Unresolved Hyperfine Structure Research Articles

Unconstrained optimization method for interpreting the concentration and temperature dependence of the linewidths of interacting nitroxide spin labels. Application to the measurement of translational diffusion coefficients of spin-labeled phospholipids in membranes

A modified Gauss-Newton optimization procedure has been used to fit an analytical expression for the total peak-to-peak, first-derivative linewidth to the concentration and temperature dependence of the ESR spectra of interacting nitroxide spin labels. The measurements were restricted to the low-concentration regime in which the contributions to the Lorentzian linewidth, ΔH pp L from both exchange and magnetic dipole-dipole interactions are linearly dependent on the spin-label concentration. The temperature dependences of these two contributions are inversely related via the translational diffusion coefficient, the temperature dependence of which was characterized by a single effective activation energy. The intrinsic Lorentzian linewidths were determined by linear regression on the concentration dependence, within the optimization procedure. The concentration dependence of the Gaussian linewidth, δH pp G, which arises from unresolved proton hyperfine structure, was characterized by an empirical switching function proposed by B. L. Bales and D. Willett ( J. Chem. Phys., 80, 2997 (1984)). This simulates the effects of exchange narrowing by turning off the Gaussian broadening and turning on an additional Lorentzian broadening in a manner which is determined by the relative sizes of the exchange frequency and the effective proton hyperfine splitting. The total peak-to-peak, δH pp tot, linewidth is then related to the Lorentzian and Gaussian contributions via the relation ( δH pp G/ δH pp tot) 2 + δH pp L/ δH pp tot = 1, (S. N. Dobryakov and Ga. S. Lebedev, Sov. Phys. Dokl., 13, 873 (1969)). The method has been applied to the determination of the translational diffusion coefficients of spin-labeled phosphatidylcholine in dioleoyl phosphatidylcholine bilayers over the concentration range 0.5–2.5 mol% spin label, and the temperature range 5–80°C, and also to study of the diffusion of a small spin-labeled fatty acid in water.

Magnetic resonance of conduction electrons in fluoranthene radical cation salts

The Overhauser shift method is a very sensitive technique to investigate the hyperfine interaction of conduction electrons with magnetic nuclei in organic conductors with a sufficiently narrow ESR line [1,2]. The high sensitivity is due to its double resonance character and is used to measure details of the proton NMR in single crystals. Despite an unresolved hyperfine structure, the average hyperfine tensor is accessible. Extension of the method uses magnetic nuclei like 13C, 19F and 2H in deuterated crystals as a local probe of the conduction electron spin density. Results for the (Fluoranthene) 2 .+PF 6 − radical cation salt are reported.

ESR determination of lipid translational diffusion coefficients at low spin-label concentrations in biological membranes, using exchange broadening, exchange narrowing, and dipole-dipole interactions

Spin-spin interactions between phosphatidylcholine nitroxide spin labels in fluid-phase dimyristoyl phosphatidylcholine bilayers have been studied in the concentration range 0.1–2.0 mol%. The total line broadening has been separated into its Gaussian and Lorentzian components. The Gaussian linewidth exhibits exchange narrowing of the unresolved proton hyperfine structure, with an approximate quadratic concentration dependence for the square of the linewidth, over this spin-label concentration range. The Lorentzian linewidth has a linear dependence on the spin-label concentration over the same range. The temperature dependence of the gradient of the Lorentzian broadening with respect to spin-label concentration is biphasic over the range 30–80°C, indicating a dominant contribution from magnetic dipole-dipole broadening at the lower temperatures. The contributions from the dipole-dipole and exchange interactions to the Lorentzian broadening have been separated by fitting the temperature dependence using an unconstrained optimization procedure and assuming a single effective activation energy for both diffusion-controlled processes. The lipid lateral diffusion coefficient has been determined from the concentration dependence of both the exchange broadening of the nitrogen hyperfine structure and the exchange narrowing of the proton hyperfine structure using a two-dimensional lattice diffusion model, and also from the concentration dependence of the dipole-dipole interaction using a three-dimensional Fick's law diffusion model. All three methods yield consistent values for the diffusion coefficient, although because of the approximate nature of some of the theoretical models the exchange broadening is expected to give the most reliable results. An experimental protocol for determining the actual spin concentration in the membrane sample is also presented.

EPR Study of Cu(L-ILE)2, a copper-amino acid salt

EPR measurements at the X-band have been performed between 1.5 and 293 K in single crystals of the copper salt of the amino acid L-isoleucine Cu(L-ILE)2. We measured the angular and temperature variations of the gyromagnetic factor g and the linewidth ΔB. The angular variation of g2(θ, φ) observed at high temperatures reflects molecular and crystallographic properties. The data indicate a dx2-y2 orbital ground state and give the direction of the molecular axis. The angular variation of the linewidth is a consequence of several different contributions. The unresolved hyperfine structure is the most characteristic and was analyzed in terms of Anderson's theory of narrowing applied to the destruction of the hyperfine structure by the exchange interaction. From the linewidth data we estimate 0.17 < |J|/k < 0.21 K for the intralayer exchange interaction. Below 50 K, a strong temperature variation of g, with an anisotropy reflecting a layered magnetic structure, and with a 1/T temperature dependence, was observed. It was interpreted as due to the non-isotropic exchange interactions between short-range ordered copper ions in Cu(L-ILE)2, but we do not have a quantitative theory to explain this effect yet.

Experimental determination of level shift and broadening by predissociation of the B3Π0· state of ICl

Abstract From the semiclassical analysis of the coupling between discrete molecular levels and continuum levels simple relations for the level shift and the level broadening are known. Laser spectroscopy on the transition is B 3 Π 0 ·−X 1 Σ + is applied to demonstrate the overall validity of these relations in the rotational ladder of the vibrational state υ′ = 3 of B 3 Π 0 ·. Deviations for low rotational quantum numbers might be attributed to the still unresolved hyperfine structure and the possible hyperfine predissociation and for very large rotational quantum numbers to the distortion through a coupling with a second continuum.

Microwave rotational spectra of alkaline earth monohalides: SrBr and SrI

The rotational structure in the 2Σ + ground state of the molecules Sr 81Br, Sr 79Br and SrI has been analysed for the first time. Precise Dunham coefficients Y 01, Y 11, Y 21 and Y 02 have been derived from microwave rotational spectra in the 100 GHz range. The accuracy of the spin-rotation interaction constant γ which can also be determined from these measurements is limited by effects of unresolved hyperfine structure.

Microwave rotational spectra of alkaline earth monohalides: CaI and BaI

Rotational spectra in the 2Σ + ground state of CaI and BaI have been measured in the 100 GHz range. Precise Dunham coefficients Y ik and potential constants a i can be derived from these spectra. Effects of unresolved hyperfine structure have to be considered in the interpretation of the observed γ-splitting.

Model calculations of nuclear hyperfine structure for anomalous muonium and their implications

Calculations of the effects of resolved and unresolved nuclear hyperfine structure for anomalous muonium have been made for a range of axial nuclear hyperfine parameters and magnetic fields. Although the spectra arc complex, their moments allow predictions about line widths (or depolarization rates) from unresolved hyperfine structure. The increase ofλ with decreasing field observed experimentally is obtained in these calculations. In addition we make predictions ofλ for zero-field spectra, angular dependences ofλ, and line shifts.

Low-field anomalous muonium depolarization in isotopically enriched germanium

The depolarization rate of anomalous muonium, Mu*, in germanium isotopically enriched in74Ge (I=0) was measured as a function of field. The concentration of73Ge (I=9/2) was about 9 times less than natural abundance. The depolarization rate at 10 K in this isotopically enriched crystal for both lines of those Mu* centers whose symmetry axes make an angle of 90° to the field is less than 1μsec−1 at all fields down to the lowest one measured, 14.5 gauss. This is in sharp contrast to the wide lines reported at low field in germanium having natural isotopic abundance. The spectrum of Mu* in the isotopically enriched Ge crystal was also seen at zero field. These results confirm that the increased depolarization rate for Mu* at low fields arises from unresolved nuclear hyperfine structure. The depolarization rates observed were consistent with an average hyperfine interaction with a single73Ge nucleus of 2.5 MHz, a value requiring nearly 1% of the spin density to be on a typical atom.

Sub-Doppler infrared–infrared double resonance spectroscopy of NH3 using Stark tuning and a diode laser

Double resonance experiments on NH3 are described in which a CO laser pumps a 6 μm ν4 transition while a tunable diode laser probes a 10 μm ν2 transition having a common lower level. Four different combinations of pump–probe transitions are studied. The CO laser is Lamb-dip stabilized on the pump transitions, which are tuned into coincidence with it using a precision intracavity Stark cell. The pump and probe beams overlap colinearly in the Stark cell. The double resonance signals appear as narrow transmission peaks on the diode laser scans. The narrowest observed widths are ≂3 MHz (FWHM), a large portion of which is due to unresolved hyperfine structure. An analysis of the various broadening mechanisms indicates that the diode laser contributes less than 1 MHz to the widths. Resonances due to velocity-preserving but state-changing collisions are seen. Asymmetries between co- and counterpropagating linewidths are shown to arise from a combination of field inhomogeneity and coherent narrowing effects. The data are recorded using a computer-aided, rapid-scan, digital signal averaging technique.

Millimeter wave spectra of the 2Σ radicals SrF and SrCl

Rotational spectra of gaseous SrF and SrCl have been measured in the ranges 89–91 and 269–285 GHz. Dunham coefficients Y ik and spin-rotation constants γ have been derived from these spectra. Effects of unresolved hyperfine structure had to be considered for a reliable determination of γ.

Effects of unresolved hyperfine structure in the microwave rotational spectra of the 2Σ-radicals CaBr and CaCl

The microwave rotational spectra of gaseous CaBr and CaCl have been measured in the 90-107 GHz and 270-290 GHz region. Precise values of the Dunham parameters Y 01, Y 11, Y 2, and Y 02 are given for the isotopic species Ca 79Br, Ca 81Br, Ca 35Cl and Ca 37Cl. It is shown that effects of unresolved hyperfine structure may cause considerable systematic errors in the determination of the spin-rotation constant Ψ even if the hyperfine structure splitting is so small that not even inhomo- geneous line broadening can be observed in the spectrum. If the hyperfine structure parameters are not known from other sources these effects will limit the accuracy for the determination of Ψ from microwave spectra.

Photonization-produced alignment of Cd

The degree of polarization of a target ion excited by (inner-shell) photoionization is shown to depend on only two parameters in Dill's angular-momentum-transfer formalism. The measured alignment of ${\mathrm{Cd}}^{+}^{2}D_{\frac{5}{2},\frac{3}{2}}$ is corrected for unresolved hyperfine structure, showing that anisotropic electron-ion interactions are substantially stronger than previously thought.

A simple, accurate method of correcting for unresolved hyperfine broadening in the EPR of nitroxide spin probes to determine the intrinsic linewidth and heisenberg spin exchange frequency

A method of correcting the EPR linewidth of a nitroxide that is inhomogeneously broadened by unresolved hyperfine structure is presented. Two simple equations are presented which allow the accurate deduction of the intrinsic linewidth and the Heisenberg spin exchange frequency, respectively, from the measured linewidths. The method allows correction for the inhomogeneous broadening in practically all cases in which the nitroxide tumbles in the motional narrowing region by using only a hand-held programmable calculator, thus eliminating the need for calibration libraries. The accuracy of the two equations is demonstrated for three nitroxides.

ESR of diphenylantimony radical in a triphenylantimony single crystal

Ultraviolet irradiation of a single crystal of triphenylantimony produces an antimony centered radical with an anisotropic 121Sb coupling having principal absolute values of 704, 352, and 352 MHz and a g tensor with the same principal axis as the hyperfine interaction and components of 1.998, 2.045, and 2.065. This radical has been assigned to the diphenyl antimony radical with approximately zero s and 0.7 p character. The linewidth of the spectra are consistent with unresolved hyperfine structure due to spin density delocalized on the phenyl rings.

ESR Studies of the Photoreduction of Quinones. II. The Reinter-pretation of Semiquinone Intermediates from Halogenated p-Benzoquinones in Ethanol

Abstract A reinterpretation was made of the electron spin resonance spectrum of semiquinone intermediates during the photolysis of a flowing solution of tetrachloro-p-benzoquinone (chloranil) in ethanol. The photoinduced spectrum with a width of 0.06 mT with an unresolved hyperfine structure was found to be identical with the spectrum of the semiquinone anion generated by reducing chloranil in an alkaline ethanol solution. This and other observations indicated that the photoinduced spectrum was due to the chloranil semiquinone anion formed primarily by the one-electron transfer to chloranil from the solvent ethanol, rather than to the semiquinone radical. The photoinduced one-electron transfer reaction was found, though qualitatively, to proceed more efficiently for chloranil and fluoranil, with a larger electron affinity, than for p-benzoquinone and dichloro-p-benzoquinones.

ESR and ENDOR study of single crystals of L-asparagine⋅H2O x-irradiated at room temperature

The ESR spectra of single crystals of L-asparagine⋅H2O and L-asparagine⋅D2O have been examined. Since these data show a great deal of unresolved hyperfine structure, a detailed ENDOR experiment was undertaken. The ENDOR experiment showed the presence of a single α-hydrogen coupling. By comparing the directions of the principal values (−33.4, −21.1, −10.7 G) of the hyperfine coupling tensor with the equivalent directions expected from various possible free radical models, it was concluded that the dominant free radical observed in L-asparagine⋅H2O at room temperature is CO(NH2) ĊHCH (N+H3)CO−2. The lack of an ENDOR signal from the β-hydrogen and the presence of a rather large β-nitrogen coupling (∼5.0 G) observed by ESR has been adaquately explained by a detailed INDO calculation.

Theory of electron spin-echoes in non-viscous liquids

A theory which describes the electron spin-echo for free radicals in non-viscous solution whose steady-state E.S.R. linewidth is inhomogeneously broadened by unresolved intramolecular hyperfine structure is presented. It is found that the two-pulse electron spin-echo for such systems decays at a rate approximately proportional to exp [- tT 2 -1], where T 2 -1 is the ‘intrinsic linewidth’ of an individual spin-packet. The presence of Heisenberg exchange is shown to a good approximation to add to the decay constant so that T 2 -1(ex) ≈ T 2 -1 (0) + ωex, where T 2(ex) is the phase memory time in the presence of exchange, T 2(0) is the phase memory time in the absence of exchange, and where ωex is the Heisenberg exchange frequency. This work indicates that the electron spin-echo technique may be a very important investigative tool for relaxation studies of spin labels in solution.

Heisenberg exchange in inhomogeneously broadened nitroxide EPR spectra

The type of analysis necessary to extract the Heisenberg exchange frequency, ω HE, of nitroxide free radicals in solution whose electron paramagnetic resonance (EPR) spectra are complicated by inhomogeneous broadening due to intramolecular proton hyperfine interactions is discussed. It is found that the EPR spectra must be simulated by use of extensions of the theories of Anderson and Freed. Significant error in calculations of the translational correlation time can result if the unresolved proton hyperfine structure is ignored.

ESR of γ-irradiated single crystals of rubidium uranyl nitrate: NO 3 radical

γ-Irradiated rubidium uranyl nitrate crystals are found to exhibit ESR spectra. In a general orientation, three anisotropic lines of equal intensity are observed. These have been attributed to identical radicals, situated at three nonequivalent sites in the crystal. The radicals are tentatively assigned to NO 3 on the basis of small, unresolved 14N hyperfine structure. The analysis of the ESR spectra suggests that the radicals are planar but do not possess a threefold axis. The directions of the principal axes of the g-tensor and their values are reported. Electronic structure of the NO 3 radical is discussed.