Selective study of paramagnetic centres in coals by e.s.r. and e.s.e. spectrometry

Electron spin resonance (e.s.r.) and electron spin echo (e.s.e.) studies of two natural and heat-treated bituminous coals are presented. In all samples, two main paramagnetic centres have been detected and studied selectively by obseving e.s.e. decays at different parts of the e.s.r. signal. These centres interact differently with neighbouring protons and the interaction is reduced in both centres by heat-treatment. The e.s.r. measurements reflect the heterogeneity of the region around the radical and are sensitive to the presence of proton hyperfine and dipolar broadening interactions. E.s.e. decays are sensitive to the presence of signals with longer spin memory time T_M. The T_M values obtained for all samples can be rationalized by assuming an exchange frequency in the range 1 × 10⁸ − 1 × 10⁹s⁻¹.     

Fragmentation of Fluorinated Model Compounds Exposed to Oxygen Radicals: Spin Trapping ESR Experiments and Implications for the Behaviour of Proton Exchange Membranes Used in Fuel Cells

Low‐molecular weight model compounds (MCs) for Nafion membranes used in fuel cells were exposed at 300 K to ^·OH radicals produced by UV irradiation of aqueous H₂O₂ solutions. The MCs contained fluorinated and partially fluorinated groups terminated by sulphonic or carboxylic acid groups. The fragmentation process in the MCs was studied by spin trapping electron spin resonance (ESR) methods, using 5,5‐dimethylpyrroline‐N‐oxide (DMPO), N‐tert‐butyl‐α‐phenylnitrone (PBN) and 2‐methyl‐2‐nitrosopropane (MNP) as the spin traps. The objective of these experiments was to assess the effect of the type of ionic groups (sulphonic or carboxylic) and of fluorine substitution on the spin adducts detected. DMPO experiments led to the detection of spin adducts of ^·OH and of carbon‐centred radicals (CCRs), and allowed the determination of the ^·OH attack site on the ionic and/or on the protiated or fluorinated groups. CCR adducts were also detected when using PBN as a spin trap; a key point in the interpretation of the PBN results was, however, the realisation that MNP is formed during PBN exposure to UV irradiation and oxygen or other oxidants such as H₂O₂. Experiments with MNP as the spin trap were the most informative in terms of structural details for adducts obtained from each MC. The results allowed the identification of CCRs present as adducts, based on large hyperfine splittings (hfs) from, and the number of, interacting ¹⁹F nuclei; in addition, oxygen‐centred radicals (OCRs) as MNP adducts were also identified, with much lower hfs from ¹⁹F nuclei. Taken together, the results deduced by spin trapping suggest that both sulphonic acid and acetic acid groups can be attacked by ^·OH radicals and confirm two possible degradation mechanisms in Nafion membranes: initiated at the backbone and at the side chain.     

On the Spin Trapping off Sulfur Dioxide Anion-Radicals (.SO2−) by nitrones

^.SO₂⁻ radicals react with nitrones to form the anion of the corresponding spin adduct. Addition of mild oxidants generates the spin adduct of ^.SO₂⁻, the e.p.r. spectrum of which exhibits additional hyperfine splitting.     

Spinmarkierte Oligopeptide als substratanaloge Inhibitoren der Leucinaminopeptidase

Spin Labeled Oligopeptides as Substrate Analogous Inhibitors of Leucine Aminopeptidase The synthesis of L-Thr(Bu^t)-(L-Phe)_n-SL peptides C-terminal labeled with the nitroxide spin label 2,2,5,5-tetramethyl-3-amino-pyrrolidin-1-oxyl, is described and e.s.r. parameters of the labeled compounds are presented. As shown by e.s.r. spectroscopy studies Thr(Bu^t)-Phe-SL is a competitive inhibitor of the leucine aminopeptidase and is bound by the enzyme with high affinity.     

Spin-Trapping von Radikalen bei der Zersetzung aromatischer Diazoniumsalze mittels Ultraschall

Spin Trapping of Radicals Formed during the Decomposition of Aromatic Diazonium Salts by Ultrasound The decomposition of aromatic diazonium compounds ArN₂^⊕X^⊖ (Ar = phenyl, 2,5-diethoxy-4-(N-morpholino)phenyl; X = BF₄, PF₆, B(C₆H₅)₄, SCN, OCN,N₃; 1a , 1c–f , 2b–f ) by ultrasonic waves has been studied by e.s.r. The spin trapping technique has been shown to be a suitable method for the detection of free radical intermediates.     

Distribution in correlation times for rotational diffusion of spin probes in polymers

Electron spin resonance spectra of poly(vinyl acetate) (PVAc) and plasticised PVAc doped with nitroxide spin probes show a double-peak feature at temperatures immediately below T_{50 G}. This can be explained on the basis that probe tumbling occurs with a distribution of correlation times τ_R arising from a distribution of free volume within the polymeric matrix. The two-component e.s.r. spectra arise because the distribution in τ_R of the probes straddles the value (c. 3 × 10^?9 s) which marks the change-over from fast- to slow-motion spectra. The implications of the existence of a distribution of τ_R values for calculations based on the assumption of a single value are examined. It is concluded that when probe tumbling is slow the error in τ_R could be significant, but for rapid tumbling (motionally narrowed spectra) the error is likely to be insignificant.     

Aromaticity of coal extract by 1H and 13C pulsed n.m.r. methods

High-resolution gated decoupling ¹³C spectra of soluble materials such as coal extracts have been analysed to evaluate the carbon aromaticity (f_a). A correlation was found between f_a and the reciprocal of the square of the shorter component in spin—spin relaxation time (T₂), which was obtained by pulsed ¹H n.m.r. in the solid state at low temperature. Values of carbon aromaticity for several coals as received were estimated by using the above correlation and compared with those by van Krevelen's densimetric method.     

Untersuchungen zur Spin-Trapping-Kinetik in Systemen von Radikalen unterschiedlicher Reaktivität. Die Photolyse von α-Phenylbenzoin in Gegenwart von Benzyliden-tert.-butylamin-N-oxid

Investigations on Spin-Trapping Kineties in Radical Systems with Different Reactivity. Photolyses of α-Phenylbenzoine in the Presence of Benzylideneter. -butylamine N-oxide As a model system to study the spin-trapping kinetics of systems forming two radicals of different reactivity α-phenylbenzoine (PhB) was chosen. The photolysis of PhB forms benzoyl (B·) and diphenylketylradikals (K·), respectively, which can be trapped by benzylidene-tert-butyl-amine N-oxide (PBN), as a spin trap. The rate constants of radical addition to PBN and their reactions with the spin adducts were determined by means of flash photolysis and kinetic calculations of the PBN concentration dependence. For B· and K· addtion rate constants to PBN of k₁ = 8,1 × 10⁵ M⁻¹s⁻¹, and k₂ = 5,8 · 10⁴ M⁻¹s⁻¹, respectively, were found. The rate constants for the reactions of B· and K· with the spin adducts are in the order of 10⁹…10¹⁰ M⁻¹s⁻¹.     

Proton spin-lattice relaxation in moist coal

Proton spin-lattice relaxation time constants (T₁) have been found to range over more than an order of magnitude for coals of different rank^1–6. Proposed relaxation mechanisms have included spin diffusion to paramagnetic centres^1–3 or to rotating methyl groups⁵, and penetration of molecular oxygen into the coal¹. Evidence is now presented for a large contribution from spin-relaxation processes in adsorbed moisture. Values of T₁ were measured indirectly from 50.3 MHz ¹³C CP/MAS n.m.r. spectra by varying the recovery delay inserted after each 5 ms data acquisition period. Signal heights were monitored for peaks at δ = 30 and 130 ppm (assigned to methylene groups and aromatic rings respectively). The signal recovery curves were approximately exponential.     

Effects of catalysts and steam gasification on e.s.r. of carbon black

The effects of temperature and steam concentration on Spheron-6 carbon black with and without impregnated K₂C0₃ and diluted with alumina have been measured by in-situ electron spin resonance (e.s.r.). For samples in flowing helium, heating from 290 to 1070 K results in a small increase in the carbon black resonance linewidth but a large and mostly irreversible increase in the resonance linewidth of carbon black with K₂C0₃ at >650 K. The effect of steam concentration on the e.s.r. spectra of the carbon black at 875 K has been studied; an effect is observed only for carbon black with K₂C0₃. Increasing the steam concentration from 0 to 6.5 vol% results in reversible narrowing of the linewidth while hardly changing the free radical concentration. The mechanism of the line-broadening of the resonance resulting from heating the K₂C0₃-impregnated carbon black may be due to one of two processes: an unresolved nuclear hyperfine interaction between the unpaired electron of the carbon free radical and potassium, or an increased mobility of charge carriers produced by the potassium. The reversible effects of steam are attributed to a modificartion of these processes and may be related to the mechanism of catalysed gasification of coal.     

Radikalabfang. III [1] ESR-spektroskopische Untersuchung von Radikalreaktionen eines bicyclischen 1-Hydroxy-3-imidazolin-3-oxids [2]

Spin Trapping. III. Studies of Radical Reactions of a Bicyclic 1-Hydroxy-3-imidazoline-3-oxide Radical addition reactions to the nitrono group and the oxidation of the 8-hydroxy-1,4,5,7-tetramethyl-6,8-diazabicyclo[3.2.1]oct-6-en-6-oxide 2b were investigated by e.s.r.-spectroscopy. Oxidation of 2b by Ag₂O, PbO₂, hydrogenperoxide, tert.-butoxy and benzoyloxy radicals, respectively, yields a bicyclic nitroxide with typical a(N)-value (1,9 mT). Testing a wide range of carbon-centered radicals we did not find, however, any spin adducts in contrast to the literature. Only H-atoms are found to add to the nitrone function of 2b giving a bicyclic nitroxide 4b (R′ = H) with a(N)-value of 1,4 mT, a(H_ax) = 2,4 mT. The relationship between a ring-chain tautomerism of 2 as previously described and the formation of radicals has been demonstrated by radical reactions of derivatives of ring opened isomers 5b .     

Effect of pyrite and pyrrhotite on free radical formation in coal

The effects of pyrite (FeS₂) and pyrrhotite (Fe₇S₈) on free radical formation in a coal sample (81% carbon content) have been investigated by electron spin resonance (e.s.r.) spectroscopy. Changes in the e.s.r. parameters (spin concentration g^-1, n, linewidth ΔH and g-value) were monitored in samples of coal, coal+8% FeS₂ and coal+8% Fe₇S₈, as these samples were heated in vacuum or in hydrogen from room temperature to 500 °C, in steps of 50 °C for a residence time of 30 min at each temperature. In vacuum heating, changes in n begin to occur at 400 °C, 350 °C and 300 °C respectively for coal, coal+8%Fe₇S₈ and coal+8% FeS₂ samples whereas in H₂, the corresponding temperatures are 250 °C, 200 °C and 150 °C. Changes in ΔH and g were also observed at these temperatures. The maximum increase in n occured for coal+8% FeS₂ samples whereas the minimum increase was observed for the pure coal sample. It is argued that enhancement in n is due to two mechanisms: the pyrite to pyrrhotite conversion and the presence of pyrrhotite itself. The detailed nature of the catalytic activity of pyrrhotite is not known.     

The Reactions of Bromide with Ozone Towards Bromate and the Hypobromite Puzzle: A Density Functional Theory Study

Taking the solvent water into account, the energetics of the reactions of O₃ with Br^? leading to BrO₃ ^? have been calculated by Density Functional Theory at the B3LYP/6–311+G(d)/SCRF =COSMO level. Br^? reversibly forms an adduct, BrOOO^?, (ΔG?=?+6 kJ mol^?1) that decays spin allowed into BrO^? and O₂(¹Δ_g) (ΔG?=?+13 kJ mol^?1). BrO^? undergoes an oxidation to BrO₂ ^? and a reduction to Br^?. This may be accounted for if two different adducts, OBrOOO^? and BrOOOO^?, decay into BrO^? plus O₂ and Br^? plus 2 O₂. After cyclization, OBrOOO^? may also lead to Br^? plus 2 O₂.     

Electron-spin echoes and saturation in coal

Free radical electron paramagnetic resonance (e.p.r.) spectra of coal are comprised of unresolved hyperfine multiplets with widths of ≈770 kHz, (T₂)^?1, at 90–110 K. Unresolved hyperfine interactions are measured in the spin echo decay modulation and locate nuclei in the coal structure relative to the radical. With two distinguishable coal e.p.r. spectra in the sample, it is demonstrated that components may be separated from one another in the spin echo spectrum by judicious choice of temperature, pulse interval and echo repetition rate. The relaxation times T₁ and T₂ are measured with the spin echo at several temperatures.     

Spin entropy enhancement tuned by spin state transition in La0.7−xYxCa0.3CoO3

The effect of spin states on the spin entropy has been studied in La_0.7−xY_xCa_0.3CoO₃ (x=0.0, 0.05, 0.1 and 0.15). Magnetothermopower results reveal that Y-doping improves the spin entropy of La_0.7Ca_0.3CoO₃. The magnetic results indicate that the Co³⁺ ions undergo a transition from an intermediate spin state (t⁵_2ge¹_g; S=1) to a low spin state (t⁶_2ge⁰_g; S=0), which results in the enhancement of the spin entropy. A suitable theoretical model is suggested to explain well the enhancement of the spin entropy induced by spin state transition. This investigation gives strong evidence that the enhanced spin entropy contribution to the thermopower is tuned by the spin state transition.     

The Chichibabin Amination of 5-Phenylpyrimidine and Phenylpyrazine [1,2]

5-Phenylpyrimidine ( 1 ) on treatment by KNH₂/NH₃, affords a mixture of 2 amino- and 4-amino-5-phenylpyrimidine. The presence of only the anionic C-4 adduct. 4-amino-5-phenyl-1 (or 3), 4-dihydropyrimidinide was shown by ¹H- and ¹³C-NMR spectroscopy of a solution of 1 in KNH₂/NH₃. Study of the amination by use of ¹⁵N-labelled KNH₂/NH₃ convincingly shows that in the formation of the 2-amino compound a ring opening-ring closure sequence is involved. The amination of phenylpyrazine ( 11 ) into 5-amino- and 3-amino-2-phenylpyrazine has been found to occur without opening of the pyrazine ring. Examination by ¹H- and ¹³C-NMR spectroscopy of a solution of 11 in KNH₂/NH₃, reveals that at −60°C three adducts are present; i.e. 3-amino-, 5-amino- and 6-amino-2-phenyl-dihydropyrazinides, at −40°C the C-3 and C-5 adducts are formed; and at room temperature only the thermodynamically most stable C-5 adduct is present.     

Optimization of the spin entropy by incorporating magnetic ion in a misfit-layered calcium cobaltite

Effects of (Lu, Ni) co-doping on spin entropy of Ca₃Co₄O_9+δ have been investigated. Results of this study show that (Lu, Ni) co-doping can be more effective than single Lu doping for optimizing the spin entropy of Ca₃Co₄O_9+δ. X-ray photo-emission spectroscopy confirms a reduction in Co⁴⁺ concentration, which leads to enhanced spin entropy from Co sites. The magnetic ions of Ni with unpaired 3d electrons can produce extra spin entropy through a new hopping model, thereby contributing to an increase in total spin entropy. Incorporating magnetic ions is an effective way to optimize the spin entropy of thermoelectric materials. This study opens a new way to broaden and design prospective thermoelectric materials.     

1H n.m.r. relaxation in bituminous coal and semicoke

Temperature and frequency-dependent proton relaxation measurements, in conjunction with spin concentrations determined by e.s.r., have shown that for fresh and oxidized coal the relaxation mechanism involves spin-diffusion to paramagnetic centres and to rotating methyl groups on alkyl chains. ¹H relaxation in semicokes is more complex; probably both direct relaxation to paramagnetic centres as well as spin-diffusion contribute.     

The Lowest Singlet and Triplet States of o-Benzyne: Spin-Coupled Interpretation of the Electronic Structure at CAS SCF Equilibrium Geometries

The electronic structure of the X̃¹A₁ and ã³B₂states of o-benzyne is analyzed by means of spin-coupled (SC) wave functions involving eight active orbitals: two at the dehydro centers, in the plane of the ring, and six π orbitals, inherited from benzene. The equilibrium geometries of the two states, at which all calculations have been carried out, result from separate full geometry optimizations employing ‘8 in 8’ complete active space self-consistent field (CAS SCF) wave functions incorporating an analogous core-valence partitioning. The diradical character of the ground state of o-benzyne, as approximated by the singlet SC wave function, is reflected in the low value of the overlap (0.46) between the two in-plane sp²-like SC orbitals at the dehydro centers, which are just 1.274 Å apart, and in the nonnegligible extent of triplet coupling between the spins of the electrons in these orbitals — ca. 6.4% of the total spin function for the valence electrons (expressed in the Serber spin basis) is made up of spin eigenfunctions, in which these electrons enter as a triplet pair. The geometry of the ground state is found to have an aryne-like character, which is further confirmed by the pattern of alternation of the overlaps between neighboring 71 valence orbitals, and by the domination of the total spin function for the valence electrons (expressed in the Rumer spin basis) by the spin-coupling patterns corresponding to an arynic structure.