Electron Paramagnetic Resonance Line Shape Research Articles

The State of the Copper Sites in Human Ceruloplasmin

The state of the various, spectroscopically distinguishable copper sites (type 1, type 2, and type 3 copper) of human ceruloplasmin was investigated by electron spin resonance (ESR) spectroscopy. The ESR measurements were performed at 100 K and at X-band during the reaction of the protein with either ascorbate or with ferricyanide. A method was developed to directly measure the contribution of type 1 and type 2 copper signals to the ESR spectrum of the native protein. A signal arising from an unperturbed type 2 copper site, obtained by aerobically treating the protein with ascorbate, allowed the estimation that the number of type 2 copper centers detectable by ESR was substantially lower than unity. A fraction of type 1 copper sites was found to be in the reduced state and could be reoxidized by treatment with ferricyanide. The data obtained were consistent with the presence of three type 1 copper sites per protein molecule. Based on the experimentally determined stoichiometries, computer simulations of the ESR lineshape were carried out which confirmed the presence of three nonequivalent type 1 copper sites and of a noninteger amount of ESR-detectable type 2 copper in human ceruloplasmin.

Magnetism and spin dynamics in MnPS3 and pyridine intercalated MnPS3: An electron paramagnetic resonance study

A detailed investigation of the magnetic properties of single crystals of the two-dimensional layered Heisenberg antiferromagnet MnPS3 and pyridine intercalated MnPS3 by static magnetic susceptibility and electron paramagnetic resonance (EPR) is reported. A detailed analysis of the angular and temperature variation of the EPR linewidths and line shapes has been carried out. While MnPS3 orders antiferromagnetically at TN=78 K, the pyridine intercalated compound exhibits isotropic weak ferromagnetism below 70 K. It is known that in the intercalated compound there is a loss of center of inversion so that a canting of spins could arise from either Dzyaloshinsky–Moriya (DM) dijSi×Sj interaction or single ion anisotropy, DSz2. The angular dependence of EPR linewidth and line shape of MnPS3 even at 300 K shows none of the features expected for a low-dimensional magnetic system at high temperatures; a (3 cos2 θ−1)2 angular dependence of linewidth and Lorentzian line shape at θ=55°. Instead, over the entire temperature range studied, the linewidth showed a 1+cos2 θ dependence and the line shapes predominantly Gaussian. The linewidth as well as the Gaussian character increased with decreasing temperature. The results have been interpreted to show that there is substantial intralayer spin–spin correlations and that critical fluctuations manifest at temperatures much above TN. The pyridine intercalated compound, however, shows the expected W-shaped angular dependence of linewidth with a minimum at θ=55° and 135°. The angular variation of linewidth could be fitted to a A+B cos2 θ+C(3 cos2 θ−1)2 dependence. However, the near-Lorentzian line shape at all angles implies that other relaxation mechanisms are operative which is also evidenced from the cos2 θ contribution to the angular variation of linewidth. Evidence for both DM as well as single ion anisotropy in the intercalated compound is presented. The presence of both of the above terms would explain the isotropic weak ferromagnetism observed in the static susceptibility of pyridine intercalated MnPS3.

Magnetic bistability and Overhauser shift of conduction electrons in gallium oxide.

We study the intrinsic magnetic bistability of conduction-electron spins in gallium oxide \ensuremath{\beta}-${\mathrm{Ga}}_{2}$${\mathrm{O}}_{3}$ by electron-paramagnetic-resonance (EPR) spectroscopy. This compound, normally an insulator, becomes an n-type semiconductor when synthesized under reducing conditions. The crystals studied in this work have a conductivity of 180--200 ${\mathrm{\ensuremath{\Omega}}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at room temperature and 140 ${\mathrm{\ensuremath{\Omega}}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at liquid-helium temperature. It has been previously shown [J. Phys. Chem. 96, 5513 (1992)] that the hyperfine interaction between conduction-electron spins and nuclear spins of gallium is responsible for a strong dynamic nuclear polarization when the EPR of conduction electrons is saturated (Overhauser effect). The resulting nuclear field acting on the electron spins was found to be bistable, which causes a hysteresis of the resonance line. In this work we demonstrate that hysteresis can be theoretically produced by three different control parameters: the external magnetic field ${\mathit{B}}_{0}$, the microwave frequency \ensuremath{\nu}, and the microwave field ${\mathit{B}}_{1}$ (or the microwave power P). A model is presented for the EPR line shape under bistable dynamic nuclear polarization, which is in fair agreement with experimental results for gallium oxide. We verify in this compound the existence of hysteresis of the EPR intensity upon positive and negative variations of the incident microwave power. The effect of sample size on bistability is also investigated. It is shown that this phenomenon can disappear if the sample size is larger than the skin depth of the compound. Bistability at room temperature in gallium oxide is found to be very sensitive to this size effect. The Overhauser shift of conduction electrons is also studied as a function of temperature. This parameter gives details on the hyperfine interaction between charge carriers and magnetic nuclei despite the extreme motional narrowing of the EPR line. The results are interpreted in terms of electronic transport at low temperatures involving an impurity band formed by oxygen vacancies acting as shallow donors.

Dysonian Line in the Electron Spin Resonance of Highly Conductive CuIn2S4 Single Crystals

Electron spin resonance (ESR) spectra have been measured in CdIn2S4 single crystals with various resistivities (ρ=106-10-3 Ω·cm). As the conductivity increases, samples vary from insulator-like semiconductors to degenerate ones. ESR spectra due to donor electrons are observed under illumination in high resistive samples, whereas Dysonian lines appear in low resistive ones. From the line shape analysis, we conclude that the variation in ESR line shapes with the concentration of donors arises from the motion of electrons in the paramagnetic centers.

Molecular motion of the morpholin-1-yl radical in CF 2ClCFCl 2 as studied by ESR. Use of residual anisotropy of powder spectra to extract dynamics

The dynamics of the deprotonated neutral morpholin-1-yl radical, trapped in halocarbon matrix, CF 2ClCFCl 2, has been studied utilizing electron spin resonance (ESR) spectroscopy. The experimental lineshapes of the radical exhibit an alterating linewidth effect in the temperature range 105–144 K. The major changes of the ESR lineshape were governed by the averaging of the nitrogen hyperfine anisotropy while no exchange of the isotropic hyperfine coupling constants was observed. Geometrical parameters specifying the restricted anisotropic rotation of the whole radical trapped in the matrix could be extracted. Two methods for simulating anisotropic exchange broadened ESR spectra, the secular and non-perturbative, were utilized to investigate the dynamics of the radical. A surprisingly simple “three-site” jump model with a barrier of ≈ 3.6 kcal/mol can be applied in the simulation of the experimental spectra. It has been shown that the secular method cannot reproduce the exchange broadened ESR spectra of systems with large hyperfine anisotropy undergoing large internal reorganisation.

Electron paramagnetic resonance line shapes of photoexcited triplets with rotational diffusion

A new line shape formula is developed for magnetic resonance on systems which are initially far from thermal equilibrium. A formalism for treating electron paramagnetic resonance (EPR) of rotationally diffusing molecules is extended to include typical triplet states, in which intramolecular interactions are not very small compared with the Zeeman interaction. The two developments are combined to a new formalism for calculating line shapes of photoexcited triplet states undergoing rotational diffusion. Approximations are made for parts of the calculation, and their range of validity is examined. Simulations based on this theory for randomly oriented samples of chlorophyll a (Chl a) demonstrate the usefulness of the theory in calculating line shapes for both equilibrated and polarized photoexcited EPR triplets undergoing rotational diffusion. In particular we study the effect of anisotropy in the molecular diffusion tensor on the line shapes. A comparison is also made between simulated and experimental line shapes for two expanded porphyrinoids, cadmium–texaphyrin complex (TxP–Cd), and zincoctaethylporphycene (ZnOEPC). Finally, we present a series of dynamic line shapes for the photoexcited triplet state of C60 (buckminsterfullerene), discussing their correspondence with available experimental results.

Characterization of ClOx radicals in vacuum-ultraviolet-irradiated high-purity silica glass.

An electron-spin-resonance (ESR) study was made on the defect centers induced by vacuum-ultraviolet (vuv) laser (7.9-eV) irradiation in high-purity silica glasses. We observed two types of ESR spectra, each consisting of a concentric pair of four-line peaks, in low-OH oxygen-surplus silica produced with plasma methods. Spin-Hamiltonian parameters determined by computer simulation of the experimental ESR line shapes are consistent with oxy-radicals of chlorine. The observed ${\mathrm{ClO}}_{\mathit{x}}$ (x=2,3) radicals are considered to be due to the reaction between chlorine and oxygen, which were both produced by vuv-laser irradiation.

Electron spin resonance line shape analysis of x-doxylstearic acid spin probes in dioctadecyldimethylammonium chloride vesicles

Electron spin resonance spectra of a serien of x-doxylstearic acids (x=5, 7, 10, 12, and 16) solubilized in cationic dioctadecyldimethylammonium chloride vesicles were recorded over 298-328 K and analyzed by employing spectral line simulation. The spectra from the vesicle phase are well reproduced by using the miaoncopic order-macroncopic disorder model of Freed and co-workers with Brownian rotational diffusion

Electron paramagnetic resonance lineshape analysis of small and large probes introduced into micellar aqueous solutions of ammonium pentadecafluorooctanoate

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTElectron paramagnetic resonance lineshape analysis of small and large probes introduced into micellar aqueous solutions of ammonium pentadecafluorooctanoateSandra Ristori and Giacomo MartiniCite this: Langmuir 1992, 8, 8, 1937–1942Publication Date (Print):August 1, 1992Publication History Published online1 May 2002Published inissue 1 August 1992https://doi.org/10.1021/la00044a010RIGHTS & PERMISSIONSArticle Views77Altmetric-Citations31LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (641 KB) Get e-Alerts Get e-Alerts

The YF161D1 mutant of Synechocystis 6803 exhibits an EPR signal from a light-induced photosystem II radical.

The currently accepted model for the location of the redox-active tyrosines, D and Z, in photosystem II suggests that they are symmetrically located on the D1 and D2 polypeptides, which are believed to form the heterodimer core of the reaction center. Z, the electron conduit from the manganese catalytic site to the primary chlorophyll donor, has been identified with tyrosine-161 of D1. The YF161D1 mutant of Synechocystis 6803 [Debus, R. J., Barry, B. A., Sithole, I., Babcock, G. T., & McIntosh, L. (1988b) Biochemistry 27, 9071-9074; Metz, J. G., Nixon, P. J., Rogner, M., Brudvig, G. W., & Diner, B. A. (1989) Biochemistry 28, 6960-6969], in which this tyrosine has been changed to a phenylalanine, should have no light-induced EPR (electron paramagnetic resonance) signal from a tyrosine radical. This negative result has indeed been obtained in analysis of one of two independently constructed mutants through the use of a non-oxygen-evolving core preparation (Metz et al., 1989). Here, we present an analysis of a YF161D1 mutant through the use of a photosystem II purification procedure that gives oxygen-evolving particles from wild-type Synechocystis cultures. In our mutant preparation, a light-induced EPR signal from a photosystem II radical is observed under conditions in which, in a wild-type preparation, we can accumulate an EPR signal from Z+. This EPR signal has a different lineshape from that of the Z+ tyrosine radical, and spin quantitation shows that this radical can be produced in up to 60% of the mutant reaction centers. The EPR lineshape of this radical suggests that photosystem II reaction centers of the YF161D1 mutant contain a redox-active amino acid.

Effects of Redox Reagents on ESR Line Shape of Synthetic Melanins

Abstract Melanins were synthesized from pyrogallol or β-(3,4-dihydroxyphenyl)alanine (dopa) and their electron spin resonance (ESR) spectra were measured. The pyrogallol melanin had four kinds of ESR signal; main and broad signals under anaerobic conditions, and inner and outer signals under aerobic conditions. The main, inner and outer signals disappeared by the reduction of the melanin by NaBH4 or ascorbic acid but the broad signal did not. By the oxidation of the melanin by hexacyanoferrate(III), the intensity of the signals decreased, and the inner and outer signals were not observed under aerobic conditions. Hydrogen peroxide decreased these signals especially under aerobic conditions at high pH. Superoxide gave the signals similar to that under aerobic conditions, but decreased the signals of the hexacyanoferrate(III)-oxidized melanin. ESR signals of dopa melanin showed similar behavior to that of pyrogallol melanin against redox reagents. The chemical structure around radicals in melanins were proposed from these results.

The photoexcited triplet state as a probe of dynamics and phase memory in a multiphase liquid crystal: Time-resolved electron paramagnetic resonance spectroscopy

Differences between mesophases and hysteresis effects can be examined by electron paramagnetic resonance (EPR) detection of light-induced transient triplets oriented in liquid crystals (LC). Time-resolved EPR spectroscopy (direct detection) has been applied to study the photoexcited triplet state dynamics of tetraphenylporphyrin (free-base) oriented in a multiphase LC, ZLI-1167, having a negative anisotropic susceptibility, Δχ<0. The feasibility of triplet detection over a temperature range of 100–370 K, enables one to monitor conspicuous changes of the triplet line shape associated with the different mesophases, i.e., cryst.↔(287 K) smec.↔(305 K) nem.↔(356 K) isot. Line shape analysis shows that all phases consist of domains having their directors spanned in a plane normal to the external magnetic field, as expected for Δχ<0. The triplet spectra in the fluid smectic phase depend on sample rotation, indicating the known rigidity of this phase. On the other hand, the spectra in the nematic phase are independent of the sample rotation but strongly depend on the temperature and sample alignment in the magnetic field. Two mechanisms of triplet dynamics are proposed; namely an inter- and intradomain triplet energy transfer between the porphyrin chromophores. In addition, the results suggest that, in a specific alignment procedure, the smectic character is maintained deep in the nematic and, to some extent, in the isotropic phases. Thus the EPR line shapes and temporal behavior of the nematic spectra, are associated with a phase memory effect (hysteresis), and the smectic phase persistence may indicate the existence of cybotactic groups within the nematic and isotropic phases.

Electron-spin-resonance studies of porous polyacenic semiconductive materials.

Electron-spin-resonance (ESR) studies of porous polyacenic semiconductive materials in the pristine and the heavily ${\mathrm{I}}_{2}$-doped regimes have been carried out. The measurement temperature has been varied from 15 K to room temperature. Two kinds of typical polyacenic semiconductive sample essentially prepared by pyrolysis at 600 \ifmmode^\circ\else\textdegree\fi{}C and 700 \ifmmode^\circ\else\textdegree\fi{}C of phenol-formaldehyde resin were selected for the ESR measurements. In particular, the temperature-dependent behavior of the peak-to-peak linewidth of the ESR line shape for all samples was discussed in detail, particularly in relation to the hopping behavior of unpaired spins.

A new ultra low-field ESR spectrometer

A CW electron spin resonance (ESR) spectrometer working at earth-field is described for the first time. The optimized sensitivity has allowed to reduce the size of the sample holder down to 100 mm3, a low value for such a low working frequency (1.845 MHz). At this frequency, only narrow ESR signals are significant (maximum sweep 125 μT or 100 A/m). The use of the Bloch configuration and the particular care to optimize the sensitivity lead to a design that partially compensates the lack of sensitivity due to low fields. Small material quantities are thus usable. Moreover, the spectrometer is low cost, portable, and easily reproducible with the announced performance. The main use of the spectrometer is to characterize materials for earth field magnetometry. For that purpose it has been necessary to define a figure of merit for magnetometry. Until this study, the only parameter discussed was the linewidth of the homogeneous ESR line (i.e., T2). The detailed discussion of the figure of merit, taking into account not only the material but the couple spectrometer material, considerably decreases the predominance of the linewidth and let emerge secondary factors such as packaging, susceptibility, ESR lineshape, T1 vs T2, etc. The figure of merit evaluated from the material parameters has been experimentally measured for different π radicals. Comparison with their performance in earth field magnetometry demonstrates the pertinence of the proposed figure of merit.

Compound I radical in site-directed mutants of cytochrome c peroxidase as probed by electron paramagnetic resonance and electron-nuclear double resonance

The reaction of ferric cytochrome c peroxidase (CcP) from Saccharomyces cerevisiae with peroxide produces compound I, characterized by both an oxyferryl iron center and a protein-based free radical. The electron paramagnetic resonance (EPR) signal of the CcP compound I radical can be resolved into a broad majority component which accounts for approximately 90% of the spin intensity and a narrow minority component which accounts for approximately 10% of the integrated spin intensity [Hori, H., & Yonetani, T. (1985) J. Biol. Chem. 260, 3549-3555]. It was shown previously that the broad component of the compound I radical signal is eliminated by mutation of Trp-191 to Phe [Scholes, C. P., Liu, Y., Fishel, L. F., Farnum, M. F., Mauro, J. M., & Kraut, J. (1989) Isr. J. Chem. 29, 85-92]. The present work probed the effect of mutations in the vicinity of this residue by EPR and electron-nuclear double resonance (ENDOR). These mutations were obtained from a plasmid-encoded form of S. cerevisiae expressed in Escherichia coli [Fishel, L. A., Villafranca, J. E., Mauro, J. M., & Kraut, J. (1987) Biochemistry 26, 351-360]. The EPR line shape and ENDOR signals of the compound I radical were perturbed only by mutations that alter Trp-191 or residues in its immediate vicinity: namely, Met-230 and Met-231, which have sulfur atoms within 4 A of the indole ring, and Asp-235, which forms a hydrogen bond with the indole nitrogen of Trp-191. Mutations of other potential oxidizable sites (tryptophan, tyrosine, methionine, and cysteine) did not alter the EPR line shapes of the compound I radical, although the integrated spin intensities were weaker in some of these mutants. Mutations at Met-230 and/or -231 perturbed the EPR line shapes of the compound I radical signal but did not eliminate it. ENDOR of these two methionine mutants showed alteration to the hyperfine couplings of several strongly coupled protons, which are characteristic of the majority compound I radical electronic structure, and a change in weaker hyperfine couplings, which suggests a different orientation of the radical with respect to its surroundings in the presence of these methionine mutations. Besides the Trp-191----Phe mutation, only the Asp-235----Asn mutation eliminated the broad component of the compound I signal. Loss of the broad compound I EPR signal coincides with both the loss of the Asp----Trp-191 hydrogen-bonding interaction and alteration of the position of the indole ring of Trp-191.(ABSTRACT TRUNCATED AT 400 WORDS)

ESR studies on luminescent ZnS:Mn films and CdS [sbnd]ZnS:Mn superlattices deposited on a GaAs(100) substrate by hot-wall epitaxy

Luminescent ZnS:Mn thin films and CdS-ZnS:Mn superlattices on a GaAs(100) substrate prepared by the hot-wall epitaxy technique have been examined by means of electron spin resonance (ESR) spectroscopy. It was observed that the lowest energy transition assigned toM 1 = −5/2 splits into either a triplet or a quintet (depending on the film thickness) when the magnetic field is applied normal to the films. Computer simulation clarified that there is no significant change in their hyperfine coupling constant and splitting parameters, while apparent differences in the ESR lineshape come from the change in linewidths of the fine structure involved. The linewidths for the transitionsM s = ±5/2↔ ±3/2 decrease markedly with increasing thickness of the ZnS:Mn film on the GaAs substrate. This phenomenon is explained in terms of uniaxial deformation of the crystal field surrounding manganese(II) due to lattice mismatch between the ZnS:Mn epitaxial layer and the GaAs substrate. For the CdS-ZnS:Mn superlattices, on the other hand, no angular dependence was observed, suggesting that the alignment of the ZnS:Mn layers sandwiched between the CdS layers is rather poor.

Characterization of epitaxially grown ZnS:Mn films on a GaAs(100) substrate prepared by the hot-wall epitaxy technique

Epitaxially grown ZnS:Mn films on a GaAs(100) substrate prepared by a hot-wall epitaxy technique have been studied using electron paramagnetic resonance (EPR) spectroscopy. The lowest-energy transition assigned to Ml=–5/2 splits into either a triplet or a quintet depending on the thickness of the film when the magnetic field is applied normal to it. Computer simulation confirmed that the EPR parameters obtained correspond to those for a single crystal of cubic structure, although the apparent lineshape is different. There is no significant difference between the hyperfine coupling constants and splitting parameters. The apparent difference in the EPR lineshape is ascribed to the change in linewidths of the fine structure involved. In particular, linewidths for the transitions Ms=±5/2 ⇌±3/2 decrease markedly with increasing thickness of the ZnS:Mn film on the GaAs(100) substrate, confirming that uniaxial deformation of the crystal field surrounding manganese(II) is derived from the lattice mismatch between them.

Base dynamics of local Z-DNA conformations as detected by electron paramagnetic resonance with spin-labeled deoxycytidine analogs

Conformation detection and base dynamics of spin-labeled Z-DNA have been investigated by electron paramagnetic resonance (EPR) spectroscopy. The two synthesized and characterized probes used in this study were C(5)-nitroxide-labeled 2'-deoxycytidine 5'-triphosphates, pppDCAT and pppDCAVAT, which serve as suitable substrates for Micrococcus luteus DNA polymerase. Enzymatic incorporation of these probes into (dG-dC)n yields the EPR-active alternating copolymers (dG-dC,DCAT)n and (dG-dC,-DCAVAT)n. These polymers assume typical B- and Z-DNA conformations under respective low (0.1 M NaCl) and high (4.5 M NaCl) salt conditions, as evidenced by their UV-circular dichroism spectra. The EPR line shape of (dG-dC,DCAT)n in Z-form is unique and significantly different from the B-form EPR spectrum. A similar observation is made for (dG-dC,DCAVAT)n. Thus, the EPR line shapes of these spin-labeled DNAs are indicative of their local conformations. The EPR spectra, analyzed with a previously published motional model [Kao, S.-C., Polnaszek, C.F., Toppin, C.R., & Bobst, A.M. (1983) Biochemistry 22, 5563-5568], indicate tau perpendicular values of 4 and 7 ns for the B- and Z-forms, respectively. Therefore, the base dynamics of Z-DNA are about two times slower than in B-DNA.

Local-magnetic-moment variation in pure and impure phases of Tl-Ca-Ba-Cu-O superconductors

In this work, we have successfully prepared Tl-Ca-Ba-Cu-O compounds with on-set temperature of 155 K and zero-resistance temperature of 123 K. Transmission electron microscopy and electron energy dispersive spectrometry analyses indicate that the major phase in this compound is tetragonal Tl 1Ca 2Ba 3Cu 4O x of P4/ mmm space-group symmetry with a = b = 0.394 nm and c = 1.97nm. Electron spin resonance (ESR) spectrometry was used to explore the local moments and hyperfine interaction of copper ions. The hyperfine lines are judged to be due to the CuO with Ca-Cu compounds intermixed between the grains. In pure samples all the spins are paired-off to give the singlet state. The vacuum annealed pure samples of Tl-Ca-Ba-Cu-O and YBa 2Cu 3O 7−δ have similar ESR lineshapes suggesting that the copper ions retrieve their original local moment due to the suppression of the super-exchange force by the decrease of oxygen atoms near the Cu-O planes.

The electron spin resonance lineshape in hydrogenated amorphous silicon germanium alloys

The electron spin-resonance lineshape of the D-center was measured in a series of hydrogenated amorphous silicon-germanium alloys (a-Si 1− x Ge x :H). The lineshape was fitted to obtain the principal g-tensor components g| and g⊥ of a cylindrically symmetric defect; inhomogeneous strains were represented by isotropic Gaussian broadening. For elemental a-Ge:H we obtained g⊥ = 2.013 and g | = 2.042; the result g | > g ⊥ is not consistent with the model that the D-center in a-Ge:H is an isolated dangling bond. The lineshape in alloys was modeled by superposing the unshifted D-center lineshapes in a-Si:H and a-Ge:H with an additional cylindrical symmetry line with arbitrary g-tensor and broadening. The parameters of this new line were fairly constant for x > 0.20. Models for these results based on delocalization onto two dangling bonds are discussed.