Chemical structural studies of natural lignin by dipolar dephasing solid-state 13C nuclear magnetic resonance

Two natural lignins, one from a gymnosperm wood the other from angiosperm wood, were examined by conventional solid-state and dipolar dephasing ¹³C nuclear magnetic resonance (NMR) techniques. The results obtained from both techniques show that the structure of natural lignins is consistent with models of softwood and hardwood lignin. The dipolar dephasing NMR data provide a measure of the degree of substitution on aromatic rings which is consistent with the models.     

31P and 27Al solid-state nuclear magnetic resonance study of taranakite

High-resolution ²⁷Al solid-state nuclear magnetic resonance (NMR) spectroscopy indicates that aluminum in taranakite is probably restricted to six-coordinate sites. High-resolution ³¹P solid-state NMR spectroscopy reveals that phosphate groups exist in two environments in taranakite. The rate of ¹H-induced dipolar dephasing of the ³¹P signals in cross-polarization, magic-angle-spinning NMR spectra of taranakite suggests that one or more oxygens of one of two phosphates are directly protonated. The same experiments suggests that the oxygens of the second form of phosphate are not directly protonated but may be hydrogen-bond receptors. The ratio of protonated phosphate to non-protonated phosphate, as measured from ³¹P single-pulse excitation, magic-angle-spinning spectra, is approximately one to three. Current address: Department of Soil Science, 1525 Observatory Drive, University of Wisconsin-Madison, Madison, WI 53706, USA     

Orientation and motion of water molecules in cordierite: A proton nuclear magnetic resonance study

Conventional and solid state proton nuclear magnetic resonance (NMR) techniques have been used to examine water molecules in the channels of a single crystal of cordierite, (Mg, Fe)₂Al₄Si₅O₁₈, as a function of temperature, magnetic field, and orientation. Only one type of water was found rather than water in two distinct rigid orientations which were indicated by earlier infrared spectral studies. However, the measured dipolar splittings indicate that this water is in rapid motion. Shifts in the dipolar doublet due to Fe²⁺ impurities indicate that the water molecules are not moving among adjacent channel sites along a channel cavity. A two-site hopping model is proposed involving the major residence time spent with the hydrogen-hydrogen vector parallel to the channels, a minor residence time spent with the hydrogen-hydrogen vector perpendicular to the channels, and a short time (<1 μs) in transit. This model fits both the present NMR data and previously reported infrared absorption data and is compared to previously reported neutron diffraction data.     

Laboratory simulation of meteoritic noble gases. I. Sorption of xenon on carbon: Trapping experiments

We have studied trapping of radioactive ¹²⁷Xe in three types of carbon: carbon black (lamp black  LB), pyrolyzed polyvinylidene chloride (PVDC), and pyrolyzed acridine (C₁₃H₉N). A total of 86 samples were exposed to Xe at T between 100 and 1000°C, for times between 5 min and 240 hours, at p_xe ~ 5 × 10^?7 atm. Excess gas phase and loosely sorbed Xe were pumped away and the remaining, tightly bound Xe was measured by γ-spectrometry.At 100°C,× >90% of the Xe desorbs within a few minutes' pumping but a small amount remains even after 4000 min. Distribution coefficients for this tightly bound Xe are ~1 × 10^?2, 1 and 10 ccSTP/g atm for LB, acridine and PVDC carbons. The tightly bound Xe consists of two components. One occurs over the entire range 100–1000°C, becoming less abundant at high T; it appears to be physisorbed. The other occurs only at T > 500°C and is probably due to volume diffusion. The adsorbed component in LB has an apparent ΔH between ?2.3 and ?5.7 kcal/mole. The diffused component, which occurs in LB and possibly in acridine carbon, has an activation energy Q = 27 ± 8 kcal/mole and a diffusion coefficient D = 1.3 × 10^?17 cm²/sec at 1000°C. These values are comparable to those found for other types of amorphous carbon (Morrisonet al., 1963; Nakai et al., 1960).The low-T component displays two paradoxical features: low ΔH_ads, in the range for Xe physisorbed on carbon, but exceedingly long adsorption or desorption times (~10³ min at 100–400 or 1000°C). Although these long times seem to suggest a high energy process such as chemisorption, our results are best explained by a model that invokes physisorption within a labyrinth of micropores—of atomic dimensions—known to exist in amorphous carbons. The long adsorption/desorption times reflect either the long distances (~5 cm) Xe atoms must migrate by random walk to enter or leave the labyrinth, or the long times needed for Xe atoms to traverse tight spots or constricted pores that connect interior and exterior surfaces of the carbon (activated entry). Both variants of this model predict long equilibration times for the observed ΔH_ads of ?2 to ?6 kcal/mole. Apparently, xenon can be tightly trapped in carbon without resorting to high-energy bonding or to exotic mechanisms.These results suggest that “planetary” type noble gases in meteorites, located at or near grain surfaces of amorphous carbon, may be trapped by adsorption in micropores, whereas components such as CCFXe, which are uniformly distributed in their carrier phases, may be trapped by mechanisms such as volume diffusion or ion implantation.     

Monitoring accelerated clogging of a model horizontal sub-surface flow constructed wetland using magnetic resonance transverse relaxation times

Horizontal sub-surface flow wetlands are essentially a bed of porous material in which suitable plants are grown to facilitate the removal of organic matter and particulates from wastewater. The aim of this study is to assess the reliability and accuracy of magnetic resonance transverse relaxation time for monitoring clogging development in a constructed wetland. In this study, three different horizontal sub-surface flow constructed wetland models have been produced using tubes packed with different sizes of glass beads with diameter 3, 8 and 14 mm. Accelerated clogging has been achieved by pumping sludge extracted from a real clogged wetland through the bead pack. A desktop MRI tomography system has been used to monitor the transverse relaxation rate as a function of position along the tube and hydraulic conductivity. To corroborate the clogging with magnetic resonance measurements, the head loss was monitored to determine the hydraulic conductivity. Using a bi-exponential fit to the spin echo train data, the slow relaxation rate contribution shows good correlation with the changing hydraulic conductivity. Both fast and slow contributions map well to the expected clog patterns for a constructed wetland. We have demonstrated that there is a linear correlation between the hydraulic conductivity and both parameters of a bi-exponential fit to R ₂ ^eff , but particularly for the case of the short T ₂ component.     

Climatic dependence of stable carbon and oxygen isotope signals recorded in speleothems: From soil water to speleothem calcite

Understanding the relationship between stable isotope signals recorded in speleothems (δ¹³C and δ¹⁸O) and the isotopic composition of the carbonate species in the soil water is of great importance for their interpretation in terms of past climate variability. Here the evolution of the carbon isotope composition of soil water on its way down to the cave during dissolution of limestone is studied for both closed and open-closed conditions with respect to CO₂.The water entering the cave flows as a thin film towards the drip site. CO₂ degasses from this film within approx. 10 s by molecular diffusion. Subsequently, chemical and isotopic equilibrium is established on a time scale of several 10-100 s. The δ¹³C value of the drip water is mainly determined by the isotopic composition of soil CO₂. The evolution of the δ¹⁸O value of the carbonate species is determined by the long exchange time T_ex, between oxygen in carbonate and water of several 10,000 s. Even if the oxygen of the CO₂ in soil water is in isotopic equilibrium with that of the water, dissolution of limestone delivers oxygen with a different isotopic composition changing the δ¹⁸O value of the carbonate species. Consequently, the δ¹⁸O value of the rainwater will only be reflected in the drip water if it has stayed in the rock for a sufficiently long time.After the water has entered the cave, the carbon and oxygen isotope composition of the drip water may be altered by CO₂-exchange with the cave air. Exchange times, , of about 3000 s are derived. Thus, only drip water, which drips in less than 3000 s onto the stalagmite surface, is suitable to imprint climatic signals into speleothem calcite deposited from it.Precipitation of calcite proceeds with time constants, τ_p, of several 100 s. Different rate constants and equilibrium concentrations for the heavy and light isotopes, respectively, result in isotope fractionation during calcite precipitation. Since T_ex ? τ_p, exchange with the oxygen in the water can be neglected, and the isotopic evolution of carbon and oxygen proceed analogously. For drip intervals T_d < 0.1τ_p the isotopic compositions of both carbon and oxygen in the solution evolve linearly in time. The calcite precipitated at the apex of the stalagmite reflects the isotopic signal of the drip water.For long drip intervals, when calcite is deposited from a stagnant water film, long drip intervals may have a significant effect on the isotopic composition of the DIC. In this case, the isotopic composition of the calcite deposited at the apex must be determined by averaging over the drip interval. Such processes must be considered when speleothems are used as proxies of past climate variability.     

Chemical structures of coal lithotypes before and after CO2 adsorption as investigated by advanced solid-state C nuclear magnetic resonance spectroscopy

Four lithotypes (vitrain, bright clarain, clarain, and fusain) of a high volatile bituminous Springfield Coal from the Illinois Basin were characterized using advanced solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The NMR techniques included quantitative direct polarization/magic angle spinning (DP/MAS), cross polarization/total sideband suppression (CP/TOSS), dipolar dephasing, CH_n selection, and recoupled C-H long-range dipolar dephasing techniques. The lithotypes that experienced high-pressure CO₂ adsorption isotherm analysis were also analyzed to determine possible changes in coal structure as a result of CO₂ saturation at high pressure and subsequent evacuation. The main carbon functionalities present in original vitrain, bright clarain, clarain and fusain were aromatic carbons (65.9%-86.1%), nonpolar alkyl groups (9.0%-28.9%), and aromatic C-O carbons (4.1%-9.5%). Among these lithotypes, aromaticity increased in the order of clarain, bright clarain, vitrain, and fusain, whereas the fraction of alkyl carbons decreased in the same order. Fusain was distinct from other three lithotypes in respect to its highest aromatic composition (86.1%) and remarkably small fraction of alkyl carbons (11.0%). The aromatic cluster size in fusain was larger than that in bright clarain. The lithotypes studied responded differently to high pressure CO₂ saturation. After exposure to high pressure CO₂, vitrain and fusain showed a decrease in aromaticity but an increase in the fraction of alkyl carbons, whereas bright clarain and clarain displayed an increase in aromaticity but a decrease in the fraction of alkyl carbons. Aromatic fused-rings were larger for bright clarain but smaller for fusain in the post-CO₂ adsorption samples compared to the original lithotypes. These observations suggested chemical CO₂-coal interactions at high pressure and the selectivity of lithotypes in response to CO₂ adsorption.     

Exploitation of relaxation in cross-polarization nuclear magnetic resonance spectroscopy of fossil fuels

The judicious choice of dipolar dephasing times or carbon magnetization holding times has been shown to improve resolution in solid state nuclear magnetic resonance (NMR) spectra of complex materials. Signals from protonated and alkylated aromatic carbons are reduced to enhance resolution of aromatic oxygenated groups. Rapidly rotating methyl groups can be resolved from other aliphatic carbon types. These techniques were used to investigate the structure of a brown coal, xylite fractions of a brown coal, a bituminous coal, an oil shale and a solvent-refined coal. The results allow estimates of the fraction of aromatic carbon that is protonated in coal to be made, and demonstrate that methyl groups in coal rotate rapidly at room temperature.     

Carbon isotopic thermometry calibrated by dolomite-calcite solvus temperatures

The temperature dependence of carbon isotopic fractionations between calcite and graphite, and between dolomite and graphite are calibrated by the calcite-dolomite solvus geothermometry using marbles collected from the contact metamorphic aureole in the Kasuga area, central Japan. The carbon isotopic fractionations (Δ¹³C_Cc-Gr and Δ¹³C_DoGr) systematically decrease with increasing metamorphic temperature. The concordant relationships between the fractionations and solvus temperatures are approximately linear with T^?2 over the temperature range. 400° to 680°C: Δ¹³C_CcGr (%.) = 5.6 × 10⁶ × T^?2 (K) ? 2.4 Δ¹³C_DoGr (%.) = 5.9 × 10⁶ × T^?2 (K) ? 1.9 These systematic relationships between fractionation and temperature suggest that carbon isotopic equilibria between carbonates and graphite were attained in many cases. The equation for the calcite-graphite system has a slope steeper than Bottinga's (1969) results. It is, however, in good agreement with that of Valley and O'Neil (1981) in the temperature range from 600° to 800°C.Because of the relatively high sensitivity to temperature, these isotopic geothermometers are useful for determining the temperatures in moderate- to high-grade metamorphosed carbonate rocks.     

Raman spectroscopy and heat capacity measurement of calcium ferrite type MgAl2O4 and CaAl2O4

Raman spectroscopy of calcium ferrite type MgAl₂O₄ and CaAl₂O₄ and heat capacity measurement of CaAl₂O₄ calcium ferrite were performed. The heat-capacity of CaAl₂O₄ calcium ferrite measured by a differential scanning calorimeter (DSC) was represented as C_P(T)=190.6–1.116 × 10⁷T^–2 + 1.491 × 10⁹T^–3 above 250 K (T in K). The obtained Raman spectra were applied to lattice dynamics calculation of heat capacity using the Kieffer model. The calculated heat capacity for CaAl₂O₄ calcium ferrite showed good agreement with that by the DSC measurement. A Kieffer model calculation for MgAl₂O₄ calcium ferrite similar to that for CaAl₂O₄ calcium ferrite was made to estimate the heat capacity of the former. The heat capacity of MgAl₂O₄ calcium ferrite was represented as C_P(T)=223.4–1352T ^–0.5 – 4.181 × 10⁶T ^–2 + 4.300 × 10⁸T ^–3 above 250 K. The calculation also gave approximated vibrational entropies at 298 K of calcium ferrite type MgAl₂O₄ and CaAl₂O₄ as 97.6 and 114.9 J mol^–1 K^–1, respectively.     

Structure of mineral glasses—III. NaAlSi3O8 supercooled liquid at 805°C and the effects of thermal history

The distribution of interatomic distances in amorphous NaAlSi₃O₈ has been determined at 805°C by X-ray radial distribution analysis to investigate structural differences between the glass (T < 763°C) and the supercooled liquid (763°C < T < 1118°C). Except for slight differences attributable to thermal expansion, no significant changes were observed. The sample crystallized during the course of the experiment, but at least one crystal-free data set was obtained. The transition from the inferred six-membered ring structure of the supercooled liquid to the four-membered ring structure of the crystal was clearly visible in radial distribution function (RDF's) determined before and after crystallization.RDF's were also determined at 25°C for two NaAlSi₃O₈ glasses with different histories. The first was derived from a melt that had been cooled slowly from 1600 to 32°C above the melting point (T_f = 1118°C) to detect possible repolymerization to a more ‘crystal-like’ structure as the melt approached T_f. The second glass was prepared by holding a single crystal of Amelia albite at 50°C above T_f to see if the crystalline four-membered ring structure was preserved in melts at temperatures just above the liquidus. No significant differences were observed between these two RDF's and one obtained from a glass quenched from 1800°C. These results suggest that in addition to the destruction of formation of a periodic structure, melting and crystallization in NaAlSi₃O₈ also involves a repolymerization of tetrahedra. This would explain the observed kinetic barrier to melting and crystallization in the anhydrous system and the catalytic effect of small amounts of water or alkali oxide.     

Diffusion and the dynamics of displacive phase transitions in cryolite (Na3AlF6) and chiolite (Na5Al3F14): Multi-nuclear NMR studies

Cryolite is a mixed-cation perovskite (Na₂(NaAl)F₆) which undergoes a monoclinic to orthorhombic displacive phase transition at ～550° C. Chiolite (Na₅Al₃F₁₄) is associated with cryolite in natural deposits, and consists of sheets of corner sharing [AlF₆] octahedra interlayered with edge-sharing [NaF₆] octahedra. Multi-nuclear NMR line shape and relaxation time (T₁) studies were performed on cryolite and chiolite in order to gain a better understanding of the atomic motions associated with the phase transition in cryolite, and Na diffusion in cryolite and chiolite. ²⁷Al, ²³Na, and ¹⁹F static NMR spectra and T₁'s in cryolite suggest that oscillatory motions of the [AlF₆] octahedra among four micro-twin and anti-phase domains in α-cryolite begin at least 150° C below the transition temperature and persist above it. Variable temperature ²³Na MAS NMR further indicates diffusional exchange at a rate of at least 13 kHz between the Na sites by the time the transition temperature is reached. ²⁷Al and ²³Na T₁'s show the same behavior with increasing temperature, indicating the same relaxation mechanisms are responsible for both. The first order nature of the cryolite transition is apparent as a jump in the ²³Na and ²⁷Al T₁'s. Above the transition temperature, the T₁'s decrease slightly indicating that the motions responsible for the drop in T₁, are still present above the transition, further supporting the dynamic nature of the high temperature phase of cryolite. Chiolite ²³Na static spectra decrease in linewidth with increasing temperature, indicating increased Na diffusion, which is interpreted as occurring within the [NaF₆] sheets in the chiolite structure, but not between the two different Na sites. ²⁷Al and ²³Na T₁'s show similar behavior as in cryolite, but there is no discontinuity due to a phase transition. ¹⁹F T₁'s are constant from room temperature to 150° C indicating no oscillatory motion of the [AlF₆] octahedra in chiolite.     

Quantitative C NMR of whole and fractionated Iowa Mollisols for assessment of organic matter composition

Both the concentrations and the stocks of soil organic carbon vary across the landscape. Do the amounts of recalcitrant components of soil organic matter (SOM) vary with landscape position? To address this question, we studied four Mollisols in central Iowa, two developed in till and two developed in loess. Two of the soils were well drained and two were poorly drained. We collected surface-horizon samples and studied organic matter in the particulate organic matter (POM) fraction, the clay fractions, and the whole, unfractionated samples. We treated the soil samples with 5 M HF at ambient temperature or at 60 °C for 30 min to concentrate the SOM. To assess the composition of the SOM, we used solid-state nuclear magnetic resonance (NMR) spectroscopy, in particular, quantitative ¹³C DP/MAS (direct-polarization/magic-angle spinning), with and without recoupled dipolar dephasing. Spin counting by correlation of the integral NMR intensity with the C concentration by elemental analysis showed that NMR was ?85% quantitative for the majority of the samples studied. For untreated whole-soil samples with <2.5 wt.% C, which is considerably less than in most previous quantitative NMR analyses of SOM, useful spectra that reflected ?65% of all C were obtained. The NMR analyses allowed us to conclude (1) that the HF treatment (with or without heat) had low impact on the organic C composition in the samples, except for protonating carboxylate anions to carboxylic acids, (2) that most organic C was observable by NMR even in untreated soil materials, (3) that esters were likely to compose only a minor fraction of SOM in these Mollisols, and (4) that the aromatic components of SOM were enriched to ∼53% in the poorly drained soils, compared with ∼48% in the well drained soils; in plant tissue and particulate organic matter (POM) the aromaticities were ∼18% and ∼32%, respectively. Nonpolar, nonprotonated aromatic C, interpreted as a proxy for charcoal C, dominated the aromatic C in all soil samples, composing 69-78% of aromatic C and 27-36% of total organic C in the whole-soil and clay-fraction samples.     

Bonding and dynamical phenomena in MgO: A high temperature 17O and 25Mg NMR study

We have measured the isotropic chemical shifts (δ_iso) and the spin-lattice relaxation times (T₁) for ¹⁷O and ²⁵Mg in MgO from room temperature up to 1300° C. The ¹⁷O chemical shifts increase linearly from 47 ppm at room temperature to 57 ppm at 1300° C, and over the same temperature range the ²⁵Mg chemical shift increases linearly from 25 to 27 ppm. These changes are not the result of changes in the bulk magnetic susceptibility of the samples, but may be due to increased orbital overlap which is the result of the increase in thermal vibration of the ions with temperature. In the case of ²⁵Mg, the shift to lower shielding with increasing temperature is opposite to that expected from simple bond length versus chemical shift trends established for the oxides at room temperature. If this is a general phenomenon, high-temperature NMR data may be biased to lower shielding. Spin-lattice relaxation times (T₁) were measured in order to study the energetics of defect motion. T₁'s for ¹⁷O and ²⁵Mg exhibit similar behavior over the range of temperatures studied. Up to 800° C, T₁'s decrease gradually, but above 800° C, T₁'s drop rapidly, with slopes corresponding to apparent activation energies of 192±9 kJ/mol (2.0±0.1 eV) for ¹⁷O and 151±6 kJ/mol (1.56±0.06 eV) for ²⁵Mg. While direct comparison of these activation energies to those derived from diffusion or conductivity measurements is complicated, the similar behavior for both nuclei suggests their relaxation phenomena are related.     

A revised model for the density and thermal expansivity of K2O-Na2O-CaO-MgO-Al2O3-SiO2 liquids from 700 to 1900 K: extension to crustal magmatic temperatures

A revised model for the volume and thermal expansivity of K₂O-Na₂O-CaO-MgO-Al₂O₃-SiO₂ liquids, which can be applied at crustal magmatic temperatures, has been derived from new low temperature (701–1092 K) density measurements on sixteen supercooled liquids, for which high temperature (1421–1896 K) liquid density data are available. These data were combined with similar measurements previously performed by the present author on eight sodium aluminosilicate samples, for which high temperature density measurements are also available. Compositions (in mol%) range from 37 to 75% SiO₂, 0 to 27% Al₂O₃, 0 to 38% MgO, 0 to 43% CaO, 0 to 33% Na₂O and 0 to 29% K₂O. The strategy employed for the low temperature density measurements is based on the assumption that the volume of a glass is equal to that of the liquid at the limiting fictive temperature, T _f ^′. The volume of the glass and liquid at T _f ^′ was obtained from the glass density at 298 K and the glass thermal expansion coefficient from 298 K to T _f ^′. The low temperature volume data were combined with the existing high temperature measurements to derive a constant thermal expansivity of each liquid over a wide temperature interval (767–1127 degrees) with a fitted 1 error of 0.5 to 5.7%. Calibration of a linear model equation leads to fitted values of Vˉ _i ±1 (cc/mol) at 1373 K for SiO₂ (26.86 ± 0.03), Al₂O₃ (37.42±0.09), MgO (10.71±0.08), CaO (15.41±0.06), Na₂O (26.57±0.06), K₂O (42.45 ± 0.09), and fitted values of dVˉ _i /dT (10⁻³ cc/mol-K) for MgO (3.27±0.17), CaO (3.74±0.12), Na₂O (7.68±0.10) and K₂O (12.08±0.20). The results indicate that neither SiO₂ nor Al₂O₃ contribute to the thermal expansivity of the liquids, and that dV/dT ^liq is independent of temperature between 701 and 1896 K over a wide range of composition. Between 59 and 78% of the thermal expansivity of the experimental liquids is derived from configurational (vs vibrational) contributions. Measured volumes and thermal expansivities can be recovered with this model with a standard deviation of 0.25% and 5.7%, respectively. Received: 2 August 1996 / Accepted: 12 June 1997     

Lignin is preserved in the fine silt fraction of an arable Luvisol

Knowledge about the fate of individual biomolecules during the decomposition process in soil is limited. We used the natural isotopic label introduced by 23 years of continuous maize cropping, together with compound specific ¹³C isotope analysis, to study lignin monomers in particle size fractions of a Luvisol. Isotope data indicated apparent decadal turnover times for lignin. A kinetic model suggests the existence of a fast and a slow decomposing lignin pool in the soil, reconciling a low stock-to-input ratio with decadal turnover times. We found new, maize-derived lignin primarily in the 63–2000 μm fraction, whereas old, C₃-derived lignin from the pre-maize vegetation had accumulated mainly in the silt (2–20 μm) fraction. This distribution of lignin differed from that of total organic carbon, which was concentrated in the <2 μm fraction. Old, C₃-derived carbon in all the soil fractions was depleted in lignin compared to new, maize-derived carbon. The observation that the 2–20 μm fraction was less depleted than the others indicates that lignin preservation is particle size specific, but the underlying mechanism controlling its preservation is not clear.     

Adsorption of dissolved organic matter on clay minerals as assessed by infra-red, CPMAS C NMR spectroscopy and low field T1 NMR relaxometry

Dissolved organic matter (DOM) is a very important environmental constituent due to its role in controlling factors for soil formation, mineral weathering and pollutant transport in the environment. Prediction of DOM physical-chemical properties is achieved by studying its chemical structure and spatial conformation. In the present study, dissolved organic matter extracted from compost obtained from the organic fraction of urban wastes (DOM-P) has been analysed by FT-IR, CPMAS ¹³C NMR spectroscopy and ¹H T₁ NMR relaxometry with fast field cycling (FFC) setup. While the first two spectroscopic techniques revealed the chemical changes of dissolved organic matter after adsorption either on kaolinite (DOM-K) or montmorillonite (DOM-S), the latter permitted the evaluation of the conformational variations as assessed by longitudinal relaxation time (T₁) distribution at the fixed magnetic field of 500 mT. Alterations of T₁ distributions from DOM-P to DOM-K and DOM-S were attributed to a decreasing molecular complexity following DOM-P adsorption on the clay minerals. This study applied for the first time solid state ¹H T₁ NMR relaxometry to dissolved organic matter from compost obtained from the organic fraction of urban wastes and revealed that this technique is very promising for studying environmentally relevant natural organic systems.     

Constraints on the structure and dynamics of the β-cristobalite polymorphs of SiO2 and AlPO4 from 31P, 27Al and 29Si NMR spectroscopy to 770 K

Nuclear magnetic resonance spectroscopic data are presented for the cristobalite polymorphs of AlPO₄ and SiO₂ from RT to 770 K, through their respective α-β transitions. The nuclear magnetic resonance (NMR) data include chemical shifts for ³¹P, ²⁷Al, and ²⁹Si, ²⁷Al quadrupole coupling parameters, and ³¹P and ²⁷Al spin-lattice relaxation rates. Also presented are electron diffraction patterns of β-cristobalite AlPO₄ that show diffuse scattering similar to that reported previously for SiO₂. For the α-phases of both AlPO₄ and SiO₂, the chemical shifts decrease approximately linearly with increasing temperature from RT to T_c and discontinuously by -2 to -3 ppm from α to β. This result is consistent with a small, continuous increase in the mean T-O-T angle (〈θ〉) of the α-phases with increasing T and an increase of 〈θ〉 by about 4° across the α-β transition for both cristobalite and its AlPO₄ analogue. Based on the ²⁹Si chemical shifts, the mean Si-O-Si angle for β-cristobalite is 152.7±1° near T_c. For AlPO₄-cristobalite, the ²⁷Al nuclear quadrupole coupling constant (C_Q) decreases approximately linearly from 1.2 MHz at RT to 0.94 MHz near T_c (493±10 K). At the α-β transition the ²⁷Al C_Q approaches zero, in agreement with the cubic average structure observed by diffraction. The satellite transitions retain a small frequency distribution above the α-β transition from electric field gradients attributed to defects. The short-range cubic symmetry of the Al-site and non-linear Al-O-P angle support a dynamically disordered model of the β-cristobalite structure. Complete averaging of the ²⁷Al quadrupole coupling in the β-phase indicates that the lifetime of any short-range ordered domains must be shorter than about 1 μs.     

Thermodynamic and magnetic properties of surface Fe<Superscript>3+</Superscript> species on quartz: effects of gamma-ray irradiation and implications for aerosol–radiation interactions

Samples of a natural amethyst, pulverized in air, and irradiated for gamma-ray doses from 0.14 to 70 kGy, have been investigated by powder electron paramagnetic resonance (EPR) spectroscopy from 90 to 294 K. The powder EPR spectra show that the surface Fe³⁺ species on the gamma-ray-irradiated quartz differ from its counterpart without irradiation in both the effective g value and the observed line shape, suggesting marked radiation effects. This suggestion is supported by quantitatively determined thermodynamic properties, magnetic susceptibility, relaxation times, and geometrical radius. In particular, the surface Fe³⁺ species on gamma-ray-irradiated quartz has larger Gibbs and activation energies than its non-irradiated counterpart, suggesting radiation-induced chemical reactions. The shorter phase-memory time (T _m) but longer spin–lattice relaxation time (T ₁) of the surface Fe³⁺ species on the gamma-ray-irradiated quartz than that without irradiation indicate stronger dipolar interactions in the former. Moreover, the calculated geometrical radius of the surface Fe³⁺ species on the gamma-ray-irradiated quartz is three orders of magnitude larger than that of its counterpart on the as-is sample. These results provide new insights into radiation-induced aerosol nucleation, with relevance to atmospheric cloud formation and global climate changes.     

Chemical and nanometer-scale structure of kerogen and its change during thermal maturation investigated by advanced solid-state C NMR spectroscopy

We have used advanced and quantitative solid-state nuclear magnetic resonance (NMR) techniques to investigate structural changes in a series of type II kerogen samples from the New Albany Shale across a range of maturity (vitrinite reflectance R₀ from 0.29% to 1.27%). Specific functional groups such as CH₃, CH₂, alkyl CH, aromatic CH, aromatic C-O, and other nonprotonated aromatics, as well as “oil prone” and “gas prone” carbons, have been quantified by ¹³C NMR; atomic H/C and O/C ratios calculated from the NMR data agree with elemental analysis. Relationships between NMR structural parameters and vitrinite reflectance, a proxy for thermal maturity, were evaluated. The aromatic cluster size is probed in terms of the fraction of aromatic carbons that are protonated (∼30%) and the average distance of aromatic C from the nearest protons in long-range H-C dephasing, both of which do not increase much with maturation, in spite of a great increase in aromaticity. The aromatic clusters in the most mature sample consist of ∼30 carbons, and of ∼20 carbons in the least mature samples. Proof of many links between alkyl chains and aromatic rings is provided by short-range and long-range ¹H-¹³C correlation NMR. The alkyl segments provide most H in the samples; even at a carbon aromaticity of 83%, the fraction of aromatic H is only 38%. While aromaticity increases with thermal maturity, most other NMR structural parameters, including the aromatic C-O fractions, decrease. Aromaticity is confirmed as an excellent NMR structural parameter for assessing thermal maturity. In this series of samples, thermal maturation mostly increases aromaticity by reducing the length of the alkyl chains attached to the aromatic cores, not by pronounced growth of the size of the fused aromatic ring clusters.