with the simplest
two-component (2cDM) model. A large set of velocity-dependent cross-section
prescriptions for elastic scattering and mass conversions, and , has been explored and the results
were compared with observational data. The results demonstrate that
self-interactions with the cross-section per particle mass evaluated at
km s being in the range of
cmg robustly suppress central cusps, thus resolving the core-cusp
problem. The core radii are controlled by the values of and the DM
cross-section's velocity-dependent power-law indices , but are
largely insensitive to the species' mass degeneracy. These values are in full
agreement with those resolving the substructure and too-big-to-fail problems.
We have also studied the evolution of halos in the 2cDM model with cosmic time.Comment: 17 pages, 13 figure
Cationic acyclic cucurbit[n]uril-type containers: synthesis and molecular recognition toward nucleotides
<p>We report the synthesis of <b>M2NH3</b> which is a tetracationic analogue of our prototypical acyclic CB[n]-type molecular container <b>M2</b>. Both <b>M1NH3</b> and <b>M2NH3</b> possess excellent solubility in D<sub>2</sub>O and do not undergo intermolecular self-association processes that would impinge on their molecular recognition properties. Compounds <b>M1NH3</b> and <b>M2NH3</b> do, however, undergo an intramolecular self-complexation process driven by ionâdipole interactions between the ureidyl C=O portals and the OCH<sub>2</sub>CH<sub>2</sub>NH<sub>3</sub> arms along with inclusion of one aromatic wall in its own hydrophobic cavity. The <i>K</i><sub>a</sub> values for <b>M1NH3</b> and <b>M2NH3</b> towards seven nucleotides were determined by <sup>1</sup>H NMR titration and found to be quite modest (<i>K</i><sub>a</sub> in the 10<sup>2</sup>â10<sup>3</sup>Â M<sup>â1</sup> range) although <b>M2NH3</b> is slightly more potent. The more highly charged guests (e.g. ATP) form stronger complexes with <b>M1NH3</b> and <b>M2NH3</b> than the less highly charged guest (e.g. ADP, AMP). The work highlights the dominant influence of the ureidyl C=O portals on the molecular recognition behaviour of acyclic CB[n]-type receptors and suggests routes (e.g. more highly charged arms) to enhance their recognition behaviour towards anions.</p
Synthesis and Recognition Properties of Cucurbit[8]uril Derivatives
A building
block approach to the synthesis of Me<sub>4</sub>CBÂ[8]
and Cy<sub>2</sub>CBÂ[8] by condensation of glycoluril hexamer <b>1</b> with bisÂ(cyclic ethers) <b>2</b> is reported. X-ray
crystallography demonstrates that the equatorial substitution results
in an ellipsoidal cavity. Me<sub>4</sub>CBÂ[8] and Cy<sub>2</sub>CBÂ[8]
display enhanced aqueous solubility and retain the ability to bind
to guests (<b>3</b>â<b>9</b>) typical of unsubstituted
CB[8]. The higher inherent solubility of Me<sub>4</sub>CBÂ[8] allowed
it to be used as a solubilizing excipient for insoluble drugs
Catalytic Asymmetric Syntheses of Quinolizidines by Dirhodium-Catalyzed Dearomatization of Isoquinolinium/Pyridinium MethylidesâThe Role of Catalyst and Carbene Source
Convenient
access to highly enantioenriched substituted quinolizidines
has been achieved by chiral dirhodiumÂ(II) carboxylate-catalyzed dearomatizing
formal [3 + 3]-cycloaddition of isoquinolinium/pyridinium methylides
and enol diazoacetates. Coordination of Lewis basic methylides to
dirhodiumÂ(II) prompts the rearrangement of the enol-carbene that is
bound to dirhodium to produce a donorâacceptor cyclopropene.
The donorâacceptor cyclopropene is in equilibrium with the
dirhodium-bound enol-carbene and undergoes both enantioselective [3
+ 3]-cycloaddition from the dirhodium-bound enol-carbene and diastereoselective
[3 + 2]-cycloaddition by uncatalyzed reaction of the cyclopropene
with isoquinolinium or pyridinium methylides. Increasing the mol %
of catalyst loading suppresses the [3 + 2]-cycloaddition pathway
Reductive C(sp<sup>2</sup>)âN Elimination from Isolated Pd(IV) Amido Aryl Complexes Prepared Using H<sub>2</sub>O<sub>2</sub> as Oxidant
Di-2-pyridyl ketone (dpk)-supported
amidoarylpalladaÂ(II)Âcycles
derived from various 2-(<i>N</i>-R-amino)Âbiphenyls (R =
H, Me, CF<sub>3</sub>CO, MeSO<sub>2</sub>, CF<sub>3</sub>SO<sub>2</sub>) react with hydrogen peroxide in MeOH, THF, MeCN or AcOH to form
the corresponding CâN coupled products, <i>N</i>-R-substituted
carbazoles, in 82â98% yield. For R = MeSO<sub>2</sub> and CF<sub>3</sub>SO<sub>2</sub>, the corresponding reaction intermediates,
amidoaryl PdÂ(IV) complexes were isolated and characterized by single
crystal X-ray diffraction and/or NMR spectroscopy. For the first time,
the CÂ(sp<sup>2</sup>)âN reductive elimination from isolated
amidoaryl PdÂ(IV) complexes has been studied in detail
Aerobic CâH Acetoxylation of 8âMethylquinoline in Pd<sup>II</sup>âPyridinecarboxylic Acid Systems: Some StructureâReactivity Relationships
Catalytic
oxidative CâH acetoxylation of 8-methylquinoline
as a model substrate with O<sub>2</sub> as oxidant was performed using
palladiumÂ(II) carboxylate catalysts derived from four different pyridinecarboxylic
acids able to form palladiumÂ(II) chelates of different size. A comparison
of the rates of the substrate CâH activation and the O<sub>2</sub> activation steps shows that the CâH activation step
is rate-limiting, whereas the O<sub>2</sub> activation occurs at a
much faster rate already at 20 °C. The chelate ring size and
the chelate ring strain of the catalytically active species are proposed
to be the key factors affecting the rate of the CâH activation
Catalytic Asymmetric Syntheses of Quinolizidines by Dirhodium-Catalyzed Dearomatization of Isoquinolinium/Pyridinium MethylidesâThe Role of Catalyst and Carbene Source
Convenient
access to highly enantioenriched substituted quinolizidines
has been achieved by chiral dirhodiumÂ(II) carboxylate-catalyzed dearomatizing
formal [3 + 3]-cycloaddition of isoquinolinium/pyridinium methylides
and enol diazoacetates. Coordination of Lewis basic methylides to
dirhodiumÂ(II) prompts the rearrangement of the enol-carbene that is
bound to dirhodium to produce a donorâacceptor cyclopropene.
The donorâacceptor cyclopropene is in equilibrium with the
dirhodium-bound enol-carbene and undergoes both enantioselective [3
+ 3]-cycloaddition from the dirhodium-bound enol-carbene and diastereoselective
[3 + 2]-cycloaddition by uncatalyzed reaction of the cyclopropene
with isoquinolinium or pyridinium methylides. Increasing the mol %
of catalyst loading suppresses the [3 + 2]-cycloaddition pathway
Aerobic CâH Acetoxylation of 8âMethylquinoline in Pd<sup>II</sup>âPyridinecarboxylic Acid Systems: Some StructureâReactivity Relationships
Catalytic
oxidative CâH acetoxylation of 8-methylquinoline
as a model substrate with O<sub>2</sub> as oxidant was performed using
palladiumÂ(II) carboxylate catalysts derived from four different pyridinecarboxylic
acids able to form palladiumÂ(II) chelates of different size. A comparison
of the rates of the substrate CâH activation and the O<sub>2</sub> activation steps shows that the CâH activation step
is rate-limiting, whereas the O<sub>2</sub> activation occurs at a
much faster rate already at 20 °C. The chelate ring size and
the chelate ring strain of the catalytically active species are proposed
to be the key factors affecting the rate of the CâH activation
NâN Bond Cleavage of Mid-Valent Ta(IV) Hydrazido and Hydrazidium Complexes Relevant to the Schrock Cycle for Dinitrogen Fixation
Chemical
reduction of the TaÂ(V) hydrazido chloride <b>1</b> generates
the open-shell, mononuclear TaÂ(IV) hydrazido complex <b>2</b>, which upon N-methylation yields the corresponding structurally
characterized TaÂ(IV) hydrazidium <b>6</b>. Chemical reduction
of <b>6</b> results in NâN bond cleavage to generate
a cis/trans mixture of the [TaÂ(V),TaÂ(V)] bisÂ(Îź-nitrido) product <b>7</b> in tetrahydrofuran and the mononuclear TaÂ(V) parent imide <b>8</b> in toluene. These results serve to establish an important
foundation for the pursuit of a group-5 metal variant of the Schrock
cycle for dinitrogen fixation
Oxidation of a Monomethylpalladium(II) Complex with O<sub>2</sub> in Water: Tuning Reaction Selectivity to Form Ethane, Methanol, or Methylhydroperoxide
Photochemical
aerobic oxidation of <i>n</i>-Pr<sub>4</sub>NÂ[(dpms)ÂPd<sup>II</sup>MeÂ(OH)] (<b>5</b>) and (dpms)ÂPd<sup>II</sup>MeÂ(OH<sub>2</sub>) (<b>8</b>) (dpms = diÂ(2-pyridyl)Âmethanesulfonate)
in water in the pH range of 6â14 at 21 °C was studied
and found to produce, in combined high yield, a mixture of MeOH, C<sub>2</sub>H<sub>6</sub>, and MeOOH along with water-soluble <i>n</i>-Pr<sub>4</sub>NÂ[(dpms)ÂPd<sup>II</sup>(OH)<sub>2</sub>]
(<b>9</b>). By changing the reaction pH and concentration of
the substrate, the oxidation reaction can be directed toward selective
production of ethane (up to 94% selectivity) or methanol (up to 54%
selective); the yield of MeOOH can be varied in the range of 0â40%.
The source of ethane was found to be an unstable dimethyl Pd<sup>IV</sup> complex (dpms)ÂPd<sup>IV</sup>Me<sub>2</sub>(OH) (<b>7</b>),
which could be generated from <b>5</b> and MeI. For shedding
light on the role of MeOOH in the aerobic reaction, oxidation of <b>5</b> and <b>8</b> with a range of hydroperoxo compounds,
including MeOOH, <i>t</i>-BuOOH, and H<sub>2</sub>O<sub>2</sub>, was carried out. The proposed mechanism of aerobic oxidation
of <b>5</b> or <b>8</b> involves predominant direct reaction
of excited methylpalladiumÂ(II) species with O<sub>2</sub> to produce
a highly electrophilic monomethyl Pd<sup>IV</sup> transient that is
involved in subsequent transfer of its methyl group to <b>5</b> or <b>8</b>, H<sub>2</sub>O, and other nucleophilic components
of the reaction mixture