Volume 45, Issue 3-4

Nuclear quadrupole resonance is an effective spectroscopic method to investigate the charge density distribution in solids. The range of compounds which may be investigated is wide, reaching from a crystal lattice built up by an ordered arrangement of neutral molecules in the crystalline material to ionic crystals and to metallic solids. First the correlations between NQR quantities and bond parameters which become apparent from the molecular orbital picture will be discussed, e.g. Hammett a, x-parameter, ionic character i, s-character, ionicity, electronegativity in their connection to NQR. Then, the correlations of NQR frequencies, respectively nuclear quadrupole coupling constants, with bond lengths and bond angles are considered. Cross correlations, e.g. of NQR frequencies with bond distances, bond distances with vibrational frequencies, and vibrational frequencies with NQR frequencies are discussed as are pK A dependencies of NQR parameters. The connection of e

The 35 CL NQR method and, in particular, the EFG asymmetry parameters at 35 Cl nuclei were used for the solution of a number of principal problems of theoretical organic and organometallic chemistry, such as the mechanism of non-induction influence of the heteroatom M on the atom Y in the non-linear Y-Z-M or Y-Z = M group, M-Cl bond multiplicity in planar molecular fragments and the asymmetry of halogen atom electron distribution in organic and organometallic molecules. In tetrahedral molecules this distribution is nearly axially symmetric for different M. This symmetry is also characteristic for axial chlorine atoms in trigonal-bipyramidal and octahedral molecules, independently of M. The deviation of the electron distribution of equatorial chlorine atoms from axial symmetry in the two latter cases depends essentially upon M and does not depend much on the peculiarity of the formation of these polyhedra and the character and number of substituents.

35 Cl NQR spectra (77 K) of 1:1 and 2:1 RR'CO-SnCl 4 are recorded and discussed. These spectra change in the course of time. At low temperatures either octahedral or trigonal-bipyramidal complexes (depending on the ratio of components and the nature of ligands) are formed. Frequently these systems contain a mixture of steric isomers or a mixture of complexes having different coordination numbers of the tin atom. In the solid state some complexes show a transition from one to another configuration.

The NQR frequencies of the 63,65 Cu nuclei in di-coordinated complex cations of Cu 1 , having the general formula CuL 2 X, where L is a substituted pyridine and X is a hard anion such as ClO 4 , are reported and discussed in the light of the known crystal structures. A departure from a strictly linear geometry of the L 2 Cu cation has but little effect on the resonance frequency, but the donor-acceptor bond between the nitrate anion and the copper atom in bis(2,6-lutidine)cuprous nitrate, which was postulated on crystallographic grounds, is confirmed by the NQR results, which also indicate a similar interaction in bis(2,6-lutidine)cuprous trifluoroacetate. A number of the complexes discussed here have not been reported previously.

Zeeman 35 Cl NQR spectroscopy has been applied to determine the magnitude and orientation of the EFG tensors at the chlorine sites in a single crystal of(CH 3 NH 3 ) 2 [(C 2 H 5 )SnCl 5 ] at 293 K. The assignment of the NQR frequencies v i ( 35 Cl), the corresponding quadrupole coupling constants eQΦ (i) zz h -1 , and the asymmetry parameters η i ( 35 Cl) to certain Cl (k) atoms is discussed. The most probable assignment is (no. i of v i ( 35 Cl), v i ( 35 Cl)/MHz, eQΦ (i) zz h -1 / MHz η i , ( 35 Cl), no. k of assigned Cl (k) , d(Sn-Cl (k) )/pm, ∢ (Φ (i) zz , d(Sn-Cl (k) )) :i = 1,17.032, 34.046,0.056, k= 1, 242.6, 3.1°; i = 2, 12.072, 24.089, 0.117, k = 3, 250.7, 1.9°; i = 3, 11.537, 22.992, 0.146, k = 2, 253.1, 2.8°; i = 4, 10.770, 21.455, 0.154, k = 4, 249.7, 2.9 °; i = 5,9.969,19.842,0.171, k=5,254.9,1.7°. The orientation of the tensor axes Φ (i) jj , j = x, y, z, i= 1 - 5 , in space is mainly determined by the geometry of the [(C 2 H 5 )SnCl 5 ] 2⊖ anion. The Φ (i) zz are nearly parallel to the assigned bond directions Sn-Cl (k) · η ( 35 CI) is low, increasing with decreasing v( 35 Cl). The reason for the asymmetry seems not to be a π-character in the Sn-Cl bond, but the charge distribution within the anion. The rule that a longer bond corresponds to a lower NQR frequency is valid as a general tendency, but not strict.

81 Br and 27 Al NQR were observed in AlBr 3 complexes with 4-XC 6 H 4 NO,(X = H, CI, Br, I, CH 3 , and C 2 H 5 ), C 6 H 5 COBr, (C 6 H 5 ) 2 CO, and (C 2 H 5 ) 2 O. In the 4-XC 6 H 4 NO 2 complexes, the 27 Al quadrupole coupling constants (QCC's) were well correlated with the Hammett er p 's of the para-substituents, i.e., electron-withdrawing groups caused reduction of the charge density of O-Al bonds which resulted in large 27 Al QCC's and vice versa. The temperature dependences of the 81 Br NQR frequencies and quadrupolar spin-lattice relaxation times showed that the 4-C 2 H 5 C 6 H 4 NO 2 and C 6 H 5 COBr complexes undergo phase transitions at 154 K and around 200 K, respectively, and show hindered rotation of the AlBr 3 groups at higher temperatures, and that the (C 2 H 5 ) 2 O complex reorients above ca. 120 K.

The 35 Cl NQR spectrum of trichloroacrylic acid, Cl 2 C = CClCOOH, was studied by single crystal Zeeman NQR spectroscopy at 303 K (high temperature phase I) and at 275 K (low temperature phase II). At 303 K for the = CCl 2 group chlorines it was found eQΦ zz h -1 ( 35 Cl (2) ) = 76.179 MHz, η ( 35 Cl) = 0.1597; eQΦ zz h -1 ( 35 Cl (3) )= 74.693 MHz, η( 35 Cl) = 0.1868 and for the = CClC - group chlorine eQΦ zz h -1 ( 35 Cl (1) ) = 75.145 MHz, η ( 35 Cl) = 0.1568. The difference in the electric field gradients at the stereochemically inequivalent chlorines Cl (2) and Cl (3) is rather large. The asymmetry parameter η ( 35 Cl (1) ) shows an unusual temperature dependence which probably originates from the change of the hydrogen dynamics and position within the dimeric H-bonded unit (Cl 2 C = CClCOOH) 2 during the phase transition

2,3,4,4-Tetrachloro-1-oxo-1,4-dihydronaphthaline, ß-TKN, crystallizes in the orthorhombic space group D 1 6 2 h - Pnma with Z = 4, and the 35 Cl NQR spectrum consists of three lines with an intensity ratio of 2:1:1. The crystal structure of 2,2,3,4-tetrachloro-1-oxo-1,2-dihydronaphthaline, α-TKN, is described in literature as belonging to the polar space group C 2 2 - P 2 1 with Z = 2. In contradiction to the reported space group and Z, we observed a three line 35 Cl NQR spectrum with an intensity ratio of 2:1:1. Therefore, the crystal structure of α-TKN was redetermined at 7 = 296 K: space group C 2 2h -P2 1 /m, Z = 2, a = 888.4 (2) pm, b = 692.3 (2) pm, c = 869.6 (2) pm, β = 91.483 (5)°. The nuclear quadrupole coupling tensors have been investigated using a 4 π Zeeman NQR goniometer and FT NQR for tracing the zero splitting cones. According to the crystal structure of β-TKN, two symmetry related zero splitting cones have been found for Cl (2) , two for Cl (3) , and four for Cl (4) ( T = 297 K). From the single crystal Zeeman split NQR measurements of α-TKN one zero splitting cone has been found for Cl (3) , one for Cl (4) , and two for CI (2) (T=290 K). The asymmetry parameters η and the direction cosines of the principal axes Φ xx , and Φ yy, and Φ zz of the nuclear quadrupole coupling tensors have been determined. The results are: Cl (j) , v ( 35 Cl (j) ), e 2 Q q h -1 ( 35 Cl (j) ), η ( 35 Cl (j) ) for β-TKN (at T = 297 K): Cl (2) , 37.442 MHz, 74.469 MHz, 0.184; Cl (3) , 36.854 MHz, 73.083 MHz, 0.160; Cl (4) , Cl (4') , 38.685 MHz, 77.307 MHz, 0.070 and for α-TKN (at T = 290 K): Cl (2) , CI (2') , 38.297 MHz, 76.501 MHz, 0.086; Cl (3) , 36.943 MHz, 73.700 MHz, 0.123; Cl (4) , 36.311 MHz, 72.314 MHz, 0.160. By comparing the crystal structures and the 35 Cl NQR results, the C-Cl bonds in the title compounds are discussed.

The 35 Cl NQR spectra of the two phases (phases I and II) of 2,3-dichloro-1,4-naphthoquinone were studied from 77 to 394 K. The crystal structure determination of the triclinic phase (phase I) was refined by single crystal X-ray diffraction. Phase I crystallizes in the space group C 1 i -P1̄. The unit cell (Z = 4) has the dimensions a = 1614.3(4) pm, 6 = 828.1 (3) pm, c = 733.2(3) pm, α = 67.02(1)°, β = 82.55(1)°, γ = 89.77(1)°. In the temperature range 439≦ TI →II /K≦447 a unidirectional phase transition I → II is observed. Phase II is orthorhombic, space group C 5 2v -Pb2 1 a. The bond distances d (C-CI) and the 35 Cl NQR frequencies correlate quite well according to the theory: v( 35 Cl) ~ (d( C-CI )) 3- The multiplicity of the 35 Cl NQR spectra is discussed. Pyroelectricity studies on phase II are reported. The pyroelectric coefficient depends little on the temperature, 8.3≦p x /μCm -2 K -1 ≦10.7 in 103 ≦T/K≦343.

A very high sensitivity continuous wave NQR spectrometer was developed to detect pure NQR transitions at low frequencies (down to 200 kHz). A signal-to-noise ratio of more than 100 to 1 has been achieved at about 1.36 MHz for crystalline B 2 0 3 . Two large n B responses have been found in vitreous B 2 0 3 (NMR detected only one site) with linewidths of less than 30 kHz. 27 A1 NQR spectra were obtained for OC-A1203 (Corundum), the mineral andalusite (a form of A1203 • Si0 2), and a glass having the composition of anorthite (CaO • A1203 • 2Si0 2). Abstract A very high sensitivity continuous wave NQR spectrometer was developed to detect pure NQR transitions at low frequencies (down to 200 kHz). A signal-to-noise ratio of more than 100 to 1 has been achieved at about 1.36 MHz for crystalline B 2 O 3 . Two large 11 B responses have been found in vitreous B 2 O 3 (NMR detected only one site) with linewidths of less than 30 kHz. 27 Al NQR spectra were obtained for α- Al 2 O 3 (Corundum), the mineral andalusite (a form of Al 2 O 3 · SiO 2 ), and a glass having the composition of anorthite (CaO · Al 2 O 3 ·2SiO 2 ).

We have employed multinuclear quadrupole resonance, using the field cycling technique, to study the structure of heterocyclic biomolecules. The electron density at nitrogen was investigated by the Townes-Dailey model of 14 N NQR. Large substituent effects were evident especially on the n orbitals. Electron transfer through H-bonds also affects the 14 N quadrupole interaction. Deuterium quadrupole resonance at exchangeable sites was also utilized. Since the deuterium quadrupole correlation coefficient is sensitive to the geometry of H-bonds, the intermolecular distance of the pyrrole-pyridine complex could be determined. The field cycling technique is sensitive enough to detect 14 N ··· 14 N double transitions in parabanic acid thanks to the long spin-lattice relaxation time. However in many biological systems, such as hydrates, the technique is hardly applicable because of the short proton relaxation times. In such cases the solid state NMR technique was utilized. The 2 H NMR spectrum of thymidine reveals interesting static and dynamical features of deuterium quadrupole interaction.

35 Cl NQR spectra of eleven hexachlorometallates (IV) A 1 [MCl 6 ] and A'[MCl 6 ], M = Sn, Te, Pb, and A + =ethylammonium, 4-picolinium, anilinium, triethylammonium, chinoxalinium, and piperi-dinium ions, and A' 2+ = 1.3-propylenediammonium ion, have been observed as a function of tem-perature. The ethylammonium hexachlorometallates(IV) (C 2 H 5 NH 3 ) 2 [MCl 6 ], M = Sn, Te, show a phase transition at 128.8 K and 204 K, respectively. Both compounds yield a single resonance line in their high temperature phases. In case of the stannate this single 35 Cl resonance line splits up into two lines at T c = 128.8 K, whereas for the tellurate no 35 Cl NQR signals could be found in the low temperature phase. A phase transition was also found for the 1.3-propylenediammonium hexa-chlorostannate(IV) at 287 K where the six line NQR spectrum of the low temperature phase changes into a four line spectrum. In contrast, the corresponding plumbate shows no transition. All other compounds studied contain distorted [MCl 6 ] 2- octahedra, and therefore they yield more than one 35 Cl resonance line. The complexes have been investigated in the temperature range temperature where the lines fade out. In case of the hexachlorostannates(IV) with the ethylammonium, the 1.3-propylenediammonium and the triethylammonium ions, the crystal structures of the compounds are known and compared with the results of the 35 Cl NQR spectroscopy.

The 35 Cl NQR frequencies in MgTeCl 6 · 6H 2 O, NiTeCl 6 · 6H 2 O, and NiIrCl 6 · 6H 2 O have been determined at various temperatures. These compounds form rhombohedral crystals with the space group R3̄ ,showing a single resonance line. The 35 Cl NQR frequencies of the Te(IV) complexes exhibit an unusual positive temperature coefficient. The deuterated analogs of the Te(IV) complexes show a small frequency shift at 77 K. These results are interpreted by the presene of O-H ··· CI type weak H-bonds. The resonance frequency of the Ir(IV) complex versus temperature curve shows a minimum and a maximum. This unusual temperature dependence is mainly explained by the same type of the H-bond as existing in the Te(IV) complexes. The π-bond formation between Ir and CI atoms is also partly responsible for the unusual temperature dependence.

In a series of MSnI 3 compounds (M = K, NH 4 , Rb, Cs, CH 3 NH 3 ) two types of coordination around the central Sn(II) were found by 127 I-NQR and powder X-ray diffraction techniques. They are square pyramidal (for M = NH 4 , Rb) and octahedral (for M = CH 3 NH 3 ). CsSnI 3 , on the other hand, showed a drastic structural change of the anion at 425 K from a square pyramid to a regular octahedron. Associated with this phase transition, the electrical conductivity increased from 4 x 10 -3 S cm -1 to about 10 2 S cm -1 . This metallic modification was characterized by 127 I-NQR and 119 Sn Mössbauer spectroscopy.

It is shown that nuclear quadrupole resonance and quadrupole perturbed magnetic resonance represent a powerful technique to determine the local polarization distribution and its second moment, the Edwards-Anderson order parameter, in proton and deuteron glasses. For quadrupolar glasses the local orientational distribution function can be determined from the inhomogeneous NMR or NQR lineshape, thus allowing for a measurement of the quadrupolar Edwards-Anderson order parameter. The comparison of the temperature dependences of the thus obtained order parameters with theoretical predictions then allows for a discrimination between the different possible models of the glassy transition.

The 35 Cl spin-lattice and spin-spin relaxation times T 1 and T 2 as functions of temperature and impurity concentration were analytically calculated and experimentally verified for the incommen-surate phase of [Rb 1-x (NH 4) x ] 2 ZnCl 4 . The results agree with the theoretical predictions for both the amplitude and Goldstone modes of fluctuations.

Dielectric measurements on single crystals and DSC of the title compound (monoclinic, space group P2 1 ) revealed a ferroelectric transition at ca. 355 K (ΔH ≈ 0.5 kJ mol -1 ) Powder x-ray diffraction indicated that the high temperature phase is also monoclinic and the change in volume at the transition is very small. A displacement of hydrogen atoms is considered as responsible for the appearance of ferroelectricity. The 35 Cl NQR signals at ca. 77 K were considerably broad, but a drastic narrowing took place after the compound had once experienced the phase transition. A tentative assignment of the NQR spectrum was made from a calculation of the NQR frequencies based on the CNDO/2 method. Another phase transition was evidenced by a small drift of the DSC curve and a slight anomaly of the dielectric behavior around 358 K. The present 1 H NMR experiments gave no indication of the onset of torsional motion of the NH 2 group reported to occur at ca. 210 K.

Pulsed NQR has been used to get information about the two phase transitions (PT) in 4,4'-di-chlorobenzophenone. The measurements of the NQR spectrum and spin-lattice relaxation suggest that the PT's correspond to a normal-incommensurate and to the lock-in of a commensurate structure at lower temperature. Strong hysteresis has been observed, which could be assigned to memory effects produced by defect migrations.

35 Cl NQR and DSC studies on phase transitions in hexachlorocyclopropane (HCCP), C 3 Cl 6 , are reported. It is found that HCCP has three solid phases: A high temperature disordered phase (Phase I) above 301 K (no NQR spectrum was observed); a metastable phase (Phase II), which exhibited 6 NQR lines from 77 to 270 K; a low temperature phase (Phase III) in which a 24-multiplet of 35 Cl NQR lines at 77 K, the most complex multiplet spectrum ever reported was observed. DSC measurement shows a A-type transition at 301 K and a broad transition of very slow rate at 285 K. The structure and mechanism of phase transitions in HCCP crystal are discussed.

The 81 Br and 127 I NQR spectra were recorded in CH 3 NH 3 HgBr 3 and CH 3 NH HgI 3 , respectively. In addition to a phase transition at 338 K, successive phase transitions take place at 127 ± 1, 184±1 and 243±5 K in CH 3 NH 3 HgBr 3 . On heating, the resonance lines of CH 3 NH 3 HgI 3 disappear near a phase transition at 328 K and one line appears above this temperature. The temperature variations of the resonance frequencies of the terminal halogen atoms in both crystals are extraordinarily steep. This indicates the large amplitude molecular motions expected for the CH 3 NH 3 cations which are linked to the terminal halogen atoms through N-H ··· X type H-bonding.

Using a superregenerative NQR spectrometer, the temperature dependence of 127 I NQR spectra in [N(CH 3 ) 4 ] 2 ZnI 4 was measured between 167 and 335 K. The measurements confirm two known structural phase transitions at 219 and 257.5 K. The correspondence between the distinct regions of iodine NQR spectra and the crystal structure is discussed. The relation of [N(CH 3 ) 4 ] 2 ZnI 4 to the incommensurate family with the general formula X 2 YZ 4 and the high temperature (paraphase) crystal space group D 16 2h (Pmcn) is studied.

The 14 N nuclear quadrupole resonance (NQR) of the cyanides in Ba[Fe(CN) s NO] · 3H 2 O (BNP) was observed between 77 and 293 K. Phase transitions exist at 118,131 (both weakly first order) and 233 K (second order). Most of the cyanides were perturbed by the structural changes except for one of the trans cyanides that was only affected by the transition at 131 K. The mechanism of the phase transitions is interpreted in terms of soft modes resulting from the ion rearrangement produced by the slowing down of the water molecules upon cooling. A comparison of the NQR spectrum of BNP with that of Na 2 [Fe(CN) 5 NO] · 3H 2 O showed that the substitution of Na + by Ba 2+ produced only minor changes in the charge distribution of the nitroprusside ion.

Ab initio calculations of electric field gradients (EFG) and conversion to nuclear quadrupole coupling constants (NQCC) are described, with particular reference to 14 N and 10,11 B. The correlation of EFG and NQCC for a series of boranes leads to the value 8.32 MHz/a.u. (0.0354 barn) for Q( 11 B). The structural variations of these microwave and single molecule calculations of 10,11 B NQCC are discussed.

The electric field gradient (EFG) in body-centered tetragonal Indium metal and hexagonal closed packed Beryllium metal is calculated on the basis of a full potential scalar relativistic augmented plane wave procedure. The various contributions to the EFG in simple metals are discussed. The total EFG in In metal found theoretically is equal to q theor = 2.67 x 10 21 Vm -2 . This value agrees well with the experimental data. Using the quadrupole moment Q deduced from the hyperfine splitting of the muon X-ray spectra one gets g exp = 2.46 x 10 21 Vm -2 . The valence electrons of Indium give a direct contribution of q el = 2.72 x 10 21 Vm -2 , whereas the direct ionic contribution q ion is much smaller and has opposite sign: q ion = -0.01 x 10 21 Vm -2 . There is a small net shielding contribution of q sh = -0.03 x 10 21 Vm -2 to the EFG composed of the ionic contribution g sh. ion = -0.39 x 10 21 Vm - 2 and the valence electron contribution q sh,ve = 0.36 x 10 21 Vm -2 .

Bertaut's method for calculating the electric field and its gradient is analyzed. The formula for the electric field is always valid, whereas that for the electric field gradient holds only for an infinitely large unit cell. We present a practical method to use the formulae.

The indirect multipole interactions of nuclei and impurity paramagnetic ions in metals and alloys via conduction electrons are investigated by means of the dielectric function method. The Hamiltonian of the indirect quadrupole-quadrupole interaction of impurity paramagnetic ions and nuclei is constructed selfconsistently, taking into account the exchange interactions and correlations of the conduction electrons as well as the antishielding effects. It is shown that the energy of these indirect quadrupole interactions of the nuclei and the paramagnetic ions decreases with the distance as R ~ 3 , oscillating with a period which is determined by the wave vector on the Fermi surface and the distance R. The influence of these indirect quadrupole-quadrupole interactions on the width and shape of the NMR lines is studied.

Recent NQR and NMR studies of Cu spin dynamics in metallic YBa 2 Cu 3 O 6+x are reviewed. The Cu nuclear relaxation rates are shown to be inconsistent with weak correlation (band) theory and the Korringa relation, but the Cu spins retain important characteristics of itinerancy. These include pairing of the Cu d-spins in the superconducting state and strong sensitivity of the Cu spin dynamics to changes in carrier concentration associated with variation of the oxygen content. Samples of reduced oxygen content (T c = 60 K) are found to contain a high concentration of Cu + Cu(1) sites associated with "full chain-empty chain" vacancy ordering. These samples also exhibit remarkable precursive suppression of planar Cu(2) nuclear relaxation above T c .

The paper reviews some copper NQR studies in the superconductors YBa 2 Cu 3 O x and YBa 2 Cu 4 O 8 performed at the University of Zürich. The following topics are discussed: the temperature and pressure dependence of the NQR frequencies; relation between linewidths and oxygen deficiency; the determination of the electric field gradients (EFG) and Knight shifts at the copper sites; discussion of some calculations of the EFG tensors; measurements of the various relaxation times in all phases. The results obtained in both structures are compared; possible interpretations are discussed.

We have performed 17 O and 63 Cu NMR measurements in YBa 2 Cu 3 O 7-δ (δ ≈ 0.15) oriented powder samples at CuO 2 plane sites Cu(2) and 0(2, 3) in the temperature range 10-300 K. The temperature dependent Knight shift K(T) and spin lattice relaxation rate T 1 -1 -(T) of O(2, 3) yield K 2 T 1 T = const in accordance with the presence of free carriers at plane oxygen sites. A sharp decrease of T 1 -1 of Cu(2) below 120 K is associated with the opening of a gap in the spectrum of antiferromagnetic spin fluctuations of localized copper 3 d 9 electrons. The comparison of different temperature dependences of spin densities at the Cu and O sites shows the presence of two nearly independent spin systems. A close similarity of YBa 2 Cu 3 O 7-δ a with heavy fermion superconductors is discussed.

Results of NQR, NMR and nuclear relaxation measurements in normal and superconducting states of high T c superconductors, (La 1-x Sr(Ba) x ) 2 CuO 4 , YBa 2 Cu 3 O 7 , (Bi - Pb) 2 Sr 2 Ca 2 Cu 3 O 10 , Tl 2 Ba 2 Ca 2 Cu 3 O 10 and (Nd 1-x Ce x ) 2 CuO 4 are reviewed.

NQR and low-field NMR spectra of 181 Ta are reported to probe the electronic environment of inequivalent tantalum sites in the low-temperature CDW state of a layered compound 2H-TaS 2 . The overall features are found to be consistent with the structure consisting of nine-atom snowflake clusters in a locally commensurate CDW state.

Powder samples of YBa 2 Cu 3 O 7-δ a were oriented by mechanically vibrating the powder at 2.4 K in a magnetic field of 1.2 T. Zeeman split NQR spectra of 63,65 Cu were obtained for the two different Cu sites. The data confirm the orientations of the principal axis system: x ∥ c for the Cu (1) site and z ∥ c for the Cu (2) site. Asymmetry parameters were determined for the two Cu sites yielding η̄ 1 = 0.95 ±0.2 and η̄ 2 = 0.008 ± 0.002. At the Cu(l) site local variations of the symmetry of the EFG tensor were observed. For the Cu sites (CuO 2 -layers) in the Bi 2 Ca 2 Sr 2 Cu 3 O y phase a nearly axially symmetric EFG tensor was found.

For YBa 2 Cu 3 O 7-δ a powder samples of different average grain sizes the temperature dependences of 63 Cu NQR frequencies and signal intensities are reported for the Cu (1) site in a temperature range of 4 300 K. Both quantities show a non-monotonic behaviour in the vicinity of the superconduction transition temperature T c . In the superconducting state the line shape of the NQR signal in zero-field depends on the strength of the dc magnetic field applied during the cooling-procedure. The resulting linewidth yields information about the internal trapped magnetic field.

We have studied 63/65 Cu NQR at the Cu(l)-chain and Cu(2)-plane site of YBa 2 (Cu 1-x M x ) 3 O 7 (M = Fe, Co). The spin-lattice relaxation rate, T 1 -1 , is enhanced at low temperatures and has in the Fe-system a peak at 2 and 4 K for x = 0.005 and 0.015, respectively. Below the peak temperatures the intensity of the Cu-NQR signal decreases rapidly. The results suggest that magnetic ordering of either Fe or Cu moments occurs without destroying the superconductivity below T c on substituting Cu by magnetic ions.

The transverse relaxation rate T 2 -1 was measured in YBa 2 Cu 3 O 7 from 4.2 to 100 K in zero magnetic field. It showed a sudden drop just below T c down to 1/3 of that above T c . A cusp-shaped sharp peak of T 2 -1 was found at 35 K for Cu(2) (plane) site, but not for Cu(l) (chain) site. Except the peak, all the behavior can be interpreted by the model of Pennington et al. that the origin of T 2 -1 at 100 K is mainly the local field fluctuation by the Cu d-electron and secondly the indirect nuclear spin-spin coupling via superexchange interaction between Cu-ions. Below T c the former is sup-pressed.

The nuclear quadrupole resonance of 139 La and 63/65 Cu in La 2-x CMxCuO 4 (M = Ba, Sr) is investigated for x up to unity. The shift of the resonance frequency of 139 La-NQR for 0.01 < x <0.05 indicates an anomalous change of the quadrupole interactions at T* c , where the frustrated magnetic phase appears. The Cu-NQR reveals clear changes of T 1 -1 and T 2 -1 and also of the profile of the Cu-NQR spectra with increasing x beyond x = 0.3 ~ 0.4, suggesting a suppression of antiferromagnetic fluctuations by hole-doping, and also a change of the electronic state of the La-system around x = 0.3 ~ 0.4.

With a high resolution time differential perturbed angular correlation spectrometer we investigated the nuclear quadrupole interaction of Ta in 1T-TaS 2 . The results obtained for the commensurate charge density wave phase lead to a revised assignment of the previously observed 181 Ta NQR resonances. Differences in the intracluster architecture of the 13-atom Star of David cluster between 1T-TaS 2 and 1T-TaSe 2 could be responsible for the metal to semiconductor transition which occurs in lT-TaS 2 but not in lT-TaSe 2 .

NQR spectra due to 63 Cu and 139 La have been recorded for La 3-x Ba 3+x Cu 6 O (x = 0, 0.75, 1). Two groups of 63 Cu NQR lines were observed (around 20-23 MHz and 29-36 MHz). The former is assigned to the Cu atoms in the CuO 4 square planar groups, and the latter to that in the CuO s square pyramidal group. For the 63 Cu NQR lines in La 3 Ba 3 Cu 6 O , the time dependent NQR spectra and the temperature dependence of NQR frequencies were observed.

The anomalous temperature dependence of NQR frequencies which is often observed in crystals in which there is reorientational motion of ions or molecules is considered. Two theories of the phenomena, the jump rotation theory and the pseudo-spin theory, are described, and their successes and failures are discussed. The importance of considering other properties such as pressure dependence of NQR, heat capacity, and elastic constants is emphasized, and it is shown that neither theory is entirely satisfactory at present.

The nuclear quadrupole interaction tensors of 14 N and 39 K in potassium nitrate at room temperature have been determined from nuclear magnetic resonance (NMR) rotation studies of single crystals at 9.4 T. Values for the coupling constants and asymmetry parameters at 296 K are: 14 N: e 2 qQ/h = 751 kHz, η = 0.022; 39 K: e 2 qQ/h = 1326kHz, η = 0.171. The temperature dependence, on approaching the order-disorder phase transition near 401 K, is linear.

Three 35 Cl NQR frequencies were observed for (Me 2 NH 2 ) 2 ZnCl 4 at room temperature, indicating the existence of three crystallographically nonequivalent chlorines in the crystal. With decreasing temperature, the frequency of the lines increased almost linearly and disappeared below ca. 220 K near the reported phase transition temperature (ca. 215 K) detected on cooling. The three NQR lines faded out above room temperature because of the occurrence of rapid anionic reorientational motions disclosed from measurements of the 35 Cl NQR spin-lattice relaxation time.

Chlorine NQR frequencies, and NQR spin-lattice and spin-spin relaxation times (T 1Q and T 2Q ) were measured for NH 4 AuCl 4 crystals at various temperatures between 4.2 and 337 K. Each of the two resonance lines observed at room temperature is split into a doublet below T c = 29 K, indicating a phase transition. A rapid decrease of T 1Q above ca. 290 K is attributable to 90° reorientational jumps of the complex anion about its pseudo C 4 axis. An activation energy of 80 kJ mol -1 was obtained for this motion. 35 Cl T 1Q exhibits a minimum at T c . The T 1Q isotope ratio T 1Q ( 37 Cl)/ T 1Q ( 35 Cl) becomes 1.6 and 1.0 on the high-and low-temperature sides of this minimum, respectively. These results were explained by fluctuations of the electric field gradient produced at the chlorine nuclei by the reorientational jumps of the NH + 4 ions. The motion of the cations is much faster and much slower than the resonance frequencies on the high-and low-temperature sides, respectively. This suggests that the phase transition is of the order-disorder type relating to the orientation of the NH + 4 ion.

Continuous wave and pulsed NQR, dielectric absorption and infrared (IR) measurements were carried out to investigate the origin of the peculiar temperature dependence of the S/N ratio of the 35 Cl NQR in the title compound. The dielectric and IR experiments evidenced a thermally activated jumping motion of the H atom participating in O-H O H-bonds. As T 1 and T 2 below 120 K were much longer than T* 2 of about 10 μs, the line broadening in this temperature region was attributed to static lattice defects. A random freezing of the jumping motion may be the origin of the defects. In the range 120-170 K, S/N was extremely low and T 2 was less than 10 μs, probably having a minimum in between. The short T 2 is considered to be responsible for the low S/N. A minimum of S/N at about 240 K is attributed to a T 1 minimum (less than 10 (μs) at that temperature.

The temperature dependence of the chlorine quadrupole spin-lattice relaxation time T 1Q was observed for one of the three 35 Cl NQR lines of (pyH) 2 MCl 6 (M = Sn, Pb, Te). Each T 1Q curve can be devided into three temperature regions. In the low-and high-temperature regions, T 1Q is dominantly determined by the relaxation mechanism due to the libration and reorientation of [MCl 6 ] 2- , respectively. In the intermediate temperature region, T 1Q results from the modulation of the electric field gradient by the motion of the neighboring pyridinium cations. This way the reorientational motion of the cation between potential wells with nonequivalent depths is precisely characterized.

The temperature dependence of the S/N ratios of three 35 Cl NQR lines (v 1 , v 2 , v 3 ) in chloral iso-butylhemiacetal shows interesting features in the range 77-300 K. 35 Cl T 1 indicates that above 220 K reorientation of CCl 3 is excited, and this results in a gradual decrease in S/N. The activation energy calculated from 35 Cl NQR T 1 results is 30 kJ mol -1 for v 1 and v 2 but 25 kJ mol -1 for v 3 . The difference seems to be attributable to the motion of H in the OH group. A minimum of 35 Cl NQR T 2 is found around 180 K but 35 Cl T 1 shows no anomaly. 35 Cl T 2 determines the S/N ratios in the range between 130-220 K. The line broadening below 130 K is attributable to an inhomogeneity of the electric field gradient at resonant nuclei produced by lattice defects.

The temperature dependences of the 35 Cl NQR frequencies v Q and the nuclear quadrupole spin-lattice relaxation times T 1Q in 2,6-lutidinium hexachlorotellurate (IV) was observed at various temperatures between 80 and 343 K. This crystal undergoes a phase transition at T c = 229 K. A single and three pairs of 35 Cl NQR frequencies were observed above and below T c , respectively. The hysteresis of the phase transition and a discontinuity in the temperature dependence of the resonance frequencies at T c indicate that this phase transition is of first order. Although the resonance frequencies of the pairs in the low temperature phase are very close to one another, T 1Q and below T c could be accurately determined by measuring the Fourier transform spectra of each line. Above ca. 250 K, T 1Q showed an exponential decrease which is attributable to the overall reorientational motion of [TeCl 6 ] 2- with an activation energy of 82 kJ mol -1

The 35 Cl NQR frequencies, linewidths, and spin-lattice relaxation times T 1 ( 35 Cl) of 1,2,3-trichlorotrideuterobenzene were measured at various temperatures. The deuterated compound shows the same bleaching out phenomenon as 1,2,3-trichlorobenzene. Single crystal 2 H NMR measurements were carried out at 295 and 193 K. The nuclear quadrupole coupling constants at room temperature are in the range of 175.8 ≦ e 2 qQh -1 (2H)/kHz ≦ 179.5, and the asymmetry parameters η in the range of 0.060 ≦ η ( 2 H) ≦ 0.073. As for the principal axes of the electric field gradient tensor, it was found that Φ zz ( 2 H) is parallel to the C - D bond, Φ yy ( 2 H) is perpendicular to the benzene ring plane and Φ xx ( 2 H) lies in the ring plane. The linewidths of the 2 H NMR satellites are idependent of temperature. For the undeuterated compound, the temperature dependence of T 1 ( 1 H) was also measured. The mechanism leading to the bleaching out of the 35 Cl NQR signals is discussed.

Solid-state activation energies for fluxional behavior in three aryl-subsituted (pentachlorocyclo-pentadienyl)phenylmercury compounds RHgC 5 Cl 5 : (pentachlorocyclopentadienyl)(pentamethyl-phenyl)mercury (I, R = C 6 (CH 3 ) 5 , E act = 19.3 kJ mol -1 ); (pentachlorocyclopentadienyl)(2,4,6-tris-(terf-butyl)phenyl)mercury (II, R = 2,4,6-C 6 H 2 (C(CH 3 ) 3 ) 3 , E act = 59.5 kJ mol -1 ); and (pentachloro-cyclopentadienyl)(phenyl)mercury (III, R = C 6 H 5 , E act = 62.8 kJ mol -1 ) have beeb obtained from 35 Cl NQR spin-lattice relaxation-time measurements. II has also been shown to be fluxional in solution by 13 C NMR spectra. II was prepared by an exchange reaction between Hg(C 5 Cl 5 ) 2 and Hg(2,4,6-C 6 H 2 (C(CH 3 ) 3 ) 3 ) 2 , which reacted readily despite the great steric hindrance present in the latter reagent.

By single crystal NMR the nuclear quadrupole coupling constants eQΦ zz h -1 ( 2 H), the asymmetry parameters η( 2 H) and the direction cosines of the principal axes of the electric field gradient tensors of the 2 H atoms of the two kinds of D 2 O molecules have been determined at 295 K in RbAl(SO 4 ) 2 · 12D 2 O. The results show that the D 2 O molecules surrounding the ion Al 3+ are “static” within the time scale of the 2 H nuclear quadrupole interaction while the D 2 O belonging to the octahedron around the Rb + undergoes fast reorientations around the twofold molecular axis. For the two kinds of D 2 O in the alum we found: eQΦ zz (1) h -1 ( 2 H) = 186.5 kHz, η (1) ,( 2 H) = 0.1234; eQΦ zz (2) h -1 (2H) = 169.7 kHz,η (2) ( 2 H) = 0.1297; eQΦ zz (3) h -1 ( 2 H) = 122.1 kHz, ( 2 H) = 0.8087. eQΦ zz (1) and eQΦ zz (2) belong to the "static" D 2 O molecules and an assignment to the two different 2 H atoms has been performed. The IR spectra of RbAl(SO 4 ) 2 · 12(H 1-x D x ) 2 O have been investigated, and for the O - D stretching frequencies values of 2145 cm -1 , 2235 cm -1 , 2470 cm -1 and 2525 cm -1 have been obtained. They agree well with the relations between ṽ OD and eQΦ zz h -1 ( 2 H) given in the literature.

35 Cl nuclear quadrupole resonance measurements on thiourea-CCl 4 inclusion complex yielded two resonance lines with intensity ratio 1:3. This indicates that the guest CCl 4 molecule occupies the three-fold symmetry site in the channel formed by the thiourea framework. The resonance frequencies closely follow Bayer's equation between 4.2 K and 27 K. At higher temperatures the resonance lines faded out. This is attributed to the onset of reorientation of the CCl 4 molecule. The activation energy for this motion was estimated to be 6 kJ mol -1 .

The temperature dependence of the 14 N-NQR spin-lattice relaxation time T 1 of a powder sample of NaNO 2 prepared from saturated aqueous solution could be fitted by a monotonic curve. When this sample was annealed, two minima of the T 1 -Tcurve appeared around 35 K and 77 K. The origin of this phenomenon is discussed.

35 Cl NQR spin-lattice relaxation times T 1Q of paramagnetic CuPtCl 6 · 6H 2 O were measured as a function of temperature. Below the cooperative Jahn-Teller phase transition at T c = 136 K, an almost temperature independent T 1Q between 0.6 and 1 ms was obtained for each line of the triplet observed. This suggests that the nuclear spin-lattice relaxation of chlorine is dominantly determined by magnetic dipolar interactions between chlorine nuclei and electron spins. No marked anomaly in the nuclear relaxation rate was detected at T c . Above T c , T 1Q for the single 35 Cl NQR line increased from 0.6 ms at T c to 1-4 ms at 365 K. The rapid T 1Q decrease observed above ca. 390 K can be attributed to a reorientational motion of the [PtCl 6 ] 2- anion with an activation energy of 1.1 x 10 2 kJ mol -1 .

The effect on the 35 Cl quadrupole relaxation of modulations of the electric field gradient by thermally activated motions of nearby nonbonding symmetric and asymmetric atomic groups has been investigated. This way the activation energy of the motions has been determined and the fraction of the electric field gradient influenced by the motions has been estimated. This fraction can be taken as a measure of the intensity of nonbonding interactions in solids.

The second-order Raman phonon process for a multilevel spin system is shown to give a quadru-polar spin-lattice relaxation rate T 1 -1 varying as T 5 at very low temperatures. This relaxation rate for quadrupole spins is similar to the one discussed for a paramagnetic spin system having a multilevel ground state. The temperature dependence of T 1 is discussed on the basis of some simplifying assumptions about the nature of the lattice vibrations in the Debye approximation. This type of relaxation process has been observed below 20 K in tetramethylammonium hydrogen bis-trichloroacetate for the 35 Cl T 1 -1 . Below 20 K the NQR frequency in the same crystal reveals a T 4 temperature dependence due to the induced modulations of the vibrational and librational coordinates by the low-frequency acoustic phonons.

Antimony nuclear quadrupole resonance frequencies in (NH 4 ) 6 Sb 4 (SO 4 ) 3 F 12 crystals were measured as a function of temperature, and no evidence of phase transition was detected between 77 K and room temperature. However, the resonance frequencies do not follow the Bayer type exactly. Such an anomaly is attributed to the effect of rotation of ammonium ions which form hydrogen bonds of types N -H ···F and N -H ···O. In fact, the spin-lattice relaxation time of 1 H and 19 F has a minimum which is ascribed to the reorientation of NH + 4 with an activation energy of 7.14 kJ mol -1 . The switch model for the electric field gradient at Sb nuclei was examined.

Cl 2 was adsorbed at 370 and 870 K on commercially available y-alumina, with BET surface area of 180m 2 /g, after dehydration at 850 K. NQR measurements were carried out between 55 and 6 MHz. The sample containing Cl 2 chemisorbed at 870 K showed the presence of a set of quite strong NQR signals with frequencies lower than 10.6 MHz, and another set of very weak NQR signals between 18.5 and 13.0 MHz. The stronger signals have been inferred as due to Al and have been assigned to Al in three distinct environments. The values of e 2 Qq/h and η determined for the signals are (1) 35.9 MHz, 0.53; (2) 35.7 MHz, 0.74; and (3) 35.3 MHz, 0.0 respectively. These values of e 2 Qq/h are found to be very high as compared to those estimated for 27 Al at distorted octahedral and tetrahedral sites of the alumina, viz. 3.5 and 4.0 MHz respectively, from MAS-NMR. Therefore, chemisorption causes 27 Al to exist in the surface region at sites with greater distortion than in the interior of the particles but much smaller than in the case of completely dehydrated samples for which no NQR signals could be observed. The weak signals have been tentatively attributed to 35 Cl. The present results demonstrate that the NQR technique can be used in studies of chemically bound species on heterogenous catalysts.

The powder lineshapes, associated with Zeeman-NQR Δ|m|= ±1 transitions for half-integer spin from 3/2 and 7/2 are derived using an optimized (θ, ϕ) sampling algorithm for the intensity response. The I = 3/2 system is examined over the full asymmetry range, whereas the I >3/2 lineshapes are calculated for η ≦0.35. The algorithm adopted is modelled on the concepts in the earlier NMR lineshape calculations of Alderman, Solum, and Grant.

New NQR methods, developed over the past years, are discussed: 1. Two frequency NQR double resonance, 2. the influence of strong off-resonance combination of rf pulses on demagnetized state in NQR, 3. remote NQR.

The Bloch-Siegert shift of the NQR frequency is calculated for the case when more than one resonance transition are excited by radiofrequency fields.

We discuss a new form of resonant response for a quadrupolar nuclear spin system subjected to applied alternating magnetic fields.

If the nuclear quadrupole interaction Ĥ Q of the ligands exceeds the magnetic hyperfine Ĥ HF and nuclear Zeeman ĤNZ interaction (Ĥ Q ≫ Ĥ HF +Ĥ NZ ), ENDOR spectra show the same behaviour as Zeeman field perturbed NQR spectra. The angular dependence of the transition frequencies and the transition probabilities of such ENDOR spectra for axial symmetry are calculated and applied to analyze the 35, 37 Cl distant ENDOR spectra of gamma-irradiated NaClO 3 single crystals.

We discuss two new two-dimensional nuclear quadrupole resonance experiments, both based on the principle of nutation spectroscopy, which can be used to determine the asymmetry parameter, and thus the full quadrupolar tensor, of spin-3/2 nuclei at zero applied magnetic field. The first experiment is a simple nutation pulse sequence in which the first time period (t 1 ) is the duration of the radiofrequency exciting pulse; and the second (t 2 ) is the normal free-precession of a quadrupolar nucleus at zero-field. After double Fourier-transformation, the result is a 2 D spectrum in which the first frequency dimension is the nutation spectrum for the quadrupolar nucleus at zero-field. For polycrystalline samples this sequence generates powder lineshapes; the position of the singularities, in these lineshapes can be used to determine the asymmetry parameters η in a very straightforward manner, η has previously only been obtainable using Zeeman perturbed NQR methods. The second sequence is the same nutation experiment with a spin-echo pulse added. The virtue of this refocussing pulse is that it allows acquisition of nutation spectra from samples with arbitrary inhomogeneous linewidth; thus, asymmetry parameters can be determined even where the quadrupolar resonance is wider than the bandwidth of the spectrometer. Experimental examples of 35 Cl, 81 Br and 63 Cu nutation and nutation-echo spectra are presented.

The density matrix approach along with fictitious spin-1/2 or tensor operator formalisms were employed to obtain general expressions for <I x >and thus the signal induced in a coil in the case of physically equivalent non-interacting spin I = 1 and 3/2 nuclei in a single crystal with a three-pulse composite pulse. The corresponding powder averages were then obtained and the dependence of the signal on the i-th pulse flip angle (θ i ) and phase (ϕ i ) studied. A numerical procedure established the optimum "π/2" composite pulse for polycrystalline samples as (90) 0 -(70) 45 -(20) 25 , where the quantity in parentheses denotes the flip angle and the subscript the phase. A two-pulse composite pulse of the form (126) 0 -(300) 80 was also inferred, but the three-pulse composite pulse is superior. These composite pulses are shown to have general validity for other spin I values.

The effect of non-resonant irradiation on the linewidth of 14 N NQR lines has been investigated in three cases with zero asymmetry parameter. The characteristic time of the free induction decay (T 2 *) in Trimethylamine (TMA), Triethylenediamine (TED) and Hexamethylenetetramine (HMT) was studied at 77 K. Circularly polarized non-resonant irradiations of 11 kHz were used with intensities of up to 24 G with a geometrical arrangement following Ito and Hashi. The largest effect, a 100% increase in T 2 *, is measured in TMA. No observable effect occurs in HMT. An intermediate result occurs in TED, where the considerable narrowing caused by the non-resonant field serves to provide a greater resolution of the fine structure first reported by Colligiani and Ambrosetti. This fine structure, presumably dipolar in origin, is not itself affected by the non-resonant irradiation. The degree to which the theory of dressing fields can account for these results is discussed.

An NQR spectrometer has been constructed using two linear integrated circuits in its oscillator-detector. This is very simple and compact and works in range 3-65 MHz. The radio frequency voltage can be varied from 10 mV p-p to 15 V p-p by changing the supply-voltage of an integrated circuit μA 733. The utility of the spectrometer is demonstrated by recording 35 Cl NQR spectra in p-C 6 H 4 Cl 2 , NaClO 3 , and KClO 3 .

This paper discusses the design of a variable-temperature high-sensitivity field cycling spectrometer for nuclear magnetic resonance and nuclear quadrupole double resonance experiments. The instrument is capable of detecting N-14 quadrupole resonance signals from samples as small as 0.1 g down to temperatures of 150 K. The magnetic field cycling is performed by mechanical transfer of the sample using a pair of switchable electromagnetic solenoids, in times of about 0.2 s.

The Zeeman NQR of 79 Br and 81 Br in samples having relatively small r\ was studied on several bromine compounds which were in the powder or polycrystal state. The absorption lines were detected in the integral form by the bridge circuit. When a static magnetic field was applied, the line shapes were broadened and usually two dips appeared symmetrically at both sides of the zero field line. The values of η were estimated from the patterns of the dips. For compounds such as NaBrO 3 , however, as expected no dips were observed.