Unprecedented Spectroscopic and Computational Evidence for Allenyl and Propargyl Titanocene(IV) Complexes: Electrophilic Quenching of Their Metallotropic Equilibrium

The synthesis and structural characterization of allenyl titanocene(IV) [TiClCp₂(CH=C=CH₂)] 3 and propargyl titanocene(IV) [TiClCp₂(CH₂?C≡C?(CH₂)₄CH₃)] 9 have been described for the first time. Advanced NMR methods including diffusion NMR methods (diffusion pulsed field gradient stimulated spin echo (PFG‐STE) and DOSY) have been applied and established that these organometallics are monomers in THF solution with hydrodynamic radii (from the Stokes–Einstein equation) of 3.5 and 4.1 Å for 3 and 9 , respectively. Full ¹H, ¹³C, Δ¹H, and Δ¹³C NMR data are given, and through the analysis of the Ramsey equation, the first electronic insights into these derivatives are provided. In solution, they are involved in their respective metallotropic allenyl–propargyl equilibria that, after quenching experiments with aromatic and aliphatic aldehydes, ketones, and protonating agents, always give the propargyl products P (when carbonyls are employed), or allenyl products A (when a proton source is added) as the major isomers. In all the cases assayed, the ratio of products suggests that the metallotropic equilibrium should be faster than the reactions of 3 and 9 with electrophiles. Indeed, DFT calculations predict lower Gibbs energy barriers for the metallotropic equilibrium, thus confirming dynamic kinetic resolution.     

Enhanced NiO Dispersion on a High Surface Area Pillared Heterostructure Covered by Niobium Leads to Optimal Behaviour in the Oxidative Dehydrogenation of Ethane

A Nb-containing siliceous porous clay heterostructure (PCH) with Nb contents from 0 to 30 wt %) was prepared from a bentonite and used as support in the preparation of supported NiO catalysts with NiO loading from 15 to 80 wt %. Supports and NiO-containing catalysts were characterised by several physicochemical techniques and tested in the oxidative dehydrogenation (ODH) of ethane. The characterisation studies on Nb-containing supports showed the presence of well-anchored Nb⁵⁺ species without the formation of Nb₂O₅ crystals. High dispersion of nickel oxide with low crystallinity was observed for the Nb-containing PCH supports. In addition, when NiO is supported on these Nb-containing porous clays, it is more effective in the ODH of ethane (ethylene selectivity of ca. 90 %) than NiO supported on the corresponding Nb-free siliceous PCH or on Nb₂O₅ (ethylene selectivities of ca. 30 and 60 %, respectively). Factors such as the NiO–Nb⁵⁺ interaction, the NiO particle size and the properties of surface Niⁿ⁺ species were shown to determine the catalytic performance.     

Pressure-Induced Enhancement of the Photoluminescence in Ca 1−x Sr x F 2 :Mn 2+

The surprising disappearance of the Mn 2+ photoluminescence (PL) on passing from CaF 2 to SrF 2 through the Ca 1 m x Sr x F 2 :Mn 2+ series is investigated through pressure spectroscopy, and as a function of the temperature. The PL quenching which is observed along the series is explained on the basis of multiphonon relaxation mechanisms, which are described by a thermally activated process. In non-luminescent crystals, PL can be recovered by applying pressure. The results are accounted for through an universal equation relating the PL lifetime as a function of P (or V ) and T , using an activation energy and a transition rate that are strongly dependent on the lattice parameter (crystal volume).     

Synthesis and optical characterization of photoactive poly(2‐phenoxyethyl acrylate) copolymers containing azobenzene units,prepared by frontal polymerization using novel ionic liquids as initiators

2‐Phenoxyethyl acrylate (2‐PEA) was polymerized alone and in the presence of an azobenzene comonomer derived from Disperse Red‐1, N‐ethyl‐N‐(2‐hydroxyethyl)‐4‐(4‐nitrophenylazo)aniline (MDR‐1), by using the frontal polymerization technique. Two novel ionic liquids, recently synthesized by us, were used as initiators: tetrabutylphosphonium persulfate (TBPPS) and trihexyltetradecylphosphonium persulfate (TETDPPS). Even if their concentrations were smaller than those found when benzoyl peroxide and terbutylperoxy neodecanoate were used, these compounds gave rise to stable propagating polymerization fronts characterized by relatively low maximum temperatures and good velocities. Moreover, at variance to these latter, TBPPS and TETDPPS prevent bubble formation, thus allowing the use of the obtained materials in optical applications. The obtained polymers were characterized by infrared spectroscopy (FTIR), their thermal properties were determined by differential scanning calorimetry, and their optical properties were studied by absorption spectroscopy in the UV–vis region. Finally, the nonlinear optical (NLO) properties of the 2‐PEA/MDR‐1 copolymers obtained with TBPPS and TETDPPS were performed according to the Z‐Scan technique with prepared film samples. It has been proven that samples with higher MDR‐1 content (0.05 mol %) exhibited outstanding cubic NLO activity with negative NLO refractive coefficients around n₂ = ?1.7 × 10^?3 esu. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Heat flow,heat capacity and thermal resistance of localized surfaces of the human body using a new calorimetric sensor

Journal of Thermal Analysis and Calorimetry - A non-invasive sensor equipped with a programmable thermostat has been developed to assess in vivo the heat flow transmitted by conduction from human...