Lars A. Bengtsson

The structure of room temperature molten polyiodides

The structure of molten polyiodides, Et3SI x (l) (Et = ethyl; x = 3, 4, 5 and 7), have been investigated by liquid x-ray scattering and Raman spectroscopy techniques at room temperature. The predominant iodine-containing species in Et3SI3(l) is a centrosymmetric I3 - ion with a closest I-I distance of 2·915(2) A. The Raman spectra indicate a large bond flexibility of the triiodide ion. The structural results of the iodine-rich melts Et3SI x (l), x > 3, are consistent with a three-dimensional network of interconnected I3 - ions and I2 molecules. The short-range order bears close similarities to that of pure liquid and solid iodine. The triiodides are on the average solvated by one, two and three iodine molecules for x = 4, 5 and 7, respectively. The coordination mode is flat-on with the I3 - and I2 units almost parallel but slightly tilted away from each other like the nearest-neighbour contacts in pure iodine. Previously published conductivity results are consistent with such a structure model.

Formation and structure of mono- and di-bismuth hydroxide and fluoride complexes in molten NH4NO3· 1.5H2O at 50 °C

The formation of bismuth hydroxide and fluoride complex species in NH4NO3· 1.5H2O has been investigated by potentiometric, NMR and Raman spectroscopy and large-angle X-ray scattering (LAXS) methods at 50 °C. Complexes with the formal composition BiOH2+, Bi2OH5+, BiF2+ and Bi2F5+ were detected. The stability constants, as derived from potentiometry, are; β*11[BiOH2+]=(5.6 ± 0.1)× 10–3 mol kg–1, β*21[Bi2OH5+]=(3.37 ± 0.02)× 10–2, β11[BiF2+]=(1.54 ± 0.02)× 104 kg mol–1 and β21[Bi2F5+]=(7.3 ± 0.5)× 103 kg2 mol–2. 19F NMR shift studies indicate that the predominant contribution to the Bi–F bonding character is most likely to be electrostatic. 14N NMR and Raman spectroscopic experiments on a number of compositions in the system Bi(NO3)3· 4.5H2O–(H,NH4)(F,NO3)· 1.5H2O indicate the presence of nitrate ions convalently bonded to bismuth. The Raman spectra show a loss of degeneracy of the internal ν3 and ν4 nitrate vibration modes in the 1300 and 700 cm–1 regions, respectively, and a residual contribution to the ν1 band of NO3– at 1048 cm–1 due to direct bismuth–nitrate interactions. A weak polarized band at 240 cm–1 is assigned to a Bi—O2NO stretching vibration. Intensity correlations in the 1050 cm–1 region yield a coordination number of four for the bismuth nitrate complex in an acidic melt. The quantitative analysis suggests that nitrate ions are released upon formation of bismuth hydroxide and fluoride complexes. Results of the structural investigation on the [Bi(NO3)4(OH2)2]– complex, supported by ab initio calculations, reveal a slightly asymmetric bidentate coordination of NO3–., dBi–O(NO3)= 2.52, 2.69 and 4.30 A, dBi–N(NO3)= 3.08 A. Rather long Bi—OH2 contacts (2.90 A) were obtained. The Bi—Bi distances in Bi2OH5+ Bi2F5+ are 3.70 A, implying a bent structure (C2v symmetry). The large Bi—Bi separations and negligible Bi—Bi orbital overlap (as revealed by ab initio calculations) indicate that these complexes may be stabilized by bridging nitrate ions. Approximate MO calculations suggest that a monodentate bridging configuration is the most stable one.

Structural studies on volatile, air and moisture sensitive liquids at ambient and elevated temperatures using liquid X-ray scattering. The structure of liquid gallium (III) chloride systems

Abstract An X-ray scattering method is presented which provides accurate structural information on air and moisture sensitive liquids at ambient and elevated temperatures using a standard θ-θ X-ray diffractometer. The method utilizes capillary glass tubes as sample containers and requires no corrections for sample container absorption or scattering, as shown by structural studies of well-known systems such as benzene, carbon tetrachloride and antimony trichloride. Artefacts produced by the sample holder are insignificant and very easy to correct for. The major drawback of the method is the long time of experiment, due to the small (compared with the standard set-up) area/volume ratio of the liquid which contributes to the intensity of the scattered radiation. However, the time required is not unduly long except for liquids containing light elements only (very low scattering power) or very heavy ones (high liner absorptivity). Liquid GaCl 3 is shown to have a dimeric structure consisting of edge-sharing GaCl 4 tetrahedra. This structure is analogous to that previously found for GaCl 3 in the gaseous and solid state and for AlCl 3 in the gaseous and liquid state. Concentrated solutions of GaCl 3 in benzene have been shown to comprise monomeric GaCl 3 units with C 3v symmetry. However, it is suggested that such units form as a result of a radiolytically induced cleavage of the Ga 2 Cl 6 moieties. No GaC correlation is resolved, which is explained by assuming a σ-type complex GaCl 3 and benzene and/or an ill-defined interaction between the GaCl 3 unit and benzene. The former sitution would most probably produce too few GaC correlation to be observable by the present method, whereas the latter situation would produce a very broad GaC correlation difficult to separate from the background. However, the deviation from the D 3h symmetry adopted by GaCl 3 in the gas phase indicates a specific interation between GaCl 3 and benzene.

The structure of (CH3)3SI3. Comparison between the structure in the solid and liquid state

Abstract The crystal structure of Me 3 SI 3 (s) has been determined. The compound is monoclinic and crystallizes in the space group Cc (No. 9) with unit cell parameters a = 7.856(2), b = 17.733(6), c = 8.259(2) A, β = 111.14(2)°, U = 1073(1) A 3 and Z = 4. The structure contains pyramidal Me 3 S &+ cations and almost centrosymmetric I 3 t- anions. The near- D ∞ h symmetry of the triiodides is verified by Raman spectroscopy, revealing one single peak at 110 cm −1 . The local structure of Me 3 SI 3 (1) at 50 °C and the sulfonium-triiodide coordination mode, as characterized by liquid X-ray scattering and Raman spectroscopy, is almost identical to that in the corresponding solid and the analogous Et 3 SI 3 melt. The difference in sulfonium hydrocarbon chains induces very minor changes in the local structure of the trialkylsulfonium triiodides both in the solid and molten state. X-ray absorption spectroscopy, XANES, of the S-K edge reveals electronically identical sulfur atoms in solid and liquid triiodides, which also implies a similar local structure.

Synthesis of main-group metal clusters in organic solvents

Oxidation of bismuth metal by GaIII in GaCl3–benzene solution produces the subvalent species Bi+ and Bi53+, which are characterized by liquid X-ray scattering, Raman and UV–VIS spectroscopy.

Speciation, structural characteristics and proton dynamics in the systems NH4NO3· 1.5H2O and NH4NO3· 1.5H2O–(HNO3, NH4F, NH3)–H2O at 50 °C

In order to obtain a good basis for exploring metal-ion complex formation in molten NH4NO3· 1.5H2O, some fundamental characteristics of the pure hydrous melt and a number of compositions in the NH4NO3· 1.5H2O–(HNO3, NH4F, NH3)–H2O system, have been investigated at 50 °C. Several aspects have been taken into consideration, e.g. thermodynamics of solvent autoprotolysis and HF formation, dynamics of proton exchange and structural properties. The acid dissociation constant of NH+4, Ka, and the equilibrium constant for formation of HF, KHF, were obtained from potentiometric measurements; Ka=(2.2 ± 0.2)× 10–9(mol kg–1)2 and KHF= 2160 ± 40 (mol kg–1)–1. Results from 19F NMR spectroscopy indicate that unprotonated fluoride, F–, probably exists as an H3NH+⋯F– ion pair in the solvent. The change in the 19F chemical shift with increasing HNO3 content in (NH4NO3–NH4F–HNO3)· 1.5H2O verifies the conclusion from potentiometric data that HF is the only proteonated fluoride species present. Raman spectroscopy and 14N NMR experiments give clear evidence for an increased tendency to NH+4⋯NO–3 ion-pair formation with decreasing water content in the systems NH4NO3–H2O. However, no loss of degeneracy of the internal ν3 and ν4 nitrate bands at 1380 and 718 cm–1, respectively, was observed. The D3h symmetry of NO–3 seems to be preserved in the NH4NO3· 1.5H2O melt. Results from Raman scattering, 1H NMR and 14N NMR experiments show significant changes in the spectra upon acidification with HNO3. These observations suggest an increase in hydrogen-bonding ability with increasing acidity. Results from large-angle X-ray scattering experiments on NH4NO3· 1.5H2O cannot be explained by a model comprising only interactions between water molecules and ions. A residual contribution to the overall radial electron density distribution at 1.8 A is tentatively assigned to remarkably short N(NH4)–O(NO3) distances. 1H NMR spectroscopy shows a strong retardation of the proton exchange between NH+4 and H2O in the acidic region. The rate constant, kH, for the proton-exchange step H3N ·HOH(OH2)s–1+ H2O → H3N ·(OH2)s+HOH, is estimated at (4.3 ± 1.5)× 107 s–1.

The structure of solutions of gallium(I) chloride in benzene

The structure of benzene solutions of Ga[GaCl4] and Ga[Ga2Cl7] has been investigated by 13C and 71Ga NMR, Raman spectroscopy and liquid X-ray scattering (LXS). Assignments of the vibrational spectra are based on a reinvestigation of the liquid Ga–GaCl3 system. The results for the Ga[GaCl4]–C6H6 system are in agreement with the view that an ion pair between Ga+ and GaCl4–, which lowers the symmetry of the GaCl4– ion from Td to C2v or lower, is formed. Spectroscopic effects indicating a complex formation between Ga+ and benzene are weak. The salt Ga[Ga2Cl7] was found to be extremely soluble in benzene ( >50% w/w). The results imply that in such solutions the Ga–Clb–Ga bridge in the Ga2Cl7– ion is bent and the ion pairing between Ga+ and Ga2Cl7– takes place via the bridging chloride ion of the latter ion. Results from LXS show that the GaIII-Clb distance is remarkably long, 2.85 A(24% longer than in solid K[Ga2Cl7]). For this system, 71Ga and 13C NMR as well as Raman spectroscopic results clearly indicate complex formation between Ga1 and benzene.

Thermodynamics of formation and the structure of polymetal alkaline-earth-metal(II) fluoride complexes in molten-nitrate media

The complexation reactions between alkaline-earth-metal ions, Mg2+, Ca2+, Sr2+ and Ba2+, and fluoride in molten equimolar (K,Na)NO3 have been studied by potentiometric titrations at temperatures between 240 and 300 °C, using a fluoride ion-selective electrode. A structural investigation at 280 °C was performed by detecting the liquid X-ray scattering from melts containing Sr2+ or Ba2+, with and without fluoride present.Equilibrium constants were determined at temperatures between 240 and 300 °C. No polymetal complexes were detected in the magnesium(II) and calcium(II) systems owing to the high stability of the polyfluoride species MgF–3, MgF2 and CaF2. The standard enthalpy and entropy changes for the stepwise formation of MmF2m– 1 in the strontium(II) and barium(II) systems have been evaluated from the temperature dependence of ΔG°m1, yielding: ΔH°11=–9.5 kJ mol–1, ΔS°11= 25 J K–1 mol–1 for Sr2++ F–→ SrF+; ΔH°21=–27 kJ mol–1, ΔS°21=–30 J K–1 mol–1 for SrF++ Sr2+→ Sr2F3+; ΔH°11=–4.7 kJ mol–1, ΔS°11= 26 J K–1 mol–1 for Ba2++ F–→ BaF+; ΔH°21=–20 kJ mol–1, ΔS°21=–21 J K–1 mol–1 for BaF++ Ba2+→ Ba2F3+. These results are compared with the predictions of the statistical quasi-lattice model for specific association in ionic melts. The structural investigation shows that new peaks arise in the radial distribution functions following the addition of fluoride to the Sr2+- and Ba2+-containing nitrate melts. The new peaks at 4.1 and 4.3 A in the Sr and Ba systems, respectively, are considered as metal–metal correlations due to the presence of the M2F3+ complexes in the melts.The thermodynamic and structural parameters indicate that the dimetal complexes M2F3+, M = Sr or Ba, might be stabilized by direct M–M interactions and/or bridging nitrates.

Spectroscopic investigation of concentrated solutions of gallium(III) chloride in mesitylene and benzene

The metal–arene interaction in concentrated binary mixtures of GaCl3 and mesitylene or benzene has been investigated using multinuclear (1H, 13C, 71Ga) NMR, Raman and UV/VIS spectroscopy as well as liquid X-ray scattering. The effects of hydrolysis and radiolysis of such mixtures have also been studied using UV/VIS, EPR and IR spectroscopy. Gallium(III) chloride has been found to be monomeric in mesitylene and to form η6 complexes with this arene. A structural determination of this complex in solution yielded a Ga–C distance of 2.20 A and a GaCl3 moiety with C3v symmetry and Ga–Cl distances of 2.10 A. In benzene, GaCl3 exists as a mixture of the Cl2Ga(µ-Cl)2GaCl2 dimer and one or several other species, containing terminal GaCl3 groups of C3v symmetry, e.g. the dimer isomer Cl2Ga(µ-Cl)GaCl3 and monomeric GaCl3. The complex with benzene is weaker than with mesitylene and the co-ordination mode cannot be unambiguously elucidated. Hydrolysis of the GaCl3–C6H6 system produced a darkly coloured precipitate showed by IR spectroscopy to be a mixture of poly(p-phenylene) and other polymeric species, probably meta-substituted, chlorinated and hydroxylated polyphenylenes. On the basis of 71Ga NMR results, it is suggested that the formation of this polymer proceeds via protonated Wheland intermediates, rather than direct oxidation of benzene by GaIII. Radiolysis of the mixtures of GaCl3 with mesitylene and benzene yielded radicals, according to EPR spectroscopy. In benzene solution the formation of radicals is accompanied by reduction of GaIII to GaI. No formation of solid polymers could be detected.

Alkaline-earth-metal(II) complexes with hydroxide and fluoride in molten NH4NO3· 1.5H2O at 50 °C

The formation of alkaline-earth-metal(II) hydroxide and fluoride complexes in molten NH4NO3· 1.5H2O has been investigated by potentiometric, NMR and Raman spectroscopic methods at 50 °C. Complexes with the formal composition MgOH+, Mg2OH3+ and CaOH+ were detected in the hydroxide systems. The formal stability constants are: β11[MgOH+]= 3.2 ± 0.1 kg mol–1, β21[Mg2OH3+]= 1.6 ± 0.5 kg2 mol–2, β11[CaOH+]= 0.8 ± 0.1 kg mol–1 obtained from potentiometric measurements. In the fluoride systems the following complexes were detected: MgF+, MgF2, Mg2F3+, CaF+, CaF2, SrF+, Sr2F3+ and BaF+ with formal stability constants: β11[MgF+]= 3.3 ± 0.5 × 102 kg mol–1, β12[MgF2]= 1.2 ± 0.2 × 106 kg2 mol–2, β21[Mg2F3+]= 2.5 ± 0.2 × 102 kg2 mol–2, β11[CaF+]= 1.0 ± 0.2 × 102 kg mol–1, β12[CaF2]= 2.9 ± 0.6 × 105 kg2 mol–2, β11[SrF+]= 1.5 ± 0.2 kg mol–1, β21[Sr2F3+]= 0.9 ± 0.2 kg2 mol–2 and β11[BaF+]= 0.7 ± 0.1 kg mol–1 . 19F NMR experiments reveal only small changes in the 19F chemical shift upon addition of Mg2+ or Ca2+, a fact which indicates a predominantly ionic M—F bonding character in the complexes formed. 14N NMR and Raman spectroscopy experiments using M(NO3)2· 3H2O–NH4NO3· 1.5H2O melts (M = Mg, Ca, Sr, Ba) indicate that nitrate ions are weakly coordinated to the metal ions and suggest the following order of increasing metal ion–nitrate interaction: Ba2+ < Sr2+ < Ca2+ < Mg2+. Difference Raman spectra of melts with magnesium or calcium ions, with and without fluoride present, show that nitrate ions, rather than water molecules, are most likely released upon metal–fluoride complex formation according to: M(H2O)x(NO3)y2–y+nF–⇌ MFn(H2O)x(NO3)y–z2 –n–y+z+zNO3–.