Diffusive diffraction observed with volume-selective STEAM MRS in 100 mu m water-filled capillaries

Diffusive diffraction patterns may be useful for probing the local environment of diffusing molecules in materials with geometrically ordered microstructure. A model system consisting in a bundle of waterfilled 100 mu m glass capillaries was probed with volume-selective stimulated echo (STEAM) MRS on a 7T Bruker PharmaScan tomograph with variable diffusion times for both, parallel and perpendicular diffusion-weighting with respect to the capillaries' axes. The precise orientation of the capillaries was determined with image processing methods. Echo attenuation curves were numerically evaluated with respect to the inner radius of the capillaries R and the diffusion coefficient D using equations given in the literature. Good agreement was found between simulation and experiment. For perpendicular diffusion-weighting and diffusion times in the order of R-2/D two diffraction minima were observed which were not present for shorter diffusion times and for parallel diffusion-weighting. In conclusion, volume-selective diffusive diffraction was observed with a standard small-animal tomograph. (C) 2008 Elsevier Inc. All rights reserved

Magnetic resonance elastography and diffusion-weighted imaging of the sol/gel phase transition in agarose

The dynamics of the sol/gel phase transition in agarose was analyzed with magnetic resonance elastography (MRE) and diffusion-weighted imaging, providing complementary information on a microstructural as well as on a macroscopic spatial scale. In thermal equilibrium, the diffusion coefficient of agarose is linearly correlated with temperature, independent of the sol/gel phase transition. In larger agarose samples, the transition from the sol to the gel state was characterized by a complex position and temperature dependency of both MIZE shear wave patterns and apparent diffusion coefficients (ADC). The position dependency of the temperature was experimentally found to be qualitatively similar to the behavior of the ADC maps. The dynamics of the temperature could be described with a simplified model that described the heat exchange between sol and gel compartments. The experiments supported the approach to derive temperature maps from the ADC maps by a linear relationship. The spatially resolved dynamics of the temperature maps were therefore employed to determine the elasticities. For this reason, experimental MRE data were simulated using a model of coupled harmonic oscillators. The calculated images agreed well with the experimentally observed MRE wave patterns. (C) 2003 Elsevier Inc. All rights reserved

Microcrystallization of benzene-d(6) in mesoporous silica revealed by H-2 solid-state nuclear magnetic resonance

Benzene-d(6) confined in a mesoscopically organized, controlled, porous-glass mesocellular foam (MCF), with a pore diameter of 30 nm, is studied by low-temperature solid-state H-2 nuclear magnetic resonance (H-2 NMR) spectroscopy in the temperature range of 90-180 K. The resulting spectra are compared to bulk benzene-d(6) and benzene confined in a mesoporous silica (SBA-15). The comparison shows that the spectra in the MCF are the superposition of an amorphous surface phase and a crystalline inner bulk phase. For the inner crystalline phase, the activation energies coincide with that of bulk benzene. The pore volume and the filling factor indicate that approximately three molecular layers of benzene are present on the inner surfaces

Stray field gradient NMR reveals effects of hydrogen bonding on diffusion coefficients of pyridine in mesoporous silica

The diffusion of pyridine confined in mesoporous silica MCM-41 (d(pore) = 3.3 nm) was studied with stray field gradient (SFG) NMR diffusometry as a function of the filling factor of the mesopores at room temperature, employing a laboratory-built SFG setup. The translational diffusion of pyridine in MCM-41 is found to be anisotropic and the diffusion parallel to the pores' cylinder axes is much faster than that perpendicular to them. The parallel diffusion coefficient depends strongly on the filling level of the guest liquid inside the pores. For a filling level of 25, which corresponds approximately to a monomolecular layer of pyridine molecules hydrogen bonded to surface-SiOH groups, a parallel diffusion coefficient of D-parallel to = 1.0 X 10(-9) m(2) s(-1) is found, which is slower than the diffusion coefficient of the bulk liquid (D = 1.6 X 10(-9) m(2) s(-1)). For higher filling factors the parallel diffusion coefficient increases and at a filling factor of 85 a diffusion coefficient of D-parallel to = 6.8 X 10(-9) m(2) s(-1) is reached. The perpendicular diffusion coefficient of D-perpendicular to = 3.7 +/- 2.0 X 10(-11) m(2) s(-1) is independent of the filling factor. Employing additional N-15 MAS data for the pyridine inside the mesopores, a microscopic model of the diffusion is proposed, which depends on the exchange of the slowly diffusing hydrogen-bonded surface pyridine molecules with fast-diffusing free pyridine molecules inside the pores. Copyright (C) 2001 John Wiley & Sons, Ltd

Hydrogen bonding of water confined in mesoporous silica MCM-41 and SBA-15 studied by H-1 solid-state NMR

The adsorption of water in two mesoporous silica materials with cylindrical pores of uniform diameter, MCM-41 and SBA-15, was studied by H-1 MAS (MAS = magic angle spinning) and static solid-state NMR spectroscopy. All observed hydrogen atoms are either surface -SiOH groups or hydrogen-bonded water molecules. Unlike MCM-41, some strongly bound water molecules exist at the inner surfaces of SBA-15 that are assigned to surface defects. At higher filling levels, a further difference between MCM-41 and SBA-15 is observed. Water molecules in MCM-41 exhibit a bimodal line distribution of chemical shifts, with one peak at the position of inner-bulk water, and the second peak at the position of water molecules in fast exchange with surface -SiOH groups. In SBA-15, a single line is observed that shifts continuously as the pore filling is increased. This result is attributed to a different pore-filling mechanism for the two silica materials. In MCM-41, due to its small pore diameter (3.3 nm), pore filling by pore condensation (axial-pore-filling mode) occurs at a low relative pressure, corresponding roughly to a single adsorbed monolayer. For SBA-15, owing to its larger pore diameter (8 nm), a gradual increase in the thickness of the adsorbed layer (radial-pore-filling mode) prevails until pore condensation takes place at a higher level of pore filling

Hydrogen deuterium isotope effect on exchange rates in eta(2) bond transition metal dihydrogen complexes revealed by H-2 solid state NMR spectroscopy

A selectively eta(2)-D-2 labeled isotopomer of the complex W(PCy3)(2) (CO)(3)) (eta(2)-D-2) has been synthesized and the H-2 NMR spectra and spin lattice relaxation rates of this complex have been measured in the temperature regime of 50 K to 300 K. The spectra have been analyzed employing a model of a combination of homonuclear dipolar D-D interaction and deuterium quadrupolar interaction and a D-D distance of 0.89 +/- 0.1 Angstrom. The line width of the spectra exhibits a weak temperature dependence at temperatures above 150 K. This temperature dependence is interpreted as a slight decrease of the quadrupolar coupling with increasing temperature, which is an indication of a change of the M-D-2 distance with changing temperatures. The spin lattice relaxation data of the complex exhibit pronounced deviations from a simple Arrhenius behavior at lower temperatures, indicating the presence of a quantum mechanical tunneling process. This process is analyzed in terms of a simple one-dimensional Bell tunnel model. A comparison with INS data from the H-2 complex reveals a strong isotope effect of 2 x 10(3) for the exchange rates of the deuterons

Evidence of microphase separation in controlled pore glasses

The phase separation of a mixture of water and isobutyric acid (iBA) confined in the pore space of Controlled Pore Glass (CPG) 10-75 has been studied by H-1 NMR relaxometry and H-1-pulsed field gradient (PFG) diffusion measurements. For an acid-rich mixture (mass fraction 54wt iBA), evidence of a phase separation process in the pores was obtained, which occurs in a temperature window between 32 and 39 degrees C, as indicated in the PFG data by an anomalous temperature dependence of the diffusion coefficient and in the relaxation data by a bi-exponential magnetization decay. The phase separation temperature of the mixture in the pore is slightly lower than in the bulk mixture of the same composition (41 degrees C) and extends over a finite temperature range. A qualitative model of the phase separation process in the pores is developed, which assumes a temperature-dependent domain-like structure of the liquid below the phase transition temperature and a breakdown of these domains upon reaching the transition temperature. (c) 2005 Elsevier Inc. All rights reserved

H-2-solid-state NMR study of benzene-d(6) confined in mesoporous silica SBA-15

Benzene-d(6) confined in the hexagonal ordered cylindrical pores of mesoporous silica SBA-15 (pore diameter 8.0 nm) was studied by low-temperature H-2-solid-state NMR spectroscopy in the temperature range between 236 and 19 K and compared to bulk benzene-d(6). The solid-state spectra of the bulk benzene-d(6) exhibit quadrupolar Pake patterns at high and low temperatures, and in the intermediate temperature regime the typical line shape changes caused by rotational jumps around the 6-fold axis. At all temperatures the benzene molecules are characterized by a single rotational correlation time. For benzene-d6 confined in SBA-15, however, these exchange dominated line shapes are not found. At all temperatures below the freezing point the spectra of benzene in the silica show the coexistence of two states with temperature-dependent intensity ratios. This behavior is the result of a Gaussian distributions of activation energies for the rotational jumps inside the pores. For the solid I-solid 11 (fast 6-fold jump to slow 6-fold jump) transition the center of the distribution is at 40 K (6.0 kJ/mol) with a width of 19.5 K (2.9 kJ/mol). For the liquid-solid I (liquidlike to fast 6-fold jump) transition the center of the distribution is at 204 K (30.6 kJ/mol) and the width is 15 K (2.2 kJ/mol). From the pore volume and the filling factor, a thickness of four molecular layers of this surface phase is estimated