Asymmetry Parameter Eta Research Articles

Ab initio based description of the unusual increase of the electric field gradient with temperature at Ti sites in rutile TiO2

Combining a precise ab initio electron band structure calculation of the TiO2 rutile structure with the temperature evolution of the Ti mean-square displacements, we reproduce a puzzling temperature increase of the electric field gradient at Ti sites in TiO2, observed experimentally. Our method employs a procedure of averaging two quadrupole electron density components (L = 2) inside a sphere vibrating with the Ti nucleus at its center, where the key factor introducing the temperature dependence is the square root of the Debye-Waller factor. Although the Debye-Waller factor always reduces the corresponding Fourier component, in TiO2 due to the interplay between terms of opposite signs, it results in a net increase of the whole sum with temperature, leading to the growth of the electric field gradient. Quantitatively, we find that the increase of electric field gradient is only half of the experimental value, which we ascribe to anharmonic effects or a strong oxygen position influence. In addition, our method reproduces the unusual temperature dependence of the asymmetry parameter eta, which first decreases with temperature, goes to zero and then increases.

The quadrupole enhanced 1H spin–lattice relaxation of the amide proton in slow tumbling proteins

An analysis, based on the stochastic Liouville approach, is presented of the R(1)-NMRD or field dependent spin-lattice relaxation rate of amide protons. The proton relaxivity, displayed as R(1)-NMRD profiles, is calculated for a reorienting (1)H-(14)N spin group, where the inter spin coupling is due to spin dipole-dipole coupling or the scalar coupling. The quadrupole nucleus (14)N has an asymmetry parameter eta = 0.4 and a quadrupole interaction which is modulated by the overall reorientational motion of the protein. In the very slow reorientational regime, omega(Q)tau(R) >> 1 and tau(R) > or = 2.0 micros, both the dipole-dipole coupling and the scalar coupling yield a T(1)-NMRD profile with three marked peaks of proton spin relaxation enhancement. These peaks appear when the proton Larmor frequency, omega(I), matches the nuclear quadrupole spin transition frequencies: omega(1) = omega(Q)2eta/3, omega(2) = omega(Q)(1-eta/3) and omega(3) = omega(Q)(1 + eta/3), and the quadrupole spin system thus acts as a relaxation sink. The relative relaxation enhancements of the peaks are different for the dipole-dipole and the scalar coupling. Considering the dipole-dipole coupling, the low frequency peak, omega(1), is small compared to the high field peaks whereas for the scalar coupling the situation is changed. For slow tumbling proteins with a correlation time of tau(R) = 400 ns, omega(2) and omega(3) are not resolved but become one relatively broad peak. At even faster reorientation, tau(R) < 60 ns, the marked peaks disappear. In this motional regime, the main effect of the cross relaxation phenomenon is a subtle perturbation of the main amide proton T(1) NMRD dispersion. The low field part of it can be approximately described by a Lorentzian function: R(DD,SC)(0.01)/(1 + (omega(I)tau(R)3/2)(2)) whereas the high field part coincides with R(DD,SC)(0.01)/(1 + (omega(I)tau(R))(2)).

1H17O nuclear quadrupole double resonance in phenylphosphinic acid and phenylphosphonic acid. 17O quadrupole coupling in P = O and POH bonds

The (17)O nuclear quadrupole resonance (NQR) frequencies have been measured in phenylphosphinic acid and phenylphosphonic acid using nuclear quadrupole double resonance. The quadrupole coupling constants have been determined with an uncertainty of +/- 10 kHz and the asymmetry parameter eta with an uncertainty of +/- 0.01. The results are compared to the published results of the theoretical calculation and the high-field solid-state NMR measurements. The position of hydrogen in the O-H...O hydrogen bond in phenylphosphinic acid has also been determined. On the basis of the present and the previously published data we show that the principal values of the electric-field-gradient tensor in P = O and P-O-H bonds correlate. A correlation between the nuclear quadrupole parameters and the length of the P-O bond is also observed.

Indirect Detection of Nitrogen‐14 in Solid‐State NMR Spectroscopy

NMR spectra of (14)N (spin I=1) are obtained by indirect detection in powders spinning at the magic angle. The method relies on the transfer of coherence from a neighboring "spy" nucleus with S=1/2, such as (13)C or (1)H, to single- or double-quantum transitions of (14)N nuclei. The transfer of coherence can occur through a combination of scalar and residual dipolar splittings (RDS); the latter are also known as second-order quadrupole-dipole cross terms. The two-dimensional NMR spectra reveal powder patterns determined by second- and third-order quadrupolar couplings. These spectra depend on the quadrupolar coupling constant C(Q) (typically a few megahertz), on the asymmetry parameter eta(Q) of the (14)N nucleus, and on the orientation of the internuclear vector r(IS) between the I ((14)N) and S (spy) nuclei with respect to the quadrupolar tensor. These parameters, which can be subject to motional averaging, can reveal valuable information about the structure and dynamics of nitrogen-containing solids. Application of this technique to various amino acids, either enriched in (13)C or with natural carbon isotope abundance, with spectra recorded at various magnetic fields, illustrates the scope of the method.

Deuterium Quadrupolar Tensors of l-Histidine Hydrochloride Monohydrate-d7

A deuterium NMR study at 14 T of a single crystal of L-histidine hydrochloride monohydrate has determined the deuteron quadrupole coupling constants CQ, asymmetry parameters eta, and electric field gradient orientation for the imidazolium and primary ammonium groups and for a water of crystallization. The imidazolium deuterons, which have very long relaxation times, have quite different coupling constants, reflecting different hydrogen bonding but nearly identical orientations, with the most distinct principal axis in both cases nearly parallel to the N-D vector. The -ND3 groups undergo 3-fold hops about the C-N bond axis and have typical quadrupole couplings; the D2O undergoes 2-fold hops, leading to a tensor with a large asymmetry parameter. With appropriate corrections for vibrational averaging, density functional cluster calculations give an excellent fit to the imidazolium tensor magnitudes and orientations.

Charge-inhomogeneity doping relations inYBa2Cu3Oydetected by angle-dependent nuclear quadrupole resonance

The origin of charge inhomogeneity in YBa_{2}Cu_{3}O_{y} is investigated using a new experimental method designed to determine the nuclear quadrupole resonance (NQR) asymmetry parameter eta for very wide NQR lines at different positions on the line. The method is based on the measurement of the echo intensity as a function of the angle between the radio frequency field H1 and the principal axis of the electric field gradient. Static charge inhomogeneity deduced from eta>0 are found in this compound, but only in conjunction with oxygen deficiency. This limits considerably the possible forms of charge inhomogeneity in bulk YBa_{2}Cu_{3}O_{y}.

Mössbauer characterization of an unusual high-spin side-on peroxo-Fe3+ species in the active site of superoxide reductase from Desulfoarculus Baarsii. Density functional calculations on related models.

Superoxide reductase (SOR) is an Fe protein that catalyzes the reduction of superoxide to give H(2)O(2). Recently, the mutation of the Glu47 residue into alanine (E47A) in the active site of SOR from Desulfoarculus baarsii has allowed the stabilization of an iron-peroxo species when quickly reacted with H(2)O(2) [Mathé et al. (2002) J. Am. Chem. Soc. 124, 4966-4967]. To further investigate this non-heme peroxo-iron species, we have carried out a Mössbauer study of the (57)Fe-enriched E47A SOR from D. baarsii reacted quickly with H(2)O(2). Considering the Mössbauer data, we conclude, in conjunction with the other spectroscopic data available and with the results of density functional calculations on related models, that this species corresponds to a high-spin side-on peroxo-Fe(3+) complex. This is one of the first examples of such a species in a biological system for which Mössbauer parameters are now available: delta(/Fe) = 0.54 (1) mm/s, DeltaE(Q) = -0.80 (5) mm/s, and the asymmetry parameter eta = 0.60 (5) mm/s. The Mössbauer and spin Hamiltonian parameters have been evaluated on a model from the side-on peroxo complex (model 2) issued from the oxidized iron center in SOR from Pyrococcus furiosus, for which structural data are available in the literature [Yeh et al. (2000) Biochemistry 39, 2499-2508]. For comparison, similar calculations have been carried out on a model derived from 2 (model 3), where the [CH(3)-S](1)(-) group has been replaced by the neutral [NH(3)](0) group [Neese and Solomon (1998) J. Am. Chem. Soc. 120, 12829-12848]. Both models 2 and 3 contain a formally high-spin Fe(3+) ion (i.e., with empty minority spin orbitals). We found, however, a significant fraction ( approximately 0.6 for 2, approximately 0.8 for 3) of spin (equivalently charge) spread over two occupied (minority spin) orbitals. The quadrupole splitting value for 2 is found to be negative and matches quite well the experimental value. The computed quadrupole tensors are rhombic in the case of 2 and axial in the case of 3. This difference originates directly from the presence of the thiolate ligand in 2. A correlation between experimental isomer shifts for Fe(3+) mononuclear complexes with computed electron densities at the iron nucleus has been built and used to evaluate the isomer shift values for 2 and 3 (0.56 and 0.63 mm/s, respectively). A significant increase of isomer shift value is found upon going from a methylthiolate to a nitrogen ligand for the Fe(3+) ion, consistent with covalency effects due to the presence of the axial thiolate ligand. Considering that the isomer shift value for 3 is likely to be in the 0.61-0.65 mm/s range [Horner et al. (2002) Eur. J. Inorg. Chem., 3278-3283], the isomer shift value for a high-spin eta(2)-O(2) Fe(3+) complex with an axial thiolate group can be estimated to be in the 0.54-0.58 mm/s range. The occurrence of a side-on peroxo intermediate in SOR is discussed in relation to the recent data published for a side-on peroxo-Fe(3+) species in another biological system [Karlsson et al. (2003) Science 299, 1039-1042].

Weakly coordinating anions: crystallographic and NQR studies of halogen-metal bonding in silver, thallium, sodium, and potassium halomethanesulfonates.

35Cl, (79,81)Br, and (127)I NQR (nuclear quadrupole resonance) spectroscopy in conjunction with X-ray crystallography is potentially one of the best ways of characterizing secondary bonding of metal cations such as Ag(+) to halogen donor atoms on the surfaces of very weakly coordinating anions. We have determined the X-ray crystal structure of Ag(O(3)SCH(2)Cl) (a = 13.241(3) A; b = 7.544(2) A; c = 4.925(2) A; orthorhombic; space group Pnma; Z = 4) and compared it with the known structure of Ag(O(3)SCH(2)Br) (Charbonnier, F.; Faure, R.; Loiseleur, H. Acta Crystallogr., Sect. B 1978, 34, 3598-3601). The halogen atom in each is apical (three-coordinate), being weakly coordinated to two silver ions. (127)I NQR studies on Ag(O(3)SCH(2)I) show the expected NQR consequences of three-coordination of iodine: substantially reduced NQR frequencies nu(1) and nu(2) and a fairly small NQR asymmetry parameter eta. The reduction of the halogen NQR frequency of the coordinating halogen atom in Ag(O(3)SCH(2)X) becomes more substantial in the series X = Cl < Br < I, indicating that the coordination to Ag(+) strengthens in this series, as expected from hard-soft acid-base principles. The numbers of electrons donated by the organic iodine atom to Ag(+) have been estimated; these indicate that the bonding to the cation is weak but not insignificant. We have not found any evidence for the bonding of these organohalogen atoms to another soft-acid metal ion, thallium. A scheme for recycling of thallium halide wastes is included.

Optical and TDPAC spectroscopy of Hg(II)-rubredoxin: model for a mononuclear tetrahedral [Hg(CysS)4]2- center. ISOLDE Collaboration.

Rubredoxins possess a well-defined mononuclear tetrahedral tetrathiolate metal binding site, a feature exploited by several investigations to study the spectroscopic characteristics and the coordination chemistry of different metal ions at this binding site. In the present work, Hg(II)-substituted rubredoxin (Rd) from Desulfovibrio gigas has been studied by electronic absorption, circular dichroism (CD), magnetic circular dichroism (MCD), and time differential perturbed angular correlation of gamma-rays (TDPAC) spectroscopies. The TDPAC spectrum of 199mHg-Rd at pH 8 exhibits a prevailing nuclear quadrupole interaction (NQI) with a precession frequency of omega1=0.09 Grad/s and an asymmetry parameter eta=0, features characteristic of a slightly distorted tetrahedral tetrathiolate metal coordination, i.e, a HgCysS4 center. In addition, three minor populated NQIs have also been detected. They may represent a trigonal HgS3 (omega1=1.13 Grad/s, eta=0.21), a digonal HgS2 (omega1= 1.34 Grad/s, eta =0.20), and a digonal Hg(II) coordination (omega = 1.58 Grad/s, eta =0.18) with unidentified ligands. Since similar studies at pH 2.5 revealed a time-dependent increase of the HgCysS4 population, the low populated sites may represent intermediate Hg(II) complexes formed prior to the generation of the thermodynamically stable structure. The metal-induced absorption envelope of Hg-Rd reveals three distinct transitions with Gaussian-resolved maxima located at 230, 257, and 284 nm, which are paralleled by dichroic features in the corresponding difference CD spectrum of Hg(II)-Rd versus apo-Rd. Based on the optical electronegativity theory of J*rgensen, the lowest energy transition has been attributed to a CysS-Hg(II) charge-transfer excitation. The Td type of metal coordination in Hg-Rd is supported by the presence of an unresolved A-term with a negative lobe at 295 nm in the difference MCD spectrum. These results point to the usefulness of optical and TDPAC spectroscopies for studying Hg(II) sites in other proteins.

Crystallographic, Electrochemical, and Pulsed EPR Study of Copper(II) Polyimidazole Complexes Relevant to the Metal Sites of Copper Proteins(,).

Copper(II) complexes of the following polyimidazole ligands have been synthesized: bis(imidazol-2-yl)methane (BIM), bis(imidazol-2-yl) ketone (BIK), 4-(imidazol-4-ylmethyl)-2-(imidazol-2-ylmethyl)imidazole (TRIM), bis[4-(imidazol-4-ylmethyl)imidazol-2-yl]methane (TIM), and bis[4-(imidazol-4-ylmethyl)imidazol-2-yl] ketone (TIK). Their crystal structures have been determined using X-ray diffraction. [Cu(ClO(4))(2)(BIM)(2)], 1, belongs to the triclinic space group P&onemacr; system, a = 7.161(4) Å, b = 7.986(6) Å, c = 9.865(3) Å, alpha = 76.73(5) degrees, beta = 71.18(3) degrees, gamma = 76.44(5) degrees, Z = 1, T = 291 K; R = 0.035, R(w) = 0.036 for 1668 reflections; Cu-N = 1.998(3) and 2.001(2) Å, Cu-O = 2.574(4) Å, in a tetragonal geometry. [Cu(BIK)(2)](ClO(4))(2), 2, belongs to the monoclinic space group C2/c system, a = 9.029(3) Å, b = 12.497(2)Å, c = 19.197(2) Å, beta = 94.59(2) degrees, Z = 4, T = 291 K; R = 0.056, R(w) = 0.061 for 1052 reflections; Cu-N = 1.961(7) and 1.954(7) Å, in a distorted tetrahedral geometry. [CuCl(TRIM)(CH(3)OH)]Cl, 6, belongs to the monoclinic space group P2(1)/n system, a = 14.192(5) Å, b = 13.832(5) Å, c = 7.913(3) Å, beta = 90.55(4) degrees, Z = 4, T = 291 K; R = 0.062, R(w) = 0.057 for 1377 reflections; Cu-N = 1.987(7), 2.007(7) and 2.007(6) Å, Cu-O = 2.521(7) Å, Cu-Cl = 2.298(2) Å, in a square pyramidal geometry. [Cu(ClO(4))(TIM)](ClO(4)), 4, belongs to the triclinic space group P&onemacr; system, a = 9.604(4) Å, b = 11.508(6) Å, c = 12.003(8) Å, alpha = 58.79(4) degrees, beta = 94.59(2) degrees, gamma = 67.43(3) degrees, Z = 2, T = 291 K; R = 0.057, R(w) = 0.062 for 2084 reflections; Cu-N = 1.985(7), 1.964(7), 1.967(7), and 1.966(7) Å, Cu-O = 2.553(8) Å, in a distorted square pyramidal geometry. [CuCl(TIK)](ClO(4)), 7, belongs to the triclinic space group P&onemacr; system, a = 7.432(3) Å, b = 12.573(3) Å, c = 12.945(2) Å, alpha = 114.94(4) degrees, beta = 92.46(2) degrees, gamma = 103.49(3) degrees, Z = 2, T = 291 K; R = 0.043, R(w) = 0.049 for 2305 reflections; Cu-N = 1.984(5), 1.989(5), 2.012(5), and 1.979(5) Å, Cu-Cl = 2.796(2) Å, in a distorted bipyramidal geometry. In methanol solution, the perchlorato complexes 1, 2, Cu(TRIM)(ClO(4))(2) (3), 4, and Cu(TIK)(ClO(4))(2) (5) exhibited redox potentials from -215 to +284 mV vs NHE together with a visible absorption from 604 to 728 nm. Electron spin-echo envelope modulation (ESEEM) spectroscopy data, particularly the nuclear quadrupole interaction (NQI) parameters e(2)qQ and eta of the remote nitrogen (N1H), were analyzed and interpreted according to the model devised by Jiang et al. (J. Am. Chem. Soc. 1990, 112, 9035) with reference to Cu(HIm)(4)(ClO(4))(2). The results are the following: (i) C2 substitution of the imidazole ring, next to the remote nitrogen (1, 2) decreases the asymmetry parameter eta to ca. 0.75 compared to 1.00 for Cu(HIm)(4)(ClO(4))(2); this effect of C2 substitution on the symmetry of the electric field gradient at N1H appears similar for both the electron-donating methylene substituents (1) and the electron-withdrawing carbonyl group (2). (ii) The electron-donating or -withdrawing properties of the substituent are reflected by the variation of the e(2)qQ parameter, increasing from 1.43 to 1.75 MHz (1) or decreasing to 1.38 MHz (2), and by the nu(+) transition shifting toward higher frequencies from 1.49 to 1.65 MHz (1, 3, 4) or to lower frequencies to 1.29 MHz (2, 5). The use of the eta and nu(+) parameters to assign the Ndelta vs Nepsilon coordination of histidine to the metal and to detect modified histidine in copper-binding proteins is discussed.

Identification of a putative histidine base and of a non-protein nitrogen ligand in the active site of Fe-hydrogenases by one-dimensional and two-dimensional electron spin-echo envelope-modulation spectroscopy.

The active H-cluster of the Fe-hydrogenases from Megasphaera elsdenii and Desulfovibrio vulgaris (strain Hildenborough) has been investigated with one- and two-dimensional pulsed EPR spectroscopy. In both complexes the coordination of a nitrogen-containing ligand was found. The unusual quadrupole interaction parameters (D. vulgaris: quadrupole coupling constant, K = 1.20 MHz, asymmetry parameter eta = 0.32, M. elsdenii: K = 1.23 MHz, eta = 0.25) indicate a non-protein type of nitrogen and are consistent with cyanide as ligand to the H-cluster. The additional interactions measured on the EPR signal of the inactivated H-cluster in D. vulgaris hydrogenase are consistent with an imidazole interaction similar to that found in Rieske-type iron-sulfur clusters. Since a His residue near the putative H-cluster binding motif of Cys residues, His371, is the only conserved His in Fe-hydrogenases, it is a likely candidate for the base that accepts the proton in the heterolytic cleavage of molecular hydrogen. The inactivation of the enzyme is accompanied by direct binding of the imidazole ring to the H-cluster.

ESEEM study of the plastoquinone anion radical (QA.bul.-) in 14N- and 15N-labeled photosystem II treated with cyanide.

The nonheme iron of the photosystem II reaction center was converted to its low-spin state (S = 0) by treatment with CN-. This allowed the study of the plastoquinone, QA- anion radical by electron spin-echo envelope modulation (ESEEM) spectroscopy. A comparative analysis of the ESEEM data of QA- in 14N- and 15N-labeled PSII demonstrates the existence of a protein nitrogen nucleus coupled to the QA-. The 14N coupling is characterized by a quadrupolar coupling constant e2qQ/4h = 0.82 MHz, an asymmetry parameter eta = 0.45, and hyperfine coupling constant A approximately 2.1 MHz. The 15N hyperfine coupling is characterized by T = 0.41 MHz and alpha iso approximately 3.3 MHz. The possible origins of the nitrogen hyperfine coupling are discussed in terms of the amino acids thought to be close to the QA- in PSII. Based on a comparison of the 14N ESEEM with 14N-NQR and 14N-ESEEM data from the literature, the most likely candidate is the amide nitrogen of the peptide backbone of Ala261 of the polypeptide D2, although the indole nitrogen of Trp254 and the imino nitrogen of His215 of D2 also remain candidates.

Nuclear magnetic resonance of 23Na in an NaMnCl3 single crystal

The nuclear magnetic resonance of 23Na (I=3/2) in an NaMnCl3 single crystal grown by the Czochralski method has been investigated by employing a Bruker FT NMR spectrometer. From the experimental data, the quadrupole coupling constant e2qQ/h=155+or-2 kHz and asymmetry parameter eta =0 are determined at room temperature. The principal Z axis of the EFG tensor of the 23Na, the largest component, is parallel to the crystallographic c axis. The temperature dependence of the quadrupole parameters is also investigated in the temperature range of 140-380 K. The quadrupole coupling constant of 23Na is found to be linearly increasing as the temperature increases.

The nuclear quadrupole interaction of 181Ta in the intermetallic compound Hf2Ni

The time-differential perturbed angular correlation technique has been used to study the electric field gradient at 181Ta impurities in the intermetallic compound Hf2Ni. The results of the measurements show the presence of a single nuclear quadrupole interaction. At room temperature the frequency of the interaction is omega Q=92(1) Mrad s-1. The electric field gradient Vzz and asymmetry parameter eta exhibit a pronounced temperature dependence in a range varying from 78 to 1125 K.

Parameters of the electric field gradient tensor determined by 57Co(57Fe) and 67Cu(67Zn) emission Mossbauer spectroscopy for La2-xSrxCuO4 copper sites

The parameters of the electric field gradient (EFG) tensor at the copper sites in the La2-xSrxCuO4 lattice (the principal component of the EFG tensor eq and the asymmetry parameter eta ) were determined experimentally by emission Mossbauer spectroscopy with 57Co(587Fe) and 67Cu(67Zn) isotopes. No qualitative conformity was found between the experimental data and the values of eq calculated in a point charge model. This disagreement is attributed to an uncertainty in the values of Sternheimer coefficients for Zn2+ and Fe3+ ions and in the charge scaling of the La2-xSrxCuO4 lattice. A comparison of the experimental and calculated dependences of eq on x shows that the holes arising from the substitution of La3+ by Sr2+ are localized mainly at the oxygen sites in the Cu-O2 plane.

Nuclear magnetic resonance of 209Bi in a BiVO4 single crystal

Nuclear magnetic resonance spectra of 209Bi(I=9/2) in the ferroelastic BiVO4 single crystal has been investigated by employing a wide-line Varian spectrometer. Only three lines due to a large quadrupole interaction were measured at a fixed frequency of 6 MHz in the principal planes at room temperature. From the experimental data the quadrupole coupling constant E2qQ/h=79.2+or-0.1 MHz and asymmetry parameter eta =0.70+or-0.01 are determined; however, these values turn out to be quite different from the previous report obtained with a ceramic sample at liquid-nitrogen temperature. The principal axes of the electric field gradient tensor are determined for the first time, the x, y and z axes being the crystallographic c+25 degrees , a+25 degrees , and b axes, respectively. Thes directions are discussed in terms of the Bi-O bonds in the Bi-O polyhedra.

Perturbed angular correlation measurements of 111Cd in cobalt (II, III) oxide

The time differential perturbed angular correlation of 111Cd has been measured after implantation of the radioactive mother isotope 111In into Co3O4. Besides a broad distribution of electric field gradients ((vQC)=105(5) MHz, eta C=0.8(1)) two fractions with vQA=0 and vQB=145 MHz, eta B<0.15 are found which have been associated with 111In on substitutional A and B sites of the spinel lattice. During isochronal annealings at various oxygen partial pressures, the distribution widths delta A and delta B and the asymmetry parameter eta B approach zero; simultaneously, vQB slightly decreases. The changes are correlated and quantitatively explained in the point charge model.

The unusual temperature dependence of the electric field gradient at titanium sites in rutile (TiO2)

47Ti and 49Ti NMR measurements were performed in single-crystal rutile between 150 and 1500 K. The NMR spectrum is due to second-order quadrupole-perturbed central transitions (m=1/2 to -1/2) and consists of a doublet for each of the isotopes resulting from the different orientations of the EFT tensor at the two Ti sites in the unit cell. From the angular dependence of the NMR spectrum the magnitudes of the largest eigenvalue Vzz and the asymmetry parameter eta of the EFG tensor were determined. At room temperature one finds: mod Vzz mod =(2.2+or-0.1)*1020 V m-2, eta =0.19+or-0.01. While Vzz is only weakly dependent on temperature the asymmetry parameter eta reveals an uncommonly strong dependence on temperature caused by the non-isotropic thermal expansion of rutile.

Nuclear quadrupole optical hole burning in stoichiometric EuAsO4

The intrinsic site and over forty defect sites have been studied by optical absorption, fluorescence and optical hole burning in EuAsO4. The hole-burning spectra have allowed determination of the nuclear electrical quadrupole parameters P and asymmetry parameters eta for the 5D0 levels for both Eu isotopes at 11 of these sites. A model is presented that explains the observed correlation between the P-values and the electronic energy shifts of the defect sites. Nuclear Zeeman effect studies of two sites using nuclear hole-burning spectroscopy in fields up to 8.8 T have allowed the determination of the orientation of their 5D0 electronic quadrupole tensors. Reasonable connection is made with two types of localised defect that are expected to occur frequently in the lattice structure. Energy transfer effects have also been examined as a function of temperature.

Non-oriented spin-1 systems

A maximal state of spin polarisation of a spin-1 system with all eight degrees of freedom (namely, three distinct axes and two non-zero scalars) is realised when a spin-1 nucleus with nonzero electric quadrupole moment is embedded in a suitable crystal lattice generating an electric quadrupole field and is exposed to an external constant magnetic field whose direction does not coincide with any of the principal axes characterising the electric quadrupole field. In the absence of the magnetic field only five of the eight degrees of freedom (i.e. two distinct axes and a nonzero scalar) are realised if the asymmetry parameter eta not=0. If eta =0 the two axes coalesce. This suggestion is made to stimulate experimental efforts to produce polarised multiaxial spin-1 targets.