1H and 19F ultra-fast MAS double-quantum single-quantum NMR correlation experiments using three-spin terms of the dipolar homonuclear Hamiltonian

Measuring internuclear distances through dipolar interaction is a major challenge for solid-state nuclear magnetic resonance (NMR) spectroscopy. Obtaining reliable interatomic distances provides an access to the local structure in ordered or disordered solids. We show that at magic angle spinning (MAS) frequencies larger than ca. 50 kHz, some of the three-spin terms of the homogeneous homonuclear dipolar Hamiltonian can be used to promote the creation of double-quantum coherences between neighbouring (1)H or (19)F spins without using dipolar recoupling pulse sequences in the Dipolar Homonuclear Homogeneous Hamiltonian (DH(3)) double-quantum/single-quantum correlation experiment. This makes it possible to probe inter-nuclear spatial proximity with limited risk of probe or sample damage from radio-frequency (RF) irradiation, and is fully appropriate for fast repetition rate offering sensitivity gains in favourable cases. Experimental demonstrations are supported by multi-spin numerical simulations, which points to new possibilities for the characterization of spin-system geometries.     

Magnetic and conduction properties in 1D organic radical materials: an ab initio inspection for a challenging quest

The chemical control of magnetic and conduction properties for organic radicals is mainly based on t, the resonance integral, and U, the on-site repulsion, used in the Hubbard model. A qualitative analysis based on the competition between the kinetic and the Coulomb contribution, and the expression of the magnetic exchange coupling suggests that U should be roughly 800 cm(-1) while the resonance integral |t| should be 200 cm(-1) to reach bifunctionality. Ab initio wavefunction-based calculations allowed us to quantitatively measure those quantities for several organic materials considered as 1D systems starting from their reported crystal structures. The extraction of t and U parameters from the exchange coupling constants between neighbouring radicals allowed us to anticipate a possible metallic behaviour. Finally, the impact of chemical changes in the constitutive units is measured to rationalize the macroscopic behaviour modifications. It is shown that the intriguing regime characterized by simultaneous itinerant and localized electrons might be achieved by molecular engineering.     

Inelastic X-ray scattering and vibrational effects at the K-edges of gaseous N2, N2O, and CO2

We report non-resonant inelastic X-ray scattering experiments of several gaseous samples in the inner-shell excitation energy range. The experimental near-edge spectra from all the K-edges of N(2), N(2)O, and CO(2) including the momentum transfer dependence are presented. The results are analyzed using density functional theory calculations that accurately reproduce the experimental spectral features. We observe vibrational effects in the measured spectrum and in the calculations the atomic motion is modeled using the Franck-Condon approximation and the linear coupling model. Our findings show that vibrational effects cannot be neglected in the analysis of high resolution inelastic X-ray scattering spectroscopy. The results also support the validity of the transition potential approximation for calculating core excited state potential energy surfaces.     

Theoretical and experimental studies of CH3X-Y2 rotational line shapes for atmospheric spectra modelling: application to room-temperature CH3Cl-O2

The case of symmetric tops CH(3)X (X = Br, Cl, F, …) perturbed by non-polar diatoms Y(2) (Y = N(2), O(2), …) is analysed from the viewpoint of theoretical collisional broadening of their rotational lines observed in atmospheric spectra. A semi-classical approach involving an exponential representation of the scattering operator and exact trajectories governed by the isotropic potential is presented. For the first time the active molecule is strictly treated as a symmetric top and the atom-atom interactions are included in the intermolecular potential model. It is shown for the CH(3)Cl-O(2) system that these interactions contribute significantly to the line width for all values of the rotational quantum numbers J and K. Additional testing of modifications required in the semi-classical formalism for a correct application of the cumulant expansion is performed and it is shown that the use of the cumulant average on the rotational states of the perturbing molecule leads to entirely negligible effects for the not very strongly interacting CH(3)Cl-O(2) system. In order to check the theoretical predictions and to extend the scarce experimental data available in the literature to higher values of the rotational quantum numbers, new measurements of room-temperature O(2)-broadened CH(3)Cl rotational lines are carried out by a photomixing continuous-wave terahertz spectrometer. The experimental line widths extracted with a Voigt profile model demonstrate an excellent agreement with theoretical results up to very high J-values (J = 31, 37, 40, 45, 50).     

A novel ruthenium(II) complex for two-photon absorption-based optical power limiting in the near-IR range

In this article, the synthesis of a novel high-conjugated ligand and its corresponding Ru(II) complex PTFTF:Ru is reported, along with the linear and nonlinear optical characterizations. Two-photon absorption based optical power limiting properties (OPL), especially in the near infrared, are described and compared to those of the analogous complexes previously published. Combined with a preliminary theoretical approach, this allows us to highlight several key parameters for OPL optimization in such molecular systems and more particularly the spectral overlap between TPA and excited-state absorption.     

Extended viologen as a source of electric oscillations

A long organic molecule 1 with five bipyridinium functions separated by benzene rings (extended viologen) undergoes a reversible multi-step electron transfer. Here we show that this decacation accepts electrons at the heterogeneous interface with the occurrence of the periodically changing electric reduction currents. According to the applied bias voltage the observed current-time dependence changes from chaotic through periodic and irregular to sinusoidal and finally to monotonous. A careful choice of the controlling parameters yields the sustained periodic sinusoidal currents lasting for a prolonged time. Oscillations stem from a mutual interplay of the heterogeneous supply of electrons and the homogeneous redox reactions (disproportionation) between the transient redox forms. In difference to many other electrochemical oscillating systems the described oscillations do not require any additional external impedance. The principle of these oscillatory currents may serve as a model of a truly 'molecular oscillator'.     

High sensitive matrix-free mass spectrometry analysis of peptides using silicon nanowires-based digital microfluidic device

We present for the first time an electrowetting on dielectric (EWOD) microfluidic system coupled to a surface-assisted laser desorption-ionization (SALDI) silicon nanowire-based interface for mass spectrometry (MS) analysis of small biomolecules. Here, the transfer of analytes has been achieved on specific locations on the SALDI interface followed by their subsequent mass spectrometry analysis without the use of an organic matrix. To achieve this purpose, a device comprising a digital microfluidic system and a patterned superhydrophobic/superhydrophilic silicon nanowire interface was developed. The digital microfluidic system serves for the displacement of the droplets containing analytes, via an electrowetting actuation, inside the superhydrophilic patterns. The nanostructured silicon interface acts as an inorganic target for matrix-free laser desorption-ionization mass spectrometry analysis of the dried analytes. The proposed device can be easily used to realize several basic operations of a Lab-on-Chip such as analyte displacement and rinsing prior to MS analysis. We have demonstrated that the analysis of low molecular weight compounds (700 m/z) can be achieved with a very high sensitivity (down to 10 fmol μL(-1)).     

Isolation of C-glycosyl xanthones from Coffea pseudozanguebariae and their location

The biochemical composition of leaves from Coffea pseudozanguebariae, a wild caffeine-free coffee species, was determined. Two phenolic compounds were extracted from leaves, separated and characterized. Their structures were elucidated by mass spectrometry, and 1D and 2D NMR spectroscopy and were shown to be mangiferin (1) and isomangiferin (2), which were the main polyphenol products. Multiphoton fluorescence imaging was performed to visualize polyphenol distribution in leaf cross sections. Consistent biochemical analysis cell imaging techniques on leaves revealed yellow fluorescence in the epidermis and parenchyma cells corresponding to xanthone compounds.     

Eulerian formulation of constrained elastica

The formulation of the constrained elastica problem proposed in this paper is predicated on two key concepts: first, the deformed elastica is described by means of the distance from the conduit axis; second, the problem is formulated in terms of the Eulerian curvilinear coordinate of the conduit rather than the natural curvilinear coordinate of the elastica. This approach is further implemented within a segmentation algorithm, which transforms the global constrained elastica problem into a sequence of analogous auxiliary problems that result from dividing the conduit and the elastica into segments limited by contacts. Each auxiliary segment entails solving a segment of elastica subject to isoperimetric constraints corresponding to the assumed positions of the segment ends along the conduit. This new formulation resolves in one stroke a series of issues that afflict the classical Lagrangian approach: (i) the contact detection is reduced to checking whether a threshold on the distance function is violated, (ii) the isoperimetric conditions are transformed into regular boundary conditions, instead of being treated as external integral constraints, (iii) the method yields a well-conditioned set of equations that does not degenerate with decreasing flexural rigidity of the elastica and/or decreasing clearance between the conduit and the elastica.     

Solutions to and Validation of Matrix-Diffusion Models

A model transport system is considered in which a pulse of tracer molecules is advected along a flow channel with porous walls. The advected tracer thus undergoes diffusion, matrix-diffusion, inside the walls, which affects the breakthrough curve of the tracer. Analytical solutions in the form of series expansions are derived for a number of situations which include a finite depth of the porous matrix, varying aperture of the flow channel, and longitudinal diffusion and Taylor dispersion of the tracer in the flow channel. Novel expansions for the Laplace transforms of the concentration in the channel facilitated closed-form expressions for the solutions. A rigorous result is also derived for the asymptotic form of the breakthrough curve for a finite depth of the porous matrix, which is very different from that for an infinite matrix. A specific experimental system was created for validation of matrix-diffusion modeling for a matrix of finite depth. A previously reported fracture-column experiment was also modeled. In both cases model solutions gave excellent fits to the measured breakthrough curves with very consistent values for the diffusion coefficients used as the fitting parameters. The matrix-diffusion modeling performed could thereby be validated.