Band 53, Heft 6-7

Our goal is to extend the sensitivity of field cycling pure quadrupole resonance (PQR) methods to be of use in biological systems. The nuclei of interest are 25 Mg, 67 Zn, 43 Ca, 11 B and 17 O. The experiment is based on a field cycling double resonance technique, in which the quadrupole resonance of a rare nucleus is found through its effect on the magnetic order of the abundant nucleus to which the rare nucleus is coupled through dipole-dipole interaction. A field-cycling NMR spectrometer has been developed, based on our existing 500 MHz high resolution spectrometer. The sample can be shuttled pneumatically from the high field of a commercial 500 MHz magnet to the magnet's top, where the residual field and its gradient is canceled out by a pair of Helmholtz coils. Low field homogeneity is within 0.5 gauss. The X H signal is observed at high field as a free induction decay (FID) after a 90° pulse. At low field the sample can be irradiated by a digitally tuned RF coil in the 300 kHz-7 MHz range. The sample has to be maintained at low temperature ( ~ 30 K) to avoid relaxation via thermal motion of methyl groups in biomolecules. For this purpose field cycling equipment is placed in a variable temperature dewar (4 - 300 K). We plan to use solutions of biomolecules in standard cryoprotective buffer, containing ~ 30% glycerol. Preliminary results on the quadrupole resonance of natural abundance 17 O in the cryoprotective buffer and of natural abundance 11 B in a protease inhibitor at 50 mM are presented.

Nuclear magnetic resonance (NMR) has been used for some 40 years to study atomic arrangements, chemical bonding, and structural groupings in borate glasses and crystalline compounds, and nuclear quadrupole resonance (NQR) has more recently increased the resolution and accuracy of the measurements. Examples are presented of the use of first-order and second-order quadrupolar effects in 11 B NMR spectra to obtain structural information, and 11 B and 10 B NQR spectra to obtain the quadrupolar parameters Q cc (the coupling constant) and ƞ (the asymmetry parameter) with accuracies of 5 or 6 significant figures, and 3 figures, respectively. Q cc and ƞ are extremely sensitive to changes in atomic rearrangements and chemical bonds, so they are excellent monitors and provide identification of bonding configurations and structural groupings in borates: Examples are also presented in which combinations of NMR and NQR data are used to extract the desired information. NQR detection of resonances at frequencies as low as 276 kHz is discussed.

The effects of off-resonance irradiation in nutation NQR experiments are demonstrated both experimentally and theoretically. The theoretical description of the off-resonance effects in 2D nutation NQR spectroscopy is given, and general exact formulas for the asymmetry parameter are obtained. It is shown that the outcome of the off-resonance nutation experiments depends on the data acquisition procedure, leading to one- or three-line nutation spectra. To explain this fact and to describe the off-resonance nutation experiment properly, the transient response theory of a quadrupolar spin system to an RF pulse was modified using the wave-function approach. The 2D separation of interactions technique has been applied to separate a static and a randomly time-fluctuating dynamic part of the quadrupole interaction in the antiferroelectric phase of poly-crystalline ammonium dihydrogen arsenate. Two-dimensional off-resonance nutation NQR spectroscopy has been used to determine the full quadrupolar tensor of spin-3/2 nuclei in several molecular crystals containing the 35 Cl and 75 As nuclei. The off-resonance phenomena in 2D NQR are very important from the practical point of view (determination of ƞ in multiple or broad-line spectrum) and also provide interesting information on the dynamic properties of a quadrupolar spin system.

The electronic structure of nitrogen atoms as impurities in an ionic TiO 2 crystal has been investigated by analyzing electric field gradients (EFGs) measured by use of short-lived β-emitting 12 N implant-ed following nuclear reactions. Conventional β-NMR and its modification, suitable for the detection of quadrupole effects in the NMR spectra, were used for the investigation of hyperfine interactions of 12 N located in substitutional sites of O atoms and interstitial sites in the crystal. In order to deduce absolute values of the EFGs from the obtained eqQ/h, the quadrupole moment of 12 N has been determined from the NMR detection of 12 N implanted in BN(hexagonal) crystal. Here the EFG at the N atom in BN was measured by detecting the FT-NMR of 14 N in the crystal. The EFGs in TiO 2 are compared with the theoretical predictions based on the ab initio band-structure calculation in the framework of the KKR method.

Double resonance level crossing detection cannot normally be used to record transitions between the quadrupole Zeeman levels. Neither can it be used if the quadrupole resonance frequency is larger than the proton NMR frequency in high field. A simple variation of the level crossing experiment is demonstrated which allows energy to be efficiently transferred from quadrupole to proton system and hence lead to detection in these two cases. In the case of the quadrupole Zeeman levels it allows transitions between the ±½ levels to be detected in a small magnetic field, allowing measurement of the asymmetry parameter (ƞ) for spin 3/2 systems. This is demonstrated for the 11 B nucleus in triethanolamine borate. Detection of high frequency quadrupole transitions is demonstrated from which relaxation information is obtained.

We studied the electric quadrupole interactions of Oxygen isotopes in a TiO 2 single crystal. For 13 O and 19 O nuclei, quadrupole coupling constants were measured by the β-NMR technique, and for the 17 O nucleus the FT-NMR technique was utilized. We synthesized a TiO 2 single crystal which was enriched in 17 O up to 5 atom % to observe NMR signals without any perturbations from impurities. Using the known quadrupole moment of 17 O, EFGs at an O site in TiO 2 and the quadrupole moments of 13 O and 19 O were determined.

NMR experiments of 11 B by both field-swept and high-resolution NMR are reported to probe the electric field gradient at boron sites and its distribution in Pyrex glass. Both spectra are successfully interpreted with the same set of parameters. It is stressed that field-swept NMR experiments to observe total powder spectrum can be helpful to get information on electric field gradients and asymmetry pa-rameters if there exist many nonequivalent sites of atoms, since satellite transitions are affected by the larger first-order quadrupole effect. Field-swept NMR of 93 Nb in superconducting NbN powder, prepared by self-propagating high-temperature synthesis, is also reported. A very broad field-swept spectrum disturbed by quadrupole interaction has been observed. The spectrum is simulated by assuming distributions in electric field gradient and Knight shift at Nb sites. It is stressed that a combination of experiments at separated frequencies is important.

For nuclei which have a nuclear spin of 5/2 and exhibit a small asymmetry parameter of the electric field gradient (ƞ) at the nuclear site, line shapes of the Powder Zeeman NQR (PZNQR) spectra based on the transition between the energy levels m 1 = ± 1/2 and ± 3/2 (the lower frequency line) were studied by means of computer simulations and experiments. (i) When an ƞvalue is very small (type 1; ƞ= 0), the line shape exhibits two shoulders like the style of the American football player. (ii) While an ƞvalue is small (type 2; 0 < ƞ< around 0.01) but not zero, the line shape has two small peaks which are symmetrically located on the shoulders, as in the case of the small ƞtype of spin 3/2. (iii) When an ƞvalue is not small (type 3; around 0.01 < ƞ), the line shape has two symmetrical dips in stead of the peaks, which are also similar to the case of not small ƞ type of spin 3/2. As the ƞvalue increases from around 0.01, the two dips grow and reach the maximum at the ƞvalue of 0.349, and then become smaller and obscure in the range of ƞlarger than 0.349. The observations of PZNQR spectra were performed for several compounds including the 127 I and/or 121 Sb nuclei to estimate the ƞvalues, and gave the results as follows: very small for 127 I (207.683 and 209.133 MHz, at 77 K) in Snl 4 ; very small for 127 I (176.496 and 177.438 MHz, at 77 K) in Gal 3 ; small for 127 I (265.102 MHz, at 77 K) in CH 3 I; 0.33 for 127 I (247.69 MHz, at 77 K) in C 2 H 5 I; 0.27 for 121 Sb (58.23 MHz, at 290 K) in SbCl 3 . The estimated ƞvalues were compared to those obtained from the frequencies of two NQR lines for spin 5/2. They were in good agreement with each other for the small region of ƞ , though somewhat large disagreements were seen in the cases of not small ƞvalues.

Time differential perturbed angular correlation (TDPAC) measurements were carried-out on PbZr 0.8 Ti 0.2 O 3 as a function of temperature. A broad distribution of electric field gradient (EFG) was observed in all cases with a second site for 181 Hf- 181 Ta probes in the R3c phase. The results were analyzed in terms of a model which simulates an attraction of the ions by the smaller Ti ions. The EFG was calculated within the point charge approximation, by averaging over a large number of configurations generated by random distribution of the Ti ions in an ensemble of lattice cells. It was verified that the model reproduces qualitatively the broadening of the TDPAC frequencies as well as explains the second component EFG observed experimentally.

.6( The nuclear quadrupole interaction at 187 W(β - ) 187 Re was determined by time differential perturbed angular correlation in WC, WS 2 , WSe 2 , WSi 2 , and CaWO 4 to be (at 300 K): v Q = 335.9(2), 1094.9(1), 10311), 1131,5( 1), and 1085.9( 1) MHz, respectively. The asymmetry parameter ƞwas zero in all cases. For WSe 2 and CaWO 4 the temperature dependence of the nculear quadrupole interaction was determined between 300 K and about 900 K. Ab initio calculations of electric field gradients, using the WIEN95-code, were carried out for WC, WS 2 , WSe 2 , and WSi 2 at W-sites and Re-impurities, and for CaWO 4 at W-sites. Good agreement with experimental data was found.

The principle of the time differential perturbed angular correlation and distribution technique which measures the nuclear quadrupole interaction is briefly described. Some examples are given to show the possibilities of this technique in microscopic studies of materials.

The quadrupole interaction in Au-In compounds of different stoichiometrics was studied with the perturbed angular correlation technique. The electric field gradients at 111 Cd probes were measured and the temperature dependences of the quadrupole frequencies were determined. A new high temperature phase of Auln above 630 K and a new metastable modification of Au 7 In 3 were found.

The quadrupole interaction of 181 Ta probes in isostructural Cu 8 Hf 3 and Cu 8 Zr 3 intermetallic compounds was studied in the temperature range 24 K -1100 K with the time-differential perturbed angular correlation technique. Two nonaxial electric field gradients corresponding to two nonequiv-alent hafnium or zirconium sites in the investigated structure were found. A linear decrease in the quadrupole interaction frequency with increase of temperature for the 8(d) sites was evidenced, while the v Q (T) dependence for the 4(c) sites is weaker and has a T 3/2 character.

Two theoretical evaluations of 57m Fe quadrupole moment (Q), based on different formalisms, namely the Hartree-Fock theory and the Linearized Augmented Plane Wave method have yielded results differing by a factor of two. In both cases, Q was obtained from experimental quadrupole interaction frequencies through investigation of the Electric Field Gradients at the nuclear site of the 57m Fe probe. It is the purpose of the present work to reexamine the earlier Hartree-Fock approach. In particular, the earlier model is extended through a more realistic description of the environment of 57m Fe in the respective experiments, as well as through inclusion of electron correlation effects.

Ab initio values of Nitrogen 14 N Nuclear Quadrupole Coupling Constants (NQCC's) are calculated for a series of methyl-substituted azoles in the Multiconfigurational SCF (MCSCF) approximation. The four triazoles and two tetrazoles studied here are all isoelectronic. This enabled us to use the same level of approximation - basis set and active space -for all the molecules. The computed NQCC's are used to estimate the relative widths of the 14 N NMR signals, assuming an identical effect of molecular tumbling for all the nuclei in a molecule and neglecting solvent effects. The linewidths for the unsubstituted N atoms are, in agreement with experiment, much larger than for the methyl-substituted N atom. For the 1-methyl-tetrazole we present also the NMR shielding and spin-spin coupling constants and discuss in some more detail the dependence of all calculated NMR properties on the basis set and correlation effects.

Ab initio determination of the electric field gradient (EFG) tensors at halogen and other centres ena-bled determination of the nuclear quadrupole coupling constants (NQCC) for a diverse set of C 2v , C 3v and other symmetry molecules of general formula MH 2 X 2 and MHX 3 , where the halogen atoms (X) are Cl, Br and I, and the heavy central atoms (M) are C and Si. The study presents results at a standardised level of calculation, triple-zeta in the valence space plus polarisation functions (TZVP) for the equilib-rium geometry stage; all-electron MP2 correlation is included in all these studies. For the bromo and iodo compounds, especially the latter, it is essential to allow core polarisation, by decontraction of the p,d-functions. This is conveniently done by initial optimization of the structure with a partly contract-ed basis, followed by reestablishment of the equilibrium structure with the decontracted basis. The NQCCs, derived from the EFGs, using the 'best' values for the atomic quadruple moments Cl, Br and I, lead to good agreement with the inertial axis (IA) data obtained from microwave spectroscopy. When the data from the present study is plotted against the values derived from the IA data, obtained by whatever approximations chosen by the MW authors, we obtain a linear regression for the data (85 points) with the slope 1.0365 and intercept -0.1737, with standard errors of 0.0042 and 0.2042, respectively; these are statistically identical results irrespective of whether the data is restricted to IA or EFG principal axis (PA) data. Since as in the C 3v MH 3 X compounds studied previously, a close correlation of the microwave spectral data with the calculations was observed using the 'best' current values for Q z , there seems no need to postulate that the values of Q Br for both 79 Br and 81 Br are seriously in error. A scaling downwards of Q z by about 5% for Br and I increases the agreement with experiment, but the contributions of relativistic effects are unknown, and could lead to further reassessment. Of the two common assumptions used in MW spectroscopy, to convert from IA to EFG-PA data, either (a) cylindrical symmetry of the NQCC along the bond direction, or (b) coincidence of the tensor principal element with the bond axis, the latter is found to be a much more realistic approximation.

Ab initio determination of the electric field gradient (EFG) tensors at halogen and other centres ena-bled determination of the nuclear quadrupole coupling constants (NQCC) for a diverse set of axially symmetric (C 3v , C ∞v , D ∞h and other symmetries) inorganic and organic molecules, where the heavy ele-ments are Cl, Br, and I with C, Si, Ge, and Sn hydrides. The latter elements are in an approximately tetrahedral environment. The study presents results at a standardised level of calculation, triple-zeta in the valence space (TZV) plus polarisation functions (TZVP) for the equilibrium geometry stage; all-electron MP2 correlation is included in all these studies. f-Orbital exponents were optimised for both Br and I centres in the methanes; the atomic populations of the f-orbital components are very small for the Br-and I-atoms, confirming their role as polarisation functions rather than having any bonding character. The EFG are determined at equilibrium with the TZVP basis set, except Sn and I centres where the basis set is TZV + MP2. For the bromo and iodo compounds, especially the latter, it is essential to allow for core polarisation, by decontraction of the p,d-functions. This is conveniently done by initial optimization of the structure with a partly contracted basis, followed by reestablishment of the equilibrium structure with the decontracted basis. A close correlation of the observed (microwave spectral) data with the calculations was observed, using the 'best' values for the atomic quadrupole moments for Cl, Br, and I; thus there seems no need to postulate that the value of Q Br for 79 Br and 81 Br are in error. The SCF and MP2 wave-functions were converted into localised molecular orbitals by the Boys Method. This allowed a study of the differing s/p/d-hybridisation ratios, and the centroid positions, to be compared with the quadrupole coupling constants. The charge distributions for the atoms were converted into local bond dipoles, which in turn are correlated with the electronegativity differences of the bonded atoms.

We present ab initio calculations of EFGs at impurities in a TiO 2 crystal. They are directly calculated from the self consistent charge distribution which is determined by the KKR method within the muffin-tin approximation and based on the local density approximation. Impurities in the crystal are simulated by the super-cell method. Considering the charge state of Sc, Ti, Nb, Cd, Ta impurities in the TiO 2 , the experimental values were well reproduced. The electronic structure of these impurities is discussed by analyzing the calculated density of states.

Electric field gradients (EFG) at Hg were determined in mercury mercaptides Hg(S(CH 2 ) i CH 3 ) 2 experimentally for i ∈ {0, 1, 2} using time differential perturbed angular correlation and theoreti-cally for i ∈ {0, 1} with the full potential linearized augmented plane wave code WIEN95. Due to the large unit cells and small hydrogen atoms not full convergence of the plane wave basis set could be reached. Nevertheless, the calculated EFGs agree with experimental values to better than 20%. Furthermore, isolated molecules for i ∈ {0, 1) were investigated theoretically, and strong differences to the values for the crystalline state, especially for the asymmetry parameters η, were found.

The full potential linearized augmented plane wave method as embodied in the program package WIEN95 was originally developed for crystalline solids and is based on crystal periodicity. The present work demonstrates that it is applicable to isolated molecules with the examples of CdCl 2 , HgF 2 , and HgCl 2 by investigating the required size of artificially enlarged unit cells and by calculating bond distances and vibrational frequencies in excellent agreement with experimental data. The dependence of electric field gradients at Cd and Hg, respectively, on bond angle is investigated. A point charge like behaviour of energetically low lying states is found, whereas large discrepancies from the point charge model occurred for the energetically high lying valence states.

From the measurements of DTA • DSC and the temperature dependences of 35 Cl NQR frequencies, phase transitions were detected at 150 K, 168 K, and 172 K for (pyH) 2 [PtCl 4 ], and at 241 K for (PyH) 2 [PdCl 4 ]. In order to elucidate the motional state of the constituent ions in the crystals in connection with the structural phase transitions, the 35 Cl NQR and 1 H NMR spin-lattice relaxation times and the second moment of the 1 H NMR line were measured as functions of temperature. For both compounds, the potential wells for the cationic reorientation are suggested to be highly nonequivalent at low temperatures. Above 168 K, the pyridinium ions in (pyH) 2 [PtCl 4 ] are expected to reorient between almost equivalent potential wells. As for (pyH) 2 [PdCl 4 ], it is expected that the orientational order of the cation still remains even above the second order transition at 241 K. A change of the potential curve from two-unequal to three-unequal wells is proposed as a possible mechanism of the second order transition. The activation energies for the cationic motion in the respective model potential are derived for both compounds at high and low temperatures.

In the present work we report a molecular dynamics investigation of the two mesogens 4,4'-bis(hexyloxy)azoxybenzene (HL) and its Pd(II) acetylacetonate derivative Azpac in their nematic phases. Deuterium Zeeman and quadrupolar spin-lattice relaxation times have been measured at 46.04 MHz on two isotopomers of Azpac (Azpac-d 4 and Azpac-d 26 , deuteriated on the aromatic core and on the alkoxy chains, respectively) and on HL-d 4 , an isotopomer of HL partially deuteriated on the aromatic core, by means of the Wimperis pulse sequence. The spectral densities obtained from the measured relaxation times are discussed in terms of internal and overall molecular motions. A small step rotational diffusion model for the overall molecular motions, superimposed on a free rotational model for internal motions, allowed diffusion coefficients for molecular spinning and tumbling and for phenyl ring rotations to be derived for HL; the same models were used in describing the dynamics of Azpac.

For the title compound a phase transition from Phase II to Phase I (low and room temperature phases, respectively) was found at ca. 217 K. The temperature dependence of the 81 Br NQR frequency and that of the dielectric constant showed anomalies at ca. 195 K that were tentatively attributed to a higher order phase transition. A similar anomaly was found at ca. 218 K for 4-chlorobenzyl alcohol which showed a II-I transition at 236 K. The dielectric dispersion observed for both compounds at low temperatures indicates an excitation of a molecular motion with the dielectric relaxation rate of ca. 1 kHz. The temperature dependence of the 81 Br NQR frequencies of 2-and 3-bromobenzyl alcohol, measured at T > 77 K, gave no evidence of phase transition in their crystals.

The spin-lattice relaxation time T 1 of 23 Na-NMR in dehydrated zeolite NaY has been measured from 26 to 300 K. The magnetization recovery curve is not single-exponential at all measured temperatures and T 1 -1 increases in proportion to the square of temperature above 200 K. The result is analyzed with a theory of the Raman process based on covalency. The value of T 1 is compared with that of NaX in which the concentration of Na is about 2 times larger than in NaY.

The temperature dependences of 2 H and 195 Pt NMR spectra and the spin-lattice relaxation time T 1 were measured for [Cu(H 2 O) 6 ][PtCl 6 ]. From the simulation of 2 H NMR spectra, the jump rate of 180° flips of the water molecules (k), the nuclear quadrupole interaction parameters (e 2 Qq/h, ƞ) and the electron-nucleon dipolar interaction parameter (v D ) were obtained. By measuring 2 H T 1 , k was estimated in the temperature range where the spectrum is insensitive to the motion of the water molecules. Above the phase transition temperature, the pre-exponential factor k 0 = 8 x10 11 s -1 and the activation energy E a = 15 kJmol -1 for 180° flips of the water molecules were obtained from the spectral simulation and T 1 . 195 Pt NMR spectra showed an axially symmetric and unsymmetric powder pattern of the chemical shift anisotropy at the high and low temperature phase, respectively. For the deuterated compound, the correlation times of the electron spin in Cu 2+ were estimated from 195 Pt T 1 and the activation energy for jumping between the different configurations of Jahn-Teller distortion Δ = 200 K was obtained.

The successive phase transitions of [Mg(H 2 O) 6 ][SiF 6 ] were studied by measuring 2 H NMR spectra. The quadrupole coupling constant e 2 Q q /h and asymmetry parameter η changed drastically at each transition temperature. 1,2 H and 19 F NMR T l were measured for this compound to study the relation between the molecular motions and the successive phase transitions. The activation energy E a and the pre-exponential factor τ 0 for the reorientation of [SiF 6 ] 2- were estimated as 28 kJmol -1 and 6.0 x 10 -14 s, and those of the 180° flip of H 2 O as 33 kJmol -1 and 4.0x 10 -14 s. These two motions occur rapidly even in phase V. For the reorientation of [Mg(H 2 O) 6 ]2 + , E a = 62 kJmol -1 and τ 0 = 1.1 x 10 -16 s were obtained from the simulation of 2 H NMR spectra. The jump rate of this motion is of the order of 10 4 -10 6 s -1 in phase II. These results suggest that the successive phase transitions are closely related to the motion of [Mg(H 2 O) 6 ] 2+ .

In order to understand the mechanism of ferroelastic phase transitions in A 2 ZnBr 4 (A = (CH 3 ) 4 N and (CH 3 ) 4 P), the temperature dependences of 79 Br NQR frequencies and the spin-lattice relaxation times were measured. The temperature dependences of the 1 H and 31 P spin-lattice relaxation times were measured as well for a possible correlation between the cation dynamics and the phase transition. Although the phase transition temperatures of these two compounds differ much ( ~ 100 K), the correlation times for the cation reorientation at the individual transition temperatures amount to some 10 -11 s for both compounds.

Li 3 InBr 6 and NaInBr 4 have been synthesized and characterized by means of DTA, 81 Br NQR, 6 Li, 7 Li, 23 Na, and 115 In NMR, and AC conductivity. These measurements revealed the presence of phase transitions and cationic diffusion in both compounds. From the spin-lattice relaxation times of 81 Br NQR and the peak widths of 7 Li and 23 Na NMR spectra, it is deduced that the conduction is due to cationic diffusion. The activity energy for the Li + diffusion was found to be 24 kJ/mol for Li 3 InBr 6 .

The temperature dependence of 2 H NMR spectra was measured at 130-430 K on [M(D 2 O) 6 ] [SnCl 6 ] (M : Mg, Ca) (rhombohedral, space group: R3̅), and neutron powder diffraction on the Mg salt was performed at room temperature. Based on the accurate orientation of D 2 O molecules in an octahedral cation, spectra with a large asymmetry parameter η, observed around 200 K, were explained by a model of 180° flip of water molecules. Another motional narrowing observed above room temperature was assigned to a cationic overall reorientation about the C 3 -axis.

The effect of temperature on the chlorine NQR spin-lattice relaxation times in CsH(ClH 2 -CCOO) 2 , KH(Cl 3 CCOO) 2 and N(CH 3 ) 4 H(ClF 2 CCOO) 2 has been studied in the temperature range 77 K to room temperature. The results were discussed on the basis of NQR relaxation theory.

Multinuclear NQR and NMR techniques have been applied in order to study the molecular dynamics in [C(NH 2 ) 3 ](ClH 2 CCOO). The 35 Cl NQR frequency was measured over a wide range of temperature. The experimental results were described by using the theories of Bayer and Brown which take into account the torsional oscillations of the CClH 2 -group of the anion. A study of the proton NMR second moment as well as relaxation times T 1 , and T 1p performed in a wide temperature range revealed an onset of the guanidinium cation reorientation around its two-fold symmetry axis. Activation parameters for this motion were determined.

This paper discusses recent NQR/NMR studies performed on Y-Ba-Cu-0 superconductors at the University of Zürich. The work is concerned with normal state properties which are still controversial, for instance the spin-gap effect, i.e. the opening of a pseudo gap in the electron spin excitation spectrum at a temperature T*, which lies above T c . We will report on the detection of “anomalies” which are dis-played in the temperature dependence of several NMR and NQR quantities measured in the normal state of YBa 2 Cu 4 O 8 . These anomalies are interpreted as an electronic crossover which involves a charge re-distribution in the CuO 2 planes and an enhancement of the charge fluctuations. As a possible mechanism of the crossover, a charge density wave instability is proposed.

The effects of interdiffusion phenomena and reactions occurring in the early stages of the interaction at 1273 K between iron and chromium monoborides were studied by transmission Mössbauer and X-ray diffraction techniques, and modifications occurring in Fe/Al multilayers during both deposition and subsequent thermal aging at 400 K in vacuum or air were studied by conversion electron Mössbauer and Auger electron depth profiling techniques. In both cases the main objective of the work was to obtain a better understanding of the mechanisms of modification of the materials under examination with a view towards improving the properties of materials and coatings for specific applications.

The pressure variation of the axial EFG at the ion sites in semimetallic arsenic and antimony, at am-bient temperature, has been investigated using pulsed NQR. A weakly nonlinear decrease of the EFG is observed in both systems. The data are analyzed in terms of the lattice contribution to the EFG, which involves a lattice of point monopoles immersed in a uniform, compensating, background charge. The pressure dependence of the EFG obtained from the present measurements is far weaker than the pres-sure dependence of the calculated lattice contribution. Our results support previous suggestions that va-lence effects are important in determining the EFG in these semimetals.

The NQR line shapes of 209 Bi in the α-Bi 2 O 3 -based mixed oxides Bi 2 O 3 • 2M 2 O 3 (M = Al, Ga). Bi 2 O 3 • 3GeO 2 , and 2Bi 2 O 3 • 3GeO 2 are recorded in zero and weak magnetic fields (H ext < 500 Oe) and compared with the results of computer simulation. Splittings and line shape asymmetry, exhibited by the resonances, suggest that internal magnetic fields, similar to those earlier reported for α-Bi 2 O 3 and Bi 3 O 4 Br, may exist in these compounds. In the spectra of single crystal Bi 4 Ge 3 O 12 , the line multiplicity in external magnetic fields is higher than simulated, which might results from domains in the crystal. In external magnetic fields a notable increase in the line intensity was observed, the effect depending on the mutual orientation of the EFG axes and the fields perturbing the nuclear spin system.

Temperature dependences of 35 Cl NQR frequencies and spin-lattice relaxation times were measured at 4.2 to 400 K for natural and deuterated ammonium hexachloropalladate. It was confirmed that only the deuterated salt undergoes a first order phase transition at 30 K. The crystal structure of the low-temperature phase is predicted to be the same as that of the deuterated ammonium hexachloroplatinate and hexachloroplumbate. The mechanism of the deuteration-induced phase transition is discussed.

The oxygen content dependence of 63 Cu NQR at the Cu(1) site and proton NMR have been measured in the antiferromagnetic phase of powdered samples of hydrogen-doped YBa 2 Cu 3 O 6+x H y (0.07 <̲ x <̲ 0.17 and y ≈ 1) from 4.2 to 90 K. The spectrum of 1 H NMR is a single line and the line width increases below 15 K due to magnetic interactions. The enhancements of T 1 -1 and T 2 -1 of 63 Cu(l)NQR occur around 35 and 15 K, respectively. These enhancements increase with increasing oxygen concentration. The maximum values of T 1 -1 and T 2 -1 for the sample with x = 0.17 reach 200 sec -1 and more than 7 msec -1 , respectively. The predominant source for the relaxation mechanism of 63 Cu(l) NQR and the line broadening of 1 H NMR are found to be the fluctuating magnetic field due to the staggered Cu 2+ moments.

NMR experiments are reported for VRuP which has an orthorhombic TiNiSi structure and shows anomalies in the temperature dependence of the electric conductivity and static susceptibility. A field-swept spectrum of 51 V perturbed by a quadrupole interaction has been observed, which shows no anomalous temperature dependence. However, the metallic spin-lattice relaxation rate decreases in a stepwise fashion with decreasing temperature at around 200 K, in accordance with the resistivity and the susceptibility data, suggesting a possible CDW formation in this material.

Some new analogues of the nonlinear optical crystal L-arginine phosphate monohydrate (LAP) (Arg • HIO 3 , Arg • 2HIO 3 , Lys • HIO 3 , Lys • 2HIO 3 , Lys • 3HIO 3 , Bet • 3HIO 3 ) were obtained and investigated by means of IR, NQR, X-ray diffraction and SHG methods. The importance of this class of crystals for revealing new nonlinear optical crystals is pointed out.

Chlorine NQR studies of solid nitrosyl chloride NOC1 between 110 and 212 K are reported. In the temperature dependence an order-disorder phase transition near 145 K, found by X-ray diffraction, is confirmed. Unusual hysteretic NQR signal behaviour indirectly proves also the low temperature phase transition below 100 K. In the measured temperature region fast relaxing mechanisms are present. The upper limit of the expected 14 N NQR frequency is estimated using the Townes-Dailey approach.

NQR spectra of copper in polycrystalline samples of iodine treated RBa 2 Cu 3 O y (R = Y, Gd) were studied. The results of all the experiments show evidence for the modifications occurring in the oxygen sublattice in treated compounds.

The 35 Cl nuclear quadrupole resonances in N-chlorocarbonylisocyanide dichloride show that it exists in two polymorphic forms. The metastable a phase has six 35 Cl NQR lines and the stable β phase has three. The latter is obtained irreversibly by annealing the α phase at 179 K. The temperature dependences of the 35 Cl NQR frequencies, spin-lattice and spin-spin relaxation times are obtained in both phases. The results of the 35 Cl NQR studies are discussed together with calculated MNDO level data and other physical properties (IR frequencies, dipole moments) of this compound.

In order to determine the electric quadrupole moment of the short-lived β-emitter 41 Sc from the quadrupole coupling constant in TiO 2 , we measured the field gradient by detecting the Fourier-Transformed-NMR of stable isotope 45 Sc doped in TiO 2 . Also, in order to study the electronic structure of impurities systematically, EFGs were measured for 45 Sc, 49 Ti and 93 Nb in a TiO 2 ingle single crystal.

35 Cl Nuclear Quadrupole Resonance (NQR) has provided a valuable tool for investigating the local structure in mixed crystals K 2 S 1-x Re x Cl 6 and Al 3+ doped crystals K 2 SnCl 6 :Al 3+ . The measured NQR line shapes and relaxation times of both kinds of impurity containing crystals in the cubic phase of the host K 2 SnCl 6 show markedly different impurity effects; static impurity effects in K 2 Sn 1-x Re x Cl 6 and dynamic effects in K 2 SnCl 6 :Al 3+ . The 35 Cl NQR spectra of K 2 SnCl 6 :Al 3+ near the transition temperature (T c ) indicate the presence of pretransition of the local structure in the high temperature cubic phase.

The temperature dependence of the 81 Br NQR frequencies of the title compound has revealed the presence of three phases (I, II and III, in the order of decreasing temperature): T cl (II -1) = (318 ± 10) K and T c2 (III -II) = (202 ± 1) K. The transitions were confirmed by thermal analysis (DTA). Both of the II-I and I-II transitions exhibited strong hysteresis (i. e., superheating and supercooling, respectively). The crystal data, determined by single crystal X-ray diffraction, are: monoclinic, space group P2/a, a = 1384.8(3), b = 934.7(4), c = 1452.0(2) pm, β = 104.10(1)°, Z= 8, R = 0.076. The crystal has two crystallographically independent cations (CH 3 ) 2 NH 2+ and an infinite chain of anions almost in the c-direction. The anion chain is considered to consist of a HgBr 2 molecule and two different HgBr 4 2- anions which are interconnected via weak intermolecular Hg···Br bonds. The splitting patterns of the 81 Br NQR spectra indicate that rearrangements of the Hg-Br bonds are slight at the III-II transition but serious at the II-I transition. Some of the 81 Br NQR lines show anomalous temperature coefficients, attributable probably to thermal motions of cations.

In an alexandrite single crystal four sets of NMR spectra for 27 Al (I=5/2) were observed in the crystallographic ab, be, and ca planes. The AI (I) center has four magnetically inequivalent Al sites, whereas the AI (II) center has two. The nuclear quadrupole coupling constant and asymmetry parameter of Al(I) and AI(II) in an alexandrite crystal were determined. Within the experimental accuracy, our parameters turned out to have the same values as those of a chrysoberyl crystal.

A large number of solid fluoride and heteroligand acidocomplex compounds of antimony(III) and bismuth(III) with single and mixed cations has been prepared from aqueous solutions. Their crystal structures and 121,123 Sb and 209 Bi NQR spectra have been investigated in a wide temperature range.

NQR spectroscopy was used to study the intercalating influence on the iodine electric field gradient of lead(ll) and cadmium(ll) iodides. We used pyridine, piperidine, aniline and quinoline, as guest substances. In the intercalates the NQR frequencies and their dependence on temperature in different crystalline phases were studied. The peculiarities of chemical bonds and crystal structures were discussed.

A pair of 35 Cl NQR spin echo signals has been observed for the mixed valence complex Cs 2 [Au I Cl 2 ] [Au III Cl 4 ] between 77 and 243 K. At 77 K, two resonance lines with the half widths Δv Q ~ 50 kHz were located at v Q1 = 17.28 MHz for the Au I -Cl chlorine and at v Q2 = 27.10 MHz for the Au III -Cl chlorine in accordance with the crystal structure. The chlorine ionic characters of the Au I -Cl and Au III -Cl bonds are estimated as 0.63 and 0.42, respectively. The central gold atom carries a fractional protonic charge of 0.26 in [Au I Cl 2 ] - and 0.68 in [Au III Cl 4 ] - . The charge distributions in the complex anions differ insignificantly from those in the isolated [AuCl 2 ] - and [AuCl 4 ] - for ordinary complexes, indicating that the charge transfer interactions between the anions are weak in the mixed valence complex. The observed linear temperature dependencies of V Q and log T lQ are well explained by the lattice vibration. When the temperature was increased from 77 K, the resonance lines became gradually weak without changing Δv Q and immeasurable above 215 K. ESR spectra taken at various temperatures revealed the presence of paramagnetic sites of ca. 5 x 10 20 mol - 1 arising from Au(II). The small but finite concentration of Au(II) or some other reason should be responsible for the fade out phenomenon and the large Δv Q observed.

The 23 Na (I = 3/2) NMR powder pattern for the m = 1/2 ↔ m = -1/2 transition, broadened by the second-order quadrupole interaction, was investigated in mixed crystals Na 1-x Ag x NO 2 (x<̱0.16). From the intensity loss of the 23 Na NMR line caused by the Ag + -impurity, the range of the quadrupole interaction in the 23 Na NMR central line was estimated. The range comprises ionic charges approximately up to the first nearest neighbour Na + site from the 23 Na nucleus, showing a large difference compared with that obtained from the 14 N NQR investigation in the same system.

The 63 Cu NQR spectra of five three-coordinated complexes of Cu(I) containing an isocyanide ligand are reported. The resonance frequencies are compared with those of other three-coordinated Cu(I) complexes using a partial coupling constant model to describe the effects of the various ligands and to take into account the effects of variations in the molecular geometries. The partial coupling constant of the isocyanide ligand is much higher than that of any other ligand studied so far and leads to the order of donor atom partial field gradients for terminal ligands C>N>Cl>P ≈ S ≈ Br>I>O.

The 35 Cl NQR frequencies, spin-lattice relaxation time and 1 H NMR relaxation time were measured on crystalline Na 2 PtCl 6 • 6H 2 O at 77-350 K. The presence of three nonequivalent chlorine sites found by X-ray diffraction measurement is in agreement with the observed three NQR lines, which have different temperature dependences attributable to differences in the direction of H-bonding with water molecules. The three NQR lines correspond to three kinds of chlorines with different Pt-Cl distances and H-bond directions.

Mössbauer spectra of various human ferrous hemoglobins in different ligand forms demonstrated some features such as non-Lorentzian line shape for oxyhemoglobins, narrower line width for carbonmonoxide, and variations of quadrupole splitting for different hemoglobins. These features were considered in relation to the variations of quadrupole splitting and heme iron stereochemistry in different hemoglobins as well as in non-equivalent subunits of hemoglobin tetramer.

As a preclude to a theoretical study of nuclear quadrupole coupling constants (NQCC), via the electric field gradients at equilibrium, we review the current state of knowledge of gas-phase data for a diverse set of axially symmetric inorganic and organic molecules with symmetries C 3v , C ∞v , D ∞h in particular, where the heavy elements are CI, Br and I with C, Si and Ge hydrides. In most of the cases, the latter elements are in an approximately tetrahedral environment.

14 N nuclear quadrupole resonance of the system (NaNO 2 ) 1-x (ANO 3 ) with A = K and Na in the x-range 0≤x<0.5 and the temperature range 77 K≤7<360 K has been studied. The 14 N NQR frequency of NaNO 2 and its line width do not change with x, the latter fact untrasting the results of other mixed systems such as Na 1-x Ag x NO 2 . This indicates that in (NaNO 2 ) 1-x (ANO 3 ) x highly mobile lattice defects exist. The dependence on x of the rate of change in the spin-lattice relaxation time T 1 near the transition temperature is discussed in terms of a correlated flipping motion of the NO - 2 ion groups.

The crystal structure of [tri(o-methoxyphenvl)phosphine] 2 Cu 2 Br 2 shows it to be a halogen-bridged dimer with three-coordinated Cu (I) sites. The 63 Cu NQR frequency of the corresponding chloride indicates that it has a similar structure. The 63 Cu NQR frequencies of the complexes of tri(p-tolyl)phosphine, L 3 Cu 2 X 2 , with cuprous chloride and bromide are consistent with their having halogen-bridged structures with both three- and four-coordinated Cu(I) sites analogous to those formed by triphenyl-phosphine and tri(m-tolyl)phosphine. Cu(I) sites. [CuBr(P(C 7 H 7 O) 3 ] 2 ; Triclinic, P l̄, Z=2, a = 8.969(1), b= 10.510(2), c=l 1.428(2) Å, α=99.46(l), β=91.95(\), γ= 104.17(1)°, R=3.5%, R ω =2.2% .

The TDPAC technique has been applied to study the stability of 111 In complexes with NTA and DTPA in solutions with different concentrations of stable In at pH = 7. A sample of In-DTPA complexes attached to microspheres of albumin (MSA) has been measured at temperatures of 293 and 130 K. The results show that the products formed after 111 ln → 111 Cd decay and following Auger-effect are determined by the stability of In (Cd)-complexes with organic ligands. The daughter Cd behaviour depends on the In: ligand mole ratio, from 1: ∞ to 1:1. The possibility of Cd-ligand complex destruction and following Cd rechelating is discussed. The results indicate that the rechelating probability correlates with the stability of the parent and daughter complexes.