Unpaired Electron Spin Research Articles

Observation of the 3(3)Sigma(+)-X1Sigma+ transition of KRb by resonance enhanced two-photon ionization in a pulsed molecular beam: Hyperfine structures of 39K85Rb and 39K87Rb isotopomers.

The 3(3)Sigma(+)-X1Sigma+ transition of KRb is observed by resonance enhanced two-photon ionization in a pulsed molecular beam. Hyperfine splittings of 39K85Rb and 39K87Rb isotopomers are observed. From the magnitude of hyperfine splittings, we found that the main hyperfine structure was dominated by the Fermi contact interaction between the Rb nuclear spin and the unpaired electron spin. The Fermi contact interaction constants were determined to be 291 MHz for 39K85Rb and 665 MHz for 39K87Rb. In the KRb 3(3)Sigma+ state the electron spin couples more strongly with the Rb nuclear spin than with other angular momenta, and the energy level structure is well described by the hyperfine angular momentum coupling scheme of the b(betaS) case. The molecular constants and the Rydberg-Klein-Rees potential energy curve of the 3(3)Sigma+ state were determined.

EPR and NMR characterization of theS=9 excited state and spin density distribution in the single-molecule magnet Fe8Br8: Implications to theS=10 model and magnetization tunneling pathways

High-sensitivity, variable-frequency, high-field electron paramagnetic resonance (EPR) measurements and nuclear magnetic resonance (NMR) measurements are reported for theS=10 single-molecule magnet Fe8Br8. We find that theS=10 state is nested with its first excited state, withS=9, located at only 24±2 K above. Also reported are some preliminary81Br NMR measurements of the unpaired electron spin density on the Br− sites. The results provide new insight and benchmarks for improved electronic and magnetic structural calculations and macroscopic tunneling pathways of this class of single-molecule magnets.

Prediction of reactive sites for the electropolymerization of p‐benzenesulfonic acid derivatives: Ab initio and experimental study

AbstractA theoretical study of the electronic structure of some p‐benzenesulfonic acid derivatives is carried out to explain the electrooxidation of these compounds, in particular the allyl ether p‐benzenesulfonic acid, sodium salt electrooxidation, and electrode surface modification observed during electrochemical studies. The effects of the alkyl side chain and of the aromatic ring during electrooxidation are considered using the UHF and B3LYP levels of theory. The changes in monomer structure after removal of an electron are explained by the nodal structure of the highest occupied molecular orbital (HOMO), α and β single occupied molecular orbitals (SOMO) and by their energies. Apart from frontier molecular orbital theory, charge reaction control, which is expected to occur at higher oxidation potentials, is also discussed. The calculations indicate that the unpaired electron spin distribution and atomic charge variation in the radical cation is very sensitive to the nature of the alkyl side chain. In fact, this distribution reflects the electronic structure of these monomers related to their oxidability. It is shown that the removal of an electron is carried out by two competing mechanisms. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Effects of air/fuel combustion ratio on the polycyclic aromatic hydrocarbon content of carbonaceous soots from selected fuels

Patterns of polycyclic aromatic hydrocarbon (PAH) content were observed from GC/MS analysis of the extracts of soots at various air/fuel combustion ratios of three commonly used fuels: n-hexane, JP-8 (Jet fuel), and diesel. With increasing air/fuel ratio, from a simple diffusion flame up to an air/fuel ratio of 3.94, there is a significant loss of high molecular weight PAHs and an increasing abundance of oxidized lower molecular weight aromatics. The formation of high molecular weight PAHs is favored for JP-8 and diesel fuels at higher air/fuel combustion ratios than is the case with n-hexane, probably due to the aromatic content in JP-8 and diesel fuels acting as centers for large aromatic and soot nucleation. The efficiency and reproducibility of two techniques, Soxhlet and supercritical fluid extraction (SFE), used for extraction of PAHs from soot were compared. Electron paramagnetic resonance (EPR) measurements were performed on the soot both before and after supercritical fluid and Soxhlet extraction, and a substantial decrease in the spin density of soot following extraction indicates that extractable molecules are associated with 40–50% of the unpaired electrons in soot. This analysis generally supports trends observed in our earlier work for surface oxidation, surface area, unpaired electron spin density, hydration, and ozone oxidation.

133Cesium HYSCORE Investigation of the Di-tert-butyl Nitroxide−Cs+ Adsorption Complex in CsNaY Zeolite

133Cs hyperfine sublevel correlation (HYSCORE) spectroscopy has been employed to characterize the structure of adsorption complexes of di-tert-butyl nitroxide (DTBN) with cesium cations in zeolite CsNaY. The experimental 133Cs HYSCORE data proved the direct coordination of the adsorbed DTBN molecules to the Cs+ ions and revealed nonambiguously the existence of two different types of adsorption complexes. Evaluation of the orientation selective 133Cs HYSCORE spectra provided the 133Cs hyperfine coupling tensors and thus information about the geometrical structure of those complexes. For one type of adsorption complexes, a complex geometry was obtained where the Cs+ ion is located within the molecular mirror plane of the DTBN radical with an oxygen−cesium cation bond length of 0.25 nm. For this, the isotropic Cs hyperfine coupling was negative. The second Cs+−DTBN complex is characterized by a bent structure. With an isotropic 133Cs hyperfine coupling of 9 MHz, the unpaired electron spin density in this Cs+...

Polymer phase of the tetrakis(dimethylamino)ethylene-C60organic ferromagnet

High-pressure electron spin resonance (ESR) measurements were performed on tetrakis(dimethylamino) ethylene $(\mathrm{TDAE})\ensuremath{-}{\mathrm{C}}_{60}$ single crystals and stability of the polymeric phase was established in the $P\ensuremath{-}T$ parameter space. At 7 kbar the system undergoes a ferromagnetic to paramagnetic phase transition due to the pressure-induced polymerization. The polymeric phase remains stable after the pressure release. The depolymerization of the pressure-induced phase was observed at a temperature of 520 K, revealing an unexpectedly high thermal stability of the polymer. Below room temperature, the polymeric phase behaves as a simple Curie-type insulator with one unpaired electron spin per chemical formula. The ${\mathrm{TDAE}}^{+}$ donor-related unpaired electron spins, formerly ESR silent, become active above a temperature of 320 K, which demonstrates that the magnetic properties are profoundly defined by miniscule reorientation of TDAE molecules.

Hyperfine structure of ESR spectra as a probe for heisenberg exchange couplings in nitroxide triradicals serving as building blocks for molecule-based ferrimagnets

A study on electron spin resonance (ESR) spectroscopic determination of exchange interactions in organic oligoradicals is given. When the intramolecular exchange couplingJ between, unpaired electron spins in nitroxide-based oligoradicals falls within the order of 10 Oe (1 mK or 10−3 cm−1 forg=2), which is on the same order as the hyperfine couplingA of magnetic nuclei such as nitrogen atoms of nitroxide radicals, the magnitude ofJ can be determined from the hyperfine splitting pattern of ESR spectra in solutions. This range of the exchange couplingJ is not detectable in conventional magnetic susceptibility measurements. We demonstrate an application of hyperfine ESR spectroscopy as a probe for the exchange coupling to a series of organic oligoradicals, which the authors have recently developed as building blocks for molecule-based magnetic materials.

The reactions of imidazol-2-ylidenes with the hydrogen atom: a theoretical study and experimental confirmation with muonium.

The possible radicals resulting from hydrogen atom addition to the imidazole rings of 1,3-bis(isopropyl)-4,5-dimethylimidazol-2-ylidene (1) and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (2) have been studied by means of density functional calculations (B3LYP). The calculations included solvent effects estimated via the polarized continuum model (PCM) and an empirical treatment of vibrational averaging of hyperfine constants. Addition of a hydrogen (or muonium) atom to the carbeneic carbon of 1,3-bis(isopropyl)-4,5-dimethylimidazol-2-ylidene was found to give a radical 60.46 kJ mol(-)(1) more stable than the radical resulting from addition to the double bond. Estimation of the activation barriers for reaction at the two sites shows that addition at the carbeneic carbon is favored. The site of addition was confirmed experimentally using muonium (Mu), which can be considered a light isotope of hydrogen. Muon spin rotation and muon level-crossing spectroscopy were used to determine muon, (13)C, and (14)N hyperfine coupling constants (hfc's) for the radical products of addition to the two carbenes. Good agreement between the experimental and calculated hfc's confirms that Mu (and hence H) adds exclusively to the carbeneic carbon. The radicals that are produced have nonplanar radical centers with most of the unpaired electron spin density localized on the alpha-carbon.

Theoretical calculations of carbon-oxygen bond dissociation enthalpies of peroxyl radicals formed in the autoxidation of lipids.

Theoretical calculations were carried out to provide a framework for understanding the free radical oxidation of unsaturated lipids. The carbon[bond]hydrogen bond dissociation enthalpies (BDEs) of organic model compounds and oxidizable lipids (R[bond]H) and the carbon[bond]oxygen bond dissociation enthalpies of peroxyl radical intermediates (R[bond]OO*) have been calculated. The carbon[bond]hydrogen BDEs correlate with the rate constant for propagation of free radical autoxidation, and the carbon[bond]oxygen BDEs of peroxyl radicals correlate with rate constants for beta-fragmentation of these intermediates. Oxygen addition to intermediate carbon radicals apparently occurs preferentially at centers having the highest spin density. The calculated spin distribution therefore provides guidance about the partitioning of oxygen to delocalized carbon radicals. Where the C[bond]H BDEs are a function of the extent of conjugation in the parent lipid and the stability of the carbon radical derived therefrom, C[bond]OO* BDEs are also affected by hyperconjugation. This gives way to different rates of beta-fragmentation of peroxyl radicals formed from oxygen addition at different sites along the same delocalized radical. We have also studied by both theory and experiment the propensity for benzylic radicals to undergo oxygen addition at their ortho and para carbons which, combined, possess an equivalent unpaired electron spin density as the benzylic position itself. We find that the intermediate peroxyl radicals in these cases have negative C[bond]OO* BDEs and, thus, have rate constants for beta-fragmentation that exceed the diffusion-controlled limit for the reaction of a carbon-centered radical with oxygen.

Blue Light Perception in Plants: DETECTION AND CHARACTERIZATION OF A LIGHT-INDUCED NEUTRAL FLAVIN RADICAL IN A C450A MUTANT OF PHOTOTROPIN

The LOV2 domain of Avena sativa phototropin and its C450A mutant were expressed as recombinant fusion proteins and were examined by optical spectroscopy, electron paramagnetic resonance, and electron-nuclear double resonance. Upon irradiation (420-480 nm), the LOV2 C450A mutant protein gave an optical absorption spectrum characteristic of a flavin radical even in the absence of exogenous electron donors, thus demonstrating that the flavin mononucleotide (FMN) cofactor in its photogenerated triplet state is a potent oxidant for redox-active amino acid residues within the LOV2 domain. The FMN radical in the LOV2 C450A mutant is N(5)-protonated, suggesting that the local pH close to the FMN is acidic enough so that the cysteine residue in the wild-type protein is likely to be also protonated. An electron paramagnetic resonance analysis of the photogenerated FMN radical gave information on the geometrical and electronic structure and the environment of the FMN cofactor. The experimentally determined hyperfine couplings of the FMN radical point to a highly restricted delocalization of the unpaired electron spin in the isoalloxazine moiety. In the light of these results a possible radical-pair mechanism for the formation of the FMN-C(4a)-cysteinyl adduct in LOV domains is discussed.

Intramembrane Polarity by Electron Spin Echo Spectroscopy of Labeled Lipids

The association of water (D 2O) with phospholipid membranes was studied by using pulsed-electron spin resonance techniques. We measured the deuterium electron spin echo modulation of spin-labeled phospholipids by D 2O in membranes of dipalmitoyl phosphatidylcholine with and without 50 mol% of cholesterol. The Fourier transform of the relaxation-corrected two-pulse echo decay curve reveals peaks, at one and two times the deuterium NMR frequency, that arise from the dipolar hyperfine interaction of the deuterium nucleus with the unpaired electron spin of the nitroxide-labeled lipid. For phosphatidylcholine spin-labeled at different positions down the sn-2 chain, the amplitude of the deuterium signal decreases toward the center of the membrane, and is reduced to zero from the C-12 atom position onward. At chain positions C-5 and C-7 closer to the phospholipid headgroups, the amplitude of the deuterium signal is greater in the presence of cholesterol than in its absence. These results are in good agreement with more indirect measurements of the transmembrane polarity profile that are based on the 14N-hyperfine splittings in the conventional continuous-wave electron spin resonance spectrum.

From High‐Spin Organic Molecules to Organic Polymers with Magnetic Ordering.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Pyrolytic carbon layers—an electron spin resonance analysis

Chemical vapour infiltration is simulated by deposition of pyrolytic carbon on planar boron nitride substrates and carbon fibers in a hot-wall tubular reactor at about 1100 °C for varied pressure and flow-velocity of methane. The degree of orientation of the deposited graphite-like domains can be monitored via orientation and temperature dependence of the electron spin resonance parameters (g-tensor, linewidth). The electronic structure of the localized defects and conduction electrons is accessible by a quantitative modelling of these parameters. The interaction of the built-in hydrogen atoms with the unpaired electron spins is analysed by electron–proton spin double resonance technique (Overhauser shift)

The effect of the carbonyl moiety on the spin density delocalization in the iminoxy radicals. Hybrid density functional studies

UB1LYP method was used to study the influence of carbonyl group in 1,2-diphenyl-1-oxoethan-2-iminoxyl and 1-oxoethan-2-iminoxyl, on radical structure and unpaired electron spin density distribution. A significant change of radical properties was found due to the second carbonyl group in 2,4-dioxopenthan-3-iminoxyl and 1,3-dioxopropan-2-iminoxyl. A specific impact of the carbonyl groups on the EPR hyperfine couplings with 1 H and 13 C nuclei (calculated at UB1LYP/EPR-III level in comparison with the experimental data) was determined for the stable isomeric forms of the radicals.

Spectroscopic and XRD studies of the air degradation of acid-reacted pyrrhotites

Monoclinic and hexagonal pyrrhotites leached in 1 mol/L HCl and exposed to the air at 100% and ∼10% relative humidity for up to 5 months were studied using X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy, Mössbauer spectroscopy, and electron paramagnetic resonance (EPR). The amorphous, nonequilibrium, iron-depleted layer (NL) produced by the leaching amounted to half of the residue mass and was composed of predominantly low-spin ferrous iron and polysulfide anions. Elemental sulfur and goethite were the only crystalline products of the NL decomposition. FTIR spectroscopy and XPS also revealed several sulfoxy species and, at low humidity, a small amount of ferric oxide. Neither alterations of the underlying pyrrhotite nor new iron sulfide phases (pyrite, pyrrhotite, etc.) crystallized from the amorphous NL were found. The NL decomposition was faster in the wet environment than in the dry one, and the oxidation of the NL was much more rapid than that of starting pyrrhotites. The intensity and quadruple split of the Mössbauer signal from the product (an isomer shift of 0.36 mm/s) were found to increase over the aging, indicating that the NL structure becomes more rigid and the singlet Fe(II) gradually converts to Fe(III). X-ray Fe Lα,β emission spectra showed the formation of intermediate, high-spin Fe(II) within the NL oxidized in the humid environment, but not in the dry air. No unpaired electron spins were detected by EPR; lines of paramagnetic Fe 3+ appeared after the samples were aged in the dry air for 49 d and even later in the humid atmosphere. These phenomena are explained in terms of the formation of defects with negative correlation energy, similar to noncrystalline semiconductor chalcogenides, and of the fast electron exchange between the iron species, respectively. Mechanisms for reactions involved with the weathering of iron sulfides, which take into consideration the NL lattice elasticity, S-S and S-O bonding, oxygen incorporation, and oxidative and spin state of iron, are discussed. It is suggested in particular that the surface layer, strongly enriched in sulfur, as well as elemental sulfur and ferric oxyhydroxides, do not inhibit sulfide oxidation and acid production under weathering conditions, but the partially oxidized, disordered, nonstoichiometric layer may be passive.

From high-spin organic molecules to organic polymers with magnetic ordering.

This concept paper outlines the design of the first pi-conjugated organic polymer with magnetic ordering. This rational, "bottom-up" macromolecular design is based on synthesis and study of polyarylmethyl polyradicals with increasing number of exchange-coupled unpaired electron spins. The prospects for attaining organic polymer magnets with stability at ambient temperature and/or higher magnetic ordering temperatures will be discussed.

Solid-state systems for the electron electric dipole moment and other fundamental measurements

In 1968, Shapiro published the suggestion that one could search for an electron dipole moment (EDM) by applying a strong electric field to a substance that has an unpaired electron spin; at low temperature, the EDM interaction would lead to a net sample magnetization that can be detected with a superconducting quantum interference device (SQUID) magnetometer. One experimental EDM search based on this technique was published, and for a number of reasons including high sample conductivity, high operating temperature, and limited SQUID technology, the result was not particularly sensitive compared to other experiments in the late 1970s. Advances in SQUID and conventional magnetometery led us to reconsider this type of experiment, which can be extended to searches and tests other than EDMs (e.g., test of Lorentz invariance). In addition, the complementary measurement of an EDM-induced sample electric polarization due to application of a magnetic field to a paramagnetic sample might be effective using modern ultrasensitive charge measurement techniques. A possible paramagnetic material is Gd-substituted yttrium iron garnet which has very low conductivity and a net enhancement (atomic enhancement times crystal screening) of order unity. Use of a reasonable volume (hundreds of ${\mathrm{cm}}^{3})$ sample of this material at 50 mK and 10 kV/cm might yield an EDM sensitivity of ${10}^{\ensuremath{-}32} \mathrm{e}\mathrm{}\mathrm{cm}$ or better, a factor of ${10}^{5}$ improvement over current experimental limits.

Inter- and intramolecular spin transfer in molecular magnetic materials. Solid-state NMR spectroscopy of paramagnetic metallocenium ions.

To shed light on the interaction in molecule-based magnetic materials, the decamethylmetallocenium hexafluorophosphates, [(C(5)Me(5))(2)M](+) [PF(6)](-) with M = Cr, Mn, Fe, Co, and Ni, as well as the tetracyanoethenides, [(C(5)Me(5))(2)M](+) [TCNE](-) with M = Cr, Mn, Fe, and Co, have been investigated in the solid state by using (1)H, (13)C, (19)F, and (31)P NMR spectroscopy under magic angle spinning (MAS). The isotropic (13)C and (1)H NMR signals cover ranges of about 1300 and 500 ppm, respectively. From the shift anisotropies of the ring carbon signal of the [(C(5)Me(5))(2)M](+) cations, the total unpaired electron spin density in the ligand pi orbitals has been calculated; it amounts up to 36% (M = Ni) and is negative for M = Cr, Mn, and Fe. The radical anion of [(C(5)Me(5))(2)M](+) [TCNE](-) shifts the (13)C NMR signals of all [(C(5)Me(5))(2)M](+) cations to high frequency, which establishes transfer of positive spin density from the anions to the cations. The (19)F and (31)P NMR signals of the paramagnetic salts [(C(5)Me(5))(2)M](+) [PF(6)](-) are shifted up to 13.5 ppm relative to diamagnetic [(C(5)Me(5))(2)Co](+) [PF(6)](-). The signs of these shifts are the same as those of the pi spin density in [(C(5)Me(5))(2)M](+). After consideration of interionic ligand- and metal-centered dipolar shifts, this establishes cation-anion spin delocalization. The mixed crystals [(C(5)Me(5))(2)M(x)Co(1-x)](+)[PF(6)](-) have been prepared for M = Cr and Ni. They are isostructural with [(C(5)Me(5))(2)Co](+) [PF(6)](-) whose single-crystal structure has been determined by X-ray diffraction. The (13)C, (19)F, and (31)P MAS NMR spectra of the mixed crystals show that the respective two closest paramagnetic ions in the lattice delocalize spin density to [(C(5)Me(5))(2)Co](+), [(C(5)Me(5))(2)Ni](+), and [PF(6)](-). In [(C(5)Me(5))(2)M](+), about 10(-4) au per carbon atom are transferred.

A New Method for Measuring the Graphite Content of Anthracite Coals and Soots

A new method for estimating the average fraction of graphitic structure (the graphite-like factor) found in carbonized materials such as anthracitic coals and soots has been developed. This method employs a Q meter, a device that can be used to measure the resistance of a coil at high frequency. The resistance, and therefore the Q response, depends on the conductive behavior of the delocalized electrons in the sample placed inside a RF coil. On the basis of the Q measurement, one can estimate the percentage of the sample that contains graphite-like regions in which delocalized conduction electrons exist. A series of measurements was made on a set of graphite and silica gel mixtures in order to calibrate the measured Q values. This method was then applied to a series of anthracitic coal samples and a number of soot samples. The Q values obtained were related to the graphitic content of the soot or coal. These results are compared with carbon-13 NMR data and ESR unpaired electron spin concentration measurem...

Spin soliton in aπ-conjugated ladder polydiacetylene

The electronic structure and the spin dynamics of a spin soliton observed in a polydiacetylene have been investigated using multifrequency pulsed and continuous-wave (cw) electron paramagnetic resonance (EPR), electron spin echo (ESE) spectroscopy, electron-nuclear double resonance (ENDOR), and electron-nuclear-nuclear triple resonance (TRIPLE). The undoped ladder polymer in which two polydiacetylene wires are linked by bridges consisting of two triple bonds exhibits a strong EPR signal. In contrast, no EPR signal is observed in the ladder-structured polydiacetylene, where the bridges consist of single bonds. The observed bridge effect on the probability of formation of paramagnetic centers is interpreted in terms of a quasidegenerate ground state due to the π-conjugated ladder structure that creates a π-bond kink corresponding to a spin soliton. High-frequency and high-field EPR reveals a small anisotropy of the g tensor (principal values g 1 =2.0037, g 2 =2.0028, and g 3 =2.0023) for the spin soliton. By nutation spectroscopy, the spin multiplicity was confirmed to be S=½. The hyperfine pattern attributed to the β protons was detected by ENDOR and TRIPLE methods, providing evidence for both the positive and negative spin densities of the soliton wave function. These experimental results led to the conclusion that the unpaired electron of the spin soliton was substantially delocalized over the two polydiacetylenes through the π-conjugation. At temperatures (T) below 50 K, the width of the EPR spectrum narrows by about 45% with increasing temperature and the width remains nearly constant for T>50 K. The pronounced change in the line width for T<50 K suggests the presence of a mobile spin soliton in the ladder-structured polydiacetylene. A temperature dependence of the electron spin system's phase memory time T M for T<50 K was also observed by ESE spectroscopy. The decay of the ESE exhibits a nonexponential behavior, with T M increasing monotonically with rising temperature These findings represent clear experimental evidence for motional narrowing phenomena. Based on a one-dimensional random-walk model for the unpaired electron spin along the ladder polymer, the soliton's diffusion rate constant was estimated from an analysis of the ESE dephasing process. An Arrhenius plot of the diffusion rate was asymptotic to 8×10 7 s - 1 at very low temperature, which is two orders of magnitude slower than that of the soliton in trans-polyacetylene.