Manfred Holz

Temperature-dependent self-diffusion coefficients of water and six selected molecular liquids for calibration in accurate 1H NMR PFG measurements

Pulsed magnetic field gradient (PFG) NMR is today a routine method for the determination of self-diffusion coefficients, D. However, a remaining goal is the improvement of the precision of the method. The best procedure for the determination of accurate diffusion coefficients by PFG NMR is a calibration with a sample of precisely known D-value. In continuation of our previous work on calibration at 25°C (M. Holz and H. Weingartner, J. Magn. Reson., 1991, 92, 115) we present reference data as a function of temperature. Since H2O plays an outstanding role as liquid and as primary standard, we carefully measured self-diffusion coefficients of water by 1H PFG NMR in the temperature range from + 5 to + 55°C and added literature data obtained from tracer methods in the range between 0 and + 100°C. This comparatively large collection of data could then be fitted to a Speedy–Angell power law, showing the excellent congruence of the results of two completely different methods and proofing the certainty of the absolute values for water. In this manner reliable primary standard values with error limits of <1% were obtained, allowing us to adapt the temperature of the standard water to the sample temperature of interest. We further give 1H PFG NMR self-diffusion reference data in the range from + 5 to + 55°C for six easily accessible solvents, which we propose as secondary calibration standards, namely cyclohexane, dioxane, dodecane, DMSO, tetradecane and pentanol, covering a large range of absolute D-values and allowing us to match in addition the absolute D-values of calibration sample and sample under investigation. Furthermore, the gained accurate self-diffusion data are suited for an elaborate check of theoretical approaches in the physics of molecular liquids.

Experimental study of dynamic isotope effects in molecular liquids: Detection of translation‐rotation coupling

How and to what extent do molecular motions in a liquid depend upon the molecular mass? Since this fundamental problem could not yet be satisfactorily resolved by theoretical approaches, we present in this paper an experimental approach. In continuation of previous work from our laboratory, the effects of isotopic H–D substitution on viscosity and self‐diffusion have been measured for twelve molecular liquids in a certain temperature range near room temperature. These liquids are the often used solvents: methanol, ethanol, formamide, N,N‐dimethylformamide, dimethylsulfoxide, acetone, acetonitrile, pyridine, nitromethane, tetrahydrofuran, dioxane, and benzene; we have found comparatively high dynamic isotopic effects between 3% and 14%. Beside the NMR self‐diffusion measurements we have, for the first time, performed measurements of the effect of mass change on the molecular rotational diffusion via 17O and 14N intramolecular quadrupole relaxation. In all the given molecular liquids, except benzene, the dy...

EPR relaxations of aqueous Ni2+ ion

The experimental data for proton NMR relaxation and shifts in aqueous Ni2+ solutions at 25°C have been extended, especially to high fields (8.4 T). Infinite‐order perturbation theory for the EPR relaxation in the Ni2+ and a generalized set of Solomon–Bloembergen equations for the proton–electron spin–spin interaction are applied to deduce the rms value Δ and the correlation time τv for the tensor D(t) in the zero field splitting or SDS term of the electron spin Hamiltonian. Both the Brownian rotation of the Ni(H2O)62+, with correlation time τϑ, and the mean lifetime τp of protons in this complex, before exchanging with the bulk, are very important in determining the functional relation of the proton relation times and shift to the corresponding EPR quantities. The best fit of the theory to the data implies the following values for the parameters for assumed values τϑ=25 psec and τp=32 μsec: Δ=2.6 cm−1, τv=2.2 psec, TlS=T2S=2.9 psec at zero magnetic field, proton contact shift in the complex=14 ppm, proton...

Isotope effect on the translational and rotational motion in liquid water and ammonia

The dynamic isotope effects (IE) on the translational and rotational motion in liquid ammonia and water are reassessed by NMR measurements. For H2O/D2O the translational and rotational IE are clearly distinct. At 298 K, 23% and 30% are obtained, respectively. Both effects as well as the slopes of the temperature dependencies increase with decreasing temperature. For NH3/ND3 a rotational IE of 37% was observed at 298 K. A small increase to 40% at 222 K could be ascertained. The translational IE is about 15% at room temperature and exhibits a stronger temperature dependence. It is suggested that the observed deviations of the IE’s from the square root of mass and square root of moments of inertia laws are caused by translation-rotation coupling as well as quantum effects. The experimental data obtained in the present paper are also of importance for the correct interpretation of all kinds of experiments on water and ammonia, where isotopic substitutions are involved.

Probing the pore space of geothermal reservoir sandstones by Nuclear Magnetic Resonance

Pulsed-Field-Gradient–Nuclear Magnetic Resonance (PFG–NMR) is an interesting method to determine microscopic but volumetrically averaged properties of pore space. In the present paper a number of sandstone samples, taken from drill cores of geothermal wells in North Germany, have been investigated. The time-dependent self-diffusion of water molecules in their confined geometry is used to probe the pore space. The short-time behaviour of the self-diffusion coefficient (anomalous diffusion) in the porous matrix allows the determination of the surface-to-pore volume ratio S/Vp. At long diffusion times, molecules scout the tortuosity of the interconnected pore space of the sandstones. The NMR results were compared with data from petrographic image analysis (PIA), adsorption experiments and electric conductivity measurements. The PFG–NMR measurements give surface-to-pore volume ratios S/Vp that are comparable to those estimated with the petrographic image analysis. The tortuosities match in most cases data from conductivity measurements, so the PFG–NMR is regarded as an appropriate tool to determine this quantity. The results are not influenced by the adherence of ‘scout-molecules’ to the pore walls. The surface-to-pore volume ratios and tortuosities were used to calculate permeabilities of the systems of interest, which were in good agreement with measured core-plug permeabilities. Results of additional NMR relaxation experiments are used to obtain adsorption isotherms for cations at active surface sites.

Nuclear magnetic resonance study of self-association of small hydrophobic solutes in water: salt effects and the lyotropic series

The self-association behaviour of the relatively small hydrophobic organic components in mixtures of dimethyl sulfoxide (DMSO)–water, of tert-butyl alcohol (ButOH)–water and of tetramethylurea(TMU)–water has been studied by the application of an NMR technique. The so-called association parameter A22, which essentially is obtained from experimental intermolecular dipole–dipole relaxation data of 1H nuclei and from NMR measurements of the self-diffusion coefficients of the organic components, has been determined. The composition dependence of A22 in all the systems under investigation shows that self-association of the organic component occurs. The association parameters Aij and the Kirkwood–Buff integrals Gij are related quantities, which might yield complementary information. Thus a qualitative comparison between A22 and G22 is given for aqueous mixtures of DMSO and of ButOH. The main aim of the present work is the study of the influence of salts on the association behaviour. The A22 parameters obtained for salt-containing mixtures show that the salts (Lil, Csl and CsAc in DMSO–water; LiBr and CsBr in ButOH–water; LiBr, LiClO4 and LiAc in TMU–water) change the attractive interaction between the organic species. For ButOH and TMU the influence of salt essentially follows the lyotropic or Hofmeister series; however, for DMSO the reverse sequence of effectiveness of the ions is found. Our results indicate hydrophobic association for ButOH and TMU, while for DMSO obviously a short-range interaction of another kind is acting. Finally, a new attempt is made to explain the well known reverse order in the Hofmeister series for cations and for anions with respect to their structure-breaking ability.

The effect of site-specific isotopic substitutions on transport coefficients of liquid methanol

A new approach is suggested for studying the nature of molecular transport in simple liquids, which makes use of site‐specific isotopic substitutions. Its application represents the first systematic experimental study of a theoretically predicted correlation between transport coefficients in liquids and molecular moments of inertia. For this purpose, we have determined the viscosities and self‐diffusion coefficients at 25 °C of normal methanol and seven isotopically labeled methanol species: CH3OD, CD3OH, CD3OD, CH2DOH, CHD2OH, CHD2OD, and 13CH3OH. Except for 12C/13C substitution, the observed isotope effects are significantly larger than predicted by a square root of mass dependence, but are well correlated with the square roots of the moments of inertia. The results give strong evidence that translation–rotation coupling influences the transport processes in methanol, thus confirming earlier interpretations of isotope effects upon the transport in water.

NMR in the presence of an electric current. Simultaneous measurements of ionic mobilities, transference numbers, and self-diffusion coefficients using an NMR pulsed-gradient experiment

Abstract To facilitate the application of NMR to electrochemistry a number of methodological and technical problems must be solved. In this connection the principles of construction of an NMR probe developed for experiments in the presence of an electric current perpendicular to the magnetic field B0 are presented. To overcome convection problems, the electrolyte solutions are stabilized by means of a gel. The obstruction effect on the microscopic translational motion of water molecules and of the cation (C2H5)4N+ caused by gels is quantitatively investigated in three different systems (Agar, Agarose IEF, and Cab-O-Sil). At the gel concentrations of interest, an obstruction effect of only 1–2% was found. Using a pulsed magnetic field gradient spin-echo experiment which was modified by the application of a pulsed electric potential gradient, it could be demonstrated for the first time that in one NMR experiment the ionic mobility u+, the transference number T+, and the self-diffusion coefficient D+ can be determined simultaneously. The NMR results of the electrical transport quantities agree within the experimental error of ±5% with results from classical experiments.

Some structural aspects in binary aqueous mixtures of simple amides from rotational molecular motions

Nuclear magnetic relaxation rates of2D and14N in binary aqueous mixtures of formamide,N-methylformamide (NMF), andN,N-dimethylformamide (DMF) are reported as a function of the mixture composition. From these intramolecular quadrupolar relaxation data separate rotational correlation times for the two components of the mixture can be determined. The relative variation of the single correlation time as a function of the composition is interpreted in terms of structural changes caused by hydrogen bonding and hydrophobic effects. The results also clearly reflect the expected characteristic variation of these effects on the rotational molecular motions in going from formamide to NMF and DMF. The maximum correlation time retardation of DMF in the aqueous mixture is compared with those of other hydrophobic solvents. A correlation between this maximum retardation and the excess enthalpy of mixing of hydrophobic solvents in aqueous solution can be established graphically.

NMR Studies on hydrophobic interactions in solution Part 3 Salt effects on the self-association of ethanol in water at two different temperatures

The salt effect on the self-association of ethanol has been investigated in water-rich aqueous mixtures at 25 and 40 °C by the NMR A-parameter method. In order to investigate the anion dependence we used salts with the common sodium cation, namely Na2SO4, CF3COONa, NaBr, NaI, NaClO4 and NaSCN at fixed concentration. The association parameter A22 was obtained by means of the measurement of intermolecular 1H–1H dipole–dipole relaxation rates and self-diffusion coefficients of the ethanol molecules in D2O. The influence of the anions on the alcohol self-association at 25 °C follows the sequence: SO42− > CF3COO− > Br− > ClO4− ≈ I− > SCN−, which corresponds exactly to the well known Hofmeister or lyotropic series determining e.g. the stabilisation and destabilisation of proteins in aqueous solutions. The two salts Na2SO4 and CF3COONa promote the alcohol self-association, whereas the other salts reduce the association tendency. Thus, small organic molecules can be used for basic investigations with respect to salt effects on biologically interesting molecules. At 40 °C the relative promotingeffect of the two salts is weakened but the opposite effect of the other salts is strengthened. At this temperature the sequence ofthe anion influence is slightly different in the sense that Br− and ClO4− exchange position. Finally, we propose consideration of a special anion–apolar group interaction in water in the explanation of the anion sequence in the Hofmeister series.