All electron calculations of open-shell polyatomic molecules. III. Perturbation treatment of the spin polarization in vinyl and methyl radicals

A first order perturbation treatment starting with SCF-MO 'S in canonical or equivalent quasi-localized form is presented for the hyperfine coupling constants of vinyl and methyl radicals. The spin-polarisation contribution to hyperfine splittings is found to be large, negative for the proton of the radical center in both radicals and positive for the β protons of vinyl.     

Fisher zeros in the Kallen–Lehmann approach to 3D Ising model

The distribution of the Fisher zeros in the Kallen–Lehmann approach to three-dimensional Ising model is studied. It is argued that the presence of a non-trivial angle (a cusp) in the distribution of zeros in the complex temperatures plane near the physical singularity is realized through a strong breaking of the 2D Ising self-duality. Remarkably, the realization of the cusp in the Fisher distribution ultimately leads to an improvement of the results of the Kallen–Lehmann ansatz. In fact, excellent agreement with Monte Carlo predictions both at high and at low temperatures is observed. Besides, agreement between both approaches is found for the predictions of the critical exponent α   and of the universal amplitude ratio Δ=A₊/A₋$\Delta =A_{+}/A_{-}$, within the 3.5% and 7% of the Monte Carlo predictions, respectively.     

Pressure-induced phase transition(s) in KMnF3 and the importance of the excess volume for phase transitions in perovskite structures

We report a pressure-dependent investigation of KMnF(3) by x-ray diffraction up to 30 GPa. The results are discussed in the framework of Landau theory and in relation to the isostructural phase transition in SrTiO(3). The phase transition temperature near 186 K in KMnF(3) shifts to room temperature at a critical pressure of P(c) = 3.4 GPa; the pressure dependence of the transition point follows ΔP(c)/ΔT(c) = 0.0315 GPa K(-1). The transition becomes second order under high pressure, close to the tricritical point. The phase transition is determined by the rotation of MnF(6) octahedra with their simultaneous expansion along the rotation axis. The rotation angle was found to increase to 10.5° at 24 GPa. An additional anomaly was observed at higher pressure around 25 GPa, suggesting a further phase transition.     

Structure électronique et spectres de resonance paramagnétique électronique des radicaux vinyle et cyclopropyle

Résumé Une méthode de calcul dérivant des théories de champ self-consistant est utilisé pour l'étude des radicaux vinyle et cyclopropyle. L'énergie électronique et les écarts hyperfins dûs aux hydrogènes et au ¹³C ont été évalués en fonction de la géométrie du carbone radicalaire. L'angle C=-H du radical vinyle a été trouvé de l'ordre de 150°. Il est montré que les radicaux vinyle et cyclopropyle ont une structure électronique similaire qui ne peut être totalement expliquée ni par un modèle ni par un modèle classique, chaque radical présentant à la fois les deux caractères.

---

A method of calculation derived from self-consistent field theories is used for a study of vinyl and cyclopropyl radicals. Electronic energy, hydrogens and ¹³C hyperfine splittings are evaluated as a function of the carbon geometry. The C=-H bond angle is found to be about 150°. It is shown that vinyl and cyclopropyl radicals have a similar electronic structure which cannot be completely interpreted neither by a classical model nor by a one, each radical presenting both and character.

---

Zusammenfassung Nach einer auf dem SCF-Verfahren basierenden Methode werden Elektronenenergie, H- und ¹³C-Hyperfeinaufspaltung in Vinyl- und Cyclopropylradikalen in Abhängigkeit vom Bindungswinkel am berechnet, der sich im ersten Fall zu 150° ergibt. Es zeigt sich, daß die Elektronenstruktur der beiden behandelten Radikale ähnlich ist und daß sie weder durch ein reines - noch ein reines -Modell zu beschreiben ist.

     

Artificial neural networks for quantification in unresolved capillary electrophoresis peaks

The application of the combination of experimental design (ED) and artificial neural networks (ANNs) for the quantification of overlapped peaks in capillary zone electrophoresis is described. When the total separation cannot be achieved by separation techniques, the use of ED-ANN can be a suitable approach. The unstability of EOF causes peak shift that has to be corrected in order to apply ED-ANN methods. In this work, normalization procedure of electropherograms with consequent application of ANNs for quantification purpose was developed. Both, spectra and electropherograms can be used as multivariate data. In general, both kinds of data were found to be suitable for unresolved peaks quantification by ED-ANN approach.     

Comprehensive photokinetic and NMR study of a biphotochromic supermolecule involving two naphthopyrans linked to a central thiophene unit through acetylenic bonds

A photophysical and photochemical study of a biphotochromic compound where two naphthopyran units are linked by an acetylene-thiophene-acetylene bridge has been carried out in toluene. Both fluorescence and intersystem crossing to the triplet manifold were found to compete with the photocoloration process. Two photoproducts (transoid-trans and transoid-cis stereoisomers), absorbing at approximately 480 nm and corresponding to the opening of a single photochromic unit, were detected by spectrophotometric analysis after short irradiation time in diluted solution and identified by 1H-nuclear magnetic resonance (NMR) spectroscopy. After prolonged irradiation at 228 K of highly concentrated solutions (up to 3 x 10(-3) mol dm(-3)), two additional isomers, absorbing at approximately 550 nm, were formed. Their NMR spectra indicate the opening of both photochromic units. An interesting effect of selective vibronic excitation was found, showing that the photoreaction is favored at excited vibronic levels to the detriment of the radiative relaxation.     

Efficient BiVO4 Photoanodes by Postsynthetic Treatment: Remarkable Improvements in Photoelectrochemical Performance from Facile Borate Modification

Water‐splitting photoanodes based on semiconductor materials typically require a dopant in the structure and co‐catalysts on the surface to overcome the problems of charge recombination and high catalytic barrier. Unlike these conventional strategies, a simple treatment is reported that involves soaking a sample of pristine BiVO₄ in a borate buffer solution. This modifies the catalytic local environment of BiVO₄ by the introduction of a borate moiety at the molecular level. The self‐anchored borate plays the role of a passivator in reducing the surface charge recombination as well as that of a ligand in modifying the catalytic site to facilitate faster water oxidation. The modified BiVO₄ photoanode, without typical doping or catalyst modification, achieved a photocurrent density of 3.5 mA cm^?2 at 1.23 V and a cathodically shifted onset potential of 250 mV. This work provides an extremely simple method to improve the intrinsic photoelectrochemical performance of BiVO₄ photoanodes.     

Embedding procedure for ab initio correlation calculations in group II metals

In order to apply ab initio wave-function-based correlation methods to metals, it is desirable to split the calculation into a mean-field part and a correlation part. Whereas the mean-field part (here Hartree-Fock) is performed in the extended periodic system, it is necessary to use for the correlation part local wave-function-based correlation methods in finite fragments of the solid. For these finite entities it is necessary to construct an embedding. The authors suggest an embedding scheme which has itself no metallic character but can mimic the metal in the internal region, where the atoms are correlated. With this embedding it is also possible to localize the metallic orbitals in the central part. The long-range nonadditive contributions of metallicity and correlation are treated with the method of increments. In this paper they present different ways to construct such an embedding and discuss the influence of the embedding on the correlation energy of the solid.     

Interactions of ADP-stimulated human platelets with PEGylated polystyrene substrates prepared by surface amidation

A study primarily focused on the interactions between ADP-stimulated human platelets and PEGylated polystyrene substrates is described in this paper. The platelet–surface interactions were investigated using colorimetric acid phosphatase assay. Two types of amine-containing polymeric hydrogel materials based on poly(ethylene glycol) (PEG), H₂N–PEG–OCH₃ and H₂N–PEG–NH₂, were used to PEGylate polystyrene surfaces derivatized with maleic anhydride by amidation at alkaline pH. In addition, comparative studies using surfaces non-covalently adsorbed by bovine serum albumin (BSA) or fibrinogen (Fg) were also conducted. The assay results showed that no significant platelet adhesion was observed when PEGylated surfaces or BSA-coated surfaces were exposed to unstimulated gel-filtered platelets (GFP). However, upon ADP-stimulation, platelet adhesion to the surfaces under investigation in this study all increased to varying degrees. Most importantly, the results showed that polystyrene surfaces PEGylated using H₂N–PEG–NH₂ were most effective in resisting platelet adhesion when assays were performed using ADP-stimulated GFP. By PEGylating the surfaces of polystyrene microtiter wells via the amidation reaction described in this paper, it is demonstrated that (i) higher degree of surface PEGylation is favored at more alkaline pH and (ii) polystyrene substrates capable of more effectively resisting the adhesion of ADP-stimulated GFP can be obtained by the PEGylation reaction carried out at pH 9.1 using H₂N–PEG–NH₂.     

Unexpected halogen substituent effects on the complex thermal relaxation of naphthopyrans after UV irradiation

The kinetics of the thermal fading of four halogenated naphthopyrans (NP) have been analyzed using NMR spectroscopy in CD(3)CN. Two photomerocyanines (TT and TC) were detected after UV irradiation. The main relaxation process TC --> NP was coupled with TT/TC isomerization. The activation parameters of the various processes were rationalized by considering electronegativity and polarizability of the halogen substituents and their selective solvation by the electronegative nitrogen of acetonitrile.