Allylstannation III. Synthesis of homoallylic alcohols and 4-chloro-2,6-dialkyl-3-methyltetrahydropyrans by reactions between (E/Z)-2-butenyldichloro-n-butyltin and aldehydes

2-Butenyldichloro-n-butyltin (in various cis/trans isomer ratios) reacts readily with neat RCHO (R = CH₃, C₂H₅, (CH₃)₂CH, and C₆H₅) at 25°C to give (a) linear alcohols, RCH(OH)CH₂CHCHCH₃ in the E and Z forms, (b) branched alcohols, RCH(OH)CH(CH₃)CHCH₂ in the threo and erythro forms, and (c) 2,3,4,6-tetra-substituted tetrahydropyrans (A) as a mixture of cis/trans isomers arising from the CH(CH₃)CHCl bond. The maximum yields of these tetrahydropyrans were obtained

by the use of 3–3.5 molar ratios RCHO/tin compound in the absence of solvent, whereas work-up after reactions in CH₂Cl₂ gave linear, alcohols as the main products. The formation of linear alcohols appears to be stereospecific, as the ratio of _E/_Z isomers obtained is the same as that in the organotin compound. Tetrahydropyrans are formed preferentially as the _trans isomers.     

Oxo-titanium and -zirconium diphosphines [(η-C5H5)2M(CH2PPh2]2O (with M = Ti,Az) as building blocks for heterobimetallic complexes

Oxo-titanium and -zirconium diphosphines [(η-C₅H₅)₂M(CH₂PPh₂)]₂ (with M = Ti, Zr) were synthesized and treated with [Rh(CO)₂Cl]₂ to give the heterobimetallic d⁰-d⁸ species O[(η-C₅H₅)₂M(CH₂PPh₂)]₂Rh(CO)Cl.     

Comparison of Different Sorbents in the QuEChERS Method for the Determination of Pesticide Residues in Strawberries by LC–MS/MS

A comparison of sample preparation based on the quick, easy, cheap, effective, rugged and safe (QuEChERS) method for analysis of pesticide residues in strawberries by LC–MS/MS was made using different sorbents in the clean-up by dispersive solid-phase extraction (d-SPE). Some sorbents were laboratory-made, prepared by depositing poly(methyloctadecylsiloxane) (PMODS), poly(methyloctylsiloxane) (PMOS), aminopropyl-terminated poly(dimethylsiloxane) (APPS) and copolymer of (52–48 %)dimethyl-(48–52 %)methylphenyl-siloxane (DMMPS) onto silica supports. The commercial sorbent primary–secondary amine (PSA) and mixtures of two sorbents, primary–secondary amine and poly(methyloctadecylsiloxane), were also used in the experiments. The performances of the sorbents were evaluated by parameters such as color of the final extract, gravimetric measurement, recovery and matrix effect at the fortification level of 100 ng g^?1 of the pesticide mixture in strawberries. The recoveries were in the range 70–120 %, and the RSD values were lower than 20 % for most of the pesticides using the modified QuEChERS method with different sorbents in the clean-up step. The strawberry extracts were cleaned more efficiently with the use of primary–secondary amine sorbent, which has the function of removing sugars, organic acids and especially pigments. The sample preparation method was efficient, and LC–MS/MS determination was optimal because of high selectivity and good detectivity for the multiresidue analyses.     

Kaon production at finite temperature and baryon density in an effective relativistic mean field model

We investigate the kaon production at finite temperature and baryon density by means of an effective relativistic mean-field model with the inclusion of the full octet of baryons. Kaons are considered taking into account of an effective chemical potential depending on the self-consistent interaction between baryons. The obtained results are compared with a minimal coupling scheme, calculated for different values of the anti-kaon optical potential.     

A combined theoretical and experimental study of the ultrafast photophysics of Rhodamine B

The ultrafast dynamics of zwitterionic and cationic Rhodamine B in ethanol have been investigated using TDDFT calculations and ultrafast transient absorption spectroscopy. The calculations show that the zwitterionic form exhibits an electronically excited dark state which could potentially quench the initially photoexcited state, while in the case of cationic form the lowest excited lying dark state is outside the energy region of interest and cannot explain its quenching. Due to similarities in the relaxation dynamics of the two molecules, it is suggested that the electronically excited dark state may not play such an important role in the quenching process of this dye as previously proposed. Experimental evidence presented suggests that a quenching mechanism is active on the picosecond timescale for both forms of Rhodamine B.     

Relation between configurational entropy and relaxation dynamics of glass-forming systems under volume and temperature reduction

The structural relaxation dynamics of two molecular glass-forming systems have been analyzed by means of dielectric spectroscopy, under cooling and compression conditions. The relation of the dynamic slowing down with the reduction of the configurational entropy, S_C, as predicted by Adam and Gibbs (AG), was also investigated. As S_C is not directly accessible by experiments, it was estimated, following a common procedure in literature, from the excess entropy S_exc of the supercooled liquid with respect to the crystal, determined from calorimetric and expansivity measurements over the same T–P range of dynamics investigation. The AG relation, predicting linear dependence between the logarithmic of structural relaxation time and the reciprocal of the product of temperature with configurational entropy, was successfully tested. Actually, a bilinear relation between S_exc and S_C was found, with different proportionality factors in isothermal and isobaric conditions. Using such results, we derived an equation for predicting the pressure dependence of the glass transition temperature, in good accordance with the experimental values in literature.     

Evidence for the applicability of a novel procedure (swelling-poling-deswelling) to produce a stable alignment of second order NLO-chromophores covalently attached to a cross-linked PMMA or polystyrene polymeric network

The “swelling-poling-deswelling” technique is a new procedure of poling crosslinked polymeric network carrying covalently attached NLO (nonlinear optical) chromophores. It is based upon a solvent-swollen crosslinked polymeric network before submission to poling. Under electrical poling, the matrix is deswelled without heating above the polymer Tg (glass transition temperature) obtaining a significant improvement of the stabilization of the alignment of the chromophores and therefore of the SHG (second harmonic generation). We determined the d₃₃ values of DR1 chromophore linked in different manner to PMMA (polymethylmethacrylate) and polystyrene. Crosslinked PMMA gives very good results, in fact the 40% of the d₃₃ value remains after 4 months respect to d₃₃ evaluated a couple of hours after poling. Interestingly the same stability in the time of the d₃₃ is observed with the crosslinked polystyrene matrix. In terms of d₃₃ after poling, the two systems carrying DR1 (Disperse Red 1) moieties covalently attached to the polystyrene matrix (side-chain and crosslinked) behave in a similar manner, but in terms of stability, the linear polymeric system is the best (75%), higher than all systems investigated.