Search results for "physical chemistry"

Structural Evolution and Vanadium Distribution in the Preparation of V4+-ZrSiO4 Solid Solutions from Gels

2004

Vanadium-containing ZrSiO4-gel precursors with nominal compositions Vx-ZrSiO4 with x= 0.0, 0.002, 0.004, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.1, and 0.2 were prepared using a previously reported procedure and thermally treated over a range of temperature up to the formation of the V4+-ZrSiO4 solid solution. The structural evolution and the V4+ location and its homogenous distribution were followed using powder X-ray diffractometry and electron spin resonance spectroscopy (ESR). Our experiments showed that a tetragonal form of V4+-ZrO2 was the first crystalline phase obtained on heating the gels. On further heating, a phase transformation to the monoclinic form of V4+-ZrO2 took place. Final…

Comparison of hard vaporization and soft ionization techniques in the mass spectrometry of some palladium(II) and platinum(II) complexes with C-bonde…

1990

Abstract Electron impact, fast atom bombardment and laser induced vaporization techniques are compared in the mass spectrometric behaviour of some palladium(II) and platinum(II) complexes with C-bonded heterocyclic ligands of the type trans -[MCl(R N )(PPh 3 ) 2 ] [M = Pd, R N = 2-pyridyl, 1 ; 2-pyrazyl, 2; 2-pyrimidyl, 3; M = Pt, R N = 2-pyridyl, 4 ].

Low-temperature methyl group dynamics of hexamethylbenzene in crystalline and glassy matrices as studied by 2H NMR

1991

Abstract 2 H NMR spectra of hexamethylbenzene (HMB) in protonated crystalline and amorphous matrices at low temperatures are presented. All spectra reveal lineshape changes which can be attributed to methyl group tunnelling. Compared to neat HMB, a drastic increase of the tunnelling frequency is found for all systems. This indicates that the hindering potential originates predominantly from intermolecular forces. We studied the temperature dependence of these spectra and the spin-lattice relaxation in order to exclude a distribution of motional correlation times describing a thermally activated process. In addition, we find a distortion of the methyl tetrahedron.

Some observations on the fluorometric determination of metallic elements in micellar media

1989

Abstract Experimental evidence is given allowing us to interpret why the detection limits obtained in micellar media are higher than those expected according to fluorescence enhancements for the niobium-lumogallion-tartrate system in the presence of different nonionic surfactants. The residual fluorescence of the surfactant solution and the competitive interaction with micelles between complexes and free ligands were studied. Near the critical micelle concentration of surfactants, remarkable changes in the emission spectra were observed in the case of both complex and free ligand solutions, which indicate the existence of interactions between the chemical system and surfactant monomers and …

Chemical modification of porous calcium hydroxyapatite surfaces by grafting phenylphosphonic and phenylphosphite acids

2006

Abstract It is well known that the incorporation of organic molecules can provide an effective route to modify the surface properties of apatite ceramics. The present study shows that phosphonates can react with calcium ions to control the formation of the hydroxyapatite structures. The grafting of C6H5PO(OH)2 (PPOH) and C6H5PO(OH)H (PPH) was used to increase the specific surface area as well as the porosity of hydroxyapatite materials. The higher specific surface area for HAp treated by PPOH is around 267 m2/g compared to the pure HAp (140 m2/g). The phenyl (C6H5) groups are removed by thermal treatment around 500 °C. After treatment at 800 °C, organoapatites (PP-HAp) are converted to stoi…

Molecular structure of p -methylaniline and its van der Waals complexes with CF 3 H, CH 4 and CF 4 studied by laser induced fluorescence spectroscopy…

2002

The UV fluorescence excitation and dispersed fluorescence spectra of jet-cooled p-methylaniline have been obtained for the S1←S0 transition. The main spectral bands have been assigned by comparison with those of other relevant substituted benzenes and with the help of computed vibrational frequencies. The structure of p-methylaniline in both the S0 and S1 states has been studied at ab initio quantum chemical calculations at MP2 and CIS levels of theory using the 6-31+G∗ basis sets. Both low- and high-resolution laser induced fluorescence spectra of p-methylaniline van der Waals complexes with CF3H, CH4 and CF4 have been characterised following jet-expansion investigations. The equilibrium g…

Properties of new low melting point quaternary ammonium salts with bis(trifluoromethanesulfonyl)imide anion

2010

Eight new monocationic quaternary ammonium (QA) salts with the bis(trifluoromethanesulfonyl)imide (TFSI) anion were prepared by metathesis using our previously reported QA halides as precursors. New salts were characterized both in liquid and solid state using 1 H and 13 C NMR techniques, mass spectroscopy and elemental analysis together with X-ray diffraction and thermoanalytical methods. In addition, residual water content, viscosity and conductivity measurements were made for three of the roomtemperature ionic liquids (RTILs). The crystal structures of three compounds were determined by X-ray single crystal diffraction. Powder diffraction was used to study the crystallinity of the solid …

Negative thermal expansion of quartz glass at low temperatures: An ab initio simulation study

2017

Abstract Using a mixed classical Molecular dynamics (MD)/ab initio simulation scheme combined with a quasi-harmonic approximation, we calculate the linear thermal expansion coefficient αL(T) in vitreous silica glasses. The systems are first cooled down by classical MD simulations. Then they are structurally relaxed by ab initio DFT calculations. The vibrational properties are calculated employing the frozen phonon method, and these results are finally used to calculate the Helmholtz free energy as a function of volume. In agreement with experiments, our simulations predict that αL(T) is negative at low temperatures up to T ≈ 150 K. In this low-temperature regime, the simulation results are …

Überschwere nuklide I. Theoretische existenzmöglichkeiten von zerfallsreihen

1970

Enantiomēru atpazīšana kristāliskā stāvoklī: izvēlētu mazu organisku molekulu enantiomēru veidotu cieto šķīdumu strukturālā un termodinamiskā daba

2018

Darbā pētītas vairākas cietos šķīdumus veidojošas enantiomēru sistēmas. Izpētīta izvēlēto savienojumu enantiotīro un racēmisko formu kristālisko cietfāžu daudzveidība (polimorfisms un solvatomorfisms). Enantiomēru sistēmas raksturotas ar divkomponentu fāžu diagrammām, norādot uz termodinamiski stabilu un metastabilu cieto šķīdumu veidošanos pilnā un nepilnā enantiomēru sastāva apgabalā. Noteiktas arī kristāliskās struktūras, atklājot cietos šķīdumus ar enantioselektīvām un stereonespecifiskām kristalogrāfiskajām pozīcijām. Noteikta pētīto savienojumu cieto šķīdumu strukturālā daba. ENANTIOMĒRU CIETIE ŠĶĪDUMI, ENANTIOMĒRU ATPAZĪŠANA KRISTĀLISKĀ STĀVOKLĪ, DIVKOMPONENTU FĀŽU DIAGRAMMAS, KVAZI−…