The Hydration of Vanadyl Doped into Magnesium Acetate: Single Crystal Proton ENDOR at 20 K

The single crystal ESR and proton ENDOR of VO²⁺ doped into magnesium acetate tetrahydrate have been studied at 20 K. The VO²⁺ ion occupies Mg²⁺ sites, replacing Mg²⁺ together with one water molecule. Hyperfine tensors for the six protons in the three coordinating water molecules have been determined and their dipolar components used to construct a model of the proton geometry. The relation of the structure to that of {VO(H₂O)₅}²⁺ in a Tutton salt and in frozen aqueous solutions of vanadyl sulphate is discussed.     

Endor studies of carbon black and carbon black-polymer composites

Electron-nuclear double resonance (ENDOR) spectra have been obtained at the free proton frequency for a carbon black and a carbon black-polymer composite, at temperatures ranging from 130 K to 350 K. The ENDOR spectra show an unusual dependence on the saturation of the associated homogeneously broadened ESR resonance, as “non-saturating” microwave power gives rise to largest ENDOR enhancements. This behaviour is attributed to a “lineshift” ENDOR mechanism. In the composite, the polymer ¹H nuclei contribute very strongly to the ENDOR signal, and this signal shows a marked dependence on molecular motion within the polymer.     

A Magic-Angle-Spinning NMR Spectroscopy Method for the Site-Specific Measurement of Proton Chemical-Shift Anisotropy in Biological and Organic Solids

Proton chemical shifts are a rich probe of structure and hydrogen-bonding environments in organic and biological molecules. Until recently, measurements of ¹H chemical-shift tensors have been restricted to either solid systems with sparse proton sites or were based on the indirect determination of anisotropic tensor components from cross-relaxation and liquid-crystal experiments. We have introduced a magic-angle-spinning approach that permits site-resolved determination of chemical-shift-anisotropy tensors of protons forming chemical bonds with labeled spin 1/2 nuclei in fully protonated solids with multiple sites, including organic molecules and proteins. This approach, originally introduced for the measurements of chemical-shift tensors of amide protons, is based on three RN-symmetry-based experiments, from which the principal components of the ¹H chemical-shift tensor can be reliably extracted by simultaneous triple fit of the data. Herein, we expand our approach to a much more challenging system involving aliphatic and aromatic protons. We start with a review of prior work on experimental NMR spectroscopy and computational quantum chemical approaches for measurements of ¹H chemical-shift tensors and relating these to the electronic structures. We then present our experimental results on U-¹³C,¹⁵N-labeled histidine and demonstrate that ¹H chemical-shift tensors can be reliably determined for the ¹H¹⁵N and ¹H¹³C spin pairs in cationic and neutral forms of histidine. Finally, we demonstrate that the experimental ¹H(C) and ¹H(N) chemical-shift tensors are in agreement with DFT calculations; therefore, establishing the usefulness of our method for the characterization of structures and hydrogen-bonding environments in organic and biological solids.     

The Structure of Pb+ Laser Centres in KMgF3 from Optically Detected Electron Spin Resonance and Electron Nuclear Double Resonance

Laser action of X-rayed lead doped KMgF₃ crystals from an unknown defect was recently reported by Hörsch and Paus.¹ By applying optical detection of ESR and ENDOR, the structure of two very similar laser active Pb centres could be determined from the selectively measured hyperfine and superhyperfine interactions. They consist of a Pb⁺ substituting for a K⁺ next to an F⁻ vacancy along <110> with a next nearest K⁺ vacancy for charge compensation in two different positions (Pb⁺(1) centres). The absorption band for the excitation of the laser emission at 1.4eV could be identified from the optical excitation spectra and from a special magneto-optical ESR technique. The structural properties, as well as the explanation of their laser properties within a crystal field model, are analogous to those of Tl⁰(1) centres in alkali halides and Pb⁺(1) centres in alkaline earth fluorides. A Pb⁺⁺ perturbed F_A centre was also observed.     

CuII Pyrazolyl-Benzimidazole Dinuclear Complexes with Remarkable Antioxidant Activity

Three dinuclear coordination complexes generated from 1-n-butyl-2-((5-methyl-1H-pyrazole-3-yl)methyl)-1H-benzimidazole ( L ), have been synthesized and characterized spectroscopically and structurally by single crystal X-ray diffraction analysis. Reaction with iron(II) chloride and then copper(II) nitrate led to a co-crystal containing 78 % of [Cu(NO₃)(μ-Cl)( L’ )]₂ ( C₁ ) and 22 % of [Cu(NO₃)(μ-NO₃)( L’ )]₂ ( C₂ ), where L was oxidized to a new ligand L^’ . A mechanism is provided. Reaction with copper chloride led to the dinuclear complex [Cu(Cl)(μ-Cl)( L) ]₂ ( C₃ ). The presence of N−H⋅⋅⋅O and C−H⋅⋅⋅O intermolecular interactions in the crystal structure of C₁ and C₂ , and C−H⋅⋅⋅N and C−H⋅⋅⋅Cl hydrogen bonding in the crystal structure of C₃ led to supramolecular structures that were confirmed by Hirshfeld surface analysis. The ligands and their complexes were tested for free radical scavenging activity and ferric reducing antioxidant power. The complex C₁ / C₂ shows remarkable antioxidant activities as compared to the ligand L and reference compounds.     

A new μ1,5-dicyanamide bridged cyclic tetranuclear copper (II) complex with 1,5,9-triazacycledodecane ligands: structure,ESR spectroscopic and magnetic properties

The tetranuclear complex [Cu₂L₂(dca)₂(ClO₄)₂]₂ (1) (L = 1,5,9-triazacyclododecane, dca = dicyanamide [N(CN)₂]⁻) has been synthesized and its crystal structure, ESR spectra and magnetic properties determined. The complex contains a tetranuclear copper (II) moiety in which two dimeric units are bridged by two dca ligands. In each dimeric moiety the two copper (II) ions are bridged by one μ_1,5-dicyanamide ligand. Magnetic susceptibilities for the complex in the solid state are measured over the temperature range 4–300 K. The complex shows a weak antiferromagnetic coupling with a best fit J value −0.436 cm⁻¹. The μ_1,5-dicyanamide bridges are the principal pathway for the super-exchange interaction and the weak antiferromagnetic coupling of the complex is interpreted in term of the μ_1,5-dicyanamide ligand behavior as a poor magnetic mediator.     

UV resonance Raman probe of heme-bound imidazole established by 15N-labeling of hemoglobin

Recent studies of Cu, Zn superoxide dismutase, and of zinc-finger peptides have established that histidine ligands can be detected in ultraviolet resonance Raman (UVRR) spectra, following NH/D exchange of the imidazole. UVRR spectroscopy therefore offers promise for monitoring histidine ligation in heme proteins. In this work, we characterize heme-bound histidine UVRR bands for N-acetyl-microperoxidase-8 (MP-8) and microperoxidase-11 (MP-11), and also for hemoglobin (Hb). The Hb UVRR spectra are dominated by tyrosine and tryptophan contributions, but a band appears at 1340 cm⁻¹ in D₂O solution, which is assigned to a mode of Fe-bound imidazole. This band shifted 24 cm⁻¹ in protein which was labeled with ¹⁵N via expression of the Hb gene in E. coli grown on ¹⁵NH₄⁺. In MP-11, the position of this band is insensitive to ligation or oxidation state changes, but it is 2 cm⁻¹ lower in deoxyHb than in the CO adduct. This shift may reflect mechanical forces on the proximal histidine in the T state, and/or changes in its H-bonding.     

Synthesis of new macrobicyclic tricompartmental ligands and site preference of metal ion: structural and electrochemical properties of mononuclear copper(II) complexes

New macrobicyclic tricompartmental ligands and their mononuclear copper(II) complexes of type [CuH₂L](ClO₄)₂ have been synthesized from the precursor compound 3,4:10,11-dibenzo-1,13[N,N^′{bis(3-formyl-2-hydroxy-5-methyl)benzyl}diaza]-5,9-dioxa-cycloheptadecane with diamines like en, pn and bn by template method. These complexes have been characterized by IR, UV spectral and X-ray studies. Complex [CuH₂L³](ClO₄)₂ (in L³ the diamine used is bn) crystallized as triclinic system with space group p-1. The crystal structure explains that the geometry around copper is distorted square planar. Among the three compartments, it is interesting to observe that the metal ion prefers to be encompassed by the compartment containing phenolic oxygens and imine nitrogens (N₂O₂). From X-ray studies the presence of perchlorate anions, acetonitrile solvent molecules and the protonation of tertiary nitrogen atoms were well understood. All the complexes show hyperfine splittings in ESR spectra. The μ_eff values observed in the range 1.69–1.72 BM are close to the spin only value of single copper(II) ion. Cyclic voltammetry studies of these complexes depict one electron quasireversible reduction in the range −0.80 to −0.90 V at negative potential. A red shift in UV–Visible spectra and shifting of reduction potential towards anodic in cyclic voltammetry were witnessing the distortion of the geometry around metal center from the planarity as the macrocyclic chain length in imine nitrogen compartment increases.     

Hypergolic Behavior of Pyridinium Salts Containing Cyanoborohydride and Dicyanamide Anions with Oxidizer RFNA

A series of low‐cost, pyridinium cation‐based hypergolic ionic liquids (HIL) containing amine, butyl, or allyl substituents with cyanoborohydride [BH₃CN]⁻ and dicyanamide [DCA]⁻ anions were developed and characterized. The investigated physicochemical properties include melting and decomposition temperature, viscosity, density, heat of formation (ΔH_f) and specific impulse (I_sp). The ignition delay (ID) of all HILs was tested with the oxidizer RFNA. The HIL, 1‐allyl 4‐amino pyridinium dicyanamide, exhibited highest density (1.139 g cm⁻³) amongst the known pyridinium HILs. The heats of formation predicted on the basis of Gaussian 09 suit programs were within the range of − 30 to 356 kJ mol⁻¹. The structure of HIL, 1‐butyl 4‐aminopyridinium cyanoborohydride, was examined by single‐crystal X‐ray diffraction, which revealed hydrogen bonding between anion and cation as N1−H1N ⋅⋅⋅ N3=2.07 Å, N1−H2N ⋅⋅⋅ H1B1=2.18 Å, and N1−H2N ⋅⋅⋅ H2B1=2.21 Å, respectively. HIL (1‐allyl 4‐aminopyridiniun cyanoborohydride) exhibited highest I_sp of 228 s amongst the designed series.     

Novel Sulfonated CNN Pincer Ligands for Facile C−H Activation at a Pt(II) Center

Two novel sulfonated CNN-pincer ligands 1b and 1c and the corresponding chloro and aqua complexes K[^CNNLPtCl] and ^CNNLPt(H₂O), 3b – 3c and 2b – 2c , were prepared and fully characterized including single crystal X-ray diffraction. Along with the previously described complexes 2a and 3a , the derivatives of a CNN pincer ligand 1a , these complexes form a family of structurally similar compounds where the pincer core rigidity increases in the series 2a (3a) < 2b (3b) < 2c ( 3c ), as deduced from their XRD data. The increased ligand rigidity affects the aqua ligand dissociation energy of the ^CNNLPt(H₂O) complexes, as it follows from DFT calculations and as is reflected in the increased reactivity of the aqua complexes 2a , 2b and 2c in processes that involve aqua ligand loss. Among these processes the formation of the presumed dinuclear complexes ^CNNL₂Pt₂ and, importantly, catalytic C−D bond cleavage in C₆D₆ were studied in 2,2,2-trifluoroethanol solutions. The C−D bond cleavage reactivity was quantified as the rate of H/D exchange between C₆D₆ and CF₃CH₂OH at 80 °C.     

Synthesis and characterization of luminescent zinc complexes containing redox-active 1-(2-pyridylazo)-2-acenaphthequinol ligands with nonlinear optical property

The redox-active ligand 1-(2-pyridylazo)-2-acenaphthequinol (PAAL) and its zinc complex Zn(PAAL)₂ were isolated and characterized by single crystal X-ray diffraction, ¹H NMR, UV–Vis spectra and TGA, as well as their fluorescence and third-order nonlinear optical properties were preliminary investigated. The electronic structure of the complex is examined by cyclic voltammogram, while density functional theory calculations also support the redox-active nature of the PAAL ligand.     

Structural characterization of nylon 4 in the solid state by high-resolution solid-state H NMR spectroscopy

High-resolution solid-state ¹H NMR spectra of nylon 4 melt-quenched sample and single crystal sample in the solid state were measured in a wide range of temperatures from room temperature to 505 K by using solid state 300 MHz NMR with the FSLG-2 homo-nuclear dipolar decoupling method combined with high speed magic angle spinning method. From the experimental results, structural characterization on the crystalline and non-crystalline components was carried out. Further, intermolecular interaction between nylon 4 and water contained in the sample was discussed.     

Potential reaction paths in NOx reduction over Cu/ZSM-5

The reaction of NO with adsorbed acetone oxime has been studied over Cu/ZSM-5 employing FTIR and MS. It is found that¹⁵NO_gas reacts with¹⁴N labelled acetone oxime and/or its hydrolysis product, hydroxylamine, to form¹⁴N¹⁵NO and¹⁴N¹⁵N at 40°C. As acetone oxime is easily formed from alkyl radicals and NO via alkylnitroso compounds, the results indicate possible reaction paths for the SCR of NO _x by hydrocarbons over zeolite based catalysts. The isotopic data suggest predominant formation of N-N bonds occurs via interaction of gaseous NO with a nitrogen containing adsorbed complex.     

Cation Radicals of Covalently Linked Porphyrin Dimers and their Monomeric Constituents: An ESR and ENDOR Study

The spin density distributions in the cation radicals of various covalently linked porphyrin dimers have been studied by liquid phase ESR and ENDOR methods to find out whether these systems show intramolecular electron delocalization. Such a delocalization is known to occur in the bacteriochlorophyll dimer (“special pair”) in the photosynthetic reaction center. The dimers that were studied in this work were derived from zinc mesotetratolylporphyrin (ZnTTP) and linked at the ortho or para positions of one phenyl ring with varying bridge lengths. ¹H and ^I4N hyperfine coupling constants could be measured for the dimer cation radicals and compared with those of monomeric ZnTTP as well as ZnTTP derivatives that carry alkoxy or hydroxy substituents to mimic the bridges of the dimers. By comparing the hyperfine data of the monomer and dimer cation radicals it is concluded that in the present dimers the unpaired electron is localized on one porphyrin unit.     

Reaction mechanism of NO decomposition over alkali metal-doped cobalt oxide catalysts

The kinetics of NO decomposition were investigated over alkali metal-doped Co₃O₄ catalysts. For all the alkali metal-doped Co₃O₄ catalysts tested, the presence of O₂ caused a decrease in the N₂ formation rate with reaction orders between −0.26 and −0.40. The reaction orders with respect to NO were between 1.21 and 1.47, which are higher than unity, suggesting that NO decomposition proceeds via a bimolecular reaction. The observation by in situ Fourier transform infrared (FT-IR) spectroscopy confirmed the presence of nitrite (NO₂⁻) species on the surface under NO decomposition conditions. Isotopic transient kinetic analysis performed using ¹⁴NO and ¹⁵NO revealed that a surface-adsorbed species, probably NO₂⁻, serves as an intermediate during NO decomposition. We proposed a reaction mechanism in which the reaction is initiated by NO adsorption onto alkali metals to form NO₂⁻ species, which migrates to the interface between the alkali metals and Co₃O₄, the active sites, and then react with the adsorbed NO species to form N₂.     

Crop nitrogen utilization and soil nitrate loss in a lettuce field

Low N use efficiency and high nitrate (NO ₃ ^- ) pollution potentials are problems in intensive vegetable production systems. The purpose of this study was to quantify N utilization by lettuce (Lactuca sativa L. cv Salinas), and identify periods of NO ₃ ^- loss in an on-farm study in the Salinas Valley in coastal California. During autumn and winter, surface moisture remained low, and NO ₃ ^- concentrations increased, reflecting high net mineralizable N, as determined by anaerobic incubation, and nitrification potential, as determined by the chlorate inhibition method. At the onset of a large winter storm, tracer levels of¹⁵NO ₃ ^- were injected in the top 5 mm of soil in 30 cm-deep cylinders. After two weeks, most of the¹⁵N was present as¹⁵NO ₃ ^- at 10–30 cm depth. By difference, losses to denitrification accounted for ~ 25% of the surface-applied¹⁵N. Leaching below 30 cm did not occur, since no¹⁵N enrichment of NO ₃ ^- -N was measured in anion-exchange resin membranes placed at the base of each cylinder. During the crop period, NO ₃ ^- losses were most pronounced after irrigation events. Uptake of N by two crops of lettuce (above- and belowground material) was approximately equal to fertilizer inputs, yet simulation of N fates by the Erosion/Productivity Impact Calculator (EPIC) model indicated losses of 14.6 g-N m^–2 by leaching and 2.5 g-N m^–2 by denitrification during the 6-month crop period. The large NO ₃ ^- losses can be attributed to accumulation of soil NO ₃ ^- during winter that was leached or denitrified during the irrigated crop period.     

Synthesis and structure of a novel tetranuclear 32-membered macrocycle of triphenyltin complex with pyridine-4-carboxylic acid N-oxide

The 32-membered stereoregular triphenyltin macrocycle: [Ph₃Sn (OCOC₅H₄NO)]₄·3(CH₂Cl₂) has been synthesized by the reaction of triphenyltin chloride with pyridine-4-carboxylic acid N-oxide. The complex was characterized by elemental analysis, FT-IR, NMR (¹H, ¹¹⁹Sn) spectra and X-ray crystallography. Single crystal X-ray diffraction data reveal that the complex is highly symmetrical tetranuclear cyclic complex with the ligand pyridine-4-carboxylic acid N-oxide bridging the adjacent tin atoms. All four tin atoms are five-coordinated resulting in trigonal bipyramidal geometry. And the complex showed good thermal stability and high antitumor activity.     

Effect of hindered piperidine light stabilizer molecular structure and UV absorber addition on the oxidation of HDPE. Part 2: Mechanistic aspects—molecular modeling and electron spin resonance spectroscopy study

The first in this series of papers explored the effect of the structural characteristics of 2,2,6,6‐tetramethylpiperidine‐based hindered amine light stabilizers (HALS) on the thermal and photostabilization of high‐density polyethylene. In the second part, the energies (stabilities) of the nitroxyl radicals and various intermediate species have been predicted using AccuModel® and related to stabilization performance and electron spin resonance (ESR) spectral data. Nitroxyl radicals with low predicted stability generally afforded improved thermal and photostabilization. ESR spectra were used to obtain values of nitroxyl radical concentration ([>N? O·]) and g‐factor as a function of pre‐aging time for combinations of pre‐aged HDPE and >N? H HALS. Demethylation reactions of > N‐methyl HALS resulted in uselessly weak ESR spectra. The HALS that afforded poor thermal oxidative stabilization gave rise to pronounced minima in [>N? O·] that coincided with a maximum in hydroperoxide concentration. The g‐factor values indicated that a predominant nitroxyl canonical form generally promoted superior thermal oxidative stabilization, whereas a predominance of the dipolar N·⁺?O^? form promoted superior photo‐oxidative stabilization. These trends may be related to greater radical‐scavenging and peroxide‐trapping effects, respectively. Molecular modeling and ESR spectra can therefore provide valuable insight into the effectiveness of HALS and stabilization mechanisms. J. Vinyl Addit. Technol. 10:159–167, 2004. © 2004 Society of Plastics Engineers.     

Measurement of nitrous oxide and di-nitrogen emissions from agricultural soils

Nitrous oxide can be produced during nitrification, denitrification, dissimilatory reduction of NO ₃ ^- to NH ₄ ⁺ and chemo-denitrification. Since soils are a mosaic of aerobic and anaerobic zones, it is likely that multiple processes are contributing simultaneously to N₂O production in a soil profile. The N₂O produced by all processes may mix to form one pool before being reduced to N₂ by denitrification. Reliable methods are needed for measuring the fluxes of N₂O and N₂ simultaneously from agricultural soils. The C₂H₂ inhibition and ¹⁵N gas-flux methods are suitable for use in undisturbed soils in the field. The main disadvantage of C₂H₂ is that as well as blocking N₂O reductase, it also blocks nitrification and dissimilatory reduction of NO ₃ ^- to NH ₄ ⁺ . Potentially the ¹⁵ N gas-flux method can give reliable measurements of the fluxes of N₂O and N₂ when all N transformation processes proceed naturally. The analysis of ¹⁵N in N₂ and N₂O is now fully automated by continuous-flow isotope-ratio mass spectrometry for 12-ml gas samples contained in septum-capped vials. Depending on the methodology, the limit of detection ranges from 4 to 11 g N ha^-1day^-1 for N₂ and 4 to 15 g N ha^-1day^-1 for N₂O. By measuring the ¹⁵N content and distribution of ¹⁵N atoms in the N₂O molecules, information can also be obtained to help diagnose the sources of N₂O and the processes producing it. Only a limited number of field studies have been done using the ¹⁵N gas-flux method on agricultural soils. The measured flux rates and mole fractions of N₂O have been highly variable. In rain-fed agricultural soils, soil temperature and water-filled pore space change with the weather and so are difficult to modify. Soil organic C, NO ₃ ^- and pH should be amenable to more control. The effect of organic C depends on the degree of anaerobiosis generated as a result of its metabolism. If conditions for denitrification are not limiting, split applications of organic C will produce more N₂O than a single application because of the time lag in the synthesis of N₂O reductase. Increasing the NO ₃ ^- concentration above the K _m value for NO ₃ ^- reductase, or decreasing soil pH from 7 to 5, will have little effect on denitrification rate but will increase the mole fraction of N₂O. The effect of NO ₃ ^- concentration on the mole fraction of N₂O is enhanced at low pH. Manipulating the interaction between NO ₃ ^- supply and soil pH offers the best hope for minimising N₂O and N₂ fluxes.     

Synthesis and characterization of nitrosyl complexes containing 4,6-dimethyl-pyrimidine-2-thiolato (‘SpymMe2’) as ligand: [Ru(‘SpymMe2’,-N,-S) (‘SpymMe2’,-S)(NO)(P–P)](PF6) (P–P = 1,2-bis(diphenylphosphino)ethane or 1,2-bis(diphenylphosphino)ethylene: X-ray structure of [Ru(‘SpymMe2’,-N,-S) (‘SpymMe2’,-S)(NO)(dppe)](PF6)

The reaction products of fac-[RuCl₃(NO)(P–P)], [P–P = dppe (1) and c-dppen (2)] with the 4,6-dimethyl-2-mercaptopyrimidine ligand in methanol are the correspondent nitrosyl derivatives [Ru(‘SpymMe₂’,-N,-S)(‘SpymMe₂’,-S)(NO)(P–P)](PF₆). These are the first reported ruthenium complexes containing both, nitrosyl and a pyrimidinethiol derivative. The X-ray crystal structure of complex 1 is reported. The intense ν_NO bands in the IR spectra (KBr pellets) of complexes 1 and 2 are at 1850 and 1860 cm⁻¹, respectively. The cyclic voltammetry of both complexes shows two reduction waves centered at the NO⁺ ligand.