This in situ study assesses the long term sorption of I in a natural peat bog, a matter that is scarcely addressed but required for safety studies such as for radioactive waste disposal. Fifteen years after the artificial contamination of a boreal peat bog, the groundwater (piezometers), the peat and the vegetation were resampled to determine I distribution with comparison to the initial situation (1989). Spectroscopic analyses (SS-NMR, electronic microscopy and EDX spectroscopy) were carried out on the peat solids to identify the sorption processes. Over the past 15a the I has been spreading mostly outwards and possibly upwards in the groundwater. Sorption of I is higher at the surface of the bog (Kd=37.6Lkg−1) than at the bottom (Kd=5.1Lkg−1), and this is attributed to the oxic/anoxic conditions of the peat layers. The average surface Kd values showed more than a 2-fold increase after 15a. TEM–EDX analyses of the surface peat showed here for the first time that I was only associated with natural polyphenolic substances contained in humified plant tissues. Plants growing in the bog have not taken up much I with the exception of sedge species ([I]sedge leaves is up to 283mgkg−1).

The suitability of high-resolution solid-state 31P NMR for a straightforward determination of the protonation state of phosphate groups as well as of their pK2 values extracted from solid state mono : dianionic ratios has been demonstrated.

High-resolution solid-state NMR spectroscopy is exploited to obtain structural constraints involving strongly hydrogen-bonded silanols in octosilicate, a prominent member of the layered hydrous sodium silicates. Proton-silicon cross-polarization dynamics reveals that octosilicate contains two types of Q(3) silicons present in hydrogen-bonded -Si-O-Hcdots, three dots, centeredO-Si- and -Si-O(-)-type sites which can only be distinguished by their different abilities to cross polarize and the different mobilities of neighboring hydrous species. The theoretical analysis of the oscillating components of the polarization transfer buildup curves suggests that the model of heteronuclear pairs is an adequate description of the quantum spin system within hydrogen-bonded -Si-O-Hcdots, three dots, centeredO-Si- fragments. We also show that dipolar modulated, slow speed magic-angle (29)Si NMR spectrum provides unique geometric information on strongly hydrogen-bonded silanols. The dipolar modulated spinning sidebands contain all the information necessary to determine the internuclear Sicdots, three dots, centeredH distances as well as the magnitude and orientation of the principal elements of the (29)Si chemical shielding tensor in the molecular frame. The data provide definite proof of the intralayer character of strongly hydrogen-bonded silanol groups in a bridging, albeit not symmetric, position between neighboring tetrahedra. The approach developed in this work may be useful to obtain structural information on related layered alkali metal silicates, silica gels as well as on other classes of microporous materials.

Investigation of phase structure of poly(vinylidene fluoride) and characterization of its NMR fingerprints from three main polymorphs are presented by exploiting C-13-{F-19,H-1} and F-19-{H} MAS NMR experiments. Selective excitation of F-19 NMR signal from different phases has been used for a straightforward identification and separation of spectroscopic signatures not only from the amorphous part but also from alpha, beta, and gamma polymorphs. A new insight into the phase structure of morphologically different samples has been gained by T-1p,(F-19) relaxation measurements in the conditions of very high spinning speeds and strong radio-frequency field. Motional heterogeneity of the reverse units has been revealed, and their spatial location in the amorphous, intermediate, and crystalline phases was quantified for the first time. The spectroscopic fingerprints along with phase structure features of PVF2 and the ways exploited in this work for their retrieval might be useful to obtain structural information on PVF2 of different origin as well as on other types of fluoropolymers.

When analyzing I --> S variable contact time cross-polarization (CP) curves, the spin dynamics are usually assumed to be describable in the "fast CP regime" in which the growth of the S spin magnetization is governed by the rate of cross polarization while its decay is governed by the rate of I spin T1rho relaxation. However, in the investigation of the structures of zeolite-sorbate and other complexes by polarization transfer this will not necessarily be the case. We discuss the measurement of I --> S CP rate constants under the "slow CP regime" in which the rate of T1rho relaxation is fast compared to the rate of cross polarization, leading to a reversal of the usual assumptions such that the rate or growth is governed by the rate of I spin T1rho relaxation while the decay is governed by the rate of cross polarization (and the S spin T1rho relaxation). It is very important to recognize when a system is in the slow CP regime, as an analysis assuming the normal fast CP will lead to erroneous data. However, even when the slow CP regime is recognized, it is difficult to obtain absolute values for the CP rate constants from fits to standard CP curves, since the CP rate constant is correlated to the scaling factor, the contribution from 29Si T1rho relaxation is ignored, and it is difficult to obtain reliable data at very long contact times. The use of a 29Si{1H} CP "drain" or "depolarization" experiment, which measures absolute values of the CP rate constants, is therefore proposed as being most appropriate for theses situations. To illustrate the importance of these observations, measurements of the 1H-29Si CP rate constants in the p-dichlorobenzene/ZSM-5 sorbate-zeolite complex by 29Si{1H} CP and CP drain magic-angle spinning (MAS) NMR experiments are presented and compared and used to determine the location of the guest sorbate molecules in the cavities of the host zeolite framework.

Two different samples of high-density polyethylene (HDPE) have been studied. One (isotropic) is extracted from the material core whereas the other (anisotropic) involves two sides which have been in contact with the injection mold. It is observed by NMR microscopy (using radiofrequency field gradients) that these two sides favor toluene penetration into the material. The distribution of toluene nuclear spin relaxation times (extracted from proton T-1 and T-2 images) exhibits likewise important differences between the two samples. These differences can be accounted for by partial molecular ordering at the vicinity of the "mold sides". Finally, in investigating the anisotropic sample (without solvent), three different phases (two amorphous and one crystalline) are revealed by C-13 chemical shift imaging experiments (performed with radiofrequency field gradients under CP/MAS conditions). Each amorphous component is preferentially present at one of the two "mold sides".

An insight into the motional heterogeneity of a series of poly(ether-block-amide) copolymers is presented and discussed in terms of its NMR fingerprints dependent on a content and length of hard and soft segments and a microphase-separated morphology. Local-field dipolar spectra carry a straightforward signature of microphase-separated morphology endowed with a strong mobility gradient. The dipolar features of the poly(tetramethylene glycol) (PTMG) and polyamide-12 (PA) components, besides visualizing the presence of separated PTMG and PA phases, also give evidence for the presence of the dynamic heterogeneity of each component. H-1 MAS spectra provide an another fingerprint of phase-separated systems. Although such spectra roughly show increasing line broadening at higher PA/PTMG ratio, they are mainly sensitive to the presence of the most mobile fragments and may give a distorted vision of intrinsic mobility of the soft component in the presence of its dynamic heterogeneity. C-13 spectra bearing proton-carbon J-coupling features appear to be more sensitive to the length of the soft blocks and seem to bring a proper visualization of their overall mobility. Special attention has been paid to characterize motional heterogeneity of the soft component by exploiting cross-polarization transfer efficiency combined with indirect T-2(H-1) relaxation measurements. The cross-polarization inversion experiments provide, from heteronuclear dipolar interaction perspective, a corroborating visualization of the extent of motional distribution of the PTMG component. The revealed fingerprints of the motional heterogeneity of the soft component and the ways exploited in this work for their retrieval might be helpful for a better assessment of its role in the mechanical properties of thermoplastic elastomers.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A straightforward, albeit still unexploited, approach to access sharp principal value features of the chemical shift tensor in slightly off-magic angle, slowly spinning powder solids is revisited. Its major advantages consist of (i) the direct visualization of axial or more general tensor/site symmetry, (ii) the absence of any rotor-synchronized or amplitude-matched radiofrequency pulse, and (iii) the use of routine single-pulse or cross-polarization acquisition modes upon modest spinning speeds on a standard cross-polarization/magic-angle spinning probe. The experiments and simulations show that the principal value features remain well resolved in an easily accessible range of chemical shift anisotropy/spinning frequency ratios and have fixed spectral positions at different spinning speeds. Several useful applications of this simple and robust approach can be envisaged in organic and inorganic solids. (C) 2004 Wiley Periodicals, Inc.

A series of l and dl forms of O-phosphorylated amino acids (serine, threonine, tyrosine) have been studied by using solid-state multinuclear NMR spectroscopy and ab initio calculations. Principal elements of the (13)C and (31)P chemical shielding tensors have been measured and discussed in relation to zwitterionic structures and intermolecular contacts. DFT calculations have been compared with experimental data showing their ability to reproduce experimentally obtained tensor values in this challenging class of compounds. The changes of orientation of (31)P chemical shielding tensor with respect to the molecular frame in the presence of hydrogen bonds have been revealed and discussed on the ground of theoretical calculations. The measurements of internuclear P...P distances, based on Zeeman magnetization exchange between (31)P spins with differing chemical shielding tensor orientations, were exploited for a clear distinction between enantiomers and racemates.

Informationen über das Vorliegen und die Struktur unterschiedlicher deprotonierter Formen der Malonsäure lassen sich durch Festkörper-13C-NMR-Spektroskopie erhalten (siehe Bild). Vermessen wurden lyophilisierte Proben, die aus Stammlösungen mit unterschiedlichen pH-Werten hergestellt wurden. Die aus Säure/Base-Verhältnissen in der festen Phase berechneten pK-Werte entsprechen den mit klassischen Methoden in Lösung bestimmten.

An approach for the investigation of layered alkali metal polysilicate hydrous materials is presented by choosing complementary multinuclear NMR methodologies. New insights into the structural and motional features of magadiite, a layered sodium silicate hydrate, have been gained using H-1, H-1, Si-29 and Na-23 one- and two-dimensional NMR experiments. Complementary H-1 MAS single- and double-quantum studies have proven to be powerful tools in a clear recognition of the nature of hydrous species involved in hydrogen bonding. 1H and H-2 MAS experiments permitted us to determine the extent of the mobility of water molecules and provided corroborating evidences of an intralayer, strongly hydrogen bonded character of silanol groups. On the basis of a novel strategy in the analysis of the cross polarization dynamics, it appeared that magadiite contains two types of structurally different Q((3)) tetrahedra, as well as two types of sodium ions. The strategy applied in this work might be useful to obtain structural and motional information in the related layered alkali metal silicates, as well as in other classes of nanoporous materials.

Based on multifield NMR relaxation measurements and quantum chemistry calculations, a strategy aiming at the determination of the chemical shielding tensor (CST) in the liquid state is described. Brownian motions in the liquid state restrict the direct observation of CST to a third of its trace (isotropic shift), and even if CST can be probed indirectly through some spin relaxation rates (specific longitudinal relaxation rates, dipolar chemical shift anisotropy (CSA) cross-correlation rates), an insufficient number of experimental parameters prevents its complete determination. This lack of information can be compensated by using quantum chemical calculations so as to obtain the molecular CST orientation even if a relatively modest level of computation is used. As relaxation parameters involve a dynamic part, a prerequisite is the determination of the molecular anisotropic reorientation which can be obtained independently from dipolar cross-relaxation rates. A polycyclic molecule exhibiting a well-characterized anisotropic reorientation serves as an example for such a study, and some (but not all) carbon-13 chemical shielding tensors can be accurately determined. A comparison with solid-state NMR data and numerous chemical quantum calculations are presented.

It is shown that enantiomers and racemates that have identical isotropic NMR chemical shift as well as anisotropic chemical-shift tensor parameters can be easily distinguished by means of the ODESSA (One Dimensional Exchange Spectroscopy by Sideband Alternation) technique. The method is based on the fact that the molecular symmetries and packing of enantiomers and racemates are usually significantly different. The power of the proposed approach is demonstrated by employing as model compounds P-chiral oxazaphosphorine derivatives, which are widely used in clinical oncology. Correlation of the amplitude of the ODESSA decay (AOD) with enantiomeric excess is also presented.

Exploration of the molecular geometry in rotating powder solids on the basis of magnetization exchange between spins with identical isotropic chemical shifts but differing chemical shielding tensor orientations is demonstrated experimentally. For this we take advantage of the potential of the ODESSA (one-dimensional exchange spectroscopy by sidebands alternation) experiment for the accurate measurement of spin exchange rate constants. We also report the observation of oscillatory behavior of the rotor-driven magnetization exchange at this so-called n=0 rotational-resonance condition which, in contrast to n=1,2,3,… rotational-resonance conditions, takes place at nearly arbitrary magic-angle spinning frequencies. The sensitivity of the longitudinal exchange decays to the relevant physical parameters of the spin system under conditions of rotor-driven and proton-driven magnetization exchange is discussed theoretically and demonstrated experimentally. Several C13 and P31 spin-exchange measurements have been performed on a series of model compounds covering a broad range of internuclear distances between carboxyl carbon atoms, and on a series of phosphorylated amino acids with different internuclear distances between phosphorus sites. The capacity of the ODESSA experiment for an unambiguous recognition of distinct internuclear distances is demonstrated. Potential applications of such measurements involve the exploration of intermolecular distances and the determination of the mutual orientation of neighboring molecular fragments in polycrystalline and noncrystalline solids.

When, in the method MAS, part of the sample is at the edge of the coil so as to experience a partly radial RF field, this part gives rise during the sample spinning to spectra shifted by -1 and +1 times the spinning frequency, which are superimposed on the normal spectra with possible sidebands due to the modulated interactions, anisotropic chemical shifts and dipolar or quadrupolar interactions. The amplitude of these shifted spectra depends on the RF field distribution over the sample and on the amplitude of the RF pulse preceding the observation of the FID. On the other hand, it is independent of the spinning frequency. The quantitative description of this effect necessitates the use of the Reciprocity Theorem. Following a simplified account of the relevant theory, experimental illustrations Of this effect are presented. The analysis of these contributions to the amplitudes of the sidebands makes it possible to determine the conditions under which the perturbation of the spectra is negligible. (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.

Hydroxyaluminosilicates (HAS) are critical intermediates in the biogeochemical cycles of aluminium and silicon. To understand the extent of their role in controlling the solubility of Al in soil and surface waters, we need to know how they are formed and why they are formed in preference to other more soluble aluminous mineral phases. We have used a number of complementary analytical techniques to demonstrate the hitherto unproven mechanism of HAS formation of varying structure and stoichiometry. HAS were formed via the competitive condensation of silicic acid, Si(OH)4, at a hydroxyaluminum template (HASA). Where Si(OH)4 was present in excess, HASA acted as a template for further reactions with Si(OH)4 (HASB). HASA and HASB had idealised Si : Al ratios of 0.5 and 1.0, respectively, and were representative of HAS found in soil horizons. This mechanism of formation of HAS could be used to both explain and predict the role of Si(OH)4 in Al solubility control in the natural environment.

Proton relaxation times in the rotating frame, as obtained from the decay of nuclear magnetization locked along a radio frequency field B-1, have been measured in a series of cross-linked natural rubber materials. These decays are monoexponential and the relevant time constants, T-1p, can be monitored as a function of the radio frequency field strength B-1. At any B-1 value, they appear to be correlated in a very sensitive way to the cross-link density. A novel interpretation of the T-1p dependence upon B-1 is presented. It is based on a theory describing the evolution of a two spin 1/2 system coupled by dipolar interaction in the presence of a spin-locking field. The measured rotating-frame relaxation rate is shown to be equal to the sum of the motionally related 1/T-1p and a term arising from the radio frequency field inhomogeneity. Both terms are proportional to the square of the dipolar interaction (the second moment in the present case where a distribution of dipolar interactions exists). Concerning changes of cross-link density, Tip measurements provide therefore a more direct information than transverse relaxation measurements (also performed in this study), the composite nature of the latter decay curves making their interpretation less straightforward. As a complement to the present relaxation study, we show the correlation of the self-diffusion coefficient of a solvent imbedded in the material with rubber T-1p values.

It is shown that chemical shift imaging can be easily adapted to high-resolution solid-state experiments by means of radio-frequency field gradient technology. (The gradient is delivered by a two-turn flat coil, the axis of which coinciding with the rotor axis.) The resulting two-dimensional diagram involves chemical shift information in one dimension and spatial information (along the rotation axis) in the other dimension. Different procedures (physical filters) have been considered in order to enhance specific properties of the crystalline and amorphous components in high-density polyethylene (hdpe). Two of them rely on differences in cross-polarization or in cross-polarization inversion processes, while a third one is based on carbon-13 longitudinal relaxation times. The latter is shown to be especially efficient for effectively obtaining separate images of the amorphous and crystalline components and ultimately unraveling their distribution within the sample. In particular, it is demonstrated that two structurally distinct amorphous phases are spatially separated in the sample under investigation.

The existence of sidebands at +/-v(r) in MAS spectra due to the radial component of the RF field at the edges of the coil is described theoretically and illustrated experimentally. The height of the radial-field sidebands does not depend on the spinning speed and may contribute significantly to the intensity of -1 spinning sideband of MAS modulated internal interactions for a sample placed in a rotor of length exceeding the solenoid coil or a small volume sample placed at the edge of the coil.