The research of molecular capsules offers high application potential and numerous benefits in various fields. With the aim of forming supramolecular capsules which can be reversibly assembled and ...dissociated by simple external stimuli, we studied interactions between calixarenes containing urea and carboxylate moieties. To this end two ureido‐derivatives of p‐tert‐butylcalix4arene comprising phenylureido‐moieties and diacetate‐calix4arenes were prepared. The binding of acetate by ureido‐derivatives of calixarene in acetonitrile was characterized, revealing high affinity of ureido‐calixarenes for carboxylates. This suggested high potential for uniting the complementary calix4arenes via H‐bonds between carboxylic groups and urea moieties. The assembly of calixarenes was examined in detail by means of UV, 1H NMR, ITC, DOSY, MS, and conductometry providing insight in the structure‐stability relationship. The tetraureido‐calixarene derivative formed the most stable heterodimers with diacetate‐calix4arenes featuring practically quantitative association upon mixing the two calixarene counterparts. The possibility of controlling the formation of the heterodimer by protonating the carboxylates, thereby hindering the interactions critical for capsule assembly, was investigated. Indeed, the reversibility of breaking and re‐forming the heterodimer by addition of an acid and base to the solution containing urea‐ and carboxylate‐derivative calix4arene was demonstrated using NMR spectroscopy.
A handshake between calixarenes based on the interactions between carboxylate and urea moieties enables formation of supramolecular capsules which can be reversibly disassembled by changing the solution acidity.
In this work the anion-binding properties of three aromatic sulfonylurea derivatives in acetonitrile and dimethyl sulfoxide were explored by means of NMR titrations. It was found that the studied ...receptors effectively bind anions of low basicity (Cl
−
, Br
−
, I
−
, NO
3
−
and HSO
4
−
). The stoichiometry of the complexes with receptors containing one binding site was 1 : 1 exclusively, whereas in the case of the receptor containing two sulfonylurea groups 1 : 2 (receptor : anion) complexes were also detected in some cases. The presence of strongly basic anions (acetate and dihydrogen phosphate) led to the deprotonation of the sulfonylurea moiety. This completely hindered its anion-binding properties in DMSO and only proton transfer occurred upon the addition of basic anions to the studied receptors. In MeCN, a complex system of equilibria including both ligand deprotonation and anion binding was established. Since ionisation of receptors was proven to be a decisive factor defining the behaviour of the sulfonylurea receptors, their p
K
a
values were determined using several deprotonation agents in both solvents. The results were interpreted in the context of receptor structures and solvent properties and applied for the identification of the interactions with basic anions.
Sulfonylurea derivatives were studied as receptors revealing that they effectively bind anions of low basicity, whereas basic anions cause receptor deprotonation.
Experimental data on K
+ meson and proton sideward flow measured with the FOPI detector at SIS/GSI in the reactions Ru+Ru at 1.69
A GeV and Ni+Ni at 1.93
A GeV are presented. The K
+ sideward flow is ...found to be anti-correlated (correlated) with the one of protons at low (high) transverse momenta. When compared to the predictions of a transport model, the data favour the existence of an in-medium repulsive K
+-nucleon potential.
An overview is given over recent measurement of flow and particle production in the energy range from 0.1 to 2 AGeV. Excitation functions for the directed sideward and the azimuthally symmetric ...transverse flow are presented and show the importance of flow phenomena in this incident energy regime. Rapidity density distributions are indicative of a system size dependence of the stopping process. The role of strange particles as a probe for the hot and dense phase of hadronic matter is discussed with respect to the production and propagation. The spectra of Kaons indicate an equilibration with the surrounding baryons during the expansion while their directed flow pattern is different from that of the nucleons.
Using the FOPI detector at GSI Darmstadt, excitation functions of collective flow components were measured for the Au+Au system, in the reaction plane and out of this plane, at seven incident ...energies ranging from 100
A MeV to 800
A MeV. The threshold energies, corresponding to the onset of sideward-flow (balance energy) and squeeze-out effect (transition energy), are extracted from extrapolations of these excitation functions toward lower beam energies for charged products with
Z ⩾ 2. The transition energy is found to be larger than the balance energy. The impact parameter dependence of both balance and transition energies, when extrapolated to central collisions, suggests comparable although slightly higher values than the threshold energy for the radial flow. The relevant parameter seems to be the energy deposited into the system in order to overcome the attractive nuclear forces.
The mass distributions of baryon resonances populated in near-central collisions of Au on Au and Ni on Ni are deduced by defolding the $p_t$ spectra of charged pions by a method which does not depend ...on a specific resonance shape. In addition the mass distributions of resonances are obtained from the invariant masses of $(p, \pi^{\pm})$ pairs. With both methods the deduced mass distributions are shifted by an average value of -60 MeV/c$^2$ relative to the mass distribution of the free $\Delta(1232)$ resonance, the distributions descent almost exponentially towards mass values of 2000 MeV/c^2. The observed differences between $(p, \pi^-)$ and $(p, \pi^+)$ pairs indicate a contribution of isospin $I = 1/2$ resonances. The attempt to consistently describe the deduced mass distributions and the reconstructed kinetic energy spectra of the resonances leads to new insights about the freeze out conditions, i.e. to rather low temperatures and large expansion velocities.
Azimuthal distributions of charged particles and intermediate mass fragments emitted in Au+Au collisions at 600
A MeV have been measured using the FOPI facility at GSI-Darmstadt. Data show a strong ...increase of the in-plane azimuthal anisotropy ratio with the charge of the detected fragment. Intermediate mass fragments are found to exhibit a strong momentum-space alignment with respect of the reaction plane. The experimental results are presented as a function of the polar centre-of-mass angle and over a broad range of impact parameters. They are compared to the predictions of the Isospin Quantum Molecular Dynamics model using three different parametrisations of the equation of state. We show that such highly accurate data provide stringent test for microscopic transport models and can potentially constrain separately the stiffness of the nuclear equation of state and the momentum dependence of the nuclear interaction.
PDF
