Rolf Sjöblom

Hydrogen bond studies. 108. A nuclear magnetic relaxation study of molecular motion in solid trimethylammonium iodide, bromide, chloride, and hydrogen oxalate

Abstract Spin-lattice relaxation times in the laboratory frame at 60.16 and 20.00 MHz are reported for trimethylammonium iodide, bromide, chloride, and hydrogen oxalate in the temperature range 150–400 K. A model that makes it possible to analyze the experimental data on the basis of different assumptions regarding the reorientational motions of the methyl groups, the cations as a whole, and the correlations between these two types of motion is developed. No conclusions are drawn about the correlation since the two models investigated result in almost equally good fits with the experimental relaxation times. However, time factors, activation energies, and scale factors for the intramethyl relaxation constants could be determined from the refinements since the results were close for the two models, and thus apparently insensitive to reasonable assumptions about the correlations.

A general procedure for the calculation of theoretical nuclear magnetic resonance second moments in dipolar solids; application to oxalic acid dihydrate

Abstract It is shown that the second moment of a dipolar solid can be written as a sum of products of real second-order spherical harmonic functions. This sum can be contracted to a quadratic form, qSq, where q depends only on the direction of the magnetic field and S, the second moment tensor, only on the crystal structure and the assumptions made concerning the molecular motions. The effects on S of symmetry, rigid-body reorientations, and librational motion are investigated quantitatively. A formula for the second moment of a powder is also given. The procedure is illustrated by refinements of the relative hydrogen positions in oxalic acid dihydrate from experimental second moments.

Hydrogen bond studies. 107. A proton magnetic resonance study of molecular motion in solid trimethylammonium iodide, bromide, and hydrogen oxalate☆

Abstract Proton magnetic resonance data are reported for polycrystalline trimethyl-ammonium iodide and bromide, and for poly and monocrystalline trimethyl-ammonium hydrogen oxalate. The second moments at the lowest temperatures correspond to values calculated for rigid structures. Line narrowings occur in the temperature region 120–220 K; the values of the second moments at higher temperatures are consistent with combined reorientations of the methyl groups and the cations as a whole around their respective threefold pseudo-symmetry axes.

A general procedure for the calculation of theoretical nuclear magnetic relaxation times in dipolar solids; applications to the trimethylammonium ion

Abstract It is shown that the relaxation time of a dipolar solid can be calculated ffrom second moment tensors if the correlation times of the motional processes are known. Expressions are derived for the relaxation times in the laboratory and in the rotating frames of mono- and polycrystalline samples in the presence of one or several motional processes. The procedure is illustrated by calculations of theoretical relaxation constants for the trimethylammonium ion.

The effect of rigid-body vibrations on the nuclear magnetic resonance signal for quadrupole, chemical shift, and dipole—dipole interactions in solids

Abstract In molecular solids, the rigid-body vibrational motions usually affect the quadrupole, the chemical shift, and the dipole-dipole interactions as well as the second moment much more strongly than other vibrations. It is shown here that the changes in these couplings and second moments due to librational motions are given by simple expressions linear in the elements of the librational tensor.

The orientation dependence of spin-lattice relaxation times in single crystals

Abstract The orientation dependence of T1−1 and T1ϱ−1 for single crystals is discussed in terms of the second moment tensor formalism applied to dipolar solids. It is shown that for long correlation times, T1−1 is a linear function of the maximum possible second moment reduction that may be caused by the motion responsible for the relaxation. Similarly, T1ϱ−1 in the vicinity of the T1ϱ−1 maximum is proportional to the same second moment reduction. For short correlation times the symmetry restrictions on the orientation dependence of T1−1 are discussed and are found to differ from the restrictions on the second moment tensor for some crystal symmetries. The nonexponentiality of the relaxation for polycrystalline samples, resulting from anisotropy of the relaxation rates for single crystals is discussed.

Electrokinetic monitoring of synthetic geoclay liners

On-going work and preliminary results are presented from impedance spectroscopy investigations of sodium bentonite. The purpose of the work is to find out if alternating electric current measurements might be utilised for monitoring the conditions of synthetic geoclay liners. Theoretical analyses as well as preliminary results indicate that sufficient alteration in the signal might be obtained for variation in moisture content, salt content and location, as desired. The results also indicate that the impedance spectroscopy method is applicable for the understanding of the microstructural aspects of bentonite.

Feasibility of compacted bentonite barriers in geological disposal of mercury-containing waste

The growing concern regarding the threat to human health posed by mercury in the environment has prompted the Swedish Government to plan for disposal of mercury-containing waste in deep geologic formations. The purpose of the present 3-year project is to determine the major prerequisites and evaluate the feasibility of utilising bentonite barriers for such a purpose. The scope of the present paper is to present the results from the work carried out during the first year. The design concept should be such that the excellent materials properties of high-density bentonite can be utilised. Also, the chemistry should be adjusted in such a way that any deterioration of bentonite becomes tolerable, and such that the mercury species in solution either appear in low concentration, or in a form that has a low mobility. It is concluded that safety analyses may be based on modelling of the speciation in combination with diffusion through bentonite in one dimension. The results of such modelling will be compared with actual migration data from actual waste forms comprising highly compacted mixtures of spent mercury-containing batteries and bentonite. D 2003 Elsevier B.V. All rights reserved.

An efficient method for evaluating the effect of lattice vibrations on the NMR second moment

Abstract An improved algorithm for the evaluation of intra- and inter-molecular contributions to the NMR second moment is presented. The final expressions are given in terms of quadratic forms of the second moment tensor which depends on the equilibrium crystal structure as well as on parameters describing average atomic and molecular motion, assuming the latter to be expressible by the rigid body motion formalism. These parameters are readily available once the crystal structure of the compound considered has been determined by X-ray or neutron diffraction methods, and may serve for the evaluation of second moments of single-crystal as well as polycrystalline specimens. The use of the second moment tensor formulation considerably reduces the computational effort which is required by such calculations when the Van Vleck formula is being directly evaluated. An explicit introduction of crystal symmetry into the second moment tensor formalism, presented in this paper, further enhances the efficiency and ease of application of the proposed method.