Measured Quadrupole Splittings Research Articles

Ab-initio, Mößbauer spectroscopy, and magnetic study of the approximant Al72Ni9Fe19 to a decagonal Al-Ni-Fe quasicrystal

We report the results of ab-initio electronic structure and electric field gradient calculations and of X-ray diffraction, 57Fe Mößbauer spectroscopy, and magnetic studies of the approximant Al72Ni9Fe19 to a decagonal Al-Ni-Fe quasicrystal. The approximant crystallizes in the hexagonal space group P63/mmc with the lattice parameters a = 7.6989(2) Å and c = 7.6724(2) Å. A pseudogap in the density of states, centered at ∼0.30 eV above the Fermi level and with a width of ∼0.35 eV, is found. Evidence for the covalent nature of the chemical bonding and good metallicity of Al72Ni9Fe19 is provided. The Mößbauer spectra and a 1/T-like dependence of the magnetic susceptibility indicate that Al72Ni9Fe19 is a paramagnet down to 2.0 K. The shape of the Mößbauer spectra is accounted for by a superposition of two quadrupole doublets corresponding to Fe atoms located at two inequivalent crystallographic sites. A sudden change of the quadrupole splittings of the two doublets at ∼200 K indicates a possible structural phase transition. Excellent agreement between the calculated and measured quadrupole splittings is observed. The Debye temperature of Al72Ni9Fe19 is found to be 431 (3) K.

Mössbauer magnetization and nuclear magnetic resonance measurements on some iridium(I) complexes with fullerene ligands

The isomer shifts of the measured Mossbauer spectra are in accordance with quantum chemical calculations and both techniques showed that the studied iridium(I) complexes have a strong covalent character, namely, there are a direct σ donation of the ligands into the 6s orbital of Ir. The coordination of further π-acceptor ligands decreases the population of 6s orbital. The obtained quadrupole splittings detect significant difference among the electric field gradients at the nucleus of iridium in the discussed coordination compounds. In this paper we present the first attempt to obtain Mossbauer parameters of 193Ir by DFT calculations. The change in the measured quadrupole splittings due to the coordiantion of tetracyanoethylene to central iridium was excellently reproduced by theoretical calculation using ZORA scheme. The magnetization measurements proved that IrI has diamagnetic, singlet electronic structure in all the studied compounds. This finding was in accordance with density functional calculations as well. Nuclear magnetic resonance investigations on 31P nucleus could detect the decrease of the electric field gradient at the nucleus of 193Ir in IrCl(CO)(PPh3)2 due to the coordination of C60 to the central iridium atom.

Electronic structure of Fe-bearing lazulites

The Fe end-members scorzalite [Fe2+Al23+(PO4)(2)(OH)(2)] and barbosalite [Fe2+Fe23+(PO4)(2)(OH)(2)] of the lazulite series have been investigated by Mossbauer and diffuse reflectance spectroscopy, and by electronic structure calculations in the local spin density approximation. The measured quadrupole splitting (DeltaE(Q) = -3.99 mm/s) in scorzalite is in quantitative agreement with the calculated value (DeltaE(Q) = -3.90 mm/s), as well as its temperature dependence. The optical spectrum of barbosalite can be resolved into three peaks at 8985 cm(-1), 10980 cm(-1), and 14110 cm-1. These positions correlate well with the two calculated spin-allowed d-d transitions at 8824 cm-1 and 11477 cm-1, and with an intervalence charge transfer transition at about 14200 cm-1. The calculated low-temperature magnetic structure of barbosalite is characterized by a strong antiferromagnetic coupling (J = -84.6 cm-1) within the octahedral Fe3+-chains, whereas a weak antiferromagnetic coupling within the trioctabedral subunit cannot be considered as conclusive. The analysis of the charge and spin densities reveals that more than 90% of the covalent part of the iron-ligand bonds arises from the Fe(4s,4p)-electrons. Clusters of at least 95 atoms are required to reproduce the available experimental data with quantitative accuracy.

Electronic Structure, Porphyrin Core Distortion, and Fluxional Behavior of Bis-Ligated Low-Spin Iron(II) Porphyrinates

Three axially bis-ligated Fe(II) porphyrinates, [PFeL2], have been investigated by Mossbauer spectroscopy over the temperature range 4.2−230 K and in an applied field of 6 T. Relatively large differences in the quadrupole splitting ΔEQ were found among [TMPFe(2-MeImH)2] (1), [TMPFe(N-MeIm)2] (2), and [OEPFe(PMe3)2] (3) (1.61, 1.07, and 0.38 mm/s, respectively). Compound 3 exhibits significant line broadening above 150 K that indicates fluxional distortion and/or ligand rotation. Molecular orbital calculations in the local density approximation yield electric field gradients (efg) in good agreement with the measured quadrupole splittings, ΔEQ, and the measured sign of the efg. The observed differences in quadrupole splittings can be ascribed to distortions (ruffling) of the porphyrin core in 1 as compared to 2 and 3 and to the differences in covalent interactions of the axial nitrogen donors of 1 and 2 and phosphorous donors of 3. The observed temperature-dependent line broadening of 3 correlates with the ...

Optically-detected nuclear quadrupole resonance of Eu 3+ in LaF 3

We report optically-detected nuclear quadrupole resonance (ODNQR) of Eu 3+ in LaF 3. The anomalous nuclear quadrupole moments of 151Eu 3+ and 153Eu 3+ isotopes have been measured by detecting r.f.-induced quadrupole resonance which enhances the 7F 0 ↔ 5D 0 optical transition. Due to significant second-order hyperfine cancellation, the lowest quadrupole splitting in the ground state is only 6.95 MHz. In analysis of the ODNQR spectrum, an effective nuclear quadrupole Hamiltonian is diagonalized and the measured quadrupole splittings are used to determine the quadrupole interaction parameters for the ground state 7F 0. For 151Eu, P 151 = 2.67 and η 151 = 0.58, and for 153Eu, P 151 = 6.80 and η 151 = 0.49.

A second order phase transition in NaNO 2 observed by 23Na NMR

The spin-lattice relaxation time (T 1) and quadropole splittings for 23Na in NaNO 2 have been investigated in the range of 140 – 400 K as a func̀tion of temperature with a pulse NMR spectrometer operating at 4.7 T. The temperature dependence of T 1 was found to have an abnormal peak at 178 K. The measured quadrupole splittings as a function of temperature also showed a slight discontinuity at this temperature. These results confirm the existence of the second order phase transition at 178 K in NaNO 2, previously reported by means of measuring the thermal expansion and the dielectric constant.

Membrane surface orientation and motion of glycolipids: 2H NMR investigations

Abstract

185Re and 187Re solid‐state NMR of metaperrhenates

Abstract127I solid‐state NMR investigations of specific metaperiodates with alkyl‐ or aryl‐substituted cations of main Group V revealed second‐order quadrupole effects, except in the case of tetraethylammonium and tetramethylphosphonium metaperiodate. In these compounds the 127I quadrupole interactions are either very small or vanish. This allowed the approximate evaluation of the electric field gradients of the analogous isomorphous metaperrhenates. Despite the very large nuclear quadrupole moments of the rhenium isotopes, the estimated values of the 185Re and 187Re quadrupole coupling constants of tetraethylammonium and tetramethylphosphonium metaperrhenate have an order of magnitude accessible to solid‐state NMR measurements. The 185Re and 187Re NMR powder spectra of rhenium compounds in non‐cubic lattice structures were measured for the first time. The complete 185Re and 187Re solid‐state NMR spectra of polycrystalline tetramethylphosphonium metaperrhenate were observed, showing temperature‐dependent second‐order quadrupole splittings. In this compound the ReO anions show very small deviations from tetrahedral symmetry, which were estimated from a point charge model and the measured quadrupole splittings. In contrast, the tetrahedral symmetry of the ReO ions of tetraethylammonium metaperrhenate is nearly undistorted, because no quadrupole splittings of the 185Re and 187Re signals were detectable in the powder NMR spectrum.

Protein-lipid interactions at membrane surfaces: a deuterium and phosphorus nuclear magnetic resonance study of the interaction between bovine rhodopsin and the bilayer head groups of dimyristoylphosphatidylcholine.

Rhodopsin, isolated from bovine retinal rod outer segment disk membranes, has been reconstituted into bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine which was deuterated in the terminal methyl groups of the choline polar head group. By use of a mixed detergent system of cholate and octyl glucoside to solubilize the phospholipid and rhodopsin, 15 membrane complexes of predetermined phospholipid to rhodopsin mole ratios of between 350:1 and 65:1 have been produced by exhaustive dialysis and studied by a variety of techniques. Electron micrographs of replicas from freeze-fractured membrane complexes showed that the majority of the lipid, for all rhodopsin:phospholipid ratios, was contained in large bilayer vesicles with diameters in excess of 400 nm. Complexes produced with rhodopsin from frozen retina produced an absorption maximum at 478 nm after photobleaching whereas rhodopsin from fresh retina could be bleached more completely to an absorption maximum at 380 nm. Deuterium nuclear magnetic resonance (NMR) spectra from the lipid head groups of bilayers above the gel to liquid-crystalline phase transition temperature were shown to be sensitive in a systematic way to the presence of rhodopsin which could be bleached to 380 nm. The measured quadrupole splittings, taken as the separation of the turning points of the recorded NMR spectra, decreased from a value of 1.28 kHz for protein-free bilayers to approximately 0.40 kHz for bilayers containing 65 molecules of phospholipid for each rhodopsin at 32 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Mössbauer spectroscopic studies of hemoglobin and its isolated subunits

Samples of 90% enriched 57Fe hemoglobin and its isolated subunits have been prepared. Mössbauer spectroscopic measurements have been made on three such samples. Sample one contained contributions of oxyhemoglobin, deoxyhemoglobin, and carbonmonoxyhemoglobin. This sample was studied from a temperature of 90 K down to 230 mK. Measurements were also made at 4.2 K using a small applied magnetic field of 1.0 T. In general, the measured quadrupole splittings and isomer shifts for each component agreed with previous measurements on single component samples in the literature, and thus demonstrated that chemically enriched hemoglobin has not been altered. The second and third samples were isolated alpha and beta subunits, respectively. We have found measurable Mössbauer spectral differences between the HbO2 sites in the alpha subunit sample and the beta subunit sample. The measured Mössbauer spectral areas indicate that the iron ion has the largest mean-square displacement at the deoxy Hb sites as compared to that at the oxy- and carbonmonoxy Hb sites. The mean-square displacement at the HbO2 sites is the smallest.

Evidence from deuterium nuclear magnetic resonance for the temperature-dependent reversible self-association of erythrocyte band 3 in dimyristoylphosphatidylcholine bilayers.

Band 3, isolated from human erythrocytes, has been reconstituted into bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) deuterated in the terminal methyl groups of the choline head group. By use of Triton X-100 for selective extraction and purification of band 3 and then cholate for subsequent solubilization with the lipid, a number of reconstituted complexes were produced by exhaustive detergent dialysis with protein:lipid weight ratios of between 0.32:1 and 1.25:1. Electron micrographs of negatively stained complexes showed that this method produced large vesicles of greater than 300-nm diameter. Deuterium nuclear magnetic resonance (NMR) spectra from the choline methyl deuterons in bilayer lipid above the liquid-crystal-gel phase transition temperature were shown to change systematically with increasing concentrations of band 3 in the bilayers. The measured quadrupole splittings, taken as the separation of the turning points in the recorded spectra, decreased from a value of 1.28 kHz for pure lipid to 0.98 kHz for bilayers with a protein:lipid ratio of 1.25:1 at 26 degrees C. At 35 degrees C, a more pronounced decrease in the quadrupole splittings was measured. The data from the complexes with protein:lipid ratios up to 0.7:1 (w/w) obey the mathematical treatment for a rapid two-site exchange between lipids at the protein-lipid interface and the bulk lipid phase. The temperature dependence of the measured quadrupole splitting with respect to the protein:lipid ratio indicates that the amount of lipid at the protein-lipid interface increases with increasing temperature.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective protein-lipid interactions at membrane surfaces: a deuterium and phosphorus nuclear magnetic resonance study of the association of myelin basic protein with the bilayer head groups of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol

Basic protein from bovine spinal cord was recon- stituted into bilayers of 1,2-dimyristoyl-sn-glycero-3- phosphocholine, 1,2-dimyristoyl-sn-glycero-3-phospho-ruc- glycerol, and equimolar mixtures of both phospholipids at various protein concentrations up to 58 wt %. The lipids were selectively deuterated at all positions in their polar head groups, the motional rate and amplitude of which were characterized by deuterium and phosphorus-3 1 nuclear magnetic resonance (NMR). Basic protein, which binds peripherally to mem- branes, did not perturb any part of the head groups in phos- phatidylcholine bilayers in such a way as to induce any sig- nificant changes in the NMR spectra. However, basic protein induced large effects on the phosphorus-3 1 and the deuterium NMR spectra of bilayers of dimyristoylphosphatidylglycerol. When compared to those of pure phosphatidylglycerol bilayers at 35 OC, the measured quadrupole splittings decreased by 36%, 28%, and 50% for the a-CD2, P-CD, and 7-CD2 glycerol segments, respectively, and the phosphorus-3 1 chemical shift anisotropy (CSA) was reduced by 34% from -39 to -25 ppm by the addition of 50 wt 7% of basic protein. Deuterium spin-lattice relaxation times T1 for the p-CD and the 7-CD2 segments decreased only slightly after the addition of basic protein. These results indicate that basic protein interacts with the acidic polar head group of phosphatidylglycerol, affecting its orientation or amplitude of motion, as well as its rate of Investigations of lipid-protein interactions in reconstituted model membranes by NMR' or ESR spin-label methods have concentrated predominantly on the characterization of the motion of lipid fatty acyl chains with the hydrophobic surface of integral membrane proteins (Marsh & Watts, 1982; Rice et al., 1979; Oldfield et al., 1978; Seelig et al., 1981). Both magnetic resonance approaches are restricted to giving localized molecular information about the environment close to the site of probe attachment, and 2H NMR spectroscopy is in principle the more versatile approach in that deuterium can substitute for protons at many specific positions in a phospholipid to give information about almost any part of the molecule. Although both methods have their own virtues and strengths in their applicability and motional time scales (Watts, 1981), 2H NMR of specifically deuterated lipids is particularly

On the ground state of Ho3+ in ferromagnetic HoAl2

The ground state of the holmium ion in ferromagnetic HoAl2 is discussed in the light of available experimental evidence, including hitherto unpublished NMR measurements on165Ho. The measured quadrupole splitting is not consistent with the ground state derived from neutron form factor experiments, but is compatible with exchange and crystal field parameters derived from magnetization measurements and neutron spectroscopy in the context of the conventional three-parameter mean-field model. A more detailed analysis of the NMR data indicates that the exchange interaction in HoAl2 is over 20% stronger than that derived, using mean-field theory, from the Curie temperature. Using the revised exchange constant and a weighted average of published crystal field parameters, we obtainμ = (9.39 ± 0.05)μ B for the moment on the Ho3+ ion atT =0. The contribution of orbitally polarized conduction electrons to the hyperfine field at the holmium nucleus is estimated to be (−1.4±2.0) T.

Headgroup interactions in mixed phospholipid bilayers.

(2)H NMR methods have been used to study how bilayer-forming phospholipids interact with each other at the membrane surface. Aqueous dispersions of dimyristoyl-sn-phosphatidylcholine (Myr(2)-PtdCho), dimyristoyl-sn-phosphatidylethanolamine (Myr(2)-PtdEtn), and dimyristoyl-sn-phosphatidyl-3-glycerol, specifically deuterated at different positions in their headgroups, give well-resolved (2)H NMR powder spectra. These spectra are characteristic of a lipid bilayer with quadrupole splittings that are sensitive to the amplitude of headgroup motion. In binary mixed bilayers of deuterated lipids with an unlabeled component, all parts of the deuterated headgroup monitor the presence of the second lipid from changes in the measured quadrupole splittings. The headgroups of the charged lipids, dimyristoyl-sn-phosphatidylserine and dimyristoyl-sn-phosphatidyl-3-glycerol, interact to the largest extent with the choline moiety of Myr(2)-PtdCho and the ethanolamine moiety of Myr(2)-PtdEtn, whereas a somewhat smaller but still marked alteration in headgroup motion was observed in Myr(2)-PtdCho/Myr(2)-PtdEtn mixtures. The large changes in the deuterium quadrupole splittings for the zwitterionic lipids after addition of a charged lipid suggest that either a strong perturbation in the hydrogen bonding occurs or changes take place in the water structure at the membrane surface, or possibly both.

Hyperfine and transferred hyperfine interaction of Cr3+ in yal and lual garnets

AbstractThe hyperfine field at the 53Cr nucleus and the transferred hyperfine field at the Al3+ neighbours at the next octahedra sites were measured with ENDOR at Cr3+‐doped YAlG and LuAlG single crystals. The electric field gradient at the octahedrally surrounded lattice sites could be determined by the quadrupole interaction with the 27Al nucleus. From the measured quadrupole splitting of 53C13+ a new value of the 53Cr quadrupole moment of |Q| =0.082 b is derived. The magnetic part of the transferred hyperfine interaction between the Cr3+ ion and the A1 nuclei at the next octahedra sites could be well described assuming a point‐dipole model.

Study of Iron Ions in NaCl Using the Mössbauer Effect. I. The CoCl2Precipitated State

Single crystals of Co${\mathrm{Cl}}_{2}$ containing tracer ${\mathrm{Co}}^{57}$ were grown by precipitation within NaCl at low temperatures. The observed M\"ossbauer spectrum for ${\mathrm{Fe}}^{57}$ indicates an isomer shift of 1.32\ifmmode\pm\else\textpm\fi{}0.04 mm/sec above stainless steel and a quadrupole splitting of 1.14\ifmmode\pm\else\textpm\fi{}0.06 mm/sec at 80\ifmmode^\circ\else\textdegree\fi{}K, which is significantly less than has been observed in ionic-${\mathrm{Fe}}^{2+}$ compounds. The small splitting can be largely explained by assuming a doubly degenerate ground state with the appropriate wave functions. The measured quadrupole splitting is reduced about 20% at room temperature which can be explained by arguments analogous to those used to interpret the temperature dependence of the splittings in iron compounds. The observed angular distribution of the intensities of the two lines demonstrates that the precipitated crystals near the surface are partially aligned in their spatial orientation. The data indicate only one charge state (${\mathrm{Fe}}^{2+}$), showing that higher charge states resulting from the emission of Auger electrons reach equilibrium in less than ${10}^{\ensuremath{-}7}$ sec.