Experimental and theoretical studies of magnetic exchange in silole-bridged diradicals

Five bis(tert-butylnitroxide) diradicals connected by a silole (7 a-d) or a thiophene (12) ring as a coupler were studied. Compound 12 crystallizes in the orthorhombic space group Pna2(1) with a = 20.752(5), b = 5.826(5), and c = 34.309(5) A. X-ray crystal structure determination, electronic spectroscopy, variable-temperature EPR spectroscopy, SQUID measurements and DFT computations (UB3LYP/6-31+G*) were used to study the molecular conformations and electronic spin coupling in this series of molecules. Whereas compounds 7 b, 7 c, and 7 d are quite stable both in solution and in the solid state, 7 a and 12 undergo a partial electronic rearrangement to both a diamagnetic quinonoidal form and a monoradical species owing to the fact that they correspond to the open form of a pi-conjugated Kekulé structure. In the solid state, magnetic measurements indicate that the diradicals are all antiferromagnetically coupled, as expected from their topology. These interactions are best reproduced by means of a "Bleaney-Bowers" model that gives values of J = -142.0 cm(-1) for 7 a, -1.8 cm(-1) for 7 b, -1.3 cm(-1) for 7 c, -4.2 cm(-1) for 7 d, and -248.0 cm(-1) for 12. The temperature dependence of the EPR half-field transition in frozen dichloromethane solutions is consistent with singlet ground states and thermally accessible triplet states for diradicals 7 b, 7 c, and 7 d with DeltaE(T-S) values of 3.48, 2.09, and 8 cm(-1), respectively. No evidence of a populated triplet state was found for diradicals 7 a and 12. Similarities between the DeltaE(T-S) and J values (DeltaE(T-S) = -2 J) clearly show the intramolecular origin of the observed antiferromagnetic interaction. Analyses of the data with a "Karplus-Conroy"-type equation enabled us to establish that the silole ring, as a whole, allows a more efficient magnetic coupling of the two nitroxide radicals attached to its 2,5-positions than the thiophene ring. This superiority probably originates from the nonaromaticity of the silole which thus permits a better magnetic interaction through it. DFT calculations also support the experimental results, indicating that the magnetic exchange pathway preferentially involves the carbon pi system of the silole.     

Transition from creeping via viscous-inertial to turbulent flow in fixed beds

This review is concerned with the analysis of flow regimes in porous media, in particular, in fixed beds of spherical particles used as reactors in engineering applications, or as separation units in liquid chromatography. A transition from creeping via viscous-inertial to turbulent flow is discussed based on macro-scale transport behaviour with respect to the pressure drop-flow rate dependence, in particular, the deviation from Darcy's law, as well as direct microscopic data which reflect concomitant changes in the pore-level hydrodynamics. In contrast to the flow behaviour in straight pipes, the transition from laminar to turbulent flow in fixed particulate beds is not sharp, but proceeds gradually through a viscous-inertial flow regime. The onset of this steady, nonlinear regime and increasing role of inertial forces is macroscopically manifested in the failure of Darcy's law to describe flow through fixed beds at higher Reynolds numbers. While the physical reasons for this failure still are not completely understood, it is not caused by turbulence which occurs at Reynolds numbers about two orders of magnitude above those for which a deviation from Darcy's law is observed. Microscopic analysis shows that this steady, nonlinear flow regime is characterized by the development of an inertial core in the pore-level profile, i.e., at increasing Reynolds number velocity profiles in individual pores become flatter towards the center of the pores, while the velocity gradient increases close to the solid-liquid interface. Further, regions with local backflow and stationary eddies are demonstrated for the laminar flow regime in fixed beds. The onset of local fluctuations (end of laminar regime) is observed at superficial Reynolds numbers on the order of 100. Complementary analysis of hydrodynamic dispersion suggests that this unsteady flow accelerates lateral equilibration between different velocities in fixed beds which, in turn, reduces spreading in the longitudial (macroscopic flow) direction.     

Performances under representative pressure and temperature conditions of the gas chromatography-mass spectrometry space experiment to investigate Titan's atmospheric composition

In the frame of the calibration of the aerosol collector and pyrolyser, and gas chromatography-mass spectrometry experiments of the Huygens probe arrived at Titan, systematic experimental studies were led to estimate the influence of the operating conditions on the analyses that should have been achieved in the Titan's atmosphere. The primary objective of this study was to estimate the influence of operating conditions variations induced by (i) instrumental modifications made shortly before the probe launch which can have changed the operating pressures; (ii) the change of the probe environmental conditions (pressure, temperature) during its descent in the atmosphere; (iii) a possible deviation of pressure and temperature regulations from their nominal values because of the long journey of the instrument in space, or of other external events. The secondary objective of this work was to create an analytical database that can be used as a reference to treat the chromatograms obtained in situ, and help to identify chromatographically the analyzed species, complementary to mass spectrometry. Beyond the application to a specific instrument, this work was also useful to experimentally estimate the fundamental evolution of the separation as a function of the changes of operating conditions with time. The obtained results show (i) the significant influence of inlet and outlet pressure variation on the time of analysis, but not on the separation power. It thus enables to significantly shorten the analysis duration, and thus to analyze more compounds within the fixed time of analysis of the instrument; (ii) the significant influence of temperature on the retention. In this frame, the enthalpies of exchange between the gas phase and the stationary phase of the species were determined to be used to retrieve the analyzed species in case of deviation of the operating temperature; (iii) that the possible aging of the columns does not have influence on the columns efficiency and separation power; (iv) the analytical capabilities of the gas chromatography-mass spectrometry experiment within operating conditions representative of those encountered in situ. Finally, in spite of possible operating condition changes, it is shown that results coming from the gas chromatograph-mass spectrometer experiment, which are currently under analysis, could bring important information on the Titan's atmosphere and its history.     

Synthesis and photophysical properties of borondipyrromethene dyes bearing aryl substituents at the boron center

Several borondipyrromethene (Bodipy) dyes bearing an aryl nucleus linked directly to the boron center have been prepared under mild conditions. The choice of Grignard or lithio organo-metallic reagents allows the isolation of B(F)(aryl) or B(aryl)2 derivatives; where aryl refers to phenyl, anisyl, naphthyl, or pyrenyl fragments. A single crystal, X-ray structure determination for the bis-anisyl compound shows that the sp3 hybridized boron center remains pseudo-tetrahedral and that the B-C bond distances are 1.615 and 1.636 A. All compounds are electrode active but replacement of the fluorine atoms by aryl fragments renders the Bodipy unit more easily oxidized by 100 mV in the B(F)(aryl) and 180 mV in the B(aryl)2 compounds whereas reduction is made more difficult by a comparable amount. Strong fluorescence is observed from the Bodipy fluorophore present in each of the new dyes, with the radiative rate constant being independent of the nature of the aryl substituent. The fluorescence quantum yields are solvent dependent and, at least in some cases (aryl = anisyl or pyrenyl), nonradiative decay from the first-excited singlet state is strongly activated. There is no indication, however, for population of a charge-transfer state, in which the aryl substituent acts as donor and the Bodipy fragment functions as acceptor, that is strongly coupled to the ground state. Instead, it is conjectured that nonradiative decay involves a conformational change driven by the solvophobic effect. Thus, the rate of nonradiative decay in any given solvent increases with increasing surface accessibility (or molar volume) of the aryl substituent. Intramolecular energy transfer from pyrene or naphthalene residues to Bodipy is quantitative.     

Dilacunary decatungstates functionalized by organometallic ruthenium(II), [{Ru(C6H6)(H2O)}{Ru(C6H6)}(gamma-XW10O36)]4- (X = Si, Ge)

The benzene-Ru(II)-supported dilacunary decatungstosilicate [{Ru(C6H6)(H2O)}{Ru(C6H6)}(gamma-SiW10O36)]4- and the isostructural decatungstogermanate [{Ru(C6H6)(H2O)}{Ru(C6H6)}(gamma-GeW10O36)]4- have been synthesized and characterized by multinuclear solution NMR, IR, elemental analysis, and electrochemistry. Single-crystal X-ray analysis was carried out on K4[{Ru(C6H6)(H2O)}{Ru(C6H6)}(gamma-SiW10O36)].9H2O (K-1), which crystallizes in the orthorhombic system, space group Pmn2(1), with a = 13.6702(3) A, b = 16.2419(4) A, c = 12.1397(2) A, and Z = 2, and on K4[{Ru(C6H6)(H2O)}{Ru(C6H6)}(gamma-GeW10O36)].7H2O (K-2), which also crystallizes in the orthorhombic system, space group Pmn2(1), with a = 13.6684(12) A, b = 16.297(2) A, c = 12.1607(13) A, and Z = 2. Polyanions 1 and 2 consist of a Ru(C6H6)(H2O) group and a Ru(C6H6) group linked to a dilacunary (gamma-XW10O36) Keggin fragment resulting in an assembly with idealized Cs symmetry. The Ru(C6H6)(H2O) group is bound at the lacunary polyanion site via two Ru-O(W) bonds, whereas the Ru(C6H6) group is bound on the side via three Ru-O(W) bonds. Polyanions 1 and 2 were synthesized in aqueous acidic medium at pH 2.5 by the reaction of [Ru(C6H6)Cl2]2 with [gamma-SiW10O36]8- and [gamma-GeW10O36]8-, respectively. The formal potentials are roughly the same for the first W waves of 1 and 2. However, important differences appear for the second W waves. These observations indicate different acid-base properties for the reduced forms of 1 and 2. Three oxidation processes were detected: the oxidation of the Ru center is followed first by irreversible electrocatalytic processes of the Ru-benzene moiety and then of the electrolyte. Comparison of this behavior with that of the precursor reagent, [Ru(C6H6)Cl2]2, was useful to understand the main oxidation processes. A ligand substitution reaction was observed upon addition of dimethyl sulfoxide (dmso) to 1, 2, or [Ru(C6H6)Cl2]2. This reaction facilitates substantially the oxidation of the Ru center. The dmso was oxidized with large electrocatalytic currents more efficiently in the presence of 1 and 2 than with [Ru(C6H6)Cl2]2.     

Neutral high-potential nickel triad bisdithiolenes: structure and solid-state NMR properties of Pt[S2C2(CF3)2]2

X-ray crystallography and solid-state NMR techniques were used to determine the structure and 195Pt NMR chemical shift (CS) tensor of Pt[S2C2(CF3)2]2. This is the first reported crystal structure of a highly oxidizing (CN- or CF3-substituted) neutral bis(dithiolene) complex of a Ni triad metal in its pure form. The 195Pt NMR CS tensor is highly anisotropic and asymmetric; the latter property is attributed to the noninnocent nature of the ligand. The tensor components and orientation are determined with density functional theory calculations.     

Pressure-induced internal redox reaction of Cs2[PdI4].I2, Cs2[PdBr4].I2, and Cs2[PdCl4].I2

The pressure-induced redox reaction within the system Cs2[Pd2+I4].I2/Cs2[Pd4+I6] was investigated by means of powder X-ray diffraction. Analogous high-pressure X-ray diffraction experiments were performed on the isostructural compounds Cs2[PdX4].I2 (X = Cl, Br). Additionally, the phase transition of Cs2[PdBr4].I2 to Cs2[PdBr4I2] was characterized by means of Raman scattering experiments as well as theoretical calculations based on density functional theory. On the basis of experimentally determined crystal structure data, a pathway for the topology of the redox reactions was developed and outlined.     

Calculation of absorption spectra for light-harvesting systems using non-Markovian approaches as well as modified Redfield theory

For an ensemble of B850 rings of the light-harvesting system LH2 of purple bacteria the linear absorption spectrum is calculated. Using different Markovian and non-Markovian, time-dependent and time-independent methods based on second-order perturbation theory in the coupling between the excitonic system and its surrounding environment as well as the modified Redfield theory, the influence of the shape of the spectral density on the linear absorption spectrum is demonstrated for single samples and in the ensemble average. For long bath correlation times non-Markovian effects clearly show up in the static absorption line shapes. Among the different spectral densities studied is one of the purple bacterium Rhodospirillum molischianum obtained by a molecular-dynamics simulation earlier. The effect of static disorder on its line shapes in the ensemble average is analyzed and the results of the present calculations are compared to experimental data.     

Optimized protocol for synthesis of cyclic gramicidin S: starting amino acid is key to high yield

[structures: see text] A simple and highly efficient Fmoc solid-phase protocol for synthesizing the antimicrobial decapeptide gramicidin S and various labeled analogues is presented. When preparing the linear precursor peptides (1a-e), a systematic permutation of the starting amino acid within the cyclic sequence gave different yields between 51% and 93%. Also the subsequent step of cyclization gave widely diverging yields between 26% and 74%, depending again on the starting amino acid. The ease of cyclization was found to correlate with the tendency of the respective linear precursor peptide to assume a preorganized conformation, as observed by circular dichroism. The overall yield is thus critically dependent on the starting amino acid and can be raised from 20% to 70% using (D)Phe. The choice of coupling agent and its counterion was found to play only a marginal role. Irrespective of being able to assume a preorganized conformation, none of the linear precursor peptides exhibited any antimicrobial or hemolytic activity. Using the optimized protocol, which involves only simple Fmoc-couplings and requires no intermittent purification steps, several gramicidin S analogues (3-8) containing 19F-labeled phenylglycine derivatives and/or 15N-labeled amino acids were synthesized for solid-state NMR structure analysis.     

Condenser for Koehler-like illumination in transmission x-ray microscopes at undulator sources

We report on a novel condenser for full-field transmission x-ray microscopes that use synchrotron radiation from an undulator source. The condenser produces a Koehler-like homogeneous intensity distribution in the sample plane and eliminates object illumination problems connected with the high degree of spatial coherence in an undulator beam. The optic consists of a large number of small linear diffraction gratings and is therefore relatively easy to manufacture. First imaging experiments with a prototype condenser were successfully performed with the Twinmic x-ray microscope at the Elettra synchrotron facility in Italy.