Solid-state 13C CPMAS Research Articles

Combining the Advantages of Powder X-ray Diffraction and NMR Crystallography in Structure Determination of the Pharmaceutical Material Cimetidine Hydrochloride

We report the crystal structure of the anhydrous phase of cimetidine hydrochloride, determined directly from powder X-ray diffraction data. The material was prepared by dehydration of the readily obtained monohydrate form of cimetidine hydrochloride, the only form for which a crystal structure has previously been reported. As such, solid-state dehydration processes typically yield the product phase as a microcrystalline powder, and structure determination was carried out directly from powder X-ray diffraction data, using the direct-space genetic algorithm technique for structure solution followed by Rietveld refinement. The structure determined from powder X-ray diffraction was further validated by calculating solid-state 13C NMR data for the crystal structure (using first-principles periodic DFT techniques within the GIPAW approach) and assessing the quality of agreement with the corresponding experimental solid-state 13C CPMAS NMR data. This strategy provides a robust vindication of the correctness of the crystal structure by assessing the quality of agreement of the structure both with experimental powder X-ray diffraction data and with experimental solid-state 13C NMR data.

Native and modified chitosan-based hydrogels as green heterogeneous organocatalysts for imine-mediated Knoevenagel condensation

A variety of methylenemalononitriles and ethyl cyanoacrylates derived from both aromatic and heteroaromatic aldehydes were synthesized by Knoevenagel condensation catalysed with native and modified chitosan-based heterogeneous catalysts. The efficiency of our hydrogel organocatalysts, chitosan hydrogel beads and ureidyl-chitosan derivative hydrogel disks, was evaluated as function of pH, temperature and catalyst concentration by considering reaction rates, conversions, E/Z stereoselectivities, and kinetic studies of a model reaction between 4-nitrobenzaldehyde and ethyl cyanoacetate. An unprecedented study by solid state 13C CP MAS NMR of the employed catalyst when reaction was quenched after a 50% of conversion, has demonstrated that an imine-chitosan intermediate is formed during this process. Analysis of E/Z ethyl cyanoacrylate isomer mixtures for determining the corresponding stereoselectivity was carried out by NMR measuring carbon-proton coupling constants (3JC,H) using a novel CLIP-HSQMCB experiment. Additionally, DFT calculations let us rationalise the observed E/Z stereoselectivities as well as to evaluate the role of ureidyl moiety on interaction with aldehydes and imine intermediate formation with chitosan derivative.

The Nature of Secondary Interactions at Electrophilic Metal Sites of Molecular and Silica-Supported Organolutetium Complexes from Solid-State NMR Spectroscopy.

Lu[CH(SiMe3)2]3 reacts with [SiO2-700] to give [(≡SiO)Lu[CH(SiMe3)2]2] and CH2(SiMe3)2. [(≡SiO)Lu[CH(SiMe3)2]2] is characterized by solid-state NMR and EXAFS spectroscopy, which show that secondary Lu···C and Lu···O interactions, involving a γ-CH3 and a siloxane bridge, are present. From X-ray crystallographic analysis, the molecular analogues Lu[CH(SiMe3)2]3-x[O-2,6-tBu-C6H3]x (x = 0-2) also have secondary Lu···C interactions. The (1)H NMR spectrum of Lu[CH(SiMe3)2]3 shows that the -SiMe3 groups are equivalent to -125 °C and inequivalent below that temperature, ΔG(⧧)(Tc=148K) = 7.1 kcal mol(-1). Both -SiMe3 groups in Lu[CH(SiMe3)2]3 have (1)JCH = 117 ± 1 Hz at -140 °C. The solid-state (13)C CPMAS NMR spectrum at 20 °C shows three chemically inequivalent resonances in the area ratio of 4:1:1 (12:3:3); the J-resolved spectra for each resonance give (1)JCH = 117 ± 2 Hz. The (29)Si CPMAS NMR spectrum shows two chemically inequivalent resonances with different values of chemical shift anisotropy. Similar observations are obtained for Lu[CH(SiMe3)2]3-x[O-2,6-tBu-C6H3]x (x = 1 and 2). The spectroscopic data point to short Lu···Cγ contacts corresponding to 3c-2e Lu···Cγ-Siβ interactions, which are supported by DFT calculations. Calculated natural bond orbital (NBO) charges show that Cγ carries a negative charge, while Lu, Hγ, and Siβ carry positive charges; as the number of O-based ligands increases so does the positive charge at Lu, which in turns shortens the Lu···Cγ distance. The change in NBO charges and the resulting changes in the spectroscopic and crystallographic properties show how ligands and surface-support sites rearrange to accommodate these changes, consistent with Pauling's electroneutrality concept.

The structure and properties of 5,6-dinitro-1H-benzotriazole

5,6-Dinitro-1H-benzotriazole crystallizes in the monoclinic system, space group P21/c. The asymmetric unit contains the planar 1H-tautomer together with a water molecule of crystallization. Each water molecule is hydrogen bonded to three adjacent 5,6-dinitrobenzotriazoles forming a tape along the b-axis of the crystal. These tapes stack along the c-axis through hydrogen bonds involving the water molecules and one of the nitro groups leading to a bidimensional structure. Solid-state 13C and 15N CPMAS NMR allow to confirm that the tautomer present is the 1H one. In DMSO-d6 solution the results are quite different and, based on GIAO/B3LYP/6-311++G(d,p) calculations, lead us to conclude that the major tautomer is the 5,6-dinitro-2H-benzotriazole, a surprising result that contradicts the rule that the major tautomer in solution coincides with the one present in the crystal. An anhydrous pseudopolymorph of 5,6-dinitro-1H-benzotriazole has been obtained as a non-crystalline form and from solid-state NMR and theoretical calculations, we conclude that it is an 1H-tautomer.

A methods study of immobilization of PONOP pincer transition metal complexes on silica polyamine composites (SPC)

Immobilization of catalytically active transition metal complexes on silica polyamine composite (SPC) surfaces offers many advantages for applications in catalysis particularly for catalyst recovery and reuse. We report here the immobilization of PONOP pincer complexes of Ru, Rh, Ni and Pd on the poly(allylamine) SPC, BP-1 using the Mannich reaction. Three different methods have been investigated for synthesizing the PONOP pincer transition metal complexes on BP-1: 1) direct reaction of the preformed pincer complexes using a two step Mannich reaction; 2) immobilization of the PONOP ligand using the Mannich reaction followed by the addition of a transition metal compound of a given metal; 3) the stepwise construction of PONOP on BP-1 followed by addition of a transition metal compound. The immobilized complexes on BP-1 were characterized by FT-IR, solid-state CPMAS 13C and 31P NMR, as well as elemental analysis. Anchoring of the complexes on BP-1 was also evaluated by the metal loading data obtained from the digestion of the loaded composites followed by Atomic Absorption Spectroscopy (AAS) or Inductively Coupled Plasma Atomic Emission Spectroscopy (ICPAES). The results showed that method 1 works better for the loading of pincer complexes on the SPC than methods 2 and 3. In the case of the Ru and Ni pincer complexes reasonable agreement with the phosphorous analysis was realized, while for the Pd complex values were high relative to the loading predicted from the phosphorus analysis, indicating the formation of the Pd nanoparticles on the surface during immobilization. For the Rh and Ru immobilized complexes with methods 2 & 3, metal loading was lower than the phosphorous analysis and this is attributed to entrained triphenylphosphine from the starting rhodium and ruthenium complexes based on the 13C and 31P CPMAS NMR data. Solution experiments using the PONOP pincer ligand and the Ru(PONOP) complex with n-butyl amine were conducted to model the site of electrophilic aromatic substitution on the pyridine ring. It was found that substitution both meta- and para-to the nitrogen takes place and this helped in the interpretation of the solid-state data.

Advanced NMR methodologies and micro-analytical techniques to investigate the stratigraphy and materials of 14th century Sienese wooden paintings

In this study a multi-analytical approach was used to characterize ancient and modern constitutive materials and the stratigraphy of a Sienese wooden painting consisting of four cusps belonging to a lost Polyptych, by Andrea di Bartolo dated back to the late 14th century, repainted in the 16th century and partially restored and cleaned in the 20th century. In order to distinguish the original layers from the other ones, a comparison with the painting Adorazione dei Magi by Bartolo di Fredi (Andrea's father) dated to the 14th century, was performed. The painting Adorazione dei Magi was found to be in a very good state of conservation and provided a model of the artistic technique used in Sienese painting in the 14th century.In situ measurements by portable NMR were performed to investigate the stratigraphy of the paintings in a non-destructive and non-invasive way. By means of this technique, information about all layers constituting the paintings were obtained and used to plan the successive micro-sampling. High resolution NMR spectroscopy was applied on a few samples to characterize binders and organic substances. Specifically, solid state 13C and 31P CPMAS NMR spectroscopy and 1H NMR spectroscopy in solution were applied to characterize original and non-original materials. Other techniques such as ToF-SIMS, HPLC, IR and Raman spectroscopy were also applied.

Coordinated copper(II) supported on silica nanospheres applied to the synthesis of α-ketoamides via oxidative amidation of methyl ketones

In the present work, we describe the design and fabrication of a novel, highly efficient, and retrievable silica nanosphere-supported copper catalytic system (SiO2@APTES@DAP–Cu) via immobilization of diacetyl pyridine (DAP) onto amine-functionalized silica nanospheres. The structure of resulting organic–inorganic hybrid SiO2@APTES@DAP–Cu nanocatalyst has been systematically affirmed using various physicochemical characterization techniques such as solid-state 13C CP-MAS NMR spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, Brunauer–Emmett–Teller surface area analysis, scanning electron microscopy, atomic absorption spectroscopy, energy-dispersive X-ray fluorescence spectroscopy, X-ray diffraction, and elemental analysis. Thereafter, the catalytic performance of the synthesized heterogeneous catalyst has been investigated in oxidative amidation of methyl ketones using hydrogen peroxide as the oxidant and iodine as an additive. It has been found that the nanostructured copper catalyst displays very high efficacy with excellent functional group tolerance and allows the generation of a wide range of amidation products under mild reaction conditions. In addition, the catalyst exhibits remarkable durability as well as good recyclability for several runs with a high consistence in its catalytic activity. Furthermore, this newly developed catalytic protocol is a better alternative to the already-existing methodologies for oxidative coupling of methyl ketones with amines as it is endowed with many outstanding features such as simple work-up procedure, high product yield, cost effectiveness, no use of toxic organic solvents, easy recovery, and reusability of the nanostructured catalyst. It is noteworthy that the use of hydrogen peroxide as a potentially green oxidant is one of the most significant attributes of the present oxidative transformation which renders it economic and environmentally sustainable as water is formed as the sole by-product.

Reactions of Cd(OAc)2·2H2O with variously substituted pyridines. Efforts to unravel the factors that determine structure/nuclearity of the products

The reactions of Cd(OAc)2·2H2O with variously substituted pyridines in methanol afforded unique one-dimensional coordination polymers (1D CPs), [Cd2(μ2-κ2:κ1-OAc)2(μ2-κ1:κ1-OAc)2L2] (L=NC5H5 (1), NC5H4Me-3 (2), and NC5H3Me2-3,5 (3)) and [Cd3(μ3-κ1:κ2-OAc)3(μ2-κ2-OAc)(μ2-κ2:κ1-OAc)2(NC5H3Me2-3,4)2] (4), and discrete and bimolecular complexes, [(Cd(OAc)2(NC5H3Me2-3,4)2(H2O)2] (5), [Cd(κ2-OAc)2(NC5H4Me-4)2(H2O)]·[Cd(κ2-OAc)2(H2O)2)] (6), [Cd(κ2-OAc)2L2L′]·xH2O (x=0, L′=H2O, L=NC5H4(OMe)-4 (7); NC5H4tBu-4 (8); x=2, L=L′=NC5H4(NMe2)-4 (9·2H2O)). The products were characterised by elemental analysis, IR, solution NMR (1H and 13C), solid-state CP-MAS 13C{1H} and 113Cd NMR, TGA/DTA analyses, and single crystal X-ray diffraction. Phase purity of 1–4 was verified by powder X-ray diffraction (PXRD). Plausible mechanisms of formation of the products are proposed based on a point zero charge model. 4 represents the first cadmium containing 1D CP that possesses a tridentate bridging (μ3-κ1:κ2) acetate coordination mode and 6 represents the first structurally characterised bimolecular cadmium(II) complex containing two different neutral cadmium(II) coordination species per formula unit. 9·2H2O was calcined at 500°C to afford CdO as confirmed by PXRD and the morphology of CdO was studied by scanning electron microscopy.

Hydrophobic poly(alkoxysilane) organogels as sorbent material for oil spill cleanup

In this study, reusable poly(alkoxysilane) organogels with high absorption capacities were synthesized by the condensation of a cyclo aliphatic glycol (UNOXOL™) and altering the chain length of the alkyltriethoxysilanes. The structural and thermal properties of cross-linked poly(alkoxysilane) polymers were determined by FTIR, solid-state 13C and 29Si CPMAS NMR and TGA. The oil absorbency of poly(alkoxysilane)s was determined through oil absorption tests, absorption and desorption kinetics. Results showed that the highest oil absorbency capacities were found to be 295% for hexane, 389% for euro diesel, 428% for crude oil, 652% for gasoline, 792% for benzene, 792% for toluene, 868% for tetrahydrofuran, and 1060% for dichloromethane for the poly(alkoxysilane) gels based on UNOXOL™ and dodecyltriethoxysilane. Owing to their hydrophobic structure, the poly(alkoxysilane) organogels can selectively absorb crude oil from water. The reusability of the absorbents was quantitatively investigated, demonstrating that absorbents can be used effectively at least nine times.

Variability in the composition of charred litter generated by wildfire in different ecosystems

In most forest ecosystems the burnt material remaining on the soil after wildfire mainly comprises non-woody char derived from the litter layer. Despite the importance of the role of this material in the carbon balance, soil conservation and ecological processes, few studies have examined the properties of this type of charred biomass, which may vary widely in different forest ecosystems and also within burnt areas. In this study, we considered three forest stands (Pinus nigra, Pinus pinaster and Eucalyptus globulus) and two shrubland systems (Erica arborea and Ulex europaeus), in which some of the properties of the litter differed (amount of fuel, composition and flammability). Changes in the composition of the litter and in chemical and thermal stability indices were studied after wildfires and were related to two visually different levels of soil burn severity (SBS). The methods used included elemental analysis, solid-state 13C CP-MAS NMR spectroscopy, FTIR and DSC-TG.In all five types of litter, the degree of aromatization increased gradually with increasing SBS, although it was lower than usually found for woody char. The thermal analysis also revealed that the heat released by combustion up to 375°C decreased with increasing SBS, whereas T50 (the temperature at which 50% of the energy stored in organic matter is released) presents the opposite behaviour. However, the different types of litter were very variable in terms of the amount (E. arborea>E. globulus>P. nigra>P. pinaster≥U. europaeus) and degree of aromatization of the charred material (E. globulus>E. arborea=U. europaeus>P. pinaster>P. nigra). The differences in composition of the charred litter may be partly due to differences in the heating conditions (determined by the fire regime, initial amount and flammability of the litter). The differences may also be due to differences in chemical degradation of lignin of each plant species during burning. The relationships between thermal properties, elemental composition and chemical-shift regions in the NMR indicate that thermal analysis may be a useful tool for characterizing organic matter properties.

Tuning the physicochemical properties of β-cyclodextrin based polyurethanes via cross-linking conditions

Polyurethanes (PUs) were prepared by cross-linking β-cyclodextrin (β-CD) with two types of diisocyanates; hexamethylene diisocyanate and 4,4′-dicyclohexyl diisocyanate, respectively. Materials with diverse structural and textural properties were obtained by varying the rate of diisocyanate addition: rapid (R) or drop-wise (D; 0.1 mL/min). Characterization of the structural and textural properties was investigated by spectroscopic (1H NMR in solution, solid state 13C CP-MAS solids NMR, dynamic light scattering, UV–vis, and IR), thermogravimetric analysis, powder X-ray diffraction, and scanning electron microscopy. The accessibility of the β-CD inclusion sites of the polymers was independently evaluated using an equilibrium dye probe adsorption method with phenolphthalein, and a kinetic dye-based uptake method was studied in parallel using p-nitrophenol in aqueous solution. The characterization methods provide evidence that the drop-wise method affords polymer materials with greater cross-linking, as compared with the rapid addition method. Herein, we report the first example of a cross-linked polyurethane containing β-CD with tunable structure and physicochemical properties, according to the mode of cross-linker addition (R versus D) to control the reaction products.

Fluorescent imino and secondary amino chitosans as potential sensing biomaterials

A variety of fluorescent imino and secondary amino chitosans were synthesized under very mild conditions by reaction of the biopolymer amino functions with aromatic aldehydes in an acidified methanolic suspension. Simultaneous reactions of several aldehydes with chitosan were successfully carried out, and kinetic studies showed that 1-pyrenecarboxaldehyde reacts the fastest among them. An unprecedented study on the evaluation of the degree of N-substitution (DS, ranging from 31.7% to 12.0%) for the chitosan Schiff bases by using solid state CPMAS 13C NMR is performed. A linear correlation between the DS obtained for the secondary amino chitosans by 1H NMR (55.3–10.2%) and those obtained by CPMAS 13C NMR (34.4–13.8%) has allowed us to calculate an empirical correlation factor that could be applied on chitosan-based aromatic systems. The new chiral-labelled chitosan derivatives exhibit a stable fluorescent behaviour, which was used to explore solvent sensoring applications.

Oligomeric complexes of some heteroaromatic ligands and aromatic diamines with rhodium and molybdenum tetracarboxylates: 13 C and 15 N CPMAS NMR and density functional theory studies

Seven new oligomeric complexes of 4,4'-bipyridine; 3,3'-bipyridine; benzene-1,4-diamine; benzene-1,3-diamine; benzene-1,2-diamine; and benzidine with rhodium tetraacetate, as well as 4,4'-bipyridine with molybdenum tetraacetate, have been obtained and investigated by elemental analysis and solid-state nuclear magnetic resonance spectroscopy, (13)C and (15)N CPMAS NMR. The known complexes of pyrazine with rhodium tetrabenzoate, benzoquinone with rhodium tetrapivalate, 4,4'-bipyridine with molybdenum tetrakistrifluoroacetate and the 1 : 1 complex of 2,2'-bipyridine with rhodium tetraacetate exhibiting axial-equatorial ligation mode have been obtained as well for comparison purposes. Elemental analysis revealed 1 : 1 complex stoichiometry of all complexes. The (15)N CPMAS NMR spectra of all new complexes consist of one narrow signal, indicating regular uniform structures. Benzidine forms a heterogeneous material, probably containing linear oligomers and products of further reactions. The complexes were characterized by the parameter complexation shift Δδ (Δδ = δcomplex - δligand). This parameter ranged from around -40 to -90 ppm in the case of heteroaromatic ligands, from around -12 to -22 ppm for diamines and from -16 to -31 ppm for the complexes of molybdenum tetracarboxylates with 4,4'-bipyridine. The experimental results have been supported by a density functional theory computation of (15)N NMR chemical shifts and complexation shifts at the non-relativistic Becke, three-parameter, Perdew-Wang 91/[6-311++G(2d,p), Stuttgart] and GGA-PBE/QZ4P levels of theory and at the relativistic scalar and spin-orbit zeroth order regular approximation/GGA-PBE/QZ4P level of theory. Nucleus-independent chemical shifts have been calculated for the selected compounds.

ChemInform Abstract: Cu(II) Complex Heterogenized on SBA‐15: A Highly Efficient and Additive‐Free Solid Catalyst for the Homocoupling of Alkynes.

A Cu(II) complex has been heterogenized on SBA-15 by functionalization of SBA-15 with (N1-propylethane-1,2-diamine) triethoxysilane (NN-SBA-15) followed by anchoring of 2-pyridinecarboxaldehyde through Schiff's base formation between the terminal –NH2 and the aldehyde group of 2-pyridinecarboxaldehyde (Py-NN-SBA-15), and finally, copper(II) was immobilized through complexation. Copper(II) complex heterogenized on SBA-15 (Cu(II)-SBA-15) has been thoroughly characterized by different techniques such as N2 sorption, small-angle X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and solid-state 13C CP-MAS NMR spectroscopy. Cu(II)-SBA-15 is an efficient heterogenized solid catalyst for the oxidative homocoupling of phenyl acetylene into 1,4-diphenylbuta-1,3-diyne with a yield of 97%, even in the absence of palladium, co-catalyst and base additives. In the case of the homocoupling of other aromatic and aliphatic terminal alkynes, the yields were 85–98%.

The effect of rotating substituent in 2,2,5,7,8-pentamethylchroman derivatives. X-ray, 13C CP MAS analysis and DFT analysis

The aim of this study was to analyze the influence of rotating substituent at position 6 in 2,2,5,7,8-pentamethylchroman derivatives on geometry of the chroman ring. Three substituents have been chosen which differed in electro donor/acceptor properties (–NO2, –OH and –NH2). The substituents were rotated by 10° step from 0° to 180° whereas the geometries were fully optimized at B3LYP/6-311G** level of theory. The rotation barrier is related with a size of the substituent, being largest for the nitro group (5.9kcal/mol) and smallest for the hydroxyl group (2.0kcal/mol). The largest changes on geometry were observed in closest proximity of the substituents in the phenyl ring (due to steric effect) and C4A–C8A–O1–O2 angle in the chroman fragment, related to the antioxidant activity of a compound. The latter varied much stronger for electron-accepting substituent (the range −12.6° to −16.3°) than for electron donating ones (the variation range −17.1° to −18.9°), showing the rather high sensitivity in the first case. Apart from theoretical studies the crystal structure of newly synthesized 6-nitro-2,2,5,7,8-pentamethylchroman was determined by X-ray diffraction and studied by 13C NMR including solid-state 13C CP-MAS NMR.

Chemical Modification of Kraft Lignin: Effect on Chemical and Thermal Properties

Esterified kraft lignins (KL) were prepared by reaction with maleic anhydride (MA), succinic anhydride (SA), and phthalic anhydride (PA) in acetone solutions. The esterified lignins were characterized using ATR-FTIR, solid state CP-MAS 13C NMR spectroscopy, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). PA modification resulted in the highest weight gain percent (WGP) when compared to MA and SA modifications. Spectroscopic analysis revealed decreases in hydroxyl content and increases in carbonyl (C=O) and ester groups of the modified KL as a result of esterification. The hydrophobic properties of the modified lignin increased. The MA- and SA-modified KL showed an increased thermal stability compared to unmodified KL. PA-modified lignin presented distinct thermal decomposition stages, which showed rapid degradation at lower temperature. The results of this study indicate that it is possible to change the basic properties of kraft lignin by anhydride modification to facilitate the production of high performance composites.

Pseudo-symmetry, rotation- and inversion-twinning of a structure with dinuclear and trinuclear Cd complexes. CP-MAS-NMR and IR spectroscopies characterisation

The structure of the complex [C18H30Cd3Cl6N18S12·2(C18H30Cd2Cl3N18S12)·CdCl4], or [L6Cd3Cl6·2(L6Cd2Cl3)·CdCl4], with L being 2-amino-5-(methylthio)-1,3,4-thiadiazole, C3H5N3S2, crystallizes in the trigonal polar space group R3. The crystal packing features three chemically distinct cadmium complex species with eight crystallographically independent Cd(II) ions distributed over two types of L ligand complexes with two and three Cd(II) centers, respectively, and a tetrachlorocadmate(II) ion. The coordination environment of the cadmium ions in the dinuclear and trinuclear complexes is a distorted octahedron. The tetrachlorocadmate(II) is disordered around a crystallographic threefold rotation axis, which is, in turn, inducing disorder onto the two methyl-thio groups in closest proximity to a CdCl4anion. The crystal under investigation was found to be twinned by rotational and inversion merohedry. In the higher symmetry setting, the trinuclear complex would feature exact inversion symmetry, and the two binuclear cationic complexes would be inversion counterparts of each other. The R3¯ symmetry is broken by a mismatch of less than 1Å between one pair of ligands L between the dinuclear cations, which feature slightly different rotational angles around the Cd ion in the otherwise symmetry equivalent complexes. This compound is also investigated by FT-IR and solid-state 13CCP-MAS NMR spectroscopies.

Decomposition of submerged plant litter in a Mediterranean reservoir: A microcosm study

Decomposition of plant debris is a critical process during the filling phase of water reservoir. Here we investigated the impact of decomposition of the plant species (Cytisus scoparius, Pteridium aquilinum and Juncus effusus) in the Alaco reservoir (southern Italy). To simulate lake ecological conditions, a microcosm experiment was carried-out for 100 days with litters incubated in water lake in aerobic conditions in the light at two temperatures (+12°C and +24°C). Litter types were characterized by classic proximate chemical analyses (total C and N, labile C, cellulose and lignin content, C/N and lignin/N ratios) and, at molecular level, by solid-state 13C CPMAS NMR. Water quality was monitored through pH, electrolytic conductivity, DOC, BOD5 and dissolved oxygen. Decomposition was more rapid for C. scoparius, intermediate for J. effusus and slower for P. aquilinum. After 100 days of decomposition for all litter type the O-alkyl-C region, associated with sugars and polysaccharides, decreased and the aliphatic alkyl-C region increased for C. scoparius and P. aquilinum. Immediately after litter submergence, DOC and BOD5 dramatically increased, reaching high values for P. aquilinum and C. scoparius. However, after 100 days of incubation, DOC concentration sharply decreased reaching values usually below 50mgl−1, and after 10 days of incubation BOD5 values dropped to values close to zero. Although carried out in laboratory, the experiment showed that submerged plant litter during decomposition produces a short-term changes of water quality that rapidly returns to background level.

Surface-Bound Ruthenium Diimine Organometallic Complexes: Excited-State Properties.

Ruthenium complexes of the general formula [Ru(CO)(H)(L2)(L′2)][PF6] (L2 = trans-2PPh3, L′ = η2-4,4′-dicarboxybipyridine (1); L2 =trans-2Ph2PCH2CH2COOH, L′2 = bipyridine (2); L2 = Ph2PCHCHPPh2, L′ = η2-5-amino-1,10-phenanthroline (3); L2 = trans-2PPh3, L′2 = η2-4-carboxaldehyde-4′-methylbipyridine (4)) have been shown to have longer emission lifetimes and higher quantum yields in solution compared with more symmetrical molecules such as [Ru(bpy)3][Cl]2. Compound 4 is obtained as a mixture with the corresponding acetal, 4′. These less symmetrical complexes have been covalently immobilized on the surface of silica polyamine composites, and their photophysical properties have been studied. The surface-bound complexes have been characterized by solid-state CPMAS 13C, 31P, and 29Si NMR, UV–vis, and FT-IR spectroscopies. Excited-state lifetime studies revealed that, in general, the lifetimes of the immobilized complexes are 1.4 to 8 times longer than in solution and are dependent on particle size (300–500 μm versus 10–20 nm average diameter silica gels), polymer structure (linear poly(allylamine) versus branched poly(ethylenimine)), and the type of surface tether. One exception to this trend is the previously reported complex [Ru(bpy)2(5-amino-1,10-phenanthroline)][PF6]2 (5), where only a slight increase in lifetime is observed. Only minor changes in emission wavelength are observed for all the complexes. This opens up the possibility for enhanced heterogeneous electron transfer in photocatalytic reactions.

Use of thermal analysis and solid-state 13C CP-MAS NMR spectroscopy to diagnose organic matter quality in relation to burn severity in Atlantic soils

Wildfire has highly variable effects on soil, and different conservation strategies are required to address different levels of soil degradation. Rapid diagnosis of soil burn severity is required to enable the design of emergency post-fire rehabilitation treatments. This study evaluated whether visually different levels of soil burn severity (SBS) reflect changes in SOM quality. Five areas recently affected by wildfires were selected for study in a fire-prone environment in an Atlantic area of Southern Europe (NW Spain). Soil organic carbon (SOC) concentration and quality of mineral soil samples (0–5cm depth) were analyzed after the fires, in relation to three visually different levels of soil burn severity (SBS): control (unburned), moderate (bare soil and soil structure unaffected) and high (bare soil and surface soil structure and color altered). The samples were collected along sampling transects, to control the spatial variability. SOM quality was characterized by 13C CP-MAS NMR spectroscopy and differential scanning calorimetry (DSC). Although large reductions in SOC concentration were found in both moderate and SBS (up to 70%), important effects on SOM quality were associated only with high SBS. NMR analysis revealed these changes as losses of O-alkyl, alkyl and carboxylic structures (variable in the different soils studied) and increased values of the aromatic structures (up to 50%). The DSC analysis revealed decreased combustion heat released up to 375°C, and increased T50 (the temperature at which 50% of the energy stored in SOM is released). Relationships between thermal properties and chemical-shift regions in the NMR helped provide a better understanding of SOM thermal properties. Discriminant analysis, using the SBS levels as classification factors, revealed that thermal parameters reliably distinguished the three visually recognized levels and correctly classified 86% of the samples. Visual classification of SBS levels could therefore be used to characterize SOM changes on the basis of SOM thermal and chemical properties.