Formation constants beta(2) of calcium and magnesium fluorides at 25 degrees C

Until now, scientific work on the reactions of the complexation in water of fluoride ions with calcium or magnesium ions considered only soluble order 1 complexes (CaF(+) and MgF(+)). The precipitation of the compounds CaF(2) and MgF(2) did not take into account the preliminary formation in solution of these order 2 complexes (CaF(2)(0) and MgF(2)(0)). We therefore studied their formation using a direct potentiometric method in the F(-) ion-selective electrode at 25 degrees C. This experiment, together with a computerized thermodynamic study, allowed us to determine the values of formation constants at ionic strength equal to zero: log beta (CaF (2)(0))=5.7 and log beta (CaF (2)(0))=3.2 , as well as limiting solubilities: S(CaF(2))=10(-4.8) and S(MgF(2))=10(-5) mol l(-1). Then, some examples were studied showing the necessity to take into account the formation constant of CaF(2): the supersaturation of solutions, the validity of results from the fluoride selective electrode, the difference between released fluoride in distilled water and in artificial saliva in restorative dentistry, the fluoride concentration in hard waters and its health consequences.     

Matrix-isolated Al2OF6(2-) ion in molten and solid LiF/NaF/KF

A Raman spectrum consistent with that expected from an Al2OF6(2-) ion was observed when Na2O was dissolved in a eutectic LiF/NaF/KF (FLINAK) melt at 500 degrees C, which contained a low concentration of either AlF3 or Na3AlF6. Furthermore, it was possible to trap the Al2OF6(2-) ion in the frozen solid and to measure its Raman and IR spectra at 25 degrees C. A number of bands have been detected; among those, the two most characteristic bands of the Al2OF6(2-) ion at 494 (polarized) and 265 cm-1 in the FLINAK melt at 500 degrees C, and those at 509 and 268 (Raman) and approximately 780 to approximately 900 (IR) cm-1 for the compound matrix isolated in solid FLINAK at 25 degrees C. In the absence of added oxide, the dissolved aluminum fluoride was in the form of the octahedral AlF6(3-) ion, which has characteristic Raman bands at 542 and 325 cm-1 in the FLINAK melt at 500 degrees C. Whereas alumina, Al2O3, was found to be essentially insoluble in FLINAK melts, it was possible to dissolve sufficient amounts of Na2O to convert most of the AlF6(3-) to the oxyfluoroaluminate, Al2OF6(2-). These solutions appeared to be metastable with respect to formation of insoluble alumina at higher temperatures. The present results can be compared to previous measurements on alumina dissolved in pure molten cryolite at much higher temperatures, where alumina solubility is low and broad bands due to oxide species are difficult to detect due to overlap with bands from AlF6(3-) and AlF4-.     

MgF(3)(-) as a transition state analog of phosphoryl transfer

The formation of complexes between small G proteins and certain of their effectors can be facilitated by aluminum fluorides. Solution studies suggest that magnesium may be able to replace aluminum in such complexes. We have determined the crystal structure of RhoA.GDP bound to RhoGAP in the presence of Mg(2+) and F(-) but without Al(3+). The metallofluoride adopts a trigonal planar arrangement instead of the square planar structure of AlF(4)(-). We have confirmed that these crystals contain magnesium and not aluminum by proton-induced X-ray emission spectroscopy. The structure adopted by GDP.MgF(-) possesses the stereochemistry and approximate charge expected for the transition state. We suggest that MgF3(-) may be the reagent of choice for studying phosphoryl transfer reactions.     

Structural description of the Na(2)B(4)O(7)-Na(3)AlF(6)-TiO(2) system. 1. IR and Raman study of the solidified melts

The pseudo-binary Na2B4O7-[Na3AlF6-TiO2]11 system has been investigated at room temperature by means of X-ray diffraction, IR, Raman, and UV-vis spectroscopies. Evolution of the different spectra with Na2B4O7 and TiO2 contents evidenced the breaking-up of the large borate rings in favor of small borate units, the diminution of the BIV fraction, and the partial substitution of oxygen by fluorine with the formation of oxyfluoride species. Two domains of compositions are described: a TiO2-rich region with 20-50% Na2B4O7 with the lowering of boron coordination and formation of Ti(O,F)6 units and a TiO2-poor region with 60-90% Na2B4O7 where the Na3AlF6 modifier behavior is predominant. The enhanced modifier effect of [Na3AlF6-TiO2]11 in comparison with pure Na3AlF6 on the vitreous network of Na2B4O7 consists of fluorine preference for binding to higher strength cations, Ti4+, over Al3+ and Na+ respectively, when TiO2 addition exceeds 5 wt %.     

Spectroscopic and thermal investigations of the fluoroaluminate complex formation in NaF-KF and LiF-NaF-KF eutectics

The dissolution and complex formation of fluoroaluminates in two eutectic alkalifluoride mixtures, NaF-KF (FNAK) and LiF-NaF-KF (FLINAK), have been investigated by Raman, NMR, and thermal analysis. Melting and dissolution took place stepwise. The eutectic alkalifluoride mixtures with minor amounts of dissolved fluoroaluminate salts started melting at around 460 and 740 degrees C for FLINAK and FNAK mixtures, respectively. Total melting/dissolution of mixtures with 9-11 mol % aluminum fluoro salts added took place near 780 degrees C in the FLINAK solvent and at approximately 900 degrees C for FNAK solutions. The solidified melts were characterized by Raman bands at 561 (nu(1)), 391 (nu(2)), and 328 cm(-1) (nu(5)) and a (27)Al NMR chemical shift near 0 ppm originating from isolated AlF(6)(3-) octahedral ions. The Raman and NMR signals due to AlF(6)(3-) were also observed at temperatures where the samples were only partly melted. Upon total melting, a pronounced dissociation of AlF(6)(3-) into AlF(5)(2-) and fluoride ions took place. At even higher temperatures, the equilibrium was displaced in favor of AlF(5)(2-) in the FNAK solvent. The AlF(5)(2-) ion was characterized by an intensive Raman band at 558 cm(-1) and an increasingly positive (27)Al chemical shift with raising temperature, e.g., of 16 ppm at 935 degrees C.     

The dissociation of fluoroaluminates in FLiNaK and CsF-KF molten mixtures: a Raman spectroscopic and solubility study

By using a new furnace design, M(3)AlF(6) (M = Na, K, Cs) and mixtures of small amounts of AlF(3) in FLiNaK (46.5 mol % LiF, 11.5 mol % NaF, 42 mol % KF) and CsF-KF eutectic have been investigated over a wide temperature range (25-1050 degrees C) by Raman spectroscopy. The peak positions and their relative intensities have been measured as a function of temperature. In FLiNaK, up to 750 degrees C, the bands shift gradually to lower wavenumbers, and their halfwidths increase in agreement with published data. However, it is shown from solubility measurements and Raman data that, in these conditions, the mixture is not totally molten and the spectra correspond mainly to AlF(6)(3-) in the solid state. When the mixture is completely molten, a new band appears clearly on the high-frequency side of the main band of the spectrum, and its intensity grows up when the temperature is increased. The present results are a clear confirmation of the dissociation of AlF(6)(3-) into AlF(5)(2-) and AlF(4)(-) that our study of the Raman bands of the fully melted systems MF-AlF(3) (M = Na, K, Li) previously suggested. On these systems, it is then important to know if the spectra belong mainly to solid or liquid fluoroaluminates before drawing any conclusion concerning the liquid phase structure.     

Multi-nuclear magnetic resonance study of Na3AlF6-AlPO4 molten and solidified mixtures

Phosphorus is one of the predominant impurities in the Hall-Heroult process for industrial aluminium production. The nature of the dissolved phosphorus species in the Na(3)AlF(6)-AlPO(4) system has been investigated by in situ high-temperature (HT) (19)F, (23)Na, (27)Al, (17)O, and (31)P NMR. The combination of these experiments enables to define the presence of PO(4)(3-), AlF(5)(2-) and (AlF(4)-O-PO(3))(4-) anions in the melt, and then the formation of Al-O-P bonding. Melts solidified at different cooling rates were characterised using various solid-state NMR techniques including multiple quantum magic angle spinning (MQMAS), rotational echo double resonance (REDOR) and heteronuclear single quantum correlation (HSQC). The glass obtained by the rapid quenching of the hypereutectic melt has been carefully described in order to better understand the structure of the melt.     

The Osmium(VIII) Oxofluoro Cations OsO(2)F(3)(+) and F(cis-OsO(2)F(3))(2)(+): Syntheses, Characterization by (19)F NMR Spectroscopy and Raman Spectroscopy, X-ray Crystal Structure of F(cis-OsO(2)F(3))(2)(+)Sb(2)F(11)(-), and Density Functional Theory Calculations of OsO(2)F(3)(+), ReO(2)F(3), and F(cis-OsO(2)F(3))(2)(+)

Osmium dioxide tetrafluoride, cis-OsO(2)F(4), reacts with the strong fluoride ion acceptors AsF(5) and SbF(5) in anhydrous HF and SbF(5) solutions to form orange salts. Raman spectra are consistent with the formation of the fluorine-bridged diosmium cation F(cis-OsO(2)F(3))(2)(+), as the AsF(6)(-) and Sb(2)F(11)(-) salts, respectively. The (19)F NMR spectra of the salts in HF solution are exchange-averaged singlets occurring at higher frequency than those of the fluorine environments of cis-OsO(2)F(4). The F(cis-OsO(2)F(3))(2)(+)Sb(2)F(11)(-) salt crystallizes in the orthorhombic space group Imma. At -107 degrees C, a = 12.838(3) ?, b = 10.667(2) ?, c = 11.323(2) ?, V = 1550.7(8) ?(3), and Z = 4. Refinement converged with R = 0.0469 [R(w) = 0.0500]. The crystal structure consists of discrete fluorine-bridged F(cis-OsO(2)F(3))(2)(+) and Sb(2)F(11)(-) ions in which the fluorine bridge of the F(cis-OsO(2)F(3))(2)(+) cation is trans to an oxygen atom (Os-O 1.676 ?) of each OsO(2)F(3) group. The angle at the bridge is 155.2(8) degrees with a bridging Os---F(b) distance of 2.086(3) ?. Two terminal fluorine atoms (Os-F 1.821 ?) are cis to the two oxygen atoms (Os-O 1.750 ?), and two terminal fluorine atoms of the OsO(2)F(3) group are trans to one another (1.813 ?). The OsO(2)F(3)(+) cation was characterized by (19)F NMR and by Raman spectroscopy in neat SbF(5) solution but was not isolable in the solid state. The NMR and Raman spectroscopic findings are consistent with a trigonal bipyramidal cation in which the oxygen atoms and a fluorine atom occupy the equatorial plane and two fluorine atoms are in axial positions. Density functional theory calculations show that the crystallographic structure of F(cis-OsO(2)F(3))(2)(+) is the energy-minimized structure and the energy-minimized structures of the OsO(2)F(3)(+) cation and ReO(2)F(3) are trigonal bipyramidal having C(2)(v)() point symmetry. Attempts to prepare the OsOF(5)(+) cation by oxidative fluorination of cis-OsO(2)F(4) with KrF(+)AsF(6)(-) in anhydrous HF proved unsuccessful.     

Perfluoroterephthalate bridged complexes with M-M quadruple bonds: ((t)BuCO(2))(3)M(2)(mu-O(2)CC(6)F(4)CO(2))M(2)(O(2)C(t)Bu)(3), where M = Mo or W. Studies of solid-state,molecular, and electronic structure and correlations with electronic and Raman spectral data

The compounds [((t)BuCO(2))(3)M(2)(mu-O(2)CC(6)F(4)CO(2))M(2)(O(2)C(t)Bu)(3)], M(4)PFT, where M = Mo or W, are shown by model fitting of the powder X-ray diffraction data to have an infinite "twisted" structure involving M.O intermolecular interactions in the solid state. The dihedral angle between the M(2) units of each molecule is 54 degrees. Electronic structure calculations employing density functional theory (Gaussian 98 and ADF2000.01, gradient corrected and time dependent) on the model compounds (HCO(2))(3)M(2)(mu-O(2)CC(6)F(4)CO(2))M(2)(O(2)CH)(3), where M = Mo or W, reveal that in the gas phase the model compounds adopt planar D(2)(h) ground-state structures wherein M(2) delta to bridge pi back-bonding is maximized. The calculations predict relatively small HOMO-LUMO gaps of 1.53 eV for M = Mo and 1.22 eV for M = W for this planar structure and that, when the "conjugation" is removed by rotation of the plane of the C(6)F(4) ring to become orthogonal to the M(4) plane, this energy gap is nearly doubled to 2.57 eV for M = Mo and 2.18 eV for M = W. The Raman and resonance Raman spectra of solid M(4)PFT and of Mo(4)PFT in THF solution are dominated by bands assigned to the bridging perfluoroterephthalate (pft) group. The intensities of certain Raman bands of solid W(4)PFT are strongly enhanced on changing the excitation line from 476.5 nm (off resonance) to 676.5 nm, which is on resonance with the W(2) delta --> CO(2) (pft) pi transition at ca. 650 nm. The resonance enhanced bands are delta(s)(CO(2)) (pft) at 518 cm(-)(1) and its first overtone at 1035 cm(-)(1), consistent with the structural change to W(4)PFT expected on excitation from the ground to this pi excited state. The electronic transitions for solid Mo(4)PFT (lowest at 410 nm) were not accessible with the available excitation lines (457.9-676.5 nm), and no resonance Raman spectra of this compound could be obtained. For Mo(4)PFT in THF solution, it is the band at 399 cm(-)(1) assigned to nu(MoMo) which is the most enhanced on approach to resonance with the electronic band at 470 nm; combination bands involving the C(6)F(4) ring-stretching mode, 8a, are also enhanced.     

Superelectrophilic tetrakis(carbonyl)palladium(II)- and -platinum(II) undecafluorodiantimonate(V), [Pd(CO)4][Sb(2)F(11)]2 and [Pt(CO)4][Sb(2)F(11)]2: syntheses, physical and spectroscopic properties, their crystal, molecular, and extended structures, and density functional calculations: an experimental, computational, and comparative study .

The salts [M(CO)(4)][Sb(2)F(11)](2), M = Pd, Pt, are prepared by reductive carbonylation of Pd[Pd(SO(3)F)(6)], Pt(SO(3)F)(4) or PtF(6) in liquid SbF(5), or HF-SbF(5). The resulting moisture-sensitive, colorless solids are thermally stable up to 140 degrees C (M = Pd) or 200 degrees C (M = Pt). Their thermal decompositions are studied by differential scanning calorimetry (DSC). Single crystals of both salts are suitable for an X-ray diffraction study at 180 K. Both isostructural salts crystallize in the monoclinic space group P2(1)/c (No. 14). The unit cell volume of [Pt(CO)(4)][Sb(2)F(11)](2) is smaller than that of [Pd(CO)(4)][Sb(2)F(11)](2) by about 0.4%. The cations [M(CO)(4)](2+), M = Pd, Pt, are square planar with only very slight angular and out-of-plane deviations from D(4)(h)() symmetry. The interatomic distances and bond angles for both cations are essentially identical. The [Sb(2)F(11)](-) anions in [M(CO)(4)][Sb(2)F(11)](2,) M = Pd, Pt, are not symmetry-related, and both pairs differ in their Sb-F-Sb bridge angles and their dihedral angles. There are in each salt four to five secondary interionic C- -F contacts per CO group. Of these, two contacts per CO group are significantly shorter than the sum of the van der Waals radii by 0.58 - 0.37 A. In addition, structural, and spectroscopic details of recently synthesized [Rh(CO)(4)][Al(2)Cl(7)] are reported. The cations [Rh(CO)(4)](+) and [M(CO)(4)](2+), M = Pd, Pt, are characterized by IR and Raman spectroscopy. Of the 16 vibrational modes (13 observable, 3 inactive) 10 (Pd, Pt) or 9 (Rh), respectively, are found experimentally. The vibrational assignments are supported by DFT calculations, which provide in addition to band positions also intensities of IR bands and Raman signals as well as internal force constants for the cations. (13)C NMR measurements complete the characterization of the square planar metal carbonyl cations. The extensive characterization of [M(CO)(4)][Sb(2)F(11)](2), M = Pd, Pt, reported here, allows a comparison to linear and octahedral [M(CO)(n)()][Sb(2)F(11)](2) salts [M = Hg (n = 2); Fe, Ru, Os (n = 6)] and their derivatives, which permit a deeper understanding of M-CO bonding in the solid state for superelectrophilic cations with [Sb(2)F(11)](-) or [SbF(6)](-) as anions.     

Composition Space of the (CuO, (1)/(2)Nb(2)O(5))/(HF)(x)().pyridine/H(2)O System. Structure and Synthesis of [pyH(+)](2)[CuNb(2)(py)(4)O(2)F(10)](2-) and CuNb(py)(4)OF(5)

Single crystals of [pyH(+)](2)[CuNb(2)(py)(4)O(2)F(10)](2)(-) and CuNb(py)(4)OF(5) were synthesized in a (HF)(x)().pyridine/pyridine/water solution (150 degrees C, 24 h, autogeneous pressure) using CuO and Nb(2)O(5) as reagents. The compound [pyH(+)](2)[CuNb(2)(py)(4)O(2)F(10)](2)(-) contains clusters of [CuNb(2)(py)(4)O(2)F(10)](2)(-) anions linked through N-H(+).F hydrogen bonds to the [pyH(+)] cations. In contrast CuNb(py)(4)OF(5) is a unidimensional compound consisting only of chains, perpendicular to the c axis, of alternating [Cu(py)(4)(O/F)(2/2)](0.5+) and [NbF(4)(O/F)(2/)(2)](0.5)(-) octahedra. The chains change direction between the [110] and [1&onemacr;0] every c/2. Crystal data for [pyH(+)](2)[CuNb(2)(py)(4)O(2)F(10)](2)(-): tetragonal, space group I4(1)22 (No. 98),with a = 11.408(3) ?, c = 30.36(1) ?, and Z = 4. Crystal data for CuNb(py)(4)OF(5): monoclinic, space group C2/c (No. 15), with a = 10.561(3) ?, b = 13.546(6) ?, c = 16.103(4) ?, beta = 97.77(2) degrees, and Z = 4.     

Synthesis,characterization, and computational study of the trans-IO2F5(2-) anion

The combination of CH(3)CN solutions of [N(CH(3))(4)][F] and a mixture of cis- and trans-[N(CH(3))(4)][IO(2)F(4)] produces the novel trans-IO(2)F(5)(2)(-) anion. Under the given conditions, only the trans-IO(2)F(4)(-) anion acts as a fluoride ion acceptor, thus allowing the separation of isomerically pure, soluble cis-IO(2)F(4)(-) from insoluble trans-IO(2)F(5)(2)(-). The trans-IO(2)F(5)(2)(-) and cis-IO(2)F(4)(-) anions were characterized by infrared and Raman spectroscopy and theoretical calculations at the LDFT and HF levels of theory. The trans-IO(2)F(5)(2)(-) anion has a pentagonal-bipyramidal geometry with the two oxygen atoms occupying the axial positions. It represents the first example of a heptacoordinated main group AO(2)X(5) species and completes the series of pentagonal-bipyramidal iodine fluoride and oxide fluoride species. The geometries of the pentagonal-bipyramidal series IO(2)F(5)(2)(-), IOF(5)(2)(-), IF(5)(2)(-), IOF(6)(-), IF(6)(-), and IF(7) and the corresponding octahedral series IO(2)F(4)(-), IOF(4)(-), IF(4)(-), IOF(5), IF(5), and IF(6)(+) were calculated by identical methods. It is shown how the ionic charge, the oxidation state of the iodine atom, the coordination number, and the replacement of fluorine ligands by either an oxygen ligand or a free valence electron pair influence the stuctures and bonding of these species.     

The reaction of Li[Al(OR)4] R = OC(CF3)2Ph, OC(CF3)3 with NO/NO2 giving NO[Al(OR)4], Li[NO3] and N2O. The synthesis of NO[Al(OR)4] from Li[Al(OR)4] and NO[SbF6] in sulfur dioxide solution

NO[Al(OC(CF(3))(2)Ph)(4)] 1 and NO[Al(OC(CF(3))(3))(4)] 2 were obtained by the metathesis reaction of NO[SbF(6)] and the corresponding Li[Al(OR)(4)] salts in liquid sulfur dioxide solution in ca 40% (1) and 85% (2) isolated yield. 1 and 2, as well as Li[NO(3)] and N(2)O, were also given by the reaction of an excess of mixture of (90 mol%) NO, (10 mol%) NO(2) with Li[Al(OR)(4)] followed by extraction with SO(2). The unfavourable disproportionation reaction of 2NO(2)(g) to [NO](+)(g) and [NO(3)](-)(g)[DeltaH degrees = +616.2 kJ mol(-1)] is more than compensated by the disproportionation energy of 3NO(g) to N(2)O(g) and NO(2)(g)[DeltaH degrees =-155.4 kJ mol(-1)] and the lattice energy of Li[NO(3)](s)[U(POT)= 862 kJ mol(-1)]. Evidence is presented that the reaction proceeds via a complex of [Li](+) with NO, NO(2)(or their dimers) and N(2)O. NO(2) and Li[Al(OC(CF(3))(3))(4)] gave [NO(3)(NO)(3)][Al(OC(CF(3))(3))(4)](2), NO[Al(OC(CF(3))(3))(4)] and (NO(2))[Al(OC(CF(3))(3))(4)] products. The aluminium complex [Li[AlF(OC(CF(3))(2)Ph)(3)]](2) 3 was prepared by the thermal decomposition of Li[Al(OC(CF(3))(2)Ph)(4)]. Compounds 1 and 3 were characterized by single crystal X-ray structural analyses, 1-3 by elemental analyses, NMR, IR, Raman and mass spectra. Solid 1 contains [Al(OC(CF(3))(2)Ph)(4)](-) and [NO](+) weakly linked via donor acceptor interactions, while in the SO(2) solution there is an equilibrium between the associated [NO](+)[Al(OC(CF(3))(2)Ph)(4)](-) and separated solvated ions. Solid 2 contains essentially ionic [NO](+) and [Al(OC(CF(3))(3))(4)](-). Complex 3 consists of two [Li[AlF(OC(CF(3))(2)Ph)(3)]] units linked via fluorine lithium contacts. Compound 1 is unstable in the SO(2) solution and decomposes to yield [AlF(OC(CF(3))(2)Ph)(3)](-), [(PhC(CF(3))(2)O)(3)Al(mu-F)Al(OC(CF(3))(2)Ph)(3)](-) anions as well as (NO)C(6)H(4)C(CF(3))(2)OH, while compound 2 is stable in liquid SO(2). The [small nu](NO(+)) in 1 and [NO](+)(toluene)[SbCl(6)] are similar, implying similar basicities of [Al(OC(CF(3))(2)Ph)(4)](-) and toluene.     

Synthesis and characterization of trifluoromethyl fluoroformyl peroxycarbonate,CF3OC(O)OOC(O)F

The synthesis of CF(3)OC(O)OOC(O)F is accomplished by the photolysis of a mixture of (CF(3)CO)(2)O, FC(O)C(O)F, CO, and O(2) at -15 degrees C using a low-pressure mercury lamp. The new peroxide is obtained in pure form in low yield after repeated trap-to-trap condensation and is characterized by NMR, IR, Raman, and UV spectroscopy. Geometrical parameters were studied by ab initio methods [B3LYP/6-311+G(d)]. At room temperature, CF(3)OC(O)OOC(O)F is stable for many days in the liquid or gaseous state. The melting point is -87 degrees C, and the boiling point is extrapolated to 45 degrees C from the vapor pressure curve log p = 8.384 - 1715/T (p/mbar, T/K). A possible mechanism for the formation of CF(3)OC(O)OOC(O)F is discussed, and its properties are compared with those of related compounds.     

Fluoride-ion acceptor properties of WSF4: synthesis, characterization, and computational study of the WSF5(-) and W2S2F9(-) anions and 19F NMR spectroscopic characterization of the W2OSF9(-) anion

The new [N(CH(3))(4)][WSF(5)] salt was synthesized by two preparative methods: (a) by reaction of WSF(4) with [N(CH(3))(4)][F] in CH(3)CN and (b) directly from WF(6) using the new sulfide-transfer reagent [N(CH(3))(4)][SSi(CH(3))(3)]. The [N(CH(3))(4)][WSF(5)] salt was characterized by Raman, IR, and (19)F NMR spectroscopy and [N(CH(3))(4)][WSF(5)]·CH(3)CN by X-ray crystallography. The reaction of WSF(4) with half an aliquot of [N(CH(3))(4)][F] yielded [N(CH(3))(4)][W(2)S(2)F(9)], which was characterized by Raman and (19)F NMR spectroscopy and by X-ray crystallography. The WSF(5)(-) and W(2)S(2)F(9)(-) anions were studied by density functional theory calculations. The novel [W(2)OSF(9)](-) anion was observed by (19)F NMR spectroscopy in a CH(3)CN solution of WOF(4) and WSF(5)(-), as well as CH(3)CN solutions of WSF(4) and WOF(5)(-).     

Raman study of the hexafluoroaluminate ion in solid and molten FLINAK

Raman spectra have been obtained for matrix-isolated AlF6(3-) in an LiF/NaF/KF (FLINAK) eutectic mixture. Three Raman bands characteristic of the hexafluoroaluminate ion were identified in the solids formed from FLINAK melts which contained small amounts (5-11 mol%) of either AlF3 or Na3AlF6. The three allowed Raman-active bands of the matrix-isolated octahedral complex ion, nu 1(A1g), nu 2(Eg), and nu 5(F2g), were observed at 560.5, 380, and 325 cm-1, respectively, for the solid sample at 25 degrees C. Wavenumbers and relative intensities were similar to those of Na3AlF6 (cryolite), K3AlF6, and K2NaAlF6 (elpasolite) and other crystals known to contain discrete, octahedral AlF6(3-) ions. Peak positions, half-widths, and relative intensities for the bands were measured for samples at temperatures different from room temperature through the melting transition and into the molten state. The transition from high-temperature solid to molten salt at about 455 degrees C occurred gradually without perceptible change in the peak positions, half-widths, or relative intensities. For a sample in molten FLINAK at 455 degrees C, the nu 1(A1g), nu 2(Eg), and nu 5(F2g) modes of the AlF6(3-) ion were observed at 542, 365, and 324 cm-1, respectively. Raman depolarization experiments were consistent with these assignments, and the low value of the depolarization ratio of the nu 1(A1g) mode at 542 cm-1 indicated that the sample was molten above 455 degrees C. Differential thermal analysis also indicated that the FLINAK samples melted at about 455 degrees C. Raman measurements were performed for samples at temperatures from 25 to 600 degrees C in a silver dish, on a hot stage, in an argon-filled atmosphere, under a microscope. Additional Raman experiments were performed on samples at temperatures from 25 to 750 degrees C in a conventional graphite windowless cell, in an argon-filled quartz tube, in a standard furnace. Over the concentration range 4.8-11 mol% AlF3 (CR 23-8.0) in FLINAK, only bands due to the AlF6(3-) ion were detected. There was no evidence to support the presence of other aluminum complexes in these melts.     

Reaction growth of MF2/a-C (M = Ca, Mg) core/shell nanowires at the interface of vapor and solid reactants

C(6)F(6) vapor is employed to react with CaC(2) and Mg(3)N(2) to grow CaF(2)/a-C and Mg(2)F(2)/a-C core/shell nanowires (tens of micrometers in length, tens to hundreds of nanometers in wire diameter, and tens of nanometers in core diameter), respectively, in high yields. The growth mechanism is proposed to proceed via a reaction at the interface of the vapor and solid reactants.     

Determination of the rate constants for the NH2(X2B1) + NH2(X2B1) and NH2(X2B1) + H Recombination reactions with collision partners CH4, C2H6, CO2, CF4, and SF6 at low pressures and 296 K. Part 2

The recombination rate constants for the reactions NH2(X2B1) + NH2(X2B1) + M → N2H4 + M and NH2(X2B1) + H + M → NH3 + M, where M was CH4, C2H6, CO2, CF4, or SF6, were measured in the same experiment over presseure ranges of 1-20 and 7-20 Torr, respectively, at 296 ± 2 K. The NH2 radical was produced by the 193 nm laser photolysis of NH3. Both NH2 and NH3 were monitored simultaneously following the photolysis laser pulse. High-resolution time-resolved absorption spectroscopy was used to monitor the temporal dependence of both species: NH2 on the (1)2(21) ← (1)3(31) rotational transition of the (0,7,0)A2A1 ← (0,0,0)X2B1 electronic transition near 675 nm and NH3 in the IR on either of the inversion doublets of the qQ3(3) rotational transition of the ν1 fundamental near 2999 nm. The NH2 self-recombination clearly exhibited falloff behavior for the third-body collision partners used in this work. The pressure dependences of the NH2 self-recombination rate constants were fit using Troe’s parametrization scheme, k(inf), k(0), and F(cent), with k(inf) = 7.9 × 10(-11) cm3 molecule(-1) s(-1), the theoretical value calculated by Klippenstein et al. (J. Phys. Chem. A113, 113, 10241). The individual Troe parameters were CH4, k(0)(CH4) = 9.4 × 10(-29) and F(cent)(CH4) = 0.61; C2H6, k(0)(C2H6) = 1.5 × 10(-28) and F(cent)(C2H6) = 0.80; CO2, k(0)(CO2) = 8.6 × 10(-29) and F(cent)(CO2) = 0.66; CF4, k(0)(CF4) = 1.1 × 10(-28) and F(cent)(CF4) = 0.55; and SF6, k(0)(SF6) = 1.9 × 10(-28) and F(cent)(SF6) = 0.52, where the units of k0 are cm6 molecule(-2) s(-1). The NH2 + H + M reaction rate constant was assumed to be in the three-body pressure regime, and the association rate constants were CH4, (6.0 ± 1.8) × 10(-30); C2H6, (1.1 ± 0.41) × 10(-29); CO2, (6.5 ± 1.8) × 10(-30); CF4, (8.3 ± 1.7) × 10(-30); and SF6, (1.4 ± 0.30) × 10(-29), with units cm6 molecule(-1) s,(-1) and the systematic and experimental errors are given at the 2σ confidence level.     

19F NMR study of the equilibria and dynamics of the Al3+/F- system

A careful reinvestigation by high-field 19F NMR (470 MHz) spectroscopy has been made of the Al3+/F- system in aqueous solution under carefully controlled conditions of pH, concentration, ionic strength (I), and temperature. The 19F NMR spectra show five distinct signals at 278 K and I = 0.6 M (TMACl) which have been attributed to the complexes AlFi(3-i)+(aq) with i < or = 5. There was no need to invoke AlFi(OH)j(3-i-j)+ mixed complexes in the model under our experimental conditions (pH < or = 6.5), nor was any evidence obtained for the formation of AlF6(3-)(aq) at very high ratios of F-/Al3+. The stepwise equilibrium constants obtained for the complexes by integration of the 19F signals are in good agreement with literature data given the differences in medium and temperature. In I = 0.6 M TMACl at 278 K and in I = 3 M KCl at 298 K the log Ki values are 6.42, 5.41, 3.99, 2.50, and 0.84 (for species i = 1-5) and 6.35, 5.25, and 4.11 (for species i = 1-3), respectively. Disappearance of the 19F NMR signals under certain conditions was shown to be due to precipitation. Certain 19F NMR signals exhibit temperature- and concentration-dependent exchange broadening. Detailed line shape analysis of the spectra and magnetization transfer measurements indicate that the kinetics are dominated by F- exchange rather than complex formation. The detected reactions and their rate constants are AlF2(2+) + *F- reversible AlF*F2+ + F- (k02 = (1.8 +/- 0.3) x 10(6) M-1 s-1), AlF3(0) + *F- reversible AlF2*F0 + F- (k03 = (3.9 +/- 0.9) x 10(6) M-1 s-1), and AlF3(0) + H*F reversible AlF2*F0 + HF (kH03 = (6.6 +/- 0.5) x 10(4) M-1 s-1). The rates of these exchange reactions increase markedly with increasing F- substitution. Thus, the reactions of AlF2+(aq) were too inert to be detected even on the T1 NMR time scale, while some of the reactions of AlF3(0)(aq) were fast, causing large line broadening. The ligand exchange appears to follow an associative interchange mechanism. The cis-trans isomerization of AlF2+(aq), consistent with octahedral geometry for that complex, is slowed sufficiently to be observed at temperatures around 270 K. Difference between the Al3+/F- system and the much studied Al3+/OH- system are briefly commented on.