Magnus Sandström

On the coordinating properties of some solvents. A vibrational spectroscopic study of mercury(II) halides and antimony(V) chloride in solution; new concepts for Lewis basicity scales of solvents

Abstract Raman and infrared spectra of the stretching vibrational frequencies of mercury(II) halides in solvents with widely different solvating abilities, have been recorded and combined with literature data. The frequencies decrease as the interaction of the solvent with the mercury atom in the HgX2 entity increases. Using published data from structure determinations by X-ray diffraction in solutions and crystals, an empirical correlation of the XHgX angle and the frequency shift is obtained. An empirical scale ranking the donor strength towards a soft acceptor is proposed for more than sixty solvents with widely varying solvating properties. The numerical donor strength DS values have been obtained as the decrease in the symmetric stretching vibration frequency of the HgBr2 molecule between the gas phase and solution. This DS scale is compared with some previously proposed scales, determined with the use of hard or borderline acceptors. The most well-known of these, the donor number DN scale based on enthalpy data of the adduct formation SbCl5·L (L = solvent molecule) in 1,2-dichloroethane is also compared with Raman measurements of the SbCl stretching frequencies of the SbCl5·L adducts in this solvent. The dependence of the measured donor strength of the solvent molecules on the properties of the acceptor and on the method used for the donor classification is discussed. An additional donor strength scale DH for hard acceptors is derived for 24 solvents from published data. The scale is based on Gibbs free energy of transfer of the sodium ion from a solvent to a reference solvent (1,2-dichloroethane). There is hardly any correlation between the soft DS and the hard DH scales, while the DS and DN scales show a fair agreement for solvents with hard donor atoms.

Vibrational spectroscopic and force field studies of N,N-dimethylthioformamide, N,N-dimethylformamide, their deuterated analogues and bis(N,N-dimethylthioformamide)mercury(II) perchlorate

Raman and infrared absorption spectra of N, N-dimethylthioformamide, N, N-dimethyIthioformamide-d(7), N, N-dimethylformamide, N, N-dimethylformamide-d(7) and bis( N, N-dimethylthioformamide)mercury ...

The hydration of the scandium(III) ion in aqueous solution and crystalline hydrates studied by XAFS spectroscopy, large-angle X-ray scattering and crystallography

The structures of the hydrated scandium(III) ion and of the hydrated dimeric hydrolysis complex, [Sc2(mu-OH)2]4+, in acidic aqueous solutions have been characterized by X-ray absorption fine structure (XAFS) and large-angle X-ray scattering (LAXS) methods. Comparisons with crystalline reference compounds containing hydrated scandium(III) ions in well characterized six-, seven- and eight-coordinated polyhedra have been used to evaluate the coordination numbers and configurations in aqueous solution. In strongly acidic aqueous solution the structure of the hydrated scandium(III) ion is found to be similar to that of the eight-coordinated scandium(III) ion with distorted bicapped trigonal prismatic coordinating geometry in the crystalline [Sc(H2O)(8.0)](CF3SO3)3 compound. The EXAFS data reveal for the solution, as for the solid, a mean Sc-O bond distance of 2.17(1) Angstrom to six strongly bound prism water molecules, 2.32(4) Angstrom to one capping position, with possibly another capping position at about 2.5 Angstrom. The LAXS study supports this structural model and shows furthermore a second hydration sphere with approximately 12 water molecules at a mean Sc...O(II) distance of 4.27(3) Angstrom. In less acidic concentrated scandium(III) aqueous solutions, the dimeric hydrolysis product, [Sc2(mu-OH)2(H2O)10]4+, is the predominating species with seven-coordinated scandium(III) ions in a double hydroxo bridge and five terminal water molecules at a mean Sc-O bond distance of 2.145 Angstrom. Hexahydrated scandium(III) ions are found in the crystal structure of the double salt [Sc(H2O)6][Sc(CH3SO3)6], which crystallizes in the trigonal space group R3[combining macron] with Z = 6 and the unit cell dimensions a = 14.019(2) and c = 25.3805(5) Angstrom. The Sc-O distances in the two crystallographically unique, but nearly identical, [Sc(H2O)6]3+ entities (both with 3[combining macron] imposed crystallographic symmetry) are 2.085(6) and 2.086(5) Angstrom, while the mean Sc-O distance in the near octahedral [Sc(OSO2CH3)6]3- entities (with three-fold symmetry) is 2.078 Angstrom.

Hydration of some large and highly charged metal ions

EXAFS studies of metal ions with hydration numbers higher than six in aqueous solution, often show asymmetric distribution of the metal-oxygen bond distances. The hydration number can be determined from a correlation with the bond distance. The mean Ca-O distance 2.46(1) A shows the calcium(II) ion to be eight-hydrated in a wide asymmetric distribution. Theoretically calculated EXAFS oscillations for individual snapshots from an MD simulation show large variations. The scandium(III) ion is surrounded by two groups of about eight water molecules, with the mean Sc-O distance 2.185(6) A. The yttrium(III) ion coordinates eight waters in an asymmetric distribution at 2.368(5) A, and the lanthanum(III) ion 6 + 3 water molecules at 2.52(2) and 2.65(3) A, respectively. For the the uranium(IV) and thorium(IV) ions, the M-O distances 2.42(1) and 2.45(1) A, respectively, indicate hydration numbers close to 10.

Vibrational spectroscopic force field studies of dimethyl sulfoxide and hexakis(dimethyl sulfoxide)scandium(III) iodide, and crystal and solution structure of the hexakis(dimethyl sulfoxide)scandium(III) ion

Hexakis(dimethyl sulfoxide)scandium(III) iodide, [Sc(OS(CH(3))(2))(6)]I(3) contains centrosymmetric hexasolvated scandium(III) ions with an Sc-O bond distance of 2.069(3) angstroms. EXAFS spectra yield a mean Sc-O bond distance of 2.09(1) angstroms for solvated scandium(III) ions in dimethyl sulfoxide solution, consistent with six-coordination. Raman and infrared absorption spectra have been recorded, also of the deuterated compound, and analysed by means of normal coordinate methods, together with spectra of dimethyl sulfoxide. The effects on the vibrational spectra of the weak intermolecular C-H...O interactions and of the dipole-dipole interactions in liquid dimethyl sulfoxide have been evaluated, in particular for the S-O stretching mode. The strong Raman band at 1043.6 cm(-1) and the intense IR absorption at 1062.6 cm(-1) have been assigned as the S-O stretching frequencies of the dominating species in liquid dimethyl sulfoxide, evaluated as centrosymmetric dimers with antiparallel polar S-O groups. The shifts of vibrational frequencies and force constants for coordinated dimethyl sulfoxide ligands in hexasolvated trivalent metal ion complexes are discussed. Hexasolvated scandium(iii) ions are found in dimethyl sulfoxide solution and in [Sc(OSMe(2))(6)]I(3). The iodide ion-dipole attraction shifts the methyl group C-H stretching frequency for (S-)C-H...I(-) more than for the intermolecular (S-)C-H...O interactions in liquid dimethyl sulfoxide.

Structure of the Solvated Strontium and Barium Ions in Aqueous, Dimethyl Sulfoxide and Pyridine Solution, and Crystal Structure of Strontium and Barium Hydroxide Octahydrate

Single crystal X-ray data, collected at 298 K, are used to redetermine the crystal structure of the non-isomorphic compounds [Sr(H2O)8](OH)2 and [Ba(H2O)8](OH)2. The eight water oxygen atoms form distorted Archimedean antiprisms around the octahydrated metal ions with mean metal ion-oxygen distances 2.62 and 2.79 Å for strontium and barium, respectively. A second coordination shell of 24 hydrogen-bonded oxygen atoms with mean metal ion-oxygen distances M…OII 4.76 and 4.80 Å, respectively, is observed. The hydroxide ions in both structures have an unusual hydrogen bond arrangement with 5 bonds accepted and one donated. The structure of the solvated strontium and barium ions in aqueous, dimethyl sulfoxide and pyridine solutions has been studied by means of large angle X-ray scattering and extended X-ray absorption fine structure spectroscopy techniques. In aqueous solution independent determinations of the first-sphere metal-oxygen coordination by the two techniques resulted in the bond lengths Sr-O 2.63 (2) and Ba-O 2.81 (3) Å, and for both metal ions a hydration number of 8.1 (3). The second coordination spheres are very diffuse with only about 13 water molecules at similar M…OII distances as in the crystal structures and 2-3 water molecules closer to the metal ions. In dimethyl sulfoxide solution both ions were found to coordinate 6.0 (5) solvent molecules with the distances Sr-O 2.54(1), Sr…S 3.77(1) Å, and Ba-O 2.76(1), Ba…S 3.99(1) Å. For the solvated ions in pyridine solution EXAFS measurements yielded the distances Sr-N 2.57 (2) and Ba-N 2.78 (3) Å, with a probable solvation number of 6. Correlations of hydration enthalpies and partial molar volumes with experimental M-O bond distances in aqueous solution are used to discuss hydration numbers and bond character for all of the alkaline earth metal ions.

Theoretical and experimental sulfur K-edge X-ray absorption spectroscopic study of cysteine, cystine, homocysteine, penicillamine, methionine and methionine sulfoxide

The experimental sulfur K-edge X-ray absorption near-edge structure (XANES) spectra of the amino acids cysteine, homocysteine, penicillamine, methionine, including the oxidation products methionine sulfoxide and the disulfide cystine, have been analyzed by transition potential DFT calculations. The absolute energies and intensities of the main pre-edge sulfur 1s electron transitions have been computed to determine the character of the receiving unoccupied molecular orbitals (MO), and to investigate the influence of external interactions, especially by introducing water molecules hydrogen-bonded to the ionic species present in different pH ranges. When the thiol group deprotonates for cysteine, homocysteine and penicillamine and also for the cysteine residue in glutathione the energy of the main transition, to an MO with antibonding sigma*(S-H) character, reduces by approximately 1.1 eV and the receiving MO obtains sigma*(S-C) character. The changes in transition energy due to hydrogen-bonding were in most cases found to be relatively small, although the transition intensities could vary significantly due to the changes induced in the molecular charge distribution, thereby affecting the shapes of the spectral features. For the cysteine and penicillamine zwitterions deconvolution of the experimental spectra allowed the microscopic acid dissociation constants to be extracted separately for the thiol and the protonated amine groups, pK(a)(S) = 8.5 +/- 0.1 and 8.2 +/- 0.1, and pK(a)(N) = 8.9 +/- 0.1 and 8.8 +/- 0.1, respectively, with the thiol group in both cases being the more acidic. Coordination of cysteine to nickel(II) or mercury(II) introduced a new low energy transition involving metal ion orbitals in the receiving LUMO. The small experimentally observed energy differences between the similar main absorption features of the cysteine and methionine zwitterions, 0.2-0.3 eV in comparable surrounding, as well as a minor difference in their intensities, are reflected in the calculated transitions. The S K-edge XANES spectrum of the disulfide cystine displays a characteristic double peak with the lower energy transition (2469.9 eV) into the antibonding sigma*(S-S) MO. The second peak, at 2471.5 eV in aqueous solution, contains several transitions into MOs with sigma*(S-C) character involving also charge transfer to the water molecules hydrating the protonated amine groups (NH(3)(+)) of cystine. For solid cystine without hydrogen bonding the experimental energy difference between the two peaks is 0.2 eV larger, while no such increase occurs for the oxidized disulfide of glutathione, with a similar -S-S- bond between its cysteine residues as in cystine, because the amine groups are engaged in peptide bonds. This study shows that externally induced changes in the intramolecular bonding, e.g., by coordination, conformation geometry or hydrogen-bonding, can significantly influence the S K-edge spectra, and emphasizes the importance of a similar chemical surrounding when choosing the model compounds for standard spectra of sulfur functional groups, used to deconvolute composite experimental spectra.

Structure of Jahn–Teller distorted solvated copper(II) ions in solution, and in solids with apparently regular octahedral coordination geometry

Regular octahedral coordination has been reported for some copper(II) complexes in the solid state on the basis of crystallographic studies, e.g. hexaaquacopper(II) bromate, [Cu(OH2)6](BrO3)2, hexaaquacopper(II) hexafluorosilicate, [Cu(OH2)6]SiF6, and hexakis(pyridine-1-oxide)copper(II) perchlorate, [Cu(ONC5H5)6](ClO4)2. These results are not consistent with the elongated octahedral configuration expected from the Jahn–Teller theorem for the d9 copper(II) ion nor, in some cases, with results from electron spin resonance studies. The present lattice-independent EXAFS study confirms that the local structure in the copper(II) complexes mentioned above is, in all cases, consistent with a Jahn–Teller induced elongation. Mean equatorial and axial Cu–O bond distances of 1.96(1) and 2.32(2) A, and 1.95(1) and 2.27(3) A, were obtained for the hexaaquacopper(II) ions in the bromate and hexafluorosilicate salts, respectively. For the hexakis(pyridine-1-oxide)copper(II) perchlorate only the equatorial mean Cu–O bond distance of 1.96(1) A could be observed. Evidently, there is orientational disorder of the tetragonally elongated octahedral complexes resulting in too high crystallographic space group symmetry and copper sites in apparently regular coordination geometry. For the hydrated copper(II) ion in aqueous solution, five- and six-coordinated models with different geometries have been evaluated by means of EXAFS and large angle X-ray diffraction (LAXS) data. The combined results are consistent with a Jahn–Teller elongated octahedral configuration with Cu–Oeq 1.95(1) A, Cu–Oax 2.29(3) A, and a distinct second hydration sphere with about eight water molecules and a mean Cu⋯OII distance of 4.17(3) A. In dimethylsulfoxide solution EXAFS and LAXS methods show the solvated copper(II) ions to have mean equatorial and axial Cu–O bond distances of 1.96(1) and 2.24(2) A, respectively. As a model compound for the EXAFS studies, the crystal structure of hexakis(dimethylsulfoxide)copper(II) perchlorate dimethylsulfoxide (1/2), [Cu(OS(CH3)2)6](ClO4)2·2(CH3)2SO, was determined.

Ni2+ hydration in perchlorate and chloride solutions

Abstract The hydration of nickel ions in a 3.80 molal solution of nickel perchlorate in (heavy) water has been investigated by the neutron first-order difference method. The coordination of water molecules around nickel ions is identical with that found in an earlier study of a nickel chloride solution of comparable ionic strength.

Structure and solvation of mercury(II) iodide, bromide, and chloride in pyridine solution; refinement of the crystal structure of di-iodobis(pyridine)mercury(II), [HgI2(py)2]

The structure of the neutral mercury(II) halides in pyridine (py) solution has been studied by X-ray diffraction methods and vibrational spectroscopy. Pseudo-tetrahedral HgX2(py)2 species (X = I, Br, or Cl) are formed in solution with approximately C2V symmetry. Relevant bond lengths are Hg–I 2.665(2), Hg–Br 2.497(2), and Hg–Cl 2.375(10)A. The IHgI angle was found to be 143(2)° and BrHgBr 151(3)°, including estimated corrections for shrinkage effects. The two pyridine molecules have Hg–N distances of 2.43(2), 2.45(2), and 2.47(2)A in the iodide, bromide, and chloride species, respectively. The crystal structure of [HgI2(py)2] has been refined to an R value of 0.031 for 744 observations. It consists of monomeric pseudo-tetrahedral [HgI2(py)2] species with Hg–I bond lengths of 2.664(1) and 2.668(1)A and an IHgI angle of 142.7(1)°. The two pyridine ligands are related by a mirror plane which contains the HgI2 entity. The Hg–N distance is 2.424(9)A. Raman and i.r. spectra of the pyridine solutions and of the HgX2(py)2 solids are consistent with structural models based on the diffraction data.