John H. Strange

Studies of chemical exchange by nuclear magnetic relaxation in the rotating frame

The exchange of protons between sites with different chemical shift is studied by means of N.M.R. relaxation time measurements (T 1ρ ) in the presence of an r.f. magnetic field. The system studied is chair-to-chair isomerization in cyclohexane. The exchange rate and chemical shift are measured in the temperature range 215°k to 250°k. The method is compared with other N.M.R. techniques for studying chemical exchange and is found to have advantages particularly at high exchange rates.

Deuteron spin lattice relaxation in benzene, bromobenzene, water and ammonia

The deuteron spin-lattice relaxation times, T 1Q, have been measured for benzene, water and ammonia from the melting point to the liquid-vapour critical temperature and for bromobenzene over a smaller temperature range. The results are compared with previously measured proton spin-lattice relaxation times. It is shown that the deuteron T 1Q values agree with the proton intramolecular T 1 values for benzene, thus confirming the separation of the dipolar and spin-rotation contributions performed by Powles and Figgins [1]. A value of the electric quadrupole coupling constant, e 2 qQ/th, is deduced, which agrees with the value for the molecule in the solid. For bromobenzene a value of e 2 qQ/h=175±10 khz is obtained in a similar way. For water and ammonia a solution is found to the classic N.M.R. problem of separating the inter and intra-dipolar interaction contributions to the dipolar relaxation rate. The two contributions are about equal, in agreement with theory. The separation of the spin-rotation contrib...

Diffusion in molecular crystals

Abstract In recent years considerable attention has been focussed on molecular motion in solids. In particular the study of diffusion in molecular crystals has progressed with the development of n.m.r. and radiotracer techniques and a substantial literature has now built up. Recent theoretical developments in the analysis of n.m.r. relaxation and refinement of the radiotracer experimental techniques have allowed comparisons to be made between studies at the microscopic and macroscopic level. While some anomalies still exist, a general picture of the diffusion process in molecular crystals involving a vacancy mechanism has formed. The energy barriers to motion for many materials together with activation volumes for some are known with resonable certainty and various attempts to correlate these with thermodynamic parameters have been made. In this review, the various experimental techniques used for diffusion studies are described and a comprehensive survey conducted of the results of the investigations that have been reported in the literature. The most commonly studied materials are those that exhibit a platic-crystal phase. An attempt has been made to discuss the meaning and significance of what is currently known about diffusion in molecular crystals and the point defect structure of these solids.

Morphology of porous media studied by nuclear magnetic resonance

The filling processes of water and cyclohexane in porous silica (40 A, 60 A and 112 A pore size samples) were studied using T2 nuclear magnetic resonance (n.m.r.) experiments. The silica pores contained water or cyclohexane and the experiments were performed at room temperature and at filling fractions ranging from 0.02 to 1.0 (that is, completely full). Two distinct processes were observed which depended on the hydrophilicity of the silica surface (or the surface adhesion of the liquid). Water was found to collect in small puddles in the silica interstices, and to form a surface layer over the silica before the remaining pore volume was filled. Water in a surface-treated porous silica and cyclohexane in regular porous silica appeared to completely fill the smaller before the larger pores, and not form a separate surface-coating layer. This work also presents the techniques used to calculate quantitative information about the filling process; specifically, determination of the volume to surface-area ratio...

Pore surface exploration by NMR

A carefully chosen set of experimental techniques applied to porous media characterization provides results that can be much greater than the sum of the individual parts. The inter-relation and complementarity of a number of techniques will be considered. NMR cryoporometry provides a valuable method of pore size measurement. An NMR method that is more widely used to assess pore dimensions relies on relaxation time analysis of a liquid that fills the pores and the enhanced relaxation that occurs in a liquid at the solid/liquid interface. Thermoporometry, a method based on the application of Differential Scanning Calorimetry (DSC), is closely related to cryoporometry, but employs a different set of assumptions to evaluate pore size distributions. Comparison of the results obtained on the same samples using all these methods together with gas adsorption serves to validate the methods and provide significantly more information about surface-fluid interaction and the behavior of nano-scale material within pores than each method employed in isolation. Technique developments will be discussed and applications of these methods to ideal silicas will be used to illustrate their power, particularly in combination.

Molecular motion in the plastic phase of pivalic acid studied by nuclear magnetic resonance

Nuclear magnetic spin relaxation times T 1, T 2 and T 1ρ have been measured for methyl and acid protons in pivalic acid throughout the plastic crystalline phase, 6·9 to 36·5°C. T 1ρ and T 2 of the methyl protons are interpreted in terms of translational self-diffusion of molecules. Diffusion coefficients are deduced which are in fair agreement with previous radiotracer measurements. T 1 measurements at 10 MHz and 20·8 MHz indicate molecular reorientation and the temperature dependence of this process is investigated. T 1ρ and T 2 measurements for the acid protons show that they move more rapidly through the lattice than the molecules. This is in agreement with previously reported tracer studies. A mechanism is proposed connecting the anomalously fast hydrogen atom diffusion and molecular reorientation, which is consistent with the N.M.R. and tracer measurements.

Internal surfaces of porous media studied by nuclear magnetic resonance cryoporometry

The filling processes of water and cyclohexane in porous silica (with a characteristic pore size of 60 A) are investigated using the nuclear magnetic resonance (NMR) technique of cryoporometry. In this technique, the liquid was frozen in the pores before the temperature was raised gradually; melting the smallest particles first and then particles of increased size. The volume of the molten liquid present was measured using the height of a T2 spin echo. The experiments were performed with filling fractions ranging from 10% to 100%. The results showed distinctly different behaviors of the fluids, which depended on the surface adhesion. It was found that water (a fluid which wets the pore surface) forms small puddles—much smaller than the smallest pore size—at low filling fractions. These puddles grow in size as more water is added until all the pore volume is filled. Cyclohexane (a non-wetting fluid) on the other hand, does not form small puddles but completely fills the pores with a preference for the smal...

NMR relaxation, ionic conductivity and the self-diffusion process in barium fluoride

Reports measurements of the electrical conductivity and 19F nuclear magnetic resonance (NMR) relaxation times T1, T2, T1 rho and T1D in BaF2 single crystals over a temperature range 300K-1200K. The dominant process responsible for the 19F spin relaxation is self-diffusion. The magnitude and temperature dependence of the relaxation times are satisfactorily interpreted using the Wolf theory. Absolute values of diffusion coefficients calculated from NMR relaxation data assuming a vacancy diffusion process are in excellent agreement (within 15%) with those calculated from ionic conductivity data using the Nernst-Einstein relation. From the temperature dependence studies, the diffusion parameters were determined. The formation enthalpy for a Frenkel pair was found to be 1.91 eV and migration enthalpies for free F- vacancies and F- interstitialcies were found to be 0.57 and 0.76 eV respectively.

An N.M.R. study of the pressure dependence of molecular self-diffusion and reorientation in organic plastic crystals

The proton N.M.R. relaxation times, T 1 and T 1ρ, of the organic plastic crystals, hexamethylethane, hexamethyldisilane, norbornane, norbornylene, norbornadiene and cyclohexane, have been measured as functions of hydrostatic pressure up to 2·8 kbar‡ Bar = 105 Pa. , and in most cases at two different temperatures. These materials provide examples of b.c.c., f.c.c. and h.c.p. crystal lattice structures. The T 1 measurements are analysed in terms of the reorientational motion of the molecules. This molecular motion is found to have a weak pressure dependence associated with low values for the activation volume. The T 1ρ measurements are analysed in terms of the translational self-diffusion of the molecules. This motion has a strong pressure dependence and activation volumes, ΔV*, obtained for this motion are found to fall in the range 0·57 to 1·2 times their respective molar volumes, V m. The nature of the pressure dependence of the self-diffusion shows a correlation with the entropy of fusion, ΔS f, of the ...

Capillary Imbibition and Pore Characterisation in Cement Pastes

The kinetics of capillary imbibition in ordinary Portland cement pastes has been studied experimentally and theoretically. Nuclear magnetic resonance stray field imaging (STRAFI) has been used to record water concentration profiles for various ingress times. The profiles follow a √t law and thus a master curve can be formed using the Boltzmann transformation. The distribution of pore sizes within the sample as measured by NMR cryoporometry shows a prominent peak at 100 Å. A computer model of the pore structure was developed consisting of a lattice of interconnecting pores with a size distribution consistent with the cryoporometry results. The Hagen–Poiseuille law was used to describe the kinetics of the water in this pore structure. The best agreement between the computer simulations and the experimental master curve was obtained by using a narrower range of pore sizes than indicated by the cryoporometry results.