Metal–metal bonded alkaline-earth distannyls

The first families of alkaline-earth stannylides [Ae(SnPh₃)₂·(thf)_x] (Ae = Ca, x = 3, 1; Sr, x = 3, 2; Ba, x = 4, 3) and [Ae{Sn(SiMe₃)₃}₂·(thf)_x] (Ae = Ca, x = 4, 4; Sr, x = 4, 5; Ba, x = 4, 6), where Ae is a large alkaline earth with direct Ae–Sn bonds, are presented. All complexes have been characterised by high-resolution solution NMR spectroscopy, including ¹¹⁹Sn NMR, and by X-ray diffraction crystallography. The molecular structures of [Ca(SnPh₃)₂·(thf)₄] (1′), [Sr(SnPh₃)₂·(thf)₄] (2′), [Ba(SnPh₃)₂·(thf)₅] (3′), 4, 5 and [Ba{Sn(SiMe₃)₃}₂·(thf)₅] (6′), most of which crystallised as higher thf solvates than their parents 1–6, were established by XRD analysis; the experimentally determined Sn–Ae–Sn′ angles lie in the range 158.10(3)–179.33(4)°. In a given series, the ¹¹⁹Sn NMR chemical shifts are slightly deshielded upon descending group 2 from Ca to Ba, while the silyl-substituted stannyls are much more shielded than the phenyl ones (δ¹¹⁹Sn/ppm: 1′, −133.4; 2′, −123.6; 3′, −95.5; 4, −856.8; 5, −848.2; 6′, −792.7). The bonding and electronic properties of these complexes were also analysed by DFT calculations. The combined spectroscopic, crystallographic and computational analysis of these complexes provide some insight into the main features of these unique families of homoleptic complexes. A comprehensive DFT study (Wiberg bond index, QTAIM and energy decomposition analysis) points at a primarily ionic Ae–Sn bonding, with a small covalent contribution, in these series of complexes; the Sn–Ae–Sn′ angle is associated with a flat energy potential surface around its minimum, consistent with the broad range of values determined by experimental and computational methods.

The complete series of heterobimetallic alkaline-earth distannyls [Ae{SnR₃}₂·(thf)_x] (Ae = Ca, Sr, Ba) have been prepared for R = Ph and SiMe₃, and their bonding and electronic properties have been comprehensively investigated.     

Mechanical, thermal, and fire properties of polylactide/starch blend/clay composites

Polylactide (PLA)/starch blend/clay and PLA/clay composites are prepared by melt blending. Structural and thermal characterizations are performed by differential scanning calorimetry, X-ray diffraction analysis, and thermogravimetric analysis. The fire properties are assessed on a dual cone calorimeter. Combustion residue and char formation is characterized by optical microscopy and attenuated total reflection infrared spectroscopy. Although the clay is not fully intercalated/exfoliated, the composites exhibit a higher thermal stability and much reduced peak heat release rate, and the PLA/starch blend composite retains its mechanical properties. For the PLA/starch blend composite, smoke release is also considerably reduced. Catalyzed, oxidative decomposition is shown to occur early in the thermal decomposition of the composites, prior to increased thermal stability. The inclusion of clay promotes char formation and increases the quantity of carbonaceous char in the combustion residue. There is minimal migration of the clay to the surface prior to ignition and char is formed mainly after ignition and during burning. During the later stages of burning some of the char formed is converted to CO₂.     

Stochastic simulation of infrared non-Gaussian natural terrain imagery

We describe a new technique for the fast simulation of natural IR imagery with non-Gaussian statistics. We pass samples of the imagery through an invertible pointwise nonlinear transform, to give samples of a new random field with Gaussian marginal probability density. We compute the correlation function for the transformed samples, and simulate Gaussian random fields (GRFs) with these correlations. Passing the simulated GRFs through the inverse transform gives our simulations of the original imagery. We discuss the effectiveness of this approach for simulating natural terrain backgrounds.     

Triplet state quenching by ruthenocene

Ruthenocene quenches triplet states of organic molecules with energies greater than 24000 cm^?1 in benzene solution at a diffusion controlled rate , (6 ± 1) × 10⁹ dm³ mol^?1 s^?1. For triplets with energies less than this the efficiency of quenching is dependent on the energy of the triplet state being quenched but drops off less acutely than expected for endothermic energy transfer following the Arrhenius equation. This is in agreement with the lowest triplet state of ruthenocene being geometrically distorted as expected from the previously observed large Stokes shift between absorption to and emission from its lowest triplet state. Similarities to ferrocene quenching of triplet states are discussed. Quenching of the triplet state of benzil by ruthenocene does not fall on the smooth curve which exists between the quenching rate constants k_q and the energy of the triplet state being quenched. Queching of triplet benzil by ruthenocene is therefore attributed to favourable charge-transfer interactions, also in this case the behaviour is analogous to quenching of triplet methylene-blue by ferrocene where at least a proportion of electron transfer following quenching has been previously established.     

Power saturation effects in thick single-element optical limiters

Optical limiters relying on nonlinear absorption or nonlinear refraction are most effective against an incoming laser beam when the active material is thicker than one Rayleigh length. We have found that a variety of thick optical limiter configurations can be modelled by integrating the formulae for optical transmission by thin media, and that the results are exact to the first order in power or irradiance. The limiter behaviour saturates as the optical power increases. For the case of thick irradiance-dependent absorbers and an incident Gaussian beam model, we have also found that the predictions of a simple saturation formula for the optical transmittance agree well with the transmittance values computed using a numerical technique. Under certain conditions this saturation formula is also suitable for describing the transmission of nonlinear refractors, thereby providing a straight-forward technique for modelling limiters over a broad parameter range.     

Bis(imino)carbazolate: A Master Key for Barium Chemistry

Reported here is a readily available bis(imino)carbazole‐based proligand that constitutes a convenient entry point into the challenging synthetic molecular chemistry of barium. It enables the preparation of rare or even, up to now, unknown, solution‐stable heteroleptic barium complexes. The syntheses and structural features for the first molecular barium fluoride and the first barium stannylide, with an unsupported Ba?Sn bond, are described, along with other carbazolate barium species: an amide (both a remarkably stable starting material and an excellent hydrophosphination precatalyst), iodide, and silanylide. DFT analysis of bonding patterns in the barium stannylide and barium silanylide highlights a prevailingly ionic barium–tetrelide bond with a small covalent contribution.