Band 50, Heft 1

Nuclear spin-lattice relaxation times, T 1 , of 7 Li in 13.8mol/kg LiCl-H 2 O and 12.3 mol/kg LiCl-D 2 O solutions have been measured in the temperature range 80K-313K. They showed similar temperature dependences with the same minimum values (27 ± 1 ms) which proved that the electric quadrupole interaction is dominant for the relaxation of 7Li-NMR. The T 1 and T 2 values of 7 Li-NMR in concentrated aqueous solutions of LiCl (8-14 mol/kg), LiBr (8-10 mol/kg) and LiSCN (8-14 mol/kg) at 80 K to 300 K have been studied to obtain information concerning the dynamic properties of Li + ions and the effect of anions. T 1 at temperatures higher than the glass transition point is governed by a tumbling motion of the hydrated lithium ions.

The Raman band of the v 1 (A 1 ) mode of SO 4 2- of Li2S04 dissolved in water and water-dimethyl-formamide mixtures was measured in the concentration range 0.01 to 0.2 mol • dm -3 . The central frequency of the sulfate ion increases with sulfate concentration. The limiting frequency at infinite dilution depends on the DMF content. The Raman band becomes asymmetric with increasing concentration of Li 2 SO 4 in 20 wt% DMF mixtures and with addition of LiCl to the aqueous solution of Li 2 SO 4 . The vibrational relaxation behavior of the v 1 (A 1 ) mode was investigated by analyzing the vibrational correlation function based on Kubo's stochastic theory.

The complexation of manganese (II), cobalt (II) and zinc(II) with thiocyanate ions has been studied in N,N-dimethylacetamide (DMA) by calorimetry and spectrophotometry at 298 K. In these metal systems the formation of a series of four isothiocyanato complexes, [M(NCS) n ] (2-n)+ (n = 1÷4; M = Mn, Co, and Zn) was established, and their formation constants, enthalpies and entropies were determined. Interestingly, the complexation behavior in DMA is significantly different from that in N,N-dimethylformamide (DMF), despite of the similarity of the physicochemical properties of these solvents. The complexation is indeed enhanced significantly in DMA over DMF and is more exothermic in the former solvent. Furthermore, with cobalt(II) an octahedral to tetrahedral coordination geometry change was found to occur in DMA at an earlier step of complexation than in DMF. These results suggest that six-coordination of DMA molecules to a metal ion is severely sterically hindered.

The possibility of the existence of a pre-preak in the structure factor of water at 0.6 ů 1 is discussed on the basis of the combination of computer simulation and integral equation theory techniques. It indicates spatial correlations in water at distances of the order of 10 Å.

Single crystal X-ray data, collected at 298 K, are used to redetermine the crystal structure of the non-isomorphic compounds [Sr(H 2 O) 8 ](OH) 2 and [Ba(H 2 O) 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 … O II 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 … O II 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.

The intermodular structures of liquid formamide, N-methylformamide and N,N-dimethylformamide at room temperature are studied by means of NVE molecular dynamics computer simulations. Newly developed flexible models are used. X-ray and neutron weighted structure and total radial pair distribution functions are computed from the simulated site-site pair distribution functions. They are compared with experimental results. The agreement is usually satisfactory as far as heavy atom pairs are concerned while the lengths of the hydrogen bonds are found to be systematically too long in the simulations.

The standard molar Gibbs free energies of transfer of divalent anions, in particular sulfate, from water into methanol, ethanol, ethylene glycol, dioxane, tetrahydrofuran, acetone, N-methylformamide, N,N-dimethylformamide, pyridine, acetonitrile, and dimethylsulfoxide and mixtures of some of them with water or some other solvent have been obtained from the literature. The tetraphenylarsonium tetraphenylborate extrathermodynamic assumption has been used as far as possible in order to obtain the single ion values. The generally unfavorable transfers from water (or protic solvents) into polar aprotic ones are interpreted in terms of the properties of the anions and of the solvents, dominated by the hydrogen-bonding acidity of water.

The aqueous solubilities of La(OH) 0.8 (CO 3 ) 1.1 .0.1 H 2 O, basic lanthanum carbonate of ancylite type, have been investigated as a function of ionic strength at 25.0 °C. The stoichiometric solubility constants defined by log* K I ps0 = log [La 3 + ] + 1.1 log p CO2 - 3 log [H + ] have the values 11.10, 11.32, 11.42, 11.63, and 11.70 for I = 0.1, 0.25, 1.0, 2.0, and 4.0 mol kg -1 (Na)ClO 4 respectively. The extrapolation to infinite dilution using the Pitzer equations resulted in a "thermodynamic" solubility constant, log* K° pz0 = 10.48 ± 0.08. This in turn led to the Gibbs energy of ancylite formation: Δ f G ⊖ 298 {La(OH) 0.8 (CO 3 ) 1.1 · 0.1 H 2 O} = - 1.531.5 kJ mol -1 .

Complex permittivity spectra in the frequency range 0.95 ≤ v/[GHz] ≤ 89 for N,N-dimethylformamide (DMF), N-methylformamide (NMF), formamide (FA) and their solutions of NaClO 4 are investigated to study the change of liquid structure and dynamics arising from the availability of one hydrogen-bond acceptor site together with no (DMF), one (NMF), or two (FA) donor sites on the same molecule. Three solvent relaxation processes are observed for NMF and two for FA and DMF. The relaxation parameters are used to determine solvation numbers. They show that ion-solvent interactions lead to a reduction of the average length of the H-bonded NMF chains but have only moderate influence on the FA structure. An additional solute relaxation process in DMF solutions is due to the diffusion-controlled formation and decomposition of solvent-shared ion pairs.

The recently obtained Mössbauer and EPR parameters of the Fe 4 S 4 3+ polymetallic center in the High Potential Iron-Sulfur Protein (HiPIP) II from E. halophila have been reproduced with models based on pure Heisenberg exchange. The role of double exchange versus ./-inequivalence is discussed. An evaluation of the upper limit of the double exchange parameter in the Fe 4 S 4 3+ bimetallic center is also presented. The present calculations shed further light on the electronic structure of the Fe 4 S 4 3+ center.

The molecular dynamics of solid (CH 3 ) 3 NBH 3 is investigated by deuterium NMR spectroscopy. Variable temperature lineshape analyses yield activation energies of 27 ± 3, 19 ± 2, and 12.5 ± 2 kJ/mol for -CH 3 , -N(CH 3 ) 3 and -BH 3 rotation, respectively. Analysis of the temperature depen­ dence of the spin-lattice relaxation times, T 1 , gives activation energies of 33 ± 3, 15 ± 1.5, and 14 ± 1.5 kJ/mol, respectively. Direct comparison of rotational exchange rates (from lineshape simu­ lations) an of rotational correlation times (from T 1 analyses) for -N(CH 3 ) 3 and -BH 3 rotation indicate that the two motions are correlated in solid (CH 3 ) 3 NBH 3 and together constitute a whole molecule reorientation about the N-B bond. This is supported by an internal rotational barrier of 18.0 kJ/mol for-BH 3 rotation, obtained from ab initio molecular orbital calculations at the MP2/6-31G* level.

91 Zr solid state NMR spectra have been observed for samples possessing different zirconium local-site environments (e.g. phosphates, fluorides). Depending on the quadrupole coupling, different NMR experiments have been utilized. In case of weak quadrupole coupling, magic angle spinning (MAS) is suitable. Otherwise, spin echo techniques are necessary to overcome “acoustic ringing’’ and receiver dead-time limitations. Using the step echo method, the detection of the very broad 91 Zr signals is not limited by non-uniform spectral excitation. The obtained 91Zr chemical shifts range from -246 ppm to 208 ppm, the quadrupole coupling frequencies from 50 kHz to 2.7 MHz..

We report the first single crystal deuteron NMR spectra of CD3 groups which display the socalled ±ß, ±(|α| ± ß) and ±(2|α| ± ß) lines characteristic of rotational tunneling in a sufficiently clear manner to allow a quantitative comparison with the respective theory developed in 1988 by the group of W. Müller-Warmuth. The molecular system we study is aspirin-CD 3 . We recorded spectra for differently oriented single crystals and measured spin-lattice relaxation times T 1 in a wide temperature range. At 12.5 K we exploit the dependence of the ±(|α| ± ß) and ±(2|α| ± ß) lines on the orientation of the applied field B 0 for determining the equilibrium orientation of the CD 3 group in the crystal lattice. The spectra display features which allow, by comparison with simulated spectra, a measurement of the tunnel frequency v t . Its low temperature limit is (2.7 ± 0.1) MHz. It allows to infer the height V 3 of the potential V(φ) in which the CD 3 group moves, provided that this potential is purely threefold. We get V 3 = (47.2 ± 0.5) meV. The transition from the tunneling to the classical, fast reorienting regime occurs in the 15 K ≲ T ≲ 35 K temperature range. In this range we observe a broadening, merging and eventually narrowing of the ± |α| and ±2|α| lines in very much the way predicted by Heuer. His theory, however, must be extended by taking into account all librational levels. The behaviour of the ± ß lines in the transition temperature range signalizes a reduction of the observable tunnel frequency with increasing temperature. This reduction allows an independent measurement of the potential height and represents a test of the assumption of a purely threefold potential. From the T 1 -data we derive the temperature dependence of the correlation time Ƭc of the reorientational jumps. The plot of log Ƭc vs. 1 /T follows a straight line for more than five decades. From its slope we get yet another independent number for the potential height. It agrees well with the other ones, which confirms the assumption of the essentially threefold potential V(φ) in aspirin-CD 3 .