MAS NMR spectra of quadrupolar nuclei in disordered solids: the Czjzek model

Structural disorder at the scale of two to three atomic positions around the probe nucleus results in variations of the EFG and thus in a distribution of the quadrupolar interaction. This distribution is at the origin of the lineshape tailing toward high fields which is often observed in the MAS NMR spectra of quadrupolar nuclei in disordered solids. The Czjzek model provides an analytical expression for the joint distribution of the NMR quadrupolar parameters upsilon(Q) and eta from which a lineshape can be predicted. This model is derived from the Central Limit Theorem and the statistical isotropy inherent to disorder. It is thus applicable to a wide range of materials as we have illustrated for 27Al spectra on selected examples of glasses (slag), spinels (alumina), and hydrates (cement aluminum hydrates). In particular, when relevant, the use of the Czjzek model allows a quantitative decomposition of the spectra and an accurate extraction of the second moment of the quadrupolar product. In this respect, it is important to realize that only rotational invariants such as the quadrupolar product can make sense to describe the quadrupolar interaction in disordered solids.     

Turnpike Property in Optimal Microbial Metabolite Production

Journal of Optimization Theory and Applications - We consider the problem of maximization of metabolite production in bacterial cells formulated as a dynamical optimal control problem (DOCP)....     

Noise reduction in a mode-locked semiconductor laser by coherent photon seeding

We have used coherent photon seeding to reduce the noise in an actively mode-locked extended-cavity semiconductor laser. Feedback is provided by an auxiliary cavity terminated either by a wedge plate or by a phase-conjugate mirror. Both conventional and phase-conjugate feedback can reduce the low-frequency noise components (0-5 MHz) by as much as 36 dB, hence drastically reducing the pulse fluctuations. A noise reduction of as much as 18 dB is also observed in the 0.01-3.0-GHz frequency range. We obtained these results while preserving the pulse duration (26 ps) and improving the time-bandwidth product.     

On defect groups for generalized blocks of the symmetric group

In a paper of 2003, Külshammer, Olsson and Robinson defined-blocks for the symmetric groups, where >1 is an arbitraryinteger. In this paper, we give a definition for the defectgroup of the principal -block. We then check that, in the Abeliancase, we have an analogue of one of Broué's conjectures.     

Framework functionalisation triggers metal complex binding

Post-synthetic derivatisation of a porous material produces a functionalized material that binds the metal complex V(O)acac2, in contrast to the unfunctionalized precursor, which is inactive for complex binding.     

Structure-activity relationships in cholapod anion carriers: enhanced transmembrane chloride transport through substituent tuning

Chloride transport by a series of steroid-based "cholapod" receptors/carriers was studied in vesicles. The principal method involved preincorporation of the cholapods in the vesicle membranes, and the use of lucigenin fluorescence quenching to detect inward-transported Cl-. The results showed a partial correlation between anion affinity and transport activity, in that changes at the steroidal 7 and 12 positions affected both properties in concert. However, changes at the steroidal 3-position yielded irregular effects. Among the new steroids investigated the bis-p-nitrophenylthiourea 3 showed unprecedented activity, giving measurable transport through membranes with a transporter/lipid ratio of 1:250 000 (an average of <2 transporter molecules per vesicle). Increasing transporter lipophilicity had no effect, and positively charged steroids had low activity. The p-nitrophenyl monourea 25 showed modest but significant activity. Measurements using a second method, requiring the addition of transporters to preformed vesicle suspensions, implied that transporter delivery was problematic in some cases. A series of measurements employing membranes of different thicknesses provided further evidence that the cholapods act as mobile anion carriers.