Selective activation of organic molecules by elemental fluorine

Reactions on tertiary hydrogen atoms, attached to unactivated saturated carbons, are very rare and usually inefficient. The electron density however, of a carbon-tertiary hydrogen bond is relatively high and therefore there is a chance it will react with a strong electrophile. One of the strongest electrophiles existing is, of course, the electrophilic fluorine. Two main sources for such an unusual species exist. One source is the various fluoroxy compounds like CF₃OF or CF₃COOF ant the other is the elemental fluorine itself.Indeed, when fluorine is allowed to react, at low temperatures, with various alkylcyclohexanol esters, a highly regio- and an absolutely stereospecific electrophilic substitution on the tertiary unactivated hydrogen takes place as for example:

The radical pathway possibility of these reactions is excluded and it is believed that they are of ionic nature. By dehydrofluorination, a double bond is introduced in sites that no other reagent is known to do, thus activating the molecule towards further chemical transformations. The influence of the electron-withdrawing group on the reaction center will also be discussed.The described reaction is not restricted only to cyclic compounds as aliphatic chains also react as expected, i.e.

The scope of these unusual reactions in both alicyclic and aliphatic fields will be evaluated.     

Finite-range effects in pionic atoms

Pionic atom data are refitted in terms of conventional π^? nuclear optical potentials, modified to allow for finite range in the underlying p-wave πN interaction. We find that level shifts and widths in light and medium-weight nuclei cannot distinguish between the various ranges tested. In heavy nuclei, and for reasonable choices of neutron radii, the data tend to exclude ranges of order 0.5 fm and larger.     

Spectroscopic study of the N = 27 nuclei by means of the (τ, α) reaction

The (τ, α) reaction on ⁴⁸Ca, ⁵⁰Ti, ⁵²Cr and ⁵⁴Fe target nuclei has been studied at 25 MeV incident energy. Angular distributions have been measured from 5° to 40° with a split-pole spectrometer in a large range of excitation energy. A local zero-range DWBA analysis has been carried out, using an isospin-dependent potential for the calculation of the neutron form factor, in order to get a coherent set of spectroscopic factors for both T_> and T_< levels in different nuclei. Assignments of l-values have been done for a large number of levels, most of them previously unknown, and energy centroids of hole states have been determined. Spectroscopic factors in ³⁹Ca, ⁴⁷Ti, ^{49, 52, 53}Cr have also been obtained for strongly excited states. A sum rule analysis has been carried out for the N = 27 nuclei: the $\text{1}\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{and}\text{2}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{T}{}_{\mathrm{<}}$ hole strengths are generally fully exhausted by the observed levels, whereas only a fraction of the $\text{1}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ strength has been evidenced. The $\text{1}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{, 1}\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{and}\text{2}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ analog states have been observed in all nuclei; in ⁵³Fe, the $\text{1}\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{and}\text{2}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ analog states appear to be split in several components. In addition, a CRC analysis has been carried out for some levels with angular distributions not accounted for by a direct pick-up process. These levels are tentatively identified with states resulting from the coupling of one $\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ neutron hole with excited states of the target nucleus. In particular, the $\text{5}\text{2}{}^{\mathrm{?}}\text{and}\text{9}\text{2}{}^{\mathrm{?}}$ members of the $\text{[2}{}^{\mathrm{+}}\text{?}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}\text{]}$ configuration have been identified in each final nucleus. Unambiguous J^π assignments are made; and the two-step (τ, α) reaction therefo e appears as a useful spectroscopic tool, especially for investigating high spin states.     

Polarization-mixing optical parametric oscillator

We report the experimental realization of a new type of optical parametric oscillator in which oscillation is achieved by polarization rotation in a linear retarder, followed by nonlinear polarization mixing. The mixing is performed by a type II degenerate parametric downconversion in a periodically poled KTP crystal pumped by a 1064 nm pulsed Nd:YAG pump. A single, linearly polarized beam, precisely at the degenerate wavelength is generated. The output spectrum has a narrow linewidth (below the instrumentation bandwidth of 1 nm) and is highly stable with respect to variations in the crystal temperature.     

A chiral molecular conductor: synthesis, structure, and physical properties of [ET]3[Sb2(L-tart)2].CH3CN (ET = bis(ethylendithio)tetrathiafulvalene; L-tart = (2R,3R)-(+)-tartrate)

The salt [ET](3)[Sb(2)(L-tart)(2)].CH(3)CN (1) has been obtained by electrocrystallization of the organic donor bis(ethylendithio)tetrathiafulvalene (ET or BEDT-TTF) in the presence of the chiral anionic complex [Sb(2)(L-tart)(2)](2-) (L-tart = (2R,3R)-(+)-tartrate). This salt crystallizes in the chiral space group P2(1)2(1)2(1) (a = 11.145(2) angstroms, b = 12.848(2) angstroms, c = 40.159(14) angstroms, V = 5750.4(14) angstroms(3), Z = 4) and is formed by alternating layers of the anions and of the organic radicals in a noncentrosymmetric alpha-type packing. This compound shows a room temperature electrical conductivity of approximately 1 S.cm(-1) and semiconducting behavior with an activation energy of approximately 85 meV. Analysis of the magnetic susceptibility and band structure, however, suggests that this compound should be a narrow band gap semiconductor.     

Design and synthesis of a novel family of semi-rigid ligands: versatile compounds for the preparation of 99mTc radiopharmaceuticals

Synthetic pathways to a range of novel semi-rigid tetradentate ligands, with sulfur, amido or amino donor groups, designed to coordinate technetium 99m have been developed. The technetium-99m complexes have been prepared and their stabilities in serum and by metathesis reaction via cysteine exchange reactions were compared. HPLC comparison of two (99m)Tc-complexes and their rhenium analogues led to the first proof of the nature of our radioactive complexes.     

Remote characterization of defects in plates with viscoelastic coatings using guided waves

The remote inspection for defects in large metallic elements such as pipes, tubes and plates is a field where guided waves are being applied with success. There are a number of situations where a surface coating is added for corrosion protection or insulation purposes. Since the coating materials are usually viscoelastic, the guided wave inspection range may be severely reduced unless a proper mode and an adequate frequency range is selected. Previous authors found the existence of low-attenuated modes at certain frequency ranges, which were used to detect and locate defects at reasonably large distances. This paper studies the potential of guided waves for not only locating but also sizing defects in plates with viscoelastic coatings. A hybrid finite element-boundary element method which explicitly includes the attenuating characteristics of the coating is used to determine Lamb and SH mode conversion factors at corrosion defects under the coating. Through parametric studies and analysis of the numerical results, some trends and features are highlighted that could be used for sizing purposes.     

A charge-transfer-induced spin transition in the discrete cyanide-bridged complex [[Co(tmphen)2]3[Fe(CN)6]2

A charge-transfer-induced spin transition (CTIST) is observed in the discrete cyanide-bridged complex, {[Co(tmphen)2]3[Fe(CN)6]2}. Single-crystal X-ray diffraction, 57Fe M?ssbauer spectroscopy, and magnetic susceptibility were used collectively to describe the oxidation states of the Co and Fe ions in this cluster as a function of temperature. This pentanuclear complex represents the first example of a CTIST at the discrete molecular level.     

Excitation and decay of the isovector giant monopole resonances via the 208Pb(3He,t p) reaction at 410 MeV

The excitation and subsequent proton decay of the isovector spin-flip giant monopole resonance (IVSGMR) is studied via the 208Pb(3He,t) reaction at 410 MeV. In the inclusive spectrum (60+/-5)% of the non-energy-weighted sum-rule strength for this 2 variant Planck's over 2h omega resonance was found in the region 29相似文献