Single Broad Line Research Articles

A novel single electron transfer in solid-state organic compounds: mechanically induced reduction of dipyridinium salts

When mechanical vibration (56 Hz) of each of seven dipyridinium dications, 1-7 (100 mg each), was conducted with a stainless steel ball (6.0-mm diameter, 890 mg) in a stainless steel-made twin-shell blender (7.8-mm diameter, 24 mm long) for 30 min at room temperature under strictly anaerobic conditions, the powder surfaces of four derivatives, 1-4, turned to a deep blue-purple in color, and nearly isotropic broad single-line ESR spectra were observed in the resulting powders, which can be ascribed to the formation of the corresponding monocation radicals in 1-4 resulting from a solid-state single electron transfer (SSET)

Motional narrowing of the 2H NMR spectra near the chain melting transition of phospholipid/D2O mixtures.

The reduction in spectral splitting, or motional narrowing, of the deuterium spectra of D2O/phospholipid mixtures near the main chain melting phase transition was studied for palmitoyloleoylphosphatidylcholine (POPC), palmitoyloleoylphosphatidylethanolamine (POPE) and equimolar mixtures of the two at 10% hydration. For POPC the splitting was about 1700 Hz in both the fluid and gel phases, dropping to zero near the phase transition (as reported previously). For POPE the splitting remained approximately constant above the phase transition. Below the phase transition the spectrum showed a single broad line whose linewidth varied between 100 Hz and 800 Hz. This was interpreted as being due to small domains of water within a weakly hydrated crystal. POPC:POPE (1:1) samples exhibited motional narrowing behaviour similar to that for POPC except that the splitting above the phase transition was approximately twice that below the transition. The relatively broad temperature range (approximately 20 K) of the transition is explained using a simple physical model involving lipid fluctuations near the phase transition.

Mechanical alloying of Fe and Cr: Structural and magnetic studies

Mechanical milling of a mixture of pure iron and chromium powders with a nominal composition of 28 at. % of Fe and 72 at. % of Cr was performed using two kinds of tools. The alloying process was followed by X ray diffraction and57Fe Mossbauer spectroscopy. The room temperature Mossbauer spectra revealed the formation of a broad single line at the expense of the area of α-Fe sextet. At 4.2 °K most of the singlet still remains unsplit. This behavior may be due to some metastable structure induced by the preparation method.

Electron-spin resonance in the spin-glass-like system Fe1-xGaxSbO4.

We report ESR measurements in the spin-glass-like system ${\mathrm{Fe}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{SbO}}_{4}$ for 0\ensuremath{\le}x\ensuremath{\le}0.8. A single broad line associated with the resonance of ${\mathrm{Fe}}^{3+}$ is observed at g=2.00(5). The temperature dependence of the ESR linewidth shows two different regimes. Between 200 and 640 K the temperature dependence is discussed in terms of a mean-field approximation for the magnetic interactions between neighboring Fe ions, and the experimental data have been correlated to the behavior of the static magnetic susceptibility. From this analysis we have determined the high-temperature limit of the linewidth, \ensuremath{\Delta}${\mathit{H}}_{\mathit{p}\mathit{p}}$(\ensuremath{\infty}), for different values of x. Below approximately 200 K a strong enhancement of the linewidth is observed, and, from the temperature dependence measured in this region, we have estimated values of the spin-freezing temperature for different concentrations of magnetic ions and determined a concentration of Ga atoms, ${\mathit{x}}_{0}$=0.285(15), above which there is no evidence of spin freezing for T>4.2 K. The large absolute value of the high-temperature linewidth for ${\mathrm{FeSbO}}_{4}$ and its increase when the system is magnetically diluted through Ga substitution indicate the importance of spin-diffusion processes in these materials. This observation, together with the comparatively high magnetic-percolation concentration determined from the x dependence of the freezing temperature, suggest that large portions of the system may present low-dimensional magnetic behavior. This possibility is analyzed in terms of the antisite cationic order proposed for these compounds.

An NMR study of structure and ordering in synthetic K2MgSi5O12, a leucite analogue

Phases with the composition K2MgSi5O12, belonging to the leucite structure group were synthesised under dry and hydrothermal conditions and studied using 29Si NMR. The 29Si spectrum for the dry-crystallised material (which is cubic) consists of a single broad line, suggesting a high degree of disorder. The hydrothermally crystallised material (which is probably monoclinic, with a distorted leucite lattice) has a 29Si spectrum which consists of ten lines of equal intensity, two of which have small chemical shift anisotropies and are therefore assigned to Q4(4Si) sites. These data have been interpreted in terms of a structure with 12 distinct tetrahedral sites over which 2 Mg atoms and 10 Si atoms are fully ordered. A 2-dimensional COSY spectrum shows correlations between some Q4(3Si) silicon atoms and two other Q4(3Si) silicon atoms. This fully constrains the topology of the unit cell. Two schemes of Si/Mg ordering over the unit cell can give good fits to the COSY spectrum. Using the tetrahedral (T) site notation defined for natural tetragonal leucite, the first of these arrangements involves Mg and Q4(4Si) silicon atoms each occupying one T1-type site and one T3-type site, and Q4(3Si) silicons occupying the remaining sites, i.e. four T2-type sites, two T1-type sites and two T3-type sites. In the second arrangement, the T2-type sites are occupied by Mg atoms and Q4(4Si) silicon atoms and all the T1-and T3-type sites are occupied by Q4(3Si) atoms.

Electron spin resonance studies of plasma-induced polystyrene radicals

Plasma-induced polystyrene radicals were first studied by electron spin resonance (ESR). The room temperature ESR spectrum was compared with those obtained by γ-irradiation, UV-irradiation, and mechanical fracture. It was found that even less than a few seconds of plasma-irradiation gave rise to a large amount of polystyrene radicals and the ESR spectrum consisted of two types of spectra, a triplet and a single broad line. The spectral feature of the triplet was nearly identical with that of γ-irradiated polystyrene. Thus, it was assigned to the structure of a cyclohexadienyl-type radical formed by a nearly random addition of a hydrogen atom to the aromatic ring. The single broad line, thought to be an outline of multicomponent spectrum, was assigned to an immobilized dangling-bond sites at the plasma-induced crosslinked portion of the polystyrene surface.

EPR of transition metals in fluoroaluminate glasses

The results of EPR study of fluoroaluminate glasses (FAG) in the AlF 3-YF 3-RF 2 system (where R = Mg, Ca, Sr, Ba) doped with small amounts of transition metals (Ti, V, Mn, Fe, Co, Cu) are presented. The EPR spectrum of Ti 3+ ions in FAG is similar to that reported for many glasses and can be described by g 1 = 1.96 and g 2 = 1.91 at 77 K. The vanadium-containing FAG exhibit EPR spectrum only when V is introduced as vanadium oxides; this spectrum belongs to VO 2+ ions and can be described by an axial spin-Hamiltonian with parameters (determined by fitting the experimental and simulated spectra): g ∥ = 1.949; g ⊥ = 1.980; A ∥ = 163 × 10 −4 cm −1; A ⊥ = 63 × 10 −4 cm −1. An X-band spectrum of Mn 2+ ions consists of a well-resolved sextet near g = 2.0 and weak background and g ≅ 4.3 resonances. The hyperfine splitting of the g ≅ 2.0 resonance measured at the Q-band is A = 98.5±0.5 Oe. Concentration dependence of the EPR spectrum permits one to assume a more uniform distribution of Mn 2+ ions in FAG than in HMFG where these ions are believed to be clustered. Iron is present in FAG predominately as Fe 2+ ions. A small portion of Fe 3+ ions gives EPR spectrum which is similar to that in oxide glasses but very different from the spectrum observed for Fe 3+ in HMFG. By contrast with fluoroberyllate glasses for which EPR signals of Co 2+ have been observed at 77 K the broad single line centered near g ∼ 4.3 is observed for Co 2+ in FAG only at 4.2 K. The EPR spectra of FAG containing copper are superpositions of the spectra of isolated Cu 2+ ions and the almost isotropic line belonging presumably to crystalline occlusions which undergo cooperative Jahn-Teller effect. The spectrum of Cu 2+ ions dissolved in FAG exhibits superhyperfine structure with a splitting of about 50 Oe from the interaction of unpaired electrons of Cu 2+ ions with 19F nuclei.

ESR study on synthetic glyceroglycolipid liposomal membranes

We previously reported that glyceroglycolipid liposomes without cholesterol activated mouse peritoneal macrophages in vivo and in vitro, whereas glyceroglycolipid liposomes containing equimolar cholesterol did not. In order to characterize the properties of the glyceroglycolipid membranes, ESR spectroscopic studies were carried out with an acyl spin-labeled galactosyl ceramide (SL-GC) or a headgroup spin-labeled phospholipid (SL-6-DPPA) in 1,2-dipalmityl[ β-cellobiosyl-(1′→ 3)]glycerol (Cel-DAG) liposomal membranes. The ESR spectrum of the SL-GC in the Cel-DAG liposomes at 37°C was a single broad line, indicating that the SL-GC molecules were excluded almost completely from Cel-DAG domains and formed clusters in the membranes. The spectrum of SL-DPPA in the Cel-DAG liposomes at 37°C showed broad resonance lines with the central peak being the highest, while that at 60°C gave narrow lines with the low-field peak being the highest. This observation and rotational correlation time analysis showed that the molecular motions of spin-label moiety of the SL-6-DPPA were extremely restricted at 37°C but not above T c. These results suggest that below T c the Cel-DAG molecules are packed tightly and restricted in motion in the membrane. Incorporation of cholesterol into the Cel-DAG liposomal membranes gave (1) the spectra of the SL-GC triplet, and (2) the spectra of the SL-6-DPPA narrow resonance with the low-field peak being the highest. These results suggest that cholesterol disturbs the rigid-packed structure of the Cel-DAG membrane and increases the molecular motions of the Cel-DAG. The DSC analysis of Cel-DAG with and without cholesterol agreed well to the results of the ESR technique. Thus we assume that peritoneal macrophages recognize the rigid-packed carbohydrate residues which are restricted in motion on the Cel-DAG membranes.

A comparative study of V 6O 13.27 and V 6O 13

A comparative study of the oxygen-enriched V 6O 13.27 and stoichiometric V 6O 13 is made from susceptibility and 51V NMR. Instead of a susceptibility peak at 150 K as in V 6O 13, a plateau was observed in this region. A single broad 51V NMR line was observed in V 6O 13.27 at room temperature, which narrowed on lowering of temperature. Close to 150 K, three different lines, corresponding to the three inequivalent sites as in V 6O 13, were observed. Though occurrence of spin-pairing at 150 K has been recognized for some time, a bipolaronic mechanism for this pairing in V 6O 13 itself is proposed for the first time. Interestingly, though susceptibilitywise V 6O 13.27 and V 6O 13 show very different behavior, bipolaronic spin-pairing possibly occurs in V 6O 13.27 also. However, at 50 K, antiferromagnetic ordering as in V 6O 13 does not occur in V 6O 13.27 as the latter has fewer V 4+ spins than required, due to the presence of excess oxygen.

Magnetic properties of Y 2Cu 2O 5

DC magnetic susceptibility of Y 2Cu 2O 5 measured between 2 K < T < 300 K and 0.5 kG < H < 50 kG, exhibits a sharp peak at 12 K. The copper ions, arranged in oxygen bridged chains, interact as a S = 1 2 Heisenberg chain with ferromagnetic intrachain exchange constant J F k B = 36 ± 4 K . The chains are weakly coupled antiferromagnetically with an interchain exchange constant of 2z AF J AF k B = -6.5 ± 1 K . The magnetization behavior ( M vs H) exhibits a change in slope at about 17 k Gauss. Its electron paramagnetic resonance signal consisted of a single broad line at both X- and Q-bands with g av = 2.108. The linewidth, Δ H pp , at X-band was 640 Gauss and temperature independent.

One-electron reduction of daunomycin, daunomycinone, and 7-deoxydaunomycinone by the xanthine/xanthine oxidase system: detection of semiquinone free radicals by electron spin resonance

High-resolution electron spin resonance spectra of semiquinones from daunomycin and its derivatives, daunomycinone and 7-deoxydaunomycinone, have been obtained during reduction by xanthine/xanthine oxidase or dithionite in partially nonaqueous media. Hyperfine coupling constants were assigned with the help of an experiment in deuterated solvent to observe the exchangeable protons and with the help of computer routines for the automatic assignment of hyperfine coupling constants. In contrast, in buffer the ESR spectrum of the daunomycin was a single broad line and that of the 7-deoxydaunomycinone exhibited axial symmetry showing strong g-factor anisotropy. The differences in the ESR spectra of the daunomycin semiquinone and its derivatives and the effects of the nonaqueous solvents ethanol and dimethyl sulfoxide are discussed. Daunomycin belongs to the widespread family of anthracycline drugs used in anticancer treatment. Because of the widespread use of daunomycin, a large number of investigations have been reported in recent years, and clinical as well as molecular aspects have been recently reviewed.'-s Since daunomycin is a quinone compound, it is a redox active molecule, one of its reaction pathways is the one-electron reduction to a semiquinone free radical. These radicals have been proposed to attack D N A site-specifically and produce strand breaks by producing reactive oxygen radicals such as the superoxide or the hydroxyl radicaL6s7 Additionally, oxygen-derived free radicals have been proposed to be responsible for the use-restricting cardiotoxicity of the drug, which has been attr ibuted to low levels of defensive enzymes against oxygen damage in the heart.* Previously, the E S R technique of spin-trapping has been employed to detect daunomycin-dependent superoxide and hydroxyl radical Since daunomycin is an anthraquinone derivative and, like most quinones, is easily reduced to its semiquinone free radical, direct E S R has been used to detect semiquinones of daunomycin and its derivative^.'^'^ Whereas the E S R spectra of enzymatically generated anthracycline semiquinones consist of a single, unresolved line,10-12 those obtained from chemical reduction of ant h r a ~ y c l i n e s ' ~ ~ ~ reveal hyperfine structure. Because of this distinct difference in E S R spectra, there exists a structural dilemma as 'National Institute for Environmental Health Sciences. Present address: Department of Chemistry, Luther College, Decorah, IA 52101. ~ ~ ~~~~~ *Present address: Laboratory of Clinical Pharmacology, National Cancer *Medical College of Wisconsin. Institute, National Institutes of Health, Bethesda, MD 20205. to the true identity of the semiquinones derived from daunomycin, especially in enzymatic systems. In an attempt to resolve this, we have now undertaken a study involving enzymatic reduction of anthracyclines in solutions of buffer and dimethyl sulfoxide or ethanol. W e have chosen the xanthine/xanthine oxidase reduction system for the following reasons: (i) xanthine oxidase has previously been shown to be involved during myocardial reduction of anthracyclines, and (ii) the xanthine oxidase was still active in the presence of up to 50% dimethyl sulfoxide. W e now report, for the first time, (1) Abdella, B. R. J.; Fisher, J . Enuiron. Health Perspecr. 198S, 64, 3-18. (2) Arcamone, F. Med. Res. Rev. 1984, 4, 153-188. (3) Aubel-Sadron, G.; Londos-Gagliardi, D. Biochimie 1984,66, 333-352. (4) Favaudon, V. Biochimie 1982, 64, 457-475. (5) Young, R. C.; Ozols, R. F.; Myers, C. E. N . Engl. J . Med. 1981,305, (6) Bates, D. A.; Winterbourn, C. C. Biochem. J . 1982, 203, 155-160. (7) Winterbourn, C. C. FEBS Lett. 1981, 136, 89-94. (8) Doroshow, J . H. Cancer Res. 1983, 43, 4543-4551. (9) Lown, J. W.; Chen, H.-H. Can. J . Chem. 1981, 59, 390-395. (IO) Gutibrrez, P. L.; Gee, M. V.; Bachur, N. R. Arch. Biochem. Biophys. (1 1) Kalyanaraman, B.; Peres-Reyez, E.; Mason, R. P. Biochim. Biophys. (12) Sinha, B . K.; Gregory, J. L. Biochem. Pharmacal. 1981, 30, (13) Kleyer, D. L.; Koch, T. H. J . Am. Chem. SOC. 1984,106,2380-2387. (14) Lown, J. W.; Chen, H.-H. Can. J . Chem. 1981, 59, 3212-3217. (15) Sinha, B. K.; Chignell, C. F. Chem.-Biol. Interact. 1979, 28, 301-308. (16) Sinha, B. K. Chem.-Biol. Interact. 1980, 30, 67-77. (17) Doroshow, J . H. Cancer Res. 1983, 43, 460-472. 139-1 53.

129I Mössbauer Spectroscopic Studies of Tetrathiafulvalene–Iodine Charge-Transfer Salts

Abstract The Mössbauer effect of 129I has been applied to investigate tetrathiafulvalene–iodine (TTF–In) charge-transfer salts at 16 K. The electrically conductive TTF–I0.71 salt shows a broad single-line with the isomer shift of −0.36 mm s−1, suggesting that the iodine atoms on the average are not completely ionic (I−1.0) but are negatively charged by about 0.9 (I−0.9) in the low temperature phase. This is due to the weak interactions of the iodine atoms with the sulfur atoms of the TTF molecules. This fact is consistent with the crystallographic data and the electron spin resonance spectra of the salt. This salt is recognized as a partially oxidized mixed-valence compound. The spectrum of the TTF–I3 salt consists of two sets of quadrupole octets, corresponding to the central and terminal iodine atoms of the slightly distorted I3− ion. The charge density localized on each iodine atom of the I3− ion is close to that of the isolated I3− ion. That is, the salt is regarded as a simple ionic compound, for which a high electrical conductivity can not be expected. It is confirmed that a small amount of the TTF–I2 salt coexists in the TTF–I0.71 phase obtained from the solution of excess TTF and iodine. In the salt the iodine atoms exist as polyiodides consisting predominantly of I3− units.

Interactions of tungsten hexachloride with esters of carboxylic acids

The interactions of tungsten chlorides and oxychlorides with methyl 10-undecenoate, methyl palmitate and methyl acetate, respectively, have been investigated by UV-VIS, IR and EPR spectroscopic methods. The primary complex formed by coordination of the carbonyl group to WCl6 is converted successively to an analogical, very stable complex WOCl4.ester under the simultaneous formation of acyl chloride and methyl chloride. In the case of methyl undecenoate, the coordination reaction is accompanied by a partial reduction of WCl6 to WCl5. The latter reacts with the ester in excess to give a complex characterized by a broad EPR single line at g = 1.767 (ΔH = 7.8 mT). At higher ester-to-tungsten molar ratios, this complex undergoes deeper reduction to products containing tungsten in a lower oxidation state, and competitively, a parallel oxygen abstraction reaction yielding WOCl3, acyl chloride and methyl chloride. The participation of these reactions in the formation of active WCl6-based catalysts for methathesis of unsaturated esters is discussed.

EPR studies of phase transitions in (NH4)3H(SO4)2single crystals

The successive phase transitions reported in triammonium hydrogen disulphate (TAHS) are investigated by EPR using Cu2+ and Mn2+ ions as probes. At room temperature, in the spontaneously strained monoclinic phase, Cu2+ and Mn2+ ions were found to be in two magnetically inequivalent sites. At the phase II (monoclinic) to (+140 degrees C) phase I (trigonal) transition the magnetic inequivalence of Cu2+ ions vanished, resulting in a broad single line at g=2.240+or-0.002 due to a possible dynamic Jahn-Teller effect that may be present for the 2E state of the Cu2+ ion in trigonal symmetry. This transition exhibited a hysteresis of about 45 degrees with the reverse transition appearing at +95 degrees C on cooling. The II to III transition resulted in the splitting of the Cu2+ resonance into two components with a temperature-dependent intensity ratio and separations. These results are consistent with the model that the Cu2+ ion occupies the NH4+ (I) position at room temperature, and takes an off-centred position in phase III with an associated electronic dipole moment, responding to the onset of antiferroelectric ordering.

A very narrow Mössbauer resonance in 63Ni

Investigations of the Mossbauer effect in nickel have been less fruitful than those on iron, because until now there has been no convenient resonance akin to the 14-keV transition (half-life 98 ns) in 57Fe. So far the only available Mossbauer resonance in nickel has been the 67-keV transition (half-life, 5ns) in 61Ni, whose natural resonance line is so broad that its width is comparable to the nuclear Zeeman splitting in ferromagnetic nickel metal. The result is a spectrum comprising a single broad line instead of a well-resolved splitting into discrete lines, as is the case for narrow resonances. We have recently succeeded in a scattering experiment in observing the Mossbauer effect in the 87-keV transition (half-life 1.7µs) in radioactive 63Ni (t1/2 = 100 yr). Because of the long lifetime of this state, its natural linewidth is smaller than that of 61Ni (×330), and the commonly used 57Fe (×17), but still larger than that of 67Zn (×0.2), which has the highest relative energy resolution of all Mossbauer resonances in use. The use of 63Ni should thus permit us in future to perform detailed Mossbauer studies in nickel.

Preparation and properties of poly(p‐phenylene) and polynaphthylene

AbstractPoly(p‐phenylene), poly(2,6‐naphthylene), poly(2,7‐naphthylene), poly(1,5‐naphthylene), and poly(1,4‐naphthylene) were prepared by nickel‐catalyzed polycondensation of Grignard reagents from the corresponding dibromides. The polymers were annealed in vacuo. Doping with SbF5 markedly increase the conductivity of the polymers. Annealed poly(2,6‐naphthylene) is the most conductive among the four isomers after SbF5 doping. The conductivity of doped poly(2,6‐naphthylene) and poly(p‐phenylene) is about 10−1 S · cm−1. Annealed poly(p‐phenylene) shows a broad single line ESR signal which changes to a narrow single line upon doping. Superposition of the two signals is observed at the initial stage of doping. Simultaneous conductivity and ESR measurements indicate that the narrow signal is closely related to conductivity.

The catalytic system [(C 5H 5) 2TiCl] 2/LiAlH 4 in aromatic solvents : I. Formation of titanocene hydride complexes; an ESR study

The reaction between (Cp 2TiCl) 2 and LiAlH 4 in aromatic solvents yields a stable complex compound which probably consists of linear, polynuclear form of Cp 2TiAlH 4 bearing Cp 2TiH side groups. It is characterized by a broad ESR single line Δ H 3.5.–4.5 mT at g = 1.990–1.991. The binuclear complexes Cp 2TiH 2AlCl 2 (I), Cp 2TiH 2Al(Cl)H (II) and Cp 2TiH 2AlH 2 (III) are successive formed as minor components. The ESR spectrum of II, consisting of 13 lines (α A1 0.78 mT, a H(bridge) 0.39 mT), and that of a new form of III (IIIb, a A1 0.36 mT, a H(bridge) 0.40 mT) were observed for the first time. Complexes, I II and IIIb show a superhyperfine splitting at low temperatures. Changes of the multiplicity of this splitting revealed that a H(outer) is close to that of cyclopentadienyl-hydrogen atoms 0.056–0.058 mT. All these hydrides have an electronic absorption band ( dd) at λ max 550 nm, ε ∼ 130 (1 cm mol −1).

Incorporation of a ganglioside and spin-labeled ganglioside analogue into cell and liposomal membranes.

When an aqueous solution of a spin-labeled "two tail" gangliosidoid was incubated with liposomes or sheep erythrocytes, the broad single resonance line in the ESR spectrum disappeared and a signal showing an anisotropic motion appeared, indicating that the spin-labeled "two tail" gangliosidoid in the micellar state was transferred to the lipid phase of the acceptor membranes. The transfer was temperature- and time-dependent, irrespective of the acceptor membranes, indicating that the rate of transfer is determined by the escape of monomers from the micelles. The kinetics and temperature-dependence of the association of ganglioside II3NeuAc-GgOse4Cer with sheep erythrocytes was very similar to that of the "two tail" gangliosidoid, indicating that parts of ganglioside II3NeuAc-GgOse4Cer could be incorporated into the lipid phase of membranes via a similar mechanism.

Magnetic resonance in a cubic Γ 3 (E) doublet system III. Electron spin resonance of Ho 3+ , Gd 3+ , Dy 3+ , Er 3+ and Yb 3+ in Cs 2 NaHoCl 6

In Cs 2 NaHoCl 6 the Ho 3+ ion occupies a site of cubic symmetry, and the ground state produced by the crystal field splitting of the 4f 10 , 5 I 8 manifold is a ‘non-magnetic’ Γ 3 doublet. When a field B is applied, the doublet is split by the quadratic Zeeman effect, and transitions between the two levels of the doublet become allowed at a frequency and with an intensity each proportional to B 2. At a frequency of 9.6 GHz the e. s. r. spectrum of the Ho 3+ ion shows a single broad line, with the unusual anisotropic behaviour predicted for a Γ 3 doublet in cubic symmetry (see paper I). No hyperfine splitting is resolved, and the linewidth parameter ∆ v is of order 10 GHz, independent of orientation. The centre of the line occurs at a field consistent with that calculated from n. m. r. measurements (paper II), after a correction is applied for the anomalous line shape. The e. s. r. spectra of Gd 3+ , Dy 3+ , Er 3+ , Yb 3+ , present as impurities, also show cubic symmetry, with spin Hamiltonian parameters close to those observed in analogous compounds such as Cs 2 NaYCl 6 .

Response of the Mössbauer spectrum of paramagneticFe3+inAl2O3to nuclear dipole fields

Experimental $^{57}\mathrm{Fe}$ M\"ossbauer (ME) spectra of ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$(${\mathrm{Fe}}^{3+}$) single crystals are presented. Measurements were made down to 60 mK and in applied magnetic fields from 0 to 100 G At 60 mK the ME spectra of the ${S}_{z}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}$ ionic ground state was isolated, and its response to weak (less than 100 G) applied magnetic fields was examined. A qualitative discussion of the anisotropic response to magnetic fields is given. In zero-applied magnetic field the spectrum is strongly influenced by nuclear dipole fields from surrounding $^{27}\mathrm{Al}$ nuclei. From a theoretical reconstruction of the zero-applied-field spectrum the average magnitude of the $^{27}\mathrm{Al}$ nuclear dipole field is determined to be 4.5 \ifmmode\pm\else\textpm\fi{} 1.5 G. Good agreement is obtained with a theoretical calculation which involves the identical lattice sum which occurs in the Van Vleck formula for the second moment of the nuclear dipole broadening of an NMR line. The connection between the dipole field observed in the ME experiment and the second-moment calculation of the broadening of an NMR line is discussed. Also considered are possible relaxation mechanisms. The spin-lattice relaxation rates are estimated using experimental data for the lattice thermal conductivity and application of the fluctuation-dissipation theorem to the reversible heat flow. It is concluded that at 60 mK relaxation effects are not observed in the ME spectra and also that broadening due to ${\mathrm{Fe}}^{3+}$-${\mathrm{Fe}}^{3+}$ interaction is negligible compared to the nuclear dipole broadening, except for a broad single-line contribution in the center of the spectrum.