H. G. Hertz

Chemical shifts of aqueous nonelectrolyte solutions: Influence of the polar and nonpolar groups on the water proton shifts at 0C

Hydroxyl-proton chemical shifts of alcohol mixtures and aqueous solutions containing some nonelectrolytes (alcohols, ketones, cyclic ethers, and amines) have been measured at 60 MHz and at 0°C. Methyl-proton resonance of the solutes was used as the internal reference and the water-proton shifts in solutions were measured with respect to pure water. Downfield shifts for some alcohols (particularly tertiary butanol and iso-propanol), cyclic ethers, and amines in the water-rich region were confirmed For alcohols and some other nonelectrolytes in water, the observed shifts were decomposed to component contributions arising from the “polar group” effect, “solute proton” effect, and “nonpolar group” effect. Polar effects are found to contribute a substantial fraction of the observed downfield shifts. After subtracting these polar contributions, however, there still remains certain amounts of downfield shifts which may be attributed to the effect of nonpolar groups on the water structure. The downfield shifts are found to be relatively large when the solutes have branched alkyl groups with nearly spherical shape and with diameters of about 5 Å. Strikingly large downfield shifts of water proton resonance were found for some secondary amines and tertiary diamines with globular shape. However, in view of the extrapolation technique employed in evaluating the “polar group” effect, the downfield “nonpolar group” effect we estimated should be considered as the upper limits.

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.

Search for hydrophobic association between small aprotic solutes from an application of the nuclear magnetic relaxation method

Proton and deuteron magnetic relaxation rates of the four solutes acetone, acetonitrile, trimethylamine, and tetramethylurea in their aqueous mixtures are reported. For the normal and deuterated organic substances the water was D2O and H2O, respectively. The intermolecular relaxation rates were determined. Experimental results for the self-diffusion coefficients of trimethylamine and tetramethylurea in their aqueous mixtures are also reported. From these results and literature data the A parameter, A=(1/T1)inter·D1/c′1 being a criterion for association, was calculated. We obtained the result that only for the largest solute molecule, i.e. tetramethylurea, A showed the typical concentration dependence indicating solute-solute association. For the other three components self-association is not outside the range of the sensitivity of the present method.

Proton magnetic relaxation study of water orientation around I− and Li+

Proton relaxation time measurements are performed for 6m aqueous solutions of7LiI and6LiI in D2O containing small amounts of H2O. The measurements are done at low temperatures and yield maxima of the relaxation rate plotted against 1/T. From the maxima of the relaxation rates, proteon-I− and proton-Li+ distances in the first coordination sphere of the ions are determined, and from the knowledge of the ion-water oxygen distance it is shown that for iodide a somewhat broadened H-bonded configuration is valid and that for Li+ the electric dipole orientation deviates from the radial direction. In order to test the reliability of the method a proton-127I interaction study is also performed in KI solution in glycerol. The I-H distance obtained is in satisfactory agreement with that found in the aqueous system.

Nuclear magnetic relaxation and ionic solvation of23Na+ and81Br− in aqueous mixtures of simple amides

Ion-solvent interactions of Na+ and Br− in binary aqueous mixtures of formamide,N-methylformamide (NMF), andN,N-dimethylformamide (DMF) are studied by use of23Na and81Br magnetic relaxation times, extrapolated to zero salt concentration. The relaxation times, which are controlled by quadrupolar interaction, have been measured over the complete mixture range and are compared with a simplified theoretical formula. It turned out that the23Na+ relaxation in H2O-formamide and H2O-NMF mixtures is in excellent agreement with theoretical predictions, implying nonpreferential solvation of Na+ in these systems. Small deviations of experimental from theoretical results in H2O+DMF possibly indicate weak selective hydration of the cation. In the case of the anionic nuclei81Br−, deviations from the theoretical curve occur which are to be expected, especially for systems where hydrophobic effects play a role. On the other hand, it is demonstrated that these deviations can easily be explained within the electrostatic theory by differences in structural details of the anionic solvation sphere in the mixtures compared to the pure solvents.

Nuclear magnetic relaxation by intermolecular quadrupole interaction, determination of the correlation time

Abstract Expressions for the correlation times corresponding to various models are presented which have been proposed in the literature. Experimental results fo

Effect of ion-solvation on the internal dynamics of the ethanol molecule as studied using NMRB′coefficients

The modification of reorientational correlation times for the various kinds of chemically non-equivalent protons of the ethanol molecule as a result of ionsolvation has been investigated with the help of NMR B′ coefficients. For this purpose, the proton spin-lattice relaxation times of specifically deuterated ethanols, containing only one type of chemically equivalent protons were measured in neat ethanols and their lithium chloride and iodide solutions at a number of concentrations. The measurements were also extended over a wide temperature range below room temperature. Some results of various other chlorides were also included for comparison. The results are discussed in the light of pertinent theories available at present. At 250 K the rotational correlation time of the molecule in pure ethanol is 64 ps and the correlation times of the CH2 and CH3 groups are 8 and 3 ps, respectively. In the solvation sphere of Li+ the correlation times for the molecule and the respective groups are about 4, 7, and 4 times as long. However, the internal reorientation times are found, in some cases, to be virtually zero which we take as an indication that the description of the internal motion by current theories is not satisfactory.

Velocity correlations in aqueous electrolyte solutions from diffusion, conductance and transference data: Application to concentrated solutions of nickel chloride and magnesium chloride

Velocity cross-correlation coefficients have been calculated for aqueous solutions of NiCl2 and MgCl2 up to concentrations of 4M. Examination of the concentration-dependence of these coefficients show that it is very similar for the two salts. There is no evidence for any special structural characteristics in NiCl2 solutions. The velocity correlation technique is apparently not sensitive enough to detect the small amount of complexation that is thought to be present in NiCl2 solutions.

A study of association by hydrogen bonding in the system phenol-CCl4 by the nuclear magnetic relaxation method

The system phenol-CCl4 was studied by measuring proton and deuteron magnetic relaxation rates and self-diffusion coefficients at 25°C. From these data intermolecular relaxation rates have been calculated. By means of an association parameter A, association of phenol molecules with respect to various parts of the molecule has been established. Closest distances of approach between protons and configurations of maximum occurrence probabilities are reported.

Study of weak Mn2+ and Cu2+ complexes by a nuclear magnetic resonance method

An NMR method is described and applied, which allows the investigation of the first coordination sphere of certain transition metal ions. It is based on the measurement of the solvent proton nuclear magnetic relaxation times both as a function of magnetic field and of concentration of an admixed diamagnetic salt. This procedure enables an unambiguous separation of dynamic effects from ion-pair formation effects, which both can influence the relaxation times. As an application of this method, the complex formation of Mn2+ and Cu2+ with Br−, I−, ClO4−, and SO42− in aqueous solution is studied. Thus the hydration numbersnH2O of these cations as a function of anion concentration are obtained, allowing the detection of weak inner-sphere or outersphere complexes. Also several complex stability constants are derived and are compared with literature data.