Substitution reaction | EPFL Graph Search

A substitution reaction (also known as single displacement reaction or single substitution reaction) is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions are of prime importance in organic chemistry. Substitution reactions in organic chemistry are classified either as electrophilic or nucleophilic depending upon the reagent involved, whether a reactive intermediate involved in the reaction is a carbocation, a carbanion or a free radical, and whether the substrate is aliphatic or aromatic. Detailed understanding of a reaction type helps to predict the product outcome in a reaction. It also is helpful for optimizing a reaction with regard to variables such as temperature and choice of solvent. A good example of a substitution reaction is halogenation. When chlorine gas (Cl2) is irradiated, some of the molecules are split into two chlorine radicals (Cl•), whose free electrons are strongly nucleoph

Reactivity of manganese, technetium and rhenium aquacarbonyl-complexes as potential precursors for radiopharmaceuticals

Pascal Grundler

Technetium is an artificial element and its isotope 99mTc is now used for several years as radio-imaging agent. Much research effort is directed towards enhanced targeting of the imaging agents in order to improve accuracy of the diagnosis and reduce the radiation doses delivered to the patient. Technetium(I) complexes are promising in this field and especially those with the fac-[(CO)3Tc]+ entity. Rhenium which is closely related to technetium, is used as model compound, but has also its own radiopharmaceutical potential with the isotopes 186Re and 188Re. The preparative chemistry of 99mTc has to be performed in water. Therefore, it is of importance to know its actual form in solution and understand its reactivity. Thus the goal of the present work was to investigate the reactivity of fac- [(CO)3Mn(H2O)3]+, fac-[(CO)3Tc(H2O)3]+, fac-[(CO)3Re(H2O)3]+ et fac- [(CO)2(NO)Re(H2O)3]2+ by multinuclear magnetic resonance. The water exchange rates on these complexes have been determined as a function of acidity and for tow of them (Mn and Tc) as a function of temperature. The water exchange rate decreases when going from manganese to technetium and then to rhenium. In the latter case it was found that the replacement of a CO ligand by an NO+ ligand decreases again the lability of the water molecules. For the two rhenium complexes an increase of the water exchange rate with decreasing acidity has been observed. Water molecules on fac-[(CO)3Mn(H2O)3]+, fac-[(CO)3Tc(H2O)3]+, fac- [(CO)3Re(H2O)3]+ complexes are easily substituted. This reaction has been investigated with selected ligands of various types. The complexes show a stronger affinity for soft ligands such as dimethylsulfide than for harder ligands like acetonitrile. Variable pressure experiments for the ligand substitution on fac- [(CO)3Re(H2O)3]+ provided activation volumes. The substitution by the soft ligand was found to be associatively activated whereas the substitution by hard ligands is dissociatively activated. Although activation volumes could not be determined for the substitution reaction on fac-[(CO)3Mn(H2O)3]+ and fac-[(CO)3Tc(H2O)3]+ their similar reactivity suggests that they exhibit the same mechanistic behaviour as fac- [(CO)3Re(H2O)3]+.

EPFL2005

Thermopower as a Unique Tool to Probe Quantum Phase Transitions

Stevan Arsenijevic

In my thesis, transport measurements such as resistivity and, more importantly, thermopower S, were used to explore the phase diagram of bad metals. Bad metals are electronically correlated systems whose ground state lies close to a quantum phase transition. By tuning the control parameters, such as temperature (T ), magnetic field (B), hydrostatic pressure (p) or chemical substitution (x), we can induce phase transitions between the various electronic, magnetic and structural phases. Here, the thermopower is presented as a unique tool for probing quantum phase transition because it is a measure of the entropy of conducting electrons. The main part of the thesis is dedicated to the study of Fe-based superconductors (FeSC) discovered in 2008. Their parent compound has an antiferromagnetic (AF) ground state, where the itinerant electrons form a spin-density wave (SDW), a periodic modulation of spin density. This coincides or is preceded by a structural, tetragonal-to-orthorhombic transition. The nesting between the electron and hole Fermi surface is believed to be the driving mechanism for the SDW state. By changing the structural or chemical properties the AF ground state of FeSC is suppressed, giving way to superconductivity (SC). The remaining antiferromagnetic fluctuations above the transition can provide a glue for SC pairing. Here, the analysis of the thermopower S/T of BaFe1−xCoxAs2 (BFCA) in the x-T phase diagram shows the signatures of the spin fluctuation which have a dome-like dependence and follow the trend of superconducting Tc . The logarithmic increase of S/T upon decreasing T is ascribed to the proximity of the spin-density-wave quantum critical point. It can be understood as an increase of entropy due to the incommensurate AF spin fluctuations. We can ascribe the high values of thermopower in BFCA at intermediate- and room-temperatures to the influence of low-T quantum criticality. To probe the response of the electronic system in FeSC to structural changes, we performed measurements under pressure of the parent compound BaFe2As2 (BFA), the SC electron-doped BFCA, and hole-doped Ba1−xKxFe2As2 (BKFA). In the parent compound pressure suppresses the structural/SDW transition, similar to the effect of doping. For doped systems, in order to describe the behavior of thermopower in the high-T range (above 100K) we used a semi-metallic two-band model which was fitted to the data in order to extract the pressure dependence of the band parameters. In both doping cases the effect of pressure was similar, an increase of the band overlap and of the effective number of charge carriers. With this model we can explain the high-T , x and p dependence of thermopower in both electron- and hole-doped BFA. In a structurally simpler Fe-chalcogenide Fe1+yTe1−xSex compound, the excess of Fe has a Kondo-like influence on the charge carriers which dramatically changes the physics of the normal state. To probe the normal state, pressure, doping, magnetic Fe-excess concentration (y) and temperature were used as control parameters. At low-T a characteristic upturn of resistivity (ρmag ) is observed, followed by an increase of thermopower (Smag ), which we identify as the magnetic contribution caused by the spin-flip scattering events. Increasing the y resulted in an increase of ρmag , and a decrease of Smag , which is in agreement with the behavior of canonical Kondo-systems. Pressure suppresses the magnetic contribution to transport, thus increasing the itinerancy of the system. MnSi is another system in which the sensitivity of thermopower to entropy brings new information related to the complex magnetic structure. Pressure was used to drive the system from a helically ordered, canonical Fermi-liquid (FL) phase with T 2-resistivity to the intrinsically disordered, non-Fermi-liquid (NFL) phase above pc with T3/2-dependence. Our contour plot of S/T demonstrated how powerful the thermopower technique is, by reproducing the whole previously established T -p phase diagram. At the phase transition from the magnetically-ordered FL phase to the disordered NFL, the thermopower is dramatically enhanced. We bring useful information about the mysterious partial order (PO) phase inside the NFL phase, previously detected only by neutron scattering. The fluctuating helices scenario can describe the observed increase of entropy/thermopower in the PO phase. At ambient pressure, close to the helical transition of MnSi, a moderate magnetic field can stabilize the skyrmion lattice - the lattice of topological magnetic whirls, vortices. We observe a signature of the skyrmion lattice as a minute drop in thermopower. It is located exactly in the same region of the T − B phase diagram where an increase in magnetoresistance and Hall effect was reported previously. This feature originates from the additional scattering of conducting electrons on magnetic vortices, while the change in S is dominated by the decrease of entropy as the stable skyrmion lattice is formed. Overall, resistivity was used to confirm the established phase diagram, while thermopower, as an interesting and not sufficiently understood technique, was used to probe the sensitive changes of the charge carriers at the Fermi surface. We explored various phases showing how useful thermopower is to probe the entropy of electronic system on the verge of quantum phase transition.

EPFL2012

Functional Engineering of Perovskite Nanosheets: Impact of Lead Substitution on Exfoliation in the Solid Solution RbCa2-xPbxNb3O10

Teresa Katharina Dennenwaldt, Filip Mateusz Podjaski

Tuning the chemical composition and structure for targeted functionality in two-dimensional (2D) nanosheets has become a major objective in the rapidly growing area of 2D materials. In the context of photocatalysis, both miniaturization and extending the light absorption of UV active photocatalysts are major assets. Here, we investigate the solid solution between two photocatalytic systems known from literature to evolve H-2 from water/methanol under UV-RbCa2Nb3O10 (E-g = 3.7 eV) -and visible light irradiation - RbPb2Nb3O10 (E-g = 3.0 eV) - by synthesizing hypothetical RbCa2-xPbxNb3O10. While the calcium niobate can easily be exfoliated into individual nanosheets via cation-proton exchange and subsequent treatment with tetra-n-butyl-ammonium hydroxide (TBAOH), the lead niobate barely yields nano-sheets. Spectroscopic and microscopic analysis suggest that this is caused by volatilization of Pb during synthesis, leading to a local 3D linkage of RbPb2Nb3O10 perovskite units with Pb deficient units. On the one hand, this linkage progressively prevents exfoliation along with an increasing Pb content. On the other hand, introducing Pb into the perovskite blocks successively leads to bandgap narrowing, thus gradually enhancing the light harvesting capability of the solid solution. Finding a compromise between this narrowing of the bandgap and the possibility of exfoliation, visible light sensitized nanosheets can be engineered in good yield for an initial molar ratio of Ca:Pb >= 1:1.

Wiley-V C H Verlag Gmbh2017