Mayr electrophilicity predicts the dual Diels-Alder and sigma-adduct formation behaviour of heteroaromatic super-electrophiles

We report on the dual reactivity, i.e. anionic Meisenheimer sigma adduct formation and Diels-Alder adduct formation, of a series of heteroaromatic super-electrophiles, including 4,6-dinitro-benzofuroxan, -N-arylbenzotriazoles (4), -benzothiadiazole and -benzoselenadiazole. Measured pK(a)(H(2)O) values for sigma adduct formation provide a quantitative measure of super-electrophilic reactivity with a satisfactory correlation between the Mayr E electrophilicity parameter and pK(a)(H(2)O): E = -0.662 pK(a)(H(2)O) (or pK(R+) -3.20 (r(2) = 0.987). The most highly electrophilic, pre-eminent super-electrophile is 4,6-dinitrotetrazolopyridine (E = -4.67, pK(a)(H(2)O) = 0.4), which supercedes the reference Meisenheimer super-electrophile, 4,6-dinitrobenzofuroxan (E = -5.06, pK(a) = 3.75), having itself an E value superior by 8 orders of magnitude compared to 1,3,5-trinitrobenzene as the benchmark normal Meisenheimer electrophile (E = -13.19, pK(a)(H(2)O) = 13.43). (For relevant kinetic parameters as well as E and pK(a) values, see .) In a parallel study we have investigated Diels-Alder (normal and inverse electron demand) reactivity of this series of heteroaromatic electrophiles and have shown that Mayr E values are valid predictors of whether DA adducts will form and how rapidly. The observed order of pericyclic reactivity corresponds to E = -8.5 as the demarcation E value, in close agreement with sigma complexation; thus pointing to a common origin for the two processes, i.e. an inverse relationship between the degree of aromaticity of the carbocyclic ring and ease of sigma complexation, or DA reactivity, respectively.     

Structural, morphological and photoluminescence properties of W-doped ZnO nanostructures

W-doped ZnO nanostructures were synthesized at substrate temperature of 600 °C by pulsed laser deposition (PLD), from different wt% of WO₃ and ZnO mixed together. The resulting nanostructures have been characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy and photoluminescence for structural, surface morphology and optical properties as function of W-doping. XRD results show that the films have preferred orientation along a c-axis (0 0 L) plane. We have observed nanorods on all samples, except that W-doped samples show perfectly aligned nanorods. The nanorods exhibit near-band-edge (NBE) ultraviolet (UV) and violet emissions with strong deep-level blue emissions and green emissions at room temperature.     

Cross section and rate coefficient calculations for electron impact excitation, ionisation and dissociation of the X 1Σg+, c 3Πu, a 3Σ g+, e 3Σu+ and B′1Σu+ states of H2

The weighted total cross section (WTCS) theory has been applied to the electron-H₂ collision to obtain excitation, ionisation and dissociation cross section and rate coefficients of the X ¹S_g⁺^{1}\!\Sigma _{g}^{+}, c ³P_u^{3}\!\Pi _{u}, a ³S_g⁺^{3}\!\Sigma _{g}^{+}, e $^{3}\!\Sigma _{u}^{+}$^{3}\!\Sigma _{u}^{+} and B^{′ 1}S_u⁺^{1}\!\Sigma _{u}^{+} states. Calculation has been performed in the temperature range 1500 K–15000 K. Rate coefficients are calculated from WTCS assuming Maxwellian energy distribution functions for electrons and heavy particles. Thermal equilibrium results are presented and fitting parameters (a, b and c) are given for each reaction rate coefficient: k(θ) = a (θ^b) exp(-c/θ).     

Le Nombre D’images Homomorphes de Certains Groupes et Algèbres de Golod

We investigate homomorphic images of Golod algebras. We prove that some quotients of certain Golod algebras are Golod algebras too. We determine classes of Golod algebras and nilalgebras not satisfying Golod properties which have a continuum of non isomorphic homomorphic images. We establish analogous results for Lie algebras and p-groups of Golod type.     

On the n—nilpotency

In this paper we prove that, for every integer n ≥ 1, d ≥ m ≥ 2, the sum of m n- nilpotent ideals of an algebra may not be d- nilpotent. This leads to a similar result in group theory; the product of m n- nilpotent normal subgroups may not be d- nilpotent. We apply these results to solve Rozhkov’s question by constructing some n- finite, infinite p- groups generated by n + 1 conjugates.     

DFT study of the structure of hydroxybenzoic acids and their reactions with OH and radicals

B3LYP/6-31++G(d) method was used for the structural study of 3,4,5-trihydroxybenzoic acid (3,4,5-THBA), 3,4-dihydroxybenzoic acid (3,4-DHBA), and 4-hydroxybenzoic acid (4-HBA). Calculated structures agree with available X-ray experimental data within 2%. The phenolic OH bond dissociation enthalpy (BDE) of all sites in each benzoic acid were determined and compared with those of phenol (for 4-HBA) and catechol (for 3,4-DHBA). The consistency between the calculated values and experimental ones are within 5.4% and 9.2%, respectively, for 4-HBA and 3,4-DHBA. The reactions of benzoic acids with and OH radicals were studied and it turns out that benzoic acids react differently with both radicals. We have shown that the reaction of hydroxybenzoic acids with the hydroxyl radical was governed by a phenolic hydrogen (H⁺ + e⁻) transfer from the acid to the radical, while for the superoxide radical, the reaction is governed by a proton (H⁺) transfer from the acid to the radical. The first reaction is evidenced by the delocalization of the radical on the entire quinone moiety, and the second reaction is evidenced by the negative NBO charge on the phenoxide moiety as well as the localization of the radical on the hydroperoxy (O₂H) moiety.     

On restarted and deflated block FOM and GMRES methods for sequences of shifted linear systems

Numerical Algorithms - The problem of shifted linear systems is an important and challenging issue in a number of research applications. Krylov subspace methods are effective techniques for...     

Photoluminescence and energy transfer of Tm doped LiIn (WO4)2 blue phosphors

Room temperature steady and time resolved emission spectra of LiIn_1−xTm_x(WO4)2 (where thulium concentration is 0, 0.5, 1, 5 and 10 at%) blue phosphors, under UV excitation energy have been investigated. The concentration quenching effect on the blue emission, due to the (WO₄)⁻² groups and ¹G₄→³H₆ emission transition of Tm³⁺ were studied. Two energy transfer mechanisms are shown. The first takes place between excited (WO₄)⁻² groups and the ¹G₄ energy level of Tm³⁺, and is mainly analyzed by phonon-assisted energy transfer. The second mechanism is due to an energy transfer from the excited Tm³⁺ ions to the surrounding ground state Tm³⁺ ions. The non-exponential decay curves of the ¹G₄ level observed for higher concentrations are analyzed by the Inokuti–Hirayama model. We think that the quenching effect between Tm³⁺ ions is mainly linked to the dipole–dipole interactions.