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41.
42.
Mario A. Gomez Jean‐Francois Le Berre Hassane Assaaoudi George P. Demopoulos 《Journal of Raman spectroscopy : JRS》2011,42(1):62-71
The Raman spectra of synthetic compounds equivalent to the variscite group: FeAsO4·2H2O AlAsO4·2H2O, GaAsO4·2H2O, and InAsO4·2H2O are reported. In particular, upon comparison of FeAsO4·2H2O to AlAsO4·2H2O, it is observed that the Type II (weak) H‐bond lengths in the latter are slightly longer, which is postulated to affect the stability (As release) in water at pH 5 and 7. Arsenate stretching and bending vibrations were found to be distinct in terms of spectral structure and therefore well suited for fingerprinting. The calculated As O bond strengths from existing crystallographic data showed no significant variations. The strongest ν1 (AsO43−) stretch was used to monitor the As O bonding interactions in the four As O M units, where a shift of 114 cm−1 was observed in the order FeAsO4·2H2O (lowest) < InAsO4·2H2O < GaAsO4·2H2O < AlAsO4·2H2O (highest); this order also followed exactly the measured arsenic release of these phases. This shift in ν1 (AsO43−) position was rationalized to stem from the differences in the electronegativities of the M3+ cations. The trends mentioned above were verified and found to also hold for the isostructural phosphate analogues strengite (FePO4·2H2O) and variscite (AlPO4·2H2O) using published data. Therefore, it is postulated that, as observed with the stability of solution complexes, there may be a correlation between the electronegativity of the M3+ cation in these isostructural phases and their stability (As or P release) in water. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
43.
C.A.A. GhummanA.M.C. Moutinho A. SantosO.M.N.D. Teodoro A. Tolstogouzov 《Applied Surface Science》2012,258(7):2490-2497
A TOF-SIMS VG Ionex IX23LS with upgraded data acquisition and control system was used to study the secondary emission of negative atomic and cluster ions of non-metallic elements (P, As and Sb) upon a 19 keV Ga+ bombardment of non-degenerated III-V semiconductors (GaP, GaAs, GaSb, InP, InAs and InSb) with prior neutral Cs deposition from a getter dispenser. It was found that surface cesiation enhances the peak intensity of all negative ion species; in the case of atomic ions, the greatest increase (360) was observed for P− emitted from InP. Such an enhancement was larger for In-based than for Ga-based compounds. We explained that in terms of an electronegativity difference between the composing atoms of III-V semiconductors. The greater electronegativity difference (bond ionicity) of In-based compounds resulted in the greater Cs-induced work function decrease leading to a higher increase in the ionization probability of secondary ions. 相似文献
44.
Serena Bigotti Alessandro Volonterio Matteo Zanda 《Journal of fluorine chemistry》2008,129(9):767-774
We describe in full-detail the synthesis of new ψ[CH(RF)NH]-peptidomimetics, having different fluoroalkyl groups RF, as peptide bond surrogates. A key step in the synthesis is a stereoselective aza-Michael addition of chiral α-amino acid esters to β-fluoroalkyl-α-nitroethenes. The diastereoselection of the process was influenced by the electronegativity, rather than by the steric bulk, of the fluorinated residue RF in the β-position of the nitroalkene acceptors. Replacement of a single F atom of RF by a hydrogen or methyl group brings about a dramatic drop of stereocontrol, whereas Br, Cl and CF3, albeit bulkier than F, provide inferior results in terms of stereocontrol. A mechanistic hypothesis is provided. 相似文献
45.
Basudeb Karmakar 《Journal of solid state chemistry》2005,178(9):2663-2672
Infrared reflection spectroscopic (IRRS), ultraviolet-visible-near infrared (UV-Vis-NIR) absorption and photoluminescence upconversion properties with special emphasis on the spectrochemistry of the oxyfluorophosphate (oxide incorporated fluorophosphates) glasses of the Ba(PO3)2-AlF3-CaF2-SrF2-MgF2-Ho2O3 system have been studied with different concentrations (0.1, 0.3 and 1.0 mol%) of Ho2O3. IRRS spectral band position and intensity of Ho3+ ion doped oxyfluorophosphate glasses have been discussed in terms of reduced mass and force constant. UV-Vis-NIR absorption band position has been justified with quantitative calculation of nephelauxetic parameter and covalent bonding characteristics of the host. NIR to visible upconversion has been investigated by exciting at 892 nm at room temperature. Three upconverted bands originated from the 5F3→5I8, (5S2, 5F4)→5I8 and 5F5→5I8 transitions have found to be centered at 491 nm (blue, medium), 543 nm (green, very strong) and 658 nm (red, weak), respectively. These bands have been justified from the evaluation of the absorption, normal (down conversion) fluorescence and excitation spectra. The upconversion processes have been explained by the excited state absorption (ESA), energy transfer (ET) and cross relaxation (CR) mechanisms involving population of the metastable (storage) energy levels by multiphonon deexcitation effect. It is evident from the IRRS study that the upconversion phenomena are expedited by the low multiphonon relaxation rate in oxyfluorophosphate glasses owing to their high intense low phonon energy (∼600 cm−1) which is very close to that of fluoride glasses (500-600 cm−1). 相似文献
46.
New chemical bonding paradigm in terms of chemical action functional and of its reformulations by means of electronegativity, linear response and density softness kernels is advanced; it makes no use of traditional molecular orbital bonding analysis while providing reliability in identifying the bonding regions through appropriate specialization of the chemical action variational (conservation) principle along the bond length. 相似文献
47.
The paradigm that the properties of the atoms determine the properties of the molecules that they form is systematically presented. To this end, three types of atomic properties are differentiated: (a) those that can be determined directly by spectroscopy, (b) those based on theoretical concepts, and (c) those that can be assigned to the atoms interacting in molecules. On the basis of the electronegativity values of the atoms, which can be determined from spectroscopic data together with the assumption that the electronegativities are equalized in bonds, partial charges of the atoms in molecules are determined. These partial charges are correlated with ESCA data and proton affinities. In addition, simple expressions are given for the reliable estimation of bond lengths, bond energies, and force constants. For corrigendum see DOI: 10.1002/anie.199607811 相似文献
48.
J. Portier H.S. Hilal I. Saadeddin S.J. Hwang M.A. Subramanian G. Campet 《Progress in Solid State Chemistry》2004,32(3-4):207-217
Thermodynamic parameters and energy-band gap of a given metal oxide, are suitable measures of its stability. For this reason, direct correlations between energy-band gap or chemical hardness, and thermodynamic parameters (e.g. standard enthalpy of formation) have been constructed for different metal oxides. Furthermore, a simple relationship to calculate values of energy-band gaps, for metal oxides, from values of their enthalpies of formation, is presented here. When tested, an appreciable number of metal oxides from s- , p- and d-blocks well obeyed the relationship, while a number of metal oxides deviated. A qualitative theoretical account for such different behaviors is presented here. 相似文献
49.
A pair of novel molecular indices has been proved to contain important information on the coupling between atomic displacement and electronic properties based on the electron density function within the Density Functional Theory: the nuclear reactivity (Φ) and nuclear stiffness (G). Appropriate calculation procedure has been developed and their role in describing anharmonicity of diatomic molecules has been demonstrated. This present work provides analysis of this effect for small molecules, unveiling the role of symmetry of molecular vibrational modes in modifying the affinity of a molecule to intermolecular electron transfer. The indices have been found to be a crucial factor determining thermal fluctuations in the molecular energy derivatives: electronegativity (χ) and hardness (η). The fluctuations of hardness play a specific role, as they bring a molecule uniquely to a critical region (η0), when molecule becomes unstable to an electron exchange process, due to its excitation in a selected destructive vibrational mode. 相似文献
50.
本文研究了五种有机锡化合物的光电子能谱图,讨论了影响锡原子内层d轨道电子结合能化学位移的几种因素,比如,取代基的影响,基团电负性的影响。 相似文献