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Steric, thermodynamic, and spectroscopic consequences of a stereochemically active lone pair (Lp) in Pb(II) complexes are discussed. The structure of Na-3[Pb(dota)]NO3 center dot 2H(2)O (dota = 1,4,7,10-tetraazacyclododecane-1,4,7...
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Steric, thermodynamic, and spectroscopic consequences of a stereochemically active lone pair (Lp) in Pb(II) complexes are discussed. The structure of Na-3[Pb(dota)]NO3 center dot 2H(2)O (dota = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) is reported: the four Pb-N bonds average 2.665 angstrom, while the four Pb-O bonds to the acetate groups average 2.772 angstrom. Normally, M-N bond lengths are longer than M-O bond lengths, as reported for the Sr(II) data complex. This reversal in bond length order is attributed to the Pb-O bonds being closer to a stereochemically active Lp on Pb(II). In line with the latter, a very long Pb center dot center dot center dot O contact of 3.022 angstrom is found to a water molecule coordinated over the proposed site of the Lp on Pb in its data complex. Steric control of whether the Lp is stereochemically active, that produces long Pb-N bonds close to 2.8 angstrom in length, can be exerted by ligands such as cryptands that prevent the N donors of the ligand from coordinating close together near the site on the Pb(II) opposite the Lp, the 'antipodal' site. The intense band that occurs in the electronic spectrum of Pb(II) complexes in the range 210-270 nm, that is attributed to a 6s(2) -> 6sp transition, is examined. The band shifts from 209 nm in the Pb2+(aq) ion in aqueous solution to about 260 nm as the number of N donors increases in a series of ligands containing only N and 0 donors. Within this shift it is found that in Pb(II) complexes with the same number of N donors, there is a shift to shorter wavelength, that appears to correlate with a change in the Lp from stereochemically inactive to stereochemically active. DFT calculations support the idea that the observed band in Pb(II) complexes is due to a transition from the filled 6s orbital to an empty 6p orbital, and suggest that the 6s orbital increasingly becomes hybridized with the 6p orbital as the Lp becomes more stereochemically active. The DFT calculations also support the interpretation that for complexes with the same number of N donors, the 6s(2) -> 6sp transition shifts to shorter wavelength as the Lp becomes stereochemically active. (C) 2015 Elsevier Ltd. All rights reserved.
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Neutron diffraction data, measured for two thallium germanate glass compositions, are presented and compared with previously published data for caesium germanate glasses. The measured coordination number, n_(Ge-O), for the 10 mol%...
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Neutron diffraction data, measured for two thallium germanate glass compositions, are presented and compared with previously published data for caesium germanate glasses. The measured coordination number, n_(Ge-O), for the 10 mol% Tl_2O thallium germanate glass is in good agreement with the average coordination number measured for the equivalent caesium germanate. However, while n_(Ge-O) declines for caesium germanate glasses as the amount of modifier increases above 18 mol% Cs_2O, n_(Ge-O) for the 30 mol% Tl_2O thallium germanate glass remains high with, on average, 4.40 ± 0.03 oxygen neighbours per germanium. The difference in behaviour of the germanium coordination, compared to caesium germanate glasses, arises from a change in the average thallium environment, as the role of the thallium changes from that of a modifier (similar to an alkali), to a role where some of the thallium atoms act as network formers. This is supported by ~(205)Tl NMR measurements which indicate the presence of two thallium environments.
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Tin(IV) trioxidotellurate(IV), SnTe3O8, is a member of the isotypic MIVTeIV3O8 (M = Ti, Zr, Hf, Sn) series crystallizing with eight formula units per unit cell in space group Ia\overline{3}. In comparison with the previous crystal...
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Tin(IV) trioxidotellurate(IV), SnTe3O8, is a member of the isotypic MIVTeIV3O8 (M = Ti, Zr, Hf, Sn) series crystallizing with eight formula units per unit cell in space group Ia\overline{3}. In comparison with the previous crystal structure model of SnTe3O8 based on powder X-ray diffraction data [Meunier & Galy (1971). Acta Cryst. B27, 602–608], the current model based on single-crystal X-ray data is improved in terms of precision and accuracy. Nearly regular [SnO6] octa-hedra (Sn site symmetry .\overline{3}.) are situated in the voids of an oxidotellurate(IV) framework built up by corner-sharing [TeO4] bis-phenoids (Te site symmetry 2..). A qu-anti-tative structural comparison revealed a very high degree of similarity for the structures with M = Ti, Zr, Sn in the MIVTe3O8 series.
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It is well established that the σ-bond in the elemental halogens is weak. It is likewise weak in peroxides, disulfides, interhalogens, noble gas monohalide cations, and other isoelectronic species. This is normally explained in t...
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It is well established that the σ-bond in the elemental halogens is weak. It is likewise weak in peroxides, disulfides, interhalogens, noble gas monohalide cations, and other isoelectronic species. This is normally explained in terms of lone pair-lone pair repulsion. Other explanations are given as well.
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High-pressure behavior of nitrogen trifluoride (NF_3) was investigated by Raman and IR spectroscopy at pressures up to 55 GPa and room temperature, as well as by periodic calculations up to 100 GPa. Experimentally, we find three s...
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High-pressure behavior of nitrogen trifluoride (NF_3) was investigated by Raman and IR spectroscopy at pressures up to 55 GPa and room temperature, as well as by periodic calculations up to 100 GPa. Experimentally, we find three solid-solid phase transitions at 9, 18, and 39.5 GPa. Vibrational spectroscopy indicates that in all observed phases NF_3 remains in the molecular form, in contrast to the behavior of compressed ammonia. This finding is confirmed by density functional theory calculations, which also indicate that the phase transitions of compressed NF_3 are governed by the interplay between lone-pair interactions and efficient molecule packing. Although nitrogen trifluoride is molecular in the whole pressure range studied, we show that it can be photodissociated by mid-IR laser radiation. This finding paves the way for the use of NF_3 as an oxidizing and fluorinating agent in high-pressure reactions.
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The synthesis of the hitherto unknown compound Rb_3PbCl_5 in the quasi-binary system RbCl/PbCl_2 and its structure determination is reported. This 3:1-phase decomposes peritectoidally at 305 deg C and crystallizes in a so far unkn...
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The synthesis of the hitherto unknown compound Rb_3PbCl_5 in the quasi-binary system RbCl/PbCl_2 and its structure determination is reported. This 3:1-phase decomposes peritectoidally at 305 deg C and crystallizes in a so far unknown structure type in the orthorhombic space group Pnma (a = 863.498(5) pm, b = 1573.11(1) pm, c = 838.875 pm), It shows typical structural characteristics of ns~2-configurated cations, although there are more noble gas-configurated cations than ns~2-configurated cations in the structure.
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The structure of(t-BuNH(3))(2)[TeBr6] determined by X-ray crystallography reveals the [TeBr6](2-) ion to be trigonally distorted with three long bonds of 2.761, 2.735 and 2,780 Angstrom and three short bonds of 2.609, 2.594 and 2....
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The structure of(t-BuNH(3))(2)[TeBr6] determined by X-ray crystallography reveals the [TeBr6](2-) ion to be trigonally distorted with three long bonds of 2.761, 2.735 and 2,780 Angstrom and three short bonds of 2.609, 2.594 and 2.632 Angstrom. The longer set are involved in hydrogen bonds to the t-BuNH(3)(+) cation. The symmetry of [TeBr6](2-) is close to C-3v and this model fits the vibrational spectrum which contains Te-Br stretching frequencies of 148, 163, 180 and 201 cm(-1) active in both IR and Raman spectra. Solutions of TeCl4 in HCl yield, with t-BuNH(2), the double salt (t-BuNH(3))(2)[TeCl6]. 4t-BuNH(3)Cl in which the [TeCl6](2-) ion is shown by the vibrational spectrum to have a regular octahedral (O-h) structure exhibiting the frequencies v(1) 287, v(2) 247, v(3) 230, v(4) 158 and v(5) 131 cm(-1). The spectra of (dapH(2))[TeX(6)] (dap = 1,3-diaminopropane; X = Cl or Br) are reported and examined in relation to known structures. [References: 17]
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Recently, we have investigated the energy landscape of PbS for many different pressures on the ab initio level by using Hartree-Fock and density functional theory to globally search for possible thermodynamically stable and metast...
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Recently, we have investigated the energy landscape of PbS for many different pressures on the ab initio level by using Hartree-Fock and density functional theory to globally search for possible thermodynamically stable and metastable structures. The perhaps most fascinating observation was that besides the experimentally known modification exhibiting the rock salt structure a second minimum exists close-by on the landscape showing the low-temperature α-GeTe-type structure. In the present study, we investigate the possible reasons for the existence of this metastable modification; in particular we address the question, whether the α-GeTe-type modification might be stabilized (and conversely the rock salt modification destabilized) by steric effects of the non-bonding electron pair. PbS lone pair: Energy landscape investigations of PbS for many different pressures on the ab initio level by using Hartree-Fock and DFT have shown that besides the experimentally known modification exhibiting the rock salt structure, a second minimum exists, showing the low-temperature α-GeTe-type structure (see figure). In this study, the possible reasons for the existence of this metastable modification and the steric effects of the non-bonding electron pair are investigated.
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Ethanol displays two stable conformers, the classic anti (or trans) form and a gauche conformation in which the hydroxyl hydrogen points toward one of the methyl hydrogens. Surprisingly, the two forms have nearly equal energies, a...
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Ethanol displays two stable conformers, the classic anti (or trans) form and a gauche conformation in which the hydroxyl hydrogen points toward one of the methyl hydrogens. Surprisingly, the two forms have nearly equal energies, and in the vapor phase the gauche form predominates because of its twofold degeneracy. An analysis of the energetics of these conformers based on natural bond orbital analysis helps to explain the apparently anomalous near degeneracy of these conformers.
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摘要 :
Ethanol displays two stable conformers, the classic anti (or trans) form and a gauche conformation in which the hydroxyl hydrogen points toward one of the methyl hydrogens. Surprisingly, the two forms have nearly equal energies, a...
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Ethanol displays two stable conformers, the classic anti (or trans) form and a gauche conformation in which the hydroxyl hydrogen points toward one of the methyl hydrogens. Surprisingly, the two forms have nearly equal energies, and in the vapor phase the gauche form predominates because of its twofold degeneracy. An analysis of the energetics of these conformers based on natural bond orbital analysis helps to explain the apparently anomalous near degeneracy of these conformers.
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