Temperature Dependence Of ESR Spectra Research Articles

Temperature dependence of ESR spectra of nitroxide spin probe in glassy polymers

AbstractTemperature dependencies of ESR spectra of nitroxide spin probes in glassy polymers near and below glass transition temperature were examined in detail. Three temperature ranges, each characterized by specific changes in spectral shape, were defined. (i) In the low temperature range, ESR spectra of nitroxide radical in glassy polymer matrix weakly depend on temperature and remain qualitatively the same. (ii) In the intermediate temperature range, significant changes in the shape of spectra are observed. (iii) A new phenomenon was revealed near and below glass transition temperature: narrowing of linewidths occurs while the ratio of amplitudes of different components varies insignificantly. Analysis of molecular rotational mobility was carried out by means of commonly used empirical approaches. It was shown that widely used formulas and empirical approaches are not applicable for characterization of molecular mobility in glassy polymers. Mechanisms of rotational molecular movements in glassy polymers are discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 563–575, 2009

Preparation and magnetic properties of zinc phthalocyanines homogeneously doped with alkaline metals

A method for preparation of homogeneous samples of metal-phthalocyanines doped with alkaline metals was developed. Polycrystalline samples of zinc phthalocyanines doped with sodium, potassium, and rubidium atoms were investigated. We measured the temperature dependence of ESR spectra of prepared samples and studied their composition and structure. It is shown that their magnetic properties depend strongly on the degree of doping as well as on the subsequent grinding and annealing.

Electron spin resonance investigations on ion beam irradiated single-wall carbon nanotubes

ESR investigations on single-wall carbon nanotubes irradiated with accelerated protons, helium ions, and neon ions are reported. All spectra were accurately simulated assuming that the resonance line is a convolution of up to 4 lines originating from catalyst residues, amorphous carbon, and electrons delocalized over the conducting domains of nanotubes. The faint line observed in irradiated nanotubes at g > 2.25 was assigned to magnetic impurities. However, there are no sufficient data to confirm that this line is connected to radiation-induced magnetism in carbon nanotubes. The generation of paramagnetic defects due to the bombardment of single-wall carbon nanotubes by accelerated ions is reported. These data correlate with previous Raman and thermal investigations on the same single-wall carbon nanotubes and reveals their sensitivity to ionizing radiation. The temperature dependence of ESR spectra in the range 25–250 K was used to identify the components of the ESR spectra.

Thermodynamic Analysis of the Chemical Exchange of β−Phosphorylated Cyclic Nitroxides by Using Two-dimensional (Temperature versus Magnetic Field) Simulation of ESR Spectra: The Impact of Labile Solvent−Solute Interactions on Molecular Dynamics

A complete thermodynamic characterization of the chair-to-chair interconversion in beta-diphosphorylated piperidine-N-oxyl radicals was achieved by means of the analysis of temperature-dependent ESR spectra. A new two-dimensional simulation method was developed with the coordinates temperature and magnetic field, in which the entire set of spectra was simulated simultaneously by adjusting the coefficients in the power expansion, giving the temperature-dependent ESR parameters and the thermodynamic and kinetic parameters determining the site populations and exchange rates, respectively. The new method promotes elimination of the ambiguities inherent in the spectroscopic determination of thermodynamic parameters. Labile solvent-solute interactions can strongly influence the chemical exchange, producing a complex network of symmetric and asymmetric interconversions. The solvent dependence of magnetic relaxation was also analyzed.

High temperature molecular magnetism caused by π-electrons: copper phthalocyanine doped with alkaline metals

Electron spin resonance spectra of copper phthalocyanine doped with alkaline metals ( A x CuPc ) have been investigated. The temperature dependence of ESR spectra indicates ferromagnetic behavior. The Curie-Weiss temperature varies from 30 to 115 K depending on the stoichiometry x of the samples. Some particles of polycrystalline samples were attracted to a weak magnet at temperatures slightly higher than 77 K. The observed magnetism is caused by unpaired π-electrons of phthalocyanine anions on the eg doubly degenerated molecular orbital. The observed ferromagnetism can be understood within the framework of the McConnell-2 model proposed for organic ferromagnetic charge-transfer complexes. The high-temperature magnetism in A x CuPc is considered to be a result of the Zener mechanism of double exchange between phthalocyanine molecular anions of different valence.

Motional heterogeneity and phase separation of functionalized polyester polyurethanes

The polyester polyurethanes, PU based on isophoronediisocyanate, polycaprolactone, and 1,4-butanediol with different amounts of functional groups introduced into the hard segments via second chain extender, 2,2′-bis-(hydroxymethyl) propionic acid, were investigated by electron spin resonance, ESR, spin label method, wide-angle X-ray diffraction, WAXD, optical microscopy and differential scanning calorimetry, DSC. The objective of this study is to clarify the effect of functional groups on the motional heterogeneity, microphase separation and crystallisation of the polyurethanes. The concentration of carboxylic groups varied from 0 to 0.45 mmol g −1. The temperature-dependent ESR spectra of spin labelled PU hard segments chain ends with stable nitroxide radical 2,2,6,6-tetramethyl-4-aminopiperidin-1-yloxyl are sensitive to the amount of functional groups attached to the hard segments. Composite ESR spectra of functionalized PU, with fast and slow component, suggest that PU hard segments are partitioned in two motionally different environments. According to the ratio of fast and slow component motional heterogeneity increases with an increase of functional groups up to 0.35 mmol g −1 and above this concentration slow component decreases indicating higher degree of phase mixing and stronger effect of soft segments. Polarized micrographs and the extent of ordering from WAXD measurements reveal the changes of phase morphology with the carboxylic groups content in a similar way as shown from the motional behaviour of spin label on the segmental level. The degree of crystallinity and the separation of spherulitic rings are decreasing above a certain concentration of functional groups. The effect of functional groups in PU on the hard and soft segment mixing is discussed in terms of additional noncovalent interactions and chain structure which at critical level of interactions lead to a formation of more open hard segment structure accessible to interaction with the soft segment.

A Study of the Incommensurate Phase of Rb 2 ZnCl 4 by ESR

The ESR absorption spectrum of Mn2+ ions at Zn positions has been studied in the incommensurate phase of Rb2ZnCl4. It is known that Rb2ZnCl4 crystals exhibit an incommensurate phase between temperatures Ti = 303 K and Tl = 192 K. The form and the temperature dependence of ESR spectra are in good agreement with the predictions of the phase soliton theory. The parameters h1 and h2 determining the magnetic resonance field show, as expected, a power law as a function of the temperature. A modified superposition model was used to estimate the modulation amplitude of the incommensurate phase, and the principal axis of the zero field splitting D.

Electron spin carriers in halogen-bridged 1-d complexes with nondegenerate cdw ground state

We measured temperature dependences of ESR spectra and 1 H NMR spin-lattice relaxation time T 1 on recently prepared mixedvalence hetero-metal complexes [Pd(en) 2 ][PtX 2 (en) 2 ](ClO 2 ) 4 (en: ethylenediamine; X: Cl, Br) which consist of a 1-D structure of-X-Pd II -X-Pt IV -X-Pd II -X-Pt IV -X-chain giving a non-degenerated electronic ground state. The observed low-concentration of paramagnetic M III sites and also long 1 H NMR T 1 compared with those in homo-metal complexes [M II (en) 2 ][M IV X 2 (en) 2 ](ClO 4 ) 4 (M: Pt, Pd; X: Cl, Br) having the degenerated ground state, can be explained well by the hindrance of the formation of domainwall type spin solitons in the hetero-metal complexes. This result supports our previous conclusion of the neutral soliton formation in the homo-metal halogen-bridged complexes.

Synthesis, Characterization and Studies on the Thermal Stability of Bis(Indenyl)-Praseodymium and Neodymium Complexes with a Ln-S Bond: (C9H7)2Lnsr(Thf)

Four new bis(indeny1)lanthanide thiolate complexes, (C9H7)2LnSR(THF) [Ln˭Nd, R=2-benzoxazolyl (I), 2-(I-methyl)imidazolyl (n) and 2-pyrimidinyl (III); Ln = Pr, R = 2-pyrimidinyl (IV)], have been first synthesized by the reaction of (C9H7)3Ln with equimolar amounts of thiols. These complexes were Characterized by elemental analyses, IR spectra and MS spectra, indicating that they are present as monomer structures with coordinated THF. The thermal stability of complex I has been studied by the temperature dependence of ESR spectra and TG and DTA measurements. The ESR spectra show that, at low temperature, the g-value (2.0038) which is close to the free electron spin value (ge, = 2.0032) may be assigned to the 4f Nd3 ion. When the temperature is higher than 400 K, the g-value, line width (AHpp) and relative intensity changed, probably due to the paramagnetism of decomposed organic ligand radicals.

ESR and NMR probes of counter-ion effects in perfluorosulfonate ionomer membranes

The properties of Nafion membranes, modified by exchange with tetraalkylammonium cations and equilibrated in methanol, were investigated using ESR spin probe techniques and 14 N NMR spectroscopy. TEMPO and TEMPOL nitroxide radicals are shown to act as probes of the methanol solvent phase of the membranes. The mobility of the probe radicals is restricted, the correlation times for rotational diffusion being reduced by an order of magnitude within the ionomer environment compared to free methanol solution. The larger tetraalkylammonium cations result in higher levels of swelling in methanol and a greater degree of nitroxide mobility. The temperature dependent ESR spectra show a gradual transition from freely diffusing to rigidly immobilized radicals as the temperature is lowered. The motion is completely frozen out at about 190 K, before the freezing point of methanol is reached. 14 N NMR spectroscopy of the tetraalkylammonium counter-ions is also reported. The NMR signals for all cations remain quite sharp and well resolved in methanol-equilibrated Nafion. The corresponding 14 N spin lattice relaxation times, T 1, were reduced by one order of magnitude compared to methanol solution. The long-chain surfactant cations showed the least reduction in T 1 on incorporation into the membrane, consistent with retention of significant rotational mobility. These results are interpreted in terms of a much weaker ion clustering in the organically modified membranes than is found in strongly hydrated Nafion membranes.

Polymer Effects on Reversible Formation of Dioxygen Adduct with Silver(II) Tetraphenylporphyrin Moieties

The anomalous behavior in the temperature dependence of ESR spectra has been discovered for the powder sample and LB film of poly[5-(acrylamidophenyl)-10,15,20-triphenylporphyrinato]silver(II) (denoted by polyAgAATPP) under oxygen atmosphere. The ESR intensity decreased with the decrease of temperature from room temperature to −150 °C. The IR and Raman spectroscopies at liquid nitrogen temperature revealed that the new IR- and Raman-active peaks appeared at 782 and 545 cm-1 for the LB film of polyAgAATPP under oxygen atmosphere, respectively. The peaks were assigned to ν(O−O) and ν(Ag−O) in the dioxygen adduct of polyAgAATPP, respectively. The linkage between AgTPP moieties and dioxygen was likely to be of the bridging type. These phenomena were not observed for monomeric AgAATPP.

ESR study of the motional dynamics of NO 2 adsorbed on Na-mordenite

The temperature dependent ESR spectra of NO 2 adsorbed on Na-mordenite have been studied in the temperature range 77–300 K. An analysis based on slow-motional ESR theory shows that the reversible change with temperature is caused by Heisenberg spin exchange. The activation energy is 0.8–1.6 kcal/mol depending on NO 2 concentration. The spin exchange is probably caused by the interaction of NO 2 molecules with each other as they diffuse along the channels of Na-mordenite.

ESR study of the LB films containing metallic domains

ESR and IR absorption measurements have been performed on Langmuir-Blodgett (LB) films based on a charge-transfer complex of bis(ethylenedioxy)tetrathiafulvalene (BO) and decyltetracyanoquinodimethane. Analyses of the anisotropic ESR and IR spectra have provided quantitative structural information of the system, which is consistent with the reported high conductivity of the films. The temperature dependent ESR spectra have clearly indicated that the system contains metallic domains, whose spin susceptibility is dominated by the Pauli susceptibility due to the conduction electrons on the BO stacks.

MNDO Calculations as a valuable tool for structure evaluation of contact ion pairs

AbstractContact ion pairs, essential intermediates of both chemical and biochemical reactions in solution, can be satisfactorily approximated by MNDO calculations as exemplified by the lithium and potassium salts of 2,5‐trimethylsilyl‐p‐benzosemiquinone radical anion and by sodium tetraphenylallyl anion diethyl ether. Topics discussed are the structural changes of p‐benzoquinone derivatives on single‐electron reduction, the dynamics of the countercations in semiquinone radical contact ion pairs measured by temperature‐dependent ESR spectra, as well as structural details of the sodium tetraphenyl‐allylanion complex isolated such as its intramolecular Na dibenzene sandwich or its diethylether solvation. © 1992 John Wiley & Sons, Inc.

Conformational Change of Benzoquinone Crown Ether Radical Anions Induced by Complexation with Alkali Metal Cation: An ESR Study

Abstract The crowned p-benzoquinone anion radicals, O5−–O8−, were generated either electrochemically or by using a reducing metal, and their ESR spectra were examined in both the absence and presence of the Na+ or K+ cation. The hyperfine coupling (hfc) constants of the methylene protons bonded to the quinone ring depend largely on the size of the crown ring; the orientation of the benzoquinone skeleton with respect to the crown ring was analyzed. The metal-free O5− has a fixed conformation in which the benzoquinone ring is nearly perpendicular to the crown ring, while O8− interconverts freely between two equivalent conformations. The critical case is O6−; the line-width alternation observed in its ESR spectra was well-analyzed using the two-jump model. Temperature-dependent ESR spectra and MNDO calculations have suggested that O6− undergoes a restricted interconversion between two conformations in which the benzoquinone ring has an orientation similar to that of O5−. An activation energy of 10.0 kcal mol−1 was determined for this motion. The ESR spectra of the ion-paired O5−–O8− anion radicals have suggested that the conformational motion of the anion radicals is fixed by complexation with the Na+ or K+cation. The hfc constants of the methyl protons in the benzoquinone ring increased upon complexation, indirectly confirming the interaction between the carbonyl oxygen of benzoquinone at the crown-ring side and the metal cation.

Electron spin resonance study on radicals stabilized by the σ‐π captodative effect1

AbstractESR spectra of l‐cyano‐2‐(trimethylsilyl)ethyl and related radicals were recorded. Temperature dependent ESR spectra of these radicals revealed that electron‐releasing trimethylsilylmethyl and electron‐withdrawing cyano groups synergetically functioned to stabilize the radical centers due to σ‐π captodative effects.

NH 2-terminal spin labeling of human hemoglobin — spin states and temperature dependence

In order to explore fully how ligand- and temperature-induced alterations in the spin states of heme iron are related to protein readjustments, the spin label 4-isothiocyanate (1) was covalently attached at β-93 cysteines and at NH 2-terminal valines of various heme-iron ligand forms of human hemoglobin. It was found that the mobility of NH 2-terminally bound spin labels depends on the magnetic moment of the heme iron. There is a an approximately linear relationship between the magnetic moment of the heme iron and the mobility of NH 2-terminally bound spin labels. In accordance with our previous results, the temperature dependence of ESR spectra of spin-labeled hemoglobin suggests the temperature-induced protein conformational change in those heme-iron ligand forms that are characterized by the equilibrium of the spin states of the heme iron. The conformational change was sensed at both spin-label-binding sites: at β-93 cysteines and at NH 2-terminally valines.

ESR Study of Glass-Like Carbons

Abstract ESR studies were carried out on a variety of glass-like carbons made from different precursors heat-treated in the range 300–2500 °C. It was found that the behaviors of unpaired electrons were nearly identical. The temperature dependence of ESR spectrum showed that the behavior of localized unpaired electrons in the glass-like carbons changed to that of delocalized ones at HTT about 2500 °C. The ESR spectrum of the glass-like carbons heat-treated at 800 °C consisted of a single line under a pressure of above 10−4 Torr but split into two components when air was admitted. This strong interaction with unpaired electrons and air (oxygen molecules) is caused by the active structure related to micropores in the glass-like carbons. We suppose that the polyene type radicals are the most suitable as the active structure to interpret the behavior of unpaired electrons in glass-like carbons heat-treated at 800 °C. As another major point in this study, it was found that there is a correlation between unpaired electron concentration and the g value.

ESR study on rotational motions of some free radicals in the supercooled and glassy organic solvents

Rotational motions of solute molecules in supercooled and glassy media were investigated with the temperature-dependent ESR spectra of the organic nitroxides and chlorine dioxide in some organic solvents. The rotational correlation times (τR’s) of the solutes were estimated by the method of Freed et al. for the nitroxides and by the spectral simulations based on the stochastic Liouville method for chlorine dioxide. From comparison between τR’s of the solutes and the dielectric relation times of the solvents or some equations which provide τR’s of molecules in supercooled liquid, it was concluded that a solute molecule rotates almost independently from motions of solvent molecules. The solvent-dependent parts of τR’s of the solute molecules obtained in the various combinations of the solute and solvents can be treated as perturbation for the rotational motion of a solute molecule. Nonlinear dependence of τR’s of the solutes on the relaxation times calculated from Stokes–Einstein’s relation shows such situation can be caused from inertial motion of a solute molecule.

Chemistry of organosilicon compounds: CXC. An ESR study of conformational transmission in bis(trimethylsilylmethyl)methyl and related radicals

Static conformations and the dynamic behavior of the radicals (MCH 2) 2ĊX (where M = SiR 3, GeR 3, or SnR 3); X = H, CH 3, or OSiMe 3) have been investigated by ESR. At the preferred conformation, the two metal groups almost eclipse the singly-occupied carbon p-orbital and are located on the opposite sides of the CH 2CXCH 2 plane. The barriers for the flip-flop motion, evaluated by the line-shape analysis of the temperature-dependent ESR spectra, are closely related to those for the metal-group rotation about the C(α)C(β) bond.