Unpaired Electron Spin Research Articles

13C hyperfine coupling constants in MuC 60

13C hyperfine coupling constants of the MuC 60 radical have been measured by muon level-crossing spectroscopy, using a dilute solution of 99% enriched 13C 60 in decalin. The signs as well as the magnitudes of the hyperfine constants were determined. The results range from 52.6 to −25.4 MHz and support those calculations which predict an extended distribution of unpaired electron spin density in radical adducts of fullerenes. The hyperfine constants are consistent with published electron spin resonance results for (CD 3) 3CC 60, but contradict a recent report for HC 60, where a considerably smaller value is reported for the largest splitting.

Far-Infrared Spectrum of HO2

We have measured the frequencies of 17 transitions in the far-infrared spectrum of the hydroperoxyl radical, HO2, from 2.5 to 5.9 THz, encompassing rotational levels up to N = 19 and K−1 = 5. These measured transition frequencies are combined with previously measured microwave and millimeter-wave transition frequencies in a complete reanalysis of the HO2 Hamiltonian for the ground vibrational state, including spin-rotation and nuclear hyperfine interactions. A revised set of molecular constants is presented. These include centrifugal distortion effects through octic dependence on the rotational angular momentum and the off-diagonal tenser component of the spin-spin dipolar interaction between the proton and the unpaired electron spin.

Multi-frequency EPR Evidence for a Binuclear Cu A Center in Cytochrome c Oxidase: Studies with a 63Cu- and 65Cu-Enriched, Soluble Domain of the Cytochrome ba3 Subunit II from Thermus thermophilus

We have recorded multi-frequency EPR spectra of 63Cu- and 65Cu-labeled, water-soluble Cu A-protein from the cytochrome ba 3 of T. thermophilus. The spectrum taken at the highest frequency (34.03 GHz) shows no hyperfine structure and is nominally axial with apparent g z ∼2.18 and g xy ∼2.00. The spectrum taken at the lowest frequency (3.93 GHz) shows a rich hyperfine structure. Analyses of the spectra show that the observed splitting arises from an interaction of the unpaired electron with two Cu nuclei and support the notion that Cu A is a mixed-valent [Cu(II)/Cu(I)] complex in which the unpaired electronic spin is distributed evenly over the two Cu ions.

13C NMR Spin-Lattice Relaxation Properties and Quantitative Analytical Methodology of 13C NMR Spectroscopy for Coals

The quantitative reliability of {sup 13}C NMR spectroscopy is examined and discussed. Studies were performed on eight Argonne Premium coals, using the nonoverlapping carbonyl peak of 4-(N-methylpyrrolidino)bicyclo[3.2.1]-octan-8-one triflate as an intensity reference. {sup 13}C spin-lattice relaxation times measured by cross-polarization (CP) detection depend on the CP contact time. A series of experiments with different CP contact times yielded observed {sup 13}C spin-lattice relaxation times that are significantly larger than those previously reported in the literature. The relaxation time characteristic of the most slowly relaxaing carbons, T{sub 1S}, was determined for the aromatic carbons and for the aliphatic carbons of each coal. Then, if one uses repetition delays that are >5T{sub 1S}, quantitative results can be expected for the NMR-observable carbons in a direct polarization (DP) experiment. Carbon spin counting experiments performed by using DP instead of CP yielded numbers for the fraction of carbon spins that are 81-97% of those present in the sample. A relationship was observed between the number of unpaired electron spins and the fraction of {sup 13}C spins detected. The role of unpaired electron spin dynamics on the effective electron magnetic moment is considered, and the corresponding influence on {sup 13}C NMR detection is discussed. 42more » refs., 9 figs., 2 tabs.« less

Electron spin dynamics in the new halogen-bridged one-dimensional complex, Ni 2(CH 3CS 2) 4I studied by 1H NMR

We measured temperature and Larmor frequency(ω) dependences of the 1H NMR spin-lattice relaxation time T 1 in solid Ni 2(CH 3CS 2) 4I which contains the one-dimensional(1-D) chain of -Ni-Ni-I-Ni-Ni-I- with no Peierls distortion. Almost temperature independent T 1 values of ca. 180 ms observed at ω = 40.98 MHz and in the range of 90–300 K are attributable to the relaxation due to the fluctuation of antiferromagnetically coupled unpaired electron spin formed at each -X-Ni-Ni- unit in consistent with the 1-D Mott-Hubbard type structure. T 1 in this temperature range showed the Larmor frequency dependence of ω 1 2 explainable by rapid spin diffusion along the 1-D chain. Applying the Devreux's treatment on the low-dimensional spin dynamics, the diffusion rate and activation energy for the 1-D hopping of electron spins were evaluated.

The dimerization and electronic properties of quasi-1D organic polymer ferromagnets with side free-radicals

The effects of side radicals on the energy levels of π-electrons and on the dimerization of main chain in quasi-one-dimensional organic polymer ferromagnets are studied. Allowing for full lattice relaxation, and considering the antiferromagnetic (AF) correlation between the π-electron spin and the residual spin of unpaired electron at side radicals, a set of self-consistent iterative equations are established to study the system. The results of calculation show that: (1) the spin degeneration of energy levels of π-electrons will be lifted; (2) with different electron-phonon coupling, this AF correlation always weakens the dimerization of the main chain.

A Grain Boundary Defect in Polycrystalline Films

ABSTRACTOur studies primarily concern an S=1/2 electron paramagnetic resonance (EPR) at g=2.0028 with a pair of weaker partially resolved satellites that has been observed in polycrystalline diamond grown by chemical vapor deposition (CVD). These satellites can be shown to result from the forbidden Δm=±l nuclear spin flips of hydrogen during the EPR ΔM∼ 2Å away from the unpaired electron spin. It is suggested that hydrogen enters a vacancy-like stretched bond at a grain boundary, allowing the carbons to relax backward, one bonding to the hydrogen, the other with an unpaired electron in its dangling bond.

Conformational studies of thiyl and selenenyl radicals

Muonium adds to the unsaturated bond in 1,3-dithiolane-2-thione or -2-selenone to form a chalcogen-centered radical. The muon hyperfine coupling constants (hfc) were measured for both compounds over a range of temperatures. In solution, the temperature dependence of the muon hfc indicates that for the thiyl radical the preferred conformation corresponds to the muon eclipsing the unpaired electronpz-orbital, while there is practically free rotation in the case of the selenenyl radical in the temperature range studied. In pure thione the hfc values show a remarkable discontinuity at the melting point. The two radical signals seen in the solid are interpreted as due to the presence of two dominant crystal orientations in the samples studied. In pure selenone data were obtained only in the solid. For both systems, the solid phase results show that interaction between the muon and the unpaired electron spin is enhanced compared to the liquid phase and/or the solution.

Theory of location and associated hyperfine properties of the positive muon in La2CuO4

The current understanding regarding the location of the positive muon, which is a valuable tool for probing the magnetic properties of copper oxide superconducting systems, will be reviewed for La2CuO4. The results of our present investigations by the Unrestricted Hartree-Fock Cluster procedure, which leads to a location for the muon of about 1.08 A away from an apical oxygen and with the O-μ+ direction making an angle of 25° with the O-Cu direction, will be discussed, including the magnitude and direction of the hyperfine field obtained from the calculated wave functions. These latter results, which are in reasonable agreement with earlier muon spin rotation data in powdered samples and recent data in single crystals, show the importance of including the local contact and dipolar contributions to the hyperfine field associated with the unpaired electron spin distribution in the neighborhood of the muon. Possible additional factors besides those included here, that will involve substantial computational efforts but could lead to a bridging of the remaining quantitative differences with experimental hyperfine data, will be discussed.

Hyperfine structure in the electronic spectra of the CdH and CdCH3 radicals

Magnetic hyperfine interactions in the ground and first excited states of CdH and CdCH3 were studied using high resolution electronic spectroscopy. Hyperfine splittings associated with the H nucleus were observed in the X state of CdH; hyperfine splittings associated with 111Cd and 113Cd were observed in both the ground and first excited states of CdH and CdCH3. The hyperfine parameters of CdH were found to be similar to the corresponding parameters of CdCH3. Comparison of the ground state molecular hyperfine constants with values determined in an Ar matrix electron paramagnetic resonance (EPR) study [L. B. Knight, Jr. and W. Weltner, Jr., J. Chem. Phys. 55, 2061 (1971)] indicates clearly a shift of unpaired electron spin density towards Cd, in the matrix. The excited state molecular hyperfine constants indicate that the unpaired electron essentially resides in a Cd 5pπ orbital.

Point-Mutations Affecting the Properties of TyrosineD in Photosystem II. Characterization by Isotopic Labeling and Spectral Simulation

The reaction center of photosystem II (PSII) contains two redox-active tyrosines, TyrD and TyrZ, which are Tyr160 and Tyr161 of the D2 and D1 proteins, respectively. We have introduced five site-directed mutations in the vicinity of TyrD to analyze the consequences of the mutations on spectral and functional properties of TyrD(ox). Characterization of three mutants, P161A and P161L (Pro161 changed to Ala and Leu, respectively) and Q164L (Gln164 mutated to Leu), is emphasized. Of these three mutants, only P161L is an obligate photoheterotroph; it is capable of oxygen evolution, but is photoinactivated rapidly. The D2 protein of this mutant migrates slower on a SDS-polyacrylamide gel. The EPR spectrum of TyrD(ox) is modified in the three mutants. The EPR spectra of TyrD(ox) in wild type and the mutants were characterized in detail by comparison of EPR spectra of thylakoids from cells grown in the presence and absence of tyrosine that was deuterated in specific positions. The experimentally obtained EPR spectra of wild type, P161A, and Q164L could be simulated satisfactorily using current theoretical models. The angle between one of the hydrogens on the beta-methylene carbon and the 2pz orbital at C1 of the tyrosine ring was found to change slightly but significantly as a function of the mutations (52 degrees in wild type, 50 degrees in P161A, and 48 degrees in Q164L). The overall electronic structure of TyrDox is quite unaffected; only minor redistribution of the unpaired electron spin is observed between the wild type and the mutated systems.(ABSTRACT TRUNCATED AT 250 WORDS)

NQR and NMR studies on the electron-spin correlation time in paramagnetic insulators.

The $^{35}\mathrm{Cl}$ NQR and $^{1}\mathrm{H}$ NMR spin-lattice relaxation times, ${\mathit{T}}_{1\mathit{Q}}$ and ${\mathit{T}}_{1\mathit{M}}$, respectively, in paramagnetic Ni(${\mathrm{H}}_{2}$O${)}_{6}$${\mathrm{SnCl}}_{6}$ crystals were measured as a function of temperature. Their temperature variations depend on the correlation time ${\mathrm{\ensuremath{\tau}}}_{\mathit{e}}$ of the unpaired electron spins in the paramagnetic ${\mathrm{Ni}}^{2+}$ ions, and ${\mathrm{\ensuremath{\tau}}}_{\mathit{e}}$ was found to be governed mainly by the electronic spin-lattice relaxation originating from the spin-phonon interaction above approximately 250 K and by electron-spin flips caused by the exchange interaction below approximately 150 K. The temperature dependence of ${\mathit{T}}_{1\mathit{Q}}$ and ${\mathit{T}}_{1\mathit{M}}$ at high temperatures shows that the electronic spin-lattice relaxation time is determined by the Raman process. The correlation time of the electron flips was evaluated to be 1.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ s from ${\mathit{T}}_{1\mathit{Q}}$. The exchange parameter between the nearest electronic spins and the paramagnetic Curie temperature were estimated to be 0.013 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and 0.075 K, respectively. It was shown that reliable information on the electron-spin dynamics can be obtained from ${\mathit{T}}_{1\mathit{Q}}$ and ${\mathit{T}}_{1\mathit{M}}$.

Time-resolved EPR of the radical pair P 865+.Q A−. in bacterial reaction centers. Observations of transient nutations, quantum beats and envelope modulation effects

The time evolution of the spin-polarized transient EPR signals of the radical pair state P 865 +.Q A −. is studied in perdeuterated reaction centers of Rhodobacter sphaeroides R-26 in which the non-heme iron has been replaced by zinc. The transients show modulations of the signal amplitude on several time scales. Fast quantum beat oscillations with frequencies around 15 MHz occur within the first 100 ns after the light excitation because the radical pair is generated in a coherent superposition of its eigenstates. Transient nutations also occur as a result of the precession of the magnetization about the applied microwave field B 1. At the maximum B 1 field available (0.125 mT) a nutation frequency of ≈ 3.5 MHz is obtained. Small variations in the nutation frequency throughout the spectrum occur as a result of the dipolar coupling in the radical pair. Additional oscillations in the frequency ranges 1.5 to 2.0 MHz and 2.5 to 3.0 MHz are also observed which are attributed to the interaction between the nuclear spins and the unpaired electron spins. Analogies and differences to well-known nuclear modulation phenomena observed in pulsed EPR experiments are discussed.

ENDOR and special triple resonance studies of chlorophyll cation radicals in photosystem 2.

Electron nuclear double resonance (ENDOR) and special triple (ST) resonance spectroscopies have been used to study the cation radicals of the primary donor, P680, and two secondary donor chlorophylls (Chl) in photosystem 2 (PS2). Two different preparations were employed, Tris-washed PS2 membranes and PS2 reaction centers (D1-D2-I-Cytb559 complex). One secondary donor Chl a cation radical, Chl1.+, was generated in the Tris-washed preparation, while the P680.+ radical cation and a further Chl a cation radical, Chl2.+, were produced in the reaction center preparation. The ENDOR spectrum of the primary donor radical cation of photosystem 1 (P700.+) is also presented for comparison. Hyperfine coupling constants for methyl groups have been measured for all three PS2 radical species and assigned by comparison with previously published spectra of Chl a radicals in vitro. Electron spin densities were calculated from these hyperfine couplings. Comparison of ENDOR spectral features with those of Chla.+ in vitro indicates similar values for Chl1.+ and Chl2.+ radicals but an apparent reduction in unpaired electron spin density for P680.+. It has been proposed from the more detailed studies of purple bacterial reaction centers that such a reduction in spin density can be interpreted as a delocalization over two Chl a molecules. Our calculations therefore suggest that P680.+ is a weakly coupled chlorophyll pair with 82% of the unpaired electron spin located on one chlorophyll of the pair at 15 K. Environmental or geometrical changes to the chlorin ring structure to give a novel monomeric primary donor are also possible.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and structure of tetracoordinated nickel(II) complexes of deprotonated chelated aminoazo ligands: X-ray crystal structure of bis[1-isopropyl-3-methyl-4-(4-methylphenylazo)-5-(4-methoxyphenylamidato)pyrazole] nickel(II)

Tetracoordinate nickel(II) complexes NiL2 derived from the deprotonated forms of aminoazoligands HL were prepared and investigated by spectroscopy and magnetic susceptibility measurements. The magnetic moments, which lie in the 3.1–3.6 B.M. range correspond to the occurrence both in solution and in the solid state of the high spin (S = 1) form and a tetrahedral configuration at the metal centre. The u.v. spectra exhibit three ligand field bands at 1020–1280 nm characteristic of high spin nickel(II) complexes. The large isotropic chemical shifts found in the 1H n.m.r. spectra are consistent with partial delocalization of unpaired electron spin density to the ligand HOMO. The X-ray single crystal structure of NiL 2 4 [L4= 1-isopropyl-3-methyl-4-(4-methylphenylazo)-5-(4-methoxyphenylamino)-pyrazole] reveals that the metal is coordinated by four nitrogen atoms in tetrahedral configuration with an angle of 90° between the N(1)NiN(2) and N(6)NiN(7) planes belonging to the different almost planar metallocycles. The rates of R⇄S interconversion of the tetrahedral configuration for NiL 2 4 and NiL 2 5 [L5 = 1-isopropyl-3-methyl-4-phenylazo-d 5-5-(4-methoxyphenylamino)pyrazole] are slow on the n.m.r. timescale. In contrast to NiL 2 1 -Ni 2 6 , NiL 2 7 ], which contains coordinated NH-groups instead of NAr-groups, is planar.

The nature of the copper ions in the membranes containing the particulate methane monooxygenase from Methylococcus capsulatus (Bath).

It is shown that the particulate methane monooxygenase (pMMO) has an obligate requirement for copper. The MMO activity in the particulate fractions obtained from Methylococcus capsulatus (Bath) cells is found to increase with increasing copper content of the membranes. The enzyme activity from membranes obtained from cells grown at low copper levels can be stimulated further by the addition of Cu(II) ions to the assay medium. The membrane-bound copper ions can exist in both Cu(II) and Cu(I) forms. EPR and magnetic susceptibility characterizations of the membranes indicate the presence of an exchange-coupled trinuclear Cu(II) cluster when the bulk of the copper ions is oxidized. However, the functional form of the enzyme is the reduced or partially reduced form. The copper ions in the membrane fractions as isolated often exhibit a high level of reduction. An EPR spectrum with one unpaired electron spin delocalized over three copper nuclei has been observed for the two-electron reduced trinuclear copper cluster. The high correlation between the copper level in the membranes and enzymatic activity as well as the high reactivity of the reduced copper clusters toward dioxygen strongly indicate that the membrane-bound copper ions constitute the active sites of the pMMO.

Field-Cycled Electron Nuclear Double Resonance

An electron nuclear double resonance experiment is described which allows the detection of zero-field nuclear quadrupole resonance transitions by observing the unpaired electron spins under field cycling conditions. The feasibility of this kind of experiment is demonstrated by the detection at 2 K of pure deuteron quadrupole transitions in diamagnetic molecules of perdeuterated malonic acid crystal irradiated by X-rays

Electron paramagnetic resonance forbidden transitions from hydrogen in polycrystalline diamond films

Investigation of polycrystalline diamond films by electron paramagnetic resonance at 9.5 and 35 GHZ has revealed the presence of forbidden transitions resulting from a simultaneous microwave induced flipping of unpaired electron spins and environmental nuclear spins. The spacing of the resonance lines identifies hydrogen as the atom neighboring the paramagnetic active site.

Local Magnetic Fields in some Bismuth Compounds. A Survey of Experimental Evidences

Abstract The splittings observed in the 209Bi nuclear quadrupole resonances of α-Bi2O3, in zero magnetic field, follow the typical pattern of a Zeeman perturbed NQR spectrum. The lineshapes in Bi3O4Br also indicate a poorly resolved splitting that points to the presence of an internal magnetic field in the order of 200 G. This exceeds notably the dipole nuclear magnetic fields (about several G), but is orders of magnitude smaller than paramagnetic fields produced by unpaired electron spins. The results obtained using a SQUID and μSR-technique can also be interpreted as indicative of magnetic properties not conventionally expected in such compounds.

Amorphous Silicon Solar Cell and Boron Phosphide Thin Films Analyzed by FTIR and ESR.

Thin films of three layers (p-type/i-type/n-type) amorphous silicon (a-Si:H) and boron phosphide (BP) were investigated by means of Fourier transform infra-red (FTIR) spectroscopy and electron spin resonance (ESR). The formation ratio of (B-H)/(P-H) bonds in BP film, as functions of SiH, and PH3 gas flows, was evaluated by magnitude of stretching vibration. As for a-Si, higher order bond of-(Si-H2)-n, which may deteriorate the efficiency of solar energy conversion (ESEC), was distinguishable from Si-H single bond. The number of unpaired electron spins formed in 5 % ESEC a-Si:H, 7.99-1013 (spin/gram), was 73% larger than that in 8% ESEC a-Si:H.