Electron paramagnetic resonance and the thermoluminescence emission mechanism of the 280 °C peak in natural andalusite crystal

Samples of natural andalusite (Al₂SiO₅) crystal have been investigated in terms of thermoluminescence (TL) and electron paramagnetic resonance (EPR) measurements. The TL glow curves of samples previously annealed at 600 °C for 30 min and subsequently gamma-irradiated gave rise to four glow peaks at 150, 210, 280 and 350 °C. The EPR spectra of natural samples heat-treated at 600 °C for 30 min show signals at g=5.94 and 2.014 that do not change after gamma irradiation and thermal treatments. However, it was observed that the appearance of a paramagnetic center at g=1.882 for the samples annealed at 600 °C for 30 min followed gamma irradiation. This line was attributed to Ti³⁺ centers. The EPR signals observed at g=5.94 and 2.014 are due to Fe³⁺. Correlations between EPR and TL results of these crystals show that the EPR line at g=1.882 and the TL peak at 280 °C can be attributed to the same defect center.     

13C NMR Chemical Shifts Substituent Effects of (E)- and (Z)-N-ethyl-N-Methylamides

Carbon-13 NMR chemical shifts of a series of (E)- and (Z)-N-ethyl-N-methylamides [RC(O)NEtMe, R=H, Me, Et, i-Pr, t-Bu, CF₃, ClCH₂, Cl₂CH, Cl₃C, BrCH₂, Br₂CH, Br₃C and ICH₂] are reported. The α-carbon and carbonyl carbon chemical shifts are correlated with the empirical α-substituent effect and Charton's electrical parameter ([sgrave]_I), respectively. The N-alkyl carbon resonances were attributed mainly to the γ- and δ-effects of R.     

13C NMR Chemical Shift of β-Alkoxyvinylketones: II. Empirical Substituent Effects in β-Aryl-β-Methoxyvinyltrihalomethylketones

Evaluation by empirically derived equations for the substituent effect (α,β,γ,δ) on the ¹³C NMR chemical shifts for C-1, C-2, C-3 and C-4 in β-aryl-β-methoxyvinylhalomethylketones 1a-g to 2a-g [R³C(O)-CH=C(Ar)-OMe, where R³ = CCl₃, CF₃ and Ar = p-YC₆H₄ (Y = H, Me, MeO, F, Cl, Br, NO₂)], taking as reference the β-ethoxyvinyltrichloromethylketone (3), is reported. From the calculated values for the α,β,γ,δ effects for each substituent it was possible to estimate the chemical shift of each carbon of the compounds 1,2. The ¹³C chemical shifts of the C-1, C-2, C-3, C-4 of these compounds, can be estimated with good to rasoable precision: 84% of the calculated chemical shifts are found to be within ±1.0ppm, and 100% are found to be within ±1.5ppm. The Y-Effects on C-3 and C-4 are compared with carbon charge densities (qr).     

Surface loving and surface avoiding modes

We theoretically study the propagation of sound waves in GaAs/AlAs superlattices focusing on periodic modes in the vicinity of the band gaps. Based on analytical and numerical calculations, we show that these modes are the product of a quickly oscillating function times a slowly varying envelope function. We carefully study the phase of the envelope function compared to the surface of a semi-infinite superlattice. Especially, the dephasing of the superlattice compared to its surface is a key parameter. We exhibit two kind of modes: Surface Avoiding and Surface Loving Modes whose envelope functions have their minima and respectively maxima in the vicinity of the surface. We finally consider the observability of such modes. While Surface Avoiding Modes have experimentally been observed [Phys. Rev. Lett. 97, 1224301 (2006)], we show that Surface Loving Modes are likely to be observable and we discuss the achievement of such experiments. The proposed approach could be easily transposed to other types of wave propagation in unidimensional semi-infinite periodic structures as photonic Bragg mirror.     

Application of the GIMP software in the analysis of birefringence images obtained in a multipass rheometer

Flow-induced birefringence is a technique with numerous potential applications in polymers, because the bright and dark fringe patterns of the birefringence images can be related to principal stress difference (PSD) profile along the flow centerline. In this work, we developed an easy-to-use methodology for birefringence image processing using the GIMP open-source software. In order to illustrate its application, images obtained in previous work during the flow of two grades of general purpose polystyrene through the slit-die geometry in the multipass rheometer MPR4, located at the University of ?Cambridge, UK, were reprocessed. The results showed that the PSD module profile along the flow centerline obtained with the proposed methodology is more accurate than the manual technique and is obtained with shorter processing time.     

Enantiospecific access to various C(9),C(10)-disubstituted camphors: scope and limitations

The valuable chiral sources C(9),C(10)-disubstituted camphors can be enantiospecifically obtained from the corresponding C(9)-substituted camphors by a general and straightforward synthetic method. This method involves the electrophilic treatment of a key 2-methylenenorbornan-1-ol intermediate, followed by a controlled tandem carbon-carbon double-bond addition-Wagner-Meerwein rearrangement of the norbornane framework. Discussion of the results presented suggests possible extensions and limitations of the methodology used. The feasibility of this method has been exemplified by the highly efficient enantiospecific preparation of several interesting C(9)-halogen-, C(10)-halogen, O-, S-, or Se-substituted camphors.     

Synthesis of new halo-containing acetylenes and their application to the synthesis of azoles

The convenient synthesis of ten halo- and an isoxazole-containing acetylenes from the reaction of acetylenes with n-butyl lithium and subsequent reaction with an electrophile agent (ethyl trichloroacetate, ethyl dichloroacetate, trifluoroacetic anhydride, 3-methylisoxazol-5-carbonyl chloride, carbon tetrachloride and 1,1,1-trifluoro-4-ethoxy-3-buten-2-one) in moderated to good yields is reported. The application of 1,1,1-trichloro-4-phenyl-3-butyn-2-one on the synthesis of two azoles is also showed.     

Haloacetylated enol ethers. 5 [5]. Heterocyclic ring closure reactions of β-alkoxyvinyl dichloromethyl ketones with hydroxylamine

The β-alkoxyvinyl dichloromethyl ketones 1a-d are cyclocondensed with hydroxylamine hydrochloride in pyridine to afford the 5-hydroxy-5-dichloromethyl-4,5-dihydroisoxazoles 2a-d in good yield. The cyclo-condensation of compound 1c gave, together with 2c , 3-cyano-2-hydroxy-2-dichloromethyltetrahydrofuran 5c. The dehydratation of compounds 2a,b , derived from acyclic enol ethers, with concentrated sulfuric acid at 30°, led the corresponding 5-dichloromethylisoxazoles 3a,b. The dehydratation of compounds 2c,d , derived from cyclic enol ethers, with concentrated sulfuric acid at 30°, led the bicyclic 4,5-dihydroisoxazoles 4c,d , and at 55°, a competitive rearrangement reaction gives the 3-cyano-2-hydroxy-2-dichloromethyl-2H-pyran 5d.     

Enol ethers and acetals: acylation with dichloroacetyl,acetyl and benzoyl chloride in ionic liquid medium

Synthesis of 4-alkoxy-1,1-dichloro-3-alken-2-ones [CHCl₂C(O)C(R²)C(R¹)-OR, where R, R¹, R² = Et, H, H; Me, Me, H; Et, H, Me; Me, –(CH₂)₂–; Me, –(CH₂)₃–; Et, Et, H; Et, Bu, H; Et, i-Pr, H; Et, i-Bu, H; Me, Ph, H; Me, thien-2-yl, H] from acylation of enol ethers and acetals with dichloroacetyl chloride, in ionic liquid ([BMIM][BF₄] or [BMIM][PF₆]) is reported. The synthesis of alkenones [R³–C(O)C(R²)C(R¹)-OR], where R/R¹/R²/R³ = Et/H/H/Ph, t-Bu/H/H/Ph, Me/-(CH₂)₄/Ph, Me/-(CH₂)₄/Me] from the reaction of enol ethers with benzoyl chloride or acetyl chloride, in ionic liquid [BMIM][BF₄], is also reported. Last products are described for the first time.