Methyl tunnelling, reorientation and NMR relaxation in solid acetates.

Rotational excitations of methyl groups in six solid acetates have been investigated by 1H nuclear magnetic resonance (NMR) relaxation time measurements at 15 MHz and 30 MHz and at temperatures between 10 K and the melting point. Hindering barriers between 1.6 kJ/mol and 3.7 kJ/mol have been found that could be correlated to the tunnelling frequencies observed by inelastic neutron scattering. A consistent description of the relaxation rate dependences from the classical regime at high temperatures to the quantum-mechanical regime at low temperatures is possible by Haupt's equation. The rotational potentials are mainly determined by inter-molecular interaction with an important influence of water of crystallization, if present.

The crystallization kinetics of sodalites grown by the hydrothermal transformation of kaolinite studied by 29Si MAS NMR.

The hydrothermal transformation of kaolinite to basic sodalite Na8[AlSiO4]6 (OH.H2O)2 and hydroxoborate sodalite Na8[AlSiO4]6 [B(OH)4]2 has been investigated at different temperatures (353 and 473 K). In the early stage of the reactions, the crystallization kinetics was studied by X-ray powder diffraction, thermogravimetry, IR spectroscopy and 29Si MAS NMR spectroscopy. Besides the crystallization of the sodalites, no further intermediate phases were formed. MAS NMR of the 29Si nucleus has been found to be a versatile tool to follow the progress of reactions from the signal ratio of the initial material and the crystallization product because the differences in chemical shifts result in well-separated signals. From these measurements, the growth rates of the sodalites could be determined quantitatively even for the very early stages of crystallization. It was found that sodium carbonate impurities in the NaOH solution used for the synthesis has an important influence on the reaction kinetics.

11B[27Al] and 27Al[11B] double resonance experiments on a glassy sodium aluminoborate.

Cross-polarization/magic-angle spinning (CP/MAS) and rotational echo double resonance (REDOR) experiments involving two half-integer quadrupolar nuclei, 11B and 27Al, are reported, to demonstrate boron-aluminum connectivities in a model aluminoborate glass. A detailed study of the spin-lock behavior of 11B and 27Al proves to be a prerequisite for successful CP/MAS experiments. Under MAS conditions, two distinct boron sites are observed, corresponding to tetrahedral BO4/2- sites (nuclear electric quadrupole coupling constant near 0.3 MHz) and trigonal BO3/2 sites (nuclear electric quadrupole coupling constant near 2.7 MHz). The BO4/2- sites are most successfully spin-locked in the adiabatic regime at high radio frequency (RF) field strengths, whereas for the BO3/2 sites optimum spin-lock conditions are achieved in the sudden regime (low RF field strengths). These differences can be exploited for spectral editing purposes in REDOR experiments. Using corresponding T1p filters, it becomes possible to measure individual REDOR dephasing curves for both types of boron sites. The results illustrate the possible utility of heteronuclear X-Y double resonance techniques in unravelling the intermediate range order in amorphous systems containing quadrupolar nuclei.

27Al magic-angle spinning nuclear magnetic resonance satellite transition spectroscopy of glasses in the system K2O-Al2O3-SiO2.

27Al magic-angle spinning nuclear magnetic resonance satellite transition spectroscopy at 78 MHz has been applied to determine (true) chemical shift and quadrupole coupling parameters of glasses in the system K2O-Al2O3-SiO2 with 60-80 mol% SiO2 and K2O concentrations between 0 and 24 mol%. The powdered crystalline aluminosilicates andalusite and sillimanite have also been examined. In the glasses, all Al appears to be tetrahedrally bound in the aluminosilicate network unless x = mol% K2O:mol% Al2O3 becomes extremely small. Upon decreasing x the distortion of the tetrahedral Al(OSi)4 units increases in steps, and possible explanations are discussed. Six-coordinated aluminum observed for x < 0.2 is connected with the occurrence of interstitial Al3+ ions which charge-compensate the AlO4 units in addition to K+.

The importance of paramagnetic impurities to the nuclear magnetic resonance relaxation of ion-conducting glasses.

Paramagnetic impurities have been shown to affect the 7Li nuclear magnetic resonance relaxation rates in cation-conducting glasses, and wrong data may then be extracted from the experiments. Frequency- and temperature-dependent T(-1)1 studies of lithium borate and thioborate glasses revealed that iron impurities cause frequency-independent relaxation and "shoulders" of the low-temperature slopes in high fields, whereas manganese produces enhanced relaxation peaks. The results of the T(-1)1 (and some T(-1)1p and T(-1)2) measurements are discussed.

Anomalous proton relaxation, rotational tunnelling and barriers to methyl group rotation in solid acetyl halides.

Rotational excitations of methyl groups attached to carbonyl in solid acetic acid, acetyl fluoride, acetyl chloride and acetyl bromide have been investigated by 1H nuclear magnetic resonance (NMR) relaxation times and field-cycling measurements at two frequencies and various temperatures. The tunnel splittings have been found to occur between 3.3 and 0.08 mu eV making quantum effects important for the relaxation behaviour. For the acetyl halides, similar tunnelling and NMR frequencies lead to an anomalous-looking temperature dependence of the relaxation rates. A consistent description by Haupt's equation is possible. The rotational potentials have been derived from the data and compared with those obtained from microwave spectra of the corresponding isolated molecules. The hindering potential is purely three-fold and the barrier is dominated by the functional group.

11B MAS NMR spectroscopy for characterizing the structure of glasses.

11B MAS NMR spectroscopy has been shown to deliver reliable parameters for the chemical shift and the quadrupole interaction in glasses if short excitation pulses, high-speed magic angle spinning and satellite transition spectroscopy are applied. It is possible, in particular, to separate the contributions for trigonal BO3 and tetrahedral BO4 units to determine the relative fractions of both, and to use the isotropic chemical shift values as a sensitive probe for the nearest and next nearest environments of the boron atoms. The potential of the method has been demonstrated for some borate and borosilicate glasses prepared by the alkoxide gel techniques.

Magnetic field-induced orientation of crystallites in powdered samples of layered compounds: explanation and application.

Magnetic field-induced orientation phenomena observed in many solid state NMR experiments of layered compounds have been explained in terms of the magnetic properties of these materials. Utilization of the effect to obtain "single crystal" spectra of powder samples leads to sensitive measurements of the NMR parameters which are otherwise not possible.

Zn-exchange and Mössbauer studies on the [Fe-Fe] derivatives of the purple acid Fe(III)-Zn(II)-phosphatase from kidney beans.

In order to perform Mössbauer studies, Zn(II) in the Fe(III)-Zn(II) purple acid phosphatase of the red kidney bean has been exchanged by incubating the semiapoenzyme with 57Fe(II). The resulting Fe(III)-57Fe(II) enzyme has 125% activity, compared with that of the Zn(II) enzyme. It can be oxidized by H2O2 or peroxydisulfate to the Fe(III)-57Fe(III) species with a 30-times lower activity. Incubation of the metal-free apoenzyme with 57Fe(II) in the presence of O2 leads to the 57Fe(III)-57Fe(II) species which is stable in dilute solutions, but partially oxidized during the concentration procedure to the 57Fe(III)-57Fe(III) enzyme. Limited reduction of the oxidized enzyme with ascorbate delivers a mixture of the Fe(II)-Fe(II)/Fe(III)-Fe(III) species, but not the mixed valent Fe(III)-Fe(II) species, indicating that after the transfer of the first electron the second electron of the ascorbate radical is immediately transferred to the second Fe(III). The Mössbauer spectra of the oxidized species show at 4.2 K two quadrupole doublets with delta of 0.51 mm/s and 0.53 mm/s and delta E of 1.46 and 0.96 mm/s indicating high spin Fe(III) in two different binding sites, obviously with a higher asymmetry in the chromophoric Fe(III) site. The values are too low for a mu-oxo bridge. The mixed-valent Fe(III)-Fe(II) species shows two quadrupole doublets with delta values of 0.55 mm/s and 1.14 mm/s and delta E values of 1.43 mm/s and 3.01 mm/s at 70 K for high spin Fe(II) and Fe(III), but the signal of the Fe(II) component shows magnetic patterns at 4.2 K indicating a half-integer spin system with antiferromagnetic coupling. The Fe(II)-Fe(II) system exhibits two quadrupole doublets with delta values of 1.18 mm/s and 1.22 mm/s and with delta E values of 3.69 mm/s and 2.68 mm/s again indicating a higher asymmetry in the originally chromophoric Fe(III)-binding site. Addition of phosphate shows only minor differences in the oxidized enzyme and in the mixed valent Fe(III)-Fe(II) system. Interaction with O2 is discussed.