Phenomenological analysis of the phase transitions sequence in the ferroelectric crystal (CH3NH3)5Bi2Cl11 (PMACB)

A phenomenological model is proposed to describe the sequence of phase transitions in Pentakis (methylammonium) Undecachlorodibismuthate (111), (CH₃NH₃)₅Bi₂Cl₁₁ (PMACB) crystal on the basis of the recent structural data. The ferroelectric phase transition at 307 K is attributed to the ordering in a sublattice of the methylammonium cations CH₃NH ₃ ⁺ } placed in centrosymmetric sites in the paraelectric phase, whereas the isomorphous anomaly at about 180 K is related to a variation of the order in the sublattice of the remaining CH₃NH ₃ ⁺ } cations. The phenomenological thermodynamic potential is constructed for this system of two nonequivalent sublattices and the numerical values of its coefficients are then estimated from the dielectric, pyroelectric and calorimetric data. The sublattices are found to be weakly coupled near the ferroelectric phase transition. The anomaly at 180 K is interpreted as a continuous trace of a first order phase transition in a field created by the cations already ordered in the ferroelectric phase transition. This is analogous to a cusp A₊₃ in the catastrophes theory. The comparison of the Curie constants with the saturation values of the spontaneous polarization suggests that the sublattices cannot be treated as consisting of simple two-states pseudospins.     

Ruthenium-Catalyzed Coupling Reactions of CO2 with C2H4 and Hydrosilanes towards Silyl Esters

A series of in situ-prepared catalytic systems incorporating Ru^II precursors and bidentate phosphine ligands has been probed in the reductive carboxylation of ethylene in the presence of triethylsilane as reductant. The catalytic production of propionate and acrylate silyl esters was evidenced by high-throughput screening (HTS) and implemented in batch reactor techniques. The most promising catalyst systems identified were made of Ru(H)(Cl)(CO)(PPh₃)₃ and 1,4-bis(dicyclohexylphosphino)butane (DCPB) or 1,1’-ferrocene-diyl-bis(cyclohexylphosphine) (DCPF). A marked influence of water on the acrylate/propionate selectivity was noted. Turnover numbers [mol mol(Ru)⁻¹] up to 16 for acrylate and up to 68 for propionate were reached under relatively mild conditions (20 bar, 100 °C, 0.5 mol % Ru, 40 mol % H₂O vs. HSiEt₃). Possible mechanisms are discussed.     

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.     

Ring-opening polymerization of 3,6-dimethyl-2,5-morpholinedione with discrete amino-alkoxy-bis(phenolate) yttrium initiators: mechanistic insights

Alkoxy-amino-bis(phenolate) yttrium amide and alkoxide complexes promote the ring-opening polymerization of (3S,6S)-dimethyl-2,5-morpholinedione at 60-100 degrees C via a coordination-insertion polymerization mechanism.     

Solving the musical orchestration problem using multiobjective constrained optimization with a genetic local search approach

In this paper a computational approach of musical orchestration is presented. We consider orchestration as the search of relevant sound combinations within large instruments sample databases and propose two cooperating metaheuristics to solve this problem. Orchestration is seen here as a particular case of finding optimal constrained multisets on a large ensemble with respect to several objectives. We suggest a generic and easily extendible formalization of orchestration as a constrained multiobjective search towards a target timbre, in which several perceptual dimensions are jointly optimized. We introduce Orchidée, a time-efficient evolutionary orchestration algorithm that allows the discovery of optimal solutions and favors the exploration of non-intuitive sound mixtures. We also define a formal framework for global constraints specification and introduce the innovative CDCSolver repair metaheuristic, thanks to which the search is led towards regions fulfilling a set of musical-related requirements. Evaluation of our approach on a wide set of real orchestration problems is also provided.     

Dynamics of the amorphous and crystalline alpha-, gamma-phases of indomethacin

Crystallization processes in indomethacin can be observed below Tg leading to different forms depending on the thermal treatment: a rapid and deep quench below Tg leads to the metastable alpha-phase and a slow cooling close to Tg gives rise to the stable gamma-phase. To understand this atypical behavior, we have studied the molecular mobility of the amorphous and crystalline forms of indomethacin by dielectric relaxation and 1H NMR spectroscopy. Two relaxations were detected in the glassy state obtained from the deeply quenched liquid. One, also present in the gamma-phase, was attributed to local rotations. The other one, of very low amplitude, was attributed to the Johari-Goldstein relaxation. The results allowed to discuss the relationship between these two relaxation processes and the crystallization properties of amorphous indomethacin.