Showing papers on "Nuclear quadrupole resonance published in 1985"

Zero field NMR and NQR

Abstract: Methods are described and demonstrated for detecting the coherent evolution of nuclear spin observables in zero magnetic field with the full sensitivity of high field NMR. The principle motivation is to provide a means of obtaining solid state spectra of the magnetic dipole and electric quadrupole interactions of disordered systems without the line broadening associated with random orientation with respect to the applied magnetic field. Comparison is made to previous frequency domain and high field methods. A general density operator formalism is given for the experiments where the evolution period is initiated by a sudden switching to zero field and is terminated by a sudden restoration of the field. Analytical expressions for the signals are given for a variety of simple dipolar and quadrupolar systems and numerical simulations are reported for up to six coupled spin-1/2 nuclei. Experimental results are reported or reviewed for 1H, 2D, 7Li, 13C, and 27Al nuclei in a variety of polycrystalline materials. The effects of molecular motion and bodily sample rotation are described. Various extensions of the method are discussed, including demagnetized initial conditions and correlation by two-dimensional Fourier transformation of zero field spectra with themselves or with high field spectra.

Nuclear quadrupole resonance detected at 30 MHz with a dc superconducting quantum interference device

Abstract: A dc superconducting quantum interference device is used as a tuned radio frequency amplifier at liquid helium temperatures to detect pulsed nuclear quadrupole resonance at ∼30 MHz. At a bath temperature of 4.2 K, a total system noise temperature of 6±1 K has been achieved, with a quality factor Q of 2500. A novel Q spoiler, consisting of a series array of Josephson tunnel junctions, reduces the ring‐down time of the tuned circuit after each pulse. The minimum number of Bohr magnetons observable from a free precession signal after a single pulse is ∼2×1016 in a bandwidth of 10 kHz.

Zero field NMR and NQR with selective pulses and indirect detection

Abstract: Zero field NMR and NQR spectra are obtained by the application of dc magnetic field pulses to a demagnetized sample. Pulsed dc fields allow for selective excitation of isotopic species and provide a means for coherent manipulation of the spin system in zero field. Using these selective pulses and level crossing techniques, indirect detection of a quadrupolar nucleus may be accomplished via protons without obtaining the proton background signal in the NQR spectrum. Experimental results from a variety of 1H, 2H, and 14N homo‐ and heteronuclear systems are presented as an illustration of these techniques.

Nuclear Magnetic Relaxation of Kondo Lattice Compound: CeCu6

Abstract: 63 Cu nuclear spin-lattice relaxation rate (1/ T 1 ) in a Kondo lattice compound CeCu 6 has been measured at zero external field down to 60 mK by the PQR technique. 1/ T 1 is almost temperature independent above 6 K, reflecting the Curie-Weiss behavior of the magnetic susceptibility. Below 6 K, 1/ T 1 becomes temperature dependent with decreasing temperature. Then below ∼0.2 K 1/ T 1 follows the T 1 T =const. relation, showing that the Fermi liquid state has set in. The cross-over from the magnetic regime to the non-magnetic Fermi liquid one has been explored in a typical Kondo lattice compound from a microscopical point of view.

Where are the lone-pair electrons in subvalent fourth-group compounds?

Abstract: Publisher Summary This chapter presents a discussion on the lone-pair electrons in subvalent fourth-group compounds. The chapter presents a table listing of compounds in which subvalent group-four atoms occupy perfectly symmetrical sites. Only two discrete molecular forms are represented, the rest being extended lattices. Certain subvalent, fourth-group compounds undergo single or a series of first-order phase transformations at increasing temperatures from their expected distorted phases to give successively more symmetrical structures. If the material does not melt first, this may include a phase in which the subvalent group-four atom occupies a site of perfect cubic symmetry. The materials which contain perfectly symmetrical sites for the subvalent fourth-group atom have regular octahedral (six) or cubic (eight-) or dodecahedra1 (12-coordinated) environments consisting of equidistant halogen or chalcogen nearest neighbors. Evidence for the cubic symmetry can come from the observation of single lines in Mossbauer (Fortin), pure nuclear quadrupole resonance (NQR), or electron-spin resonance (ESR) spectra, or from X-ray or neutron diffraction studies. The transformations to the cubic phase are accompanied by isotropic expansion of the lattices and large thermal parameters for the fourth-group atom. The resulting cubic phases are often intensely colored and electrically conducting.

Nitrogen and oxygen nuclear quadrupole and nuclear magnetic resonance spectroscopic study of NO bonding in pyridine 1‐oxides

Abstract: "N nuclear quadrupole resonance (NQR) spectra of the NO functional group in a series of 4-substituted pyridine 1-oxides (R=H, CH,, C,H,, OCH,, OCH,C,H,, CI, CN, NOz) and the natural abundance I7O NQR and nuclear magnetic resonance (NMR) solution spectra of a number of selected systems were obtained. The orientations of the principal axes of the '"N electric field gradient tensor (efg) in the NO group were determined using a Townes-Dailey treatment of the NQR results, combined with a -C crosspolarization magic-angle sample-spinning NMR line shape analysis of carbons bonded directly to nitrogen. The major principal axis (Z) is coincident with the N-0 bond. The orientation of the principal (Z) axis of the I7O efg tensor also lies along the N-0 bond, as determined by Townes-Dailey analysis. The N-0 m-bond orders show considerable sensitivity to the nature of the para-substituent, ranging from 0.17 to 0.31. "0 and -N NMR chemical shifts parallel these changes in m-bond order. Aromatic heterocyclic N-oxides have elicited considerable chemical, pharmacological and theoretical interest.14 Fundamental to their chemistry is the dual rr -electron donor-acceptor role of the NO moiety, through resonance with the aromatic ~ystem.~ Thus, the N-0 rr bond is especially sensitive to the nature of ring substituents. Qualitative information concerning the N-0 bond character has been obtained using various spectroscopic techniques, including UV,6 IR,7 photoelectron,' dipole moments,' gas-phase electron diffraction," microwave," x-ray diffraction12-16 and carbonlnitrogen NMR method^.'^,^^ We believe, however, that there remains a need for a more quantitative probe of the electronic environment of the NO moiety, and that nuclear quadrupole resonance (NQR) spectroscopy of the 14N and 170 nuclei can provide such information, since the quadrupole resonance spectrum is determined by the electric field gradient (efg) tensor at the quadrupolar nucleus, and the field gradient, in turn, is determined mainly by the valence shell p-orbital populations of the atom containing the nucleus in question.

Nuclear cooling and the quadrupole interaction of indium

Abstract: Indium has a number of properties that make it an attractive material for nuclear cooling. A small test refrigerator of indium has been constructed and has cooled melting /sup 3/He to 0.9 mK. Its performance demonstrates the favorable comparison of indium with PrNi/sub 5/ for two-stage refrigerators. From specific heat measurements between 1 and 10 mK, the quadrupole interaction has been determined. A value of e/sup 2/qQ/h = -47.8 MHz is obtained for the quadrupole coupling, with a negative electric field gradient.

Cubic to tetragonal structural phase transition in K2OsCl6 as studied by chlorine nuclear quadrupole resonance: precursor effects in the cubic phase

Abstract: Chlorine nuclear quadrupole resonance absorption lineshapes are reported for powder and single-crystal samples of the antifluorite K2OsCl6 for a range of temperatures above the cubic to tetragonal phase transition. The temperature dependence of the lineshape and its integrated intensity, as well as the isotope independence of the results, provide clear evidence of precursor effects associated with nonlinear anharmonic behaviour. The results suggest that the detailed dynamics associated with the precursor clusters is sensitive to whether point defects or dislocations are dominant as the primary cause of line broadening.

Critical behavior of hydrogen bonds observed in high-pressure nuclear quadrupole resonance studies

Abstract: A summary of our NMR results is presented in Table I . Integration of the line shapes of the sample exposed at room temperature yields a total spin density of 1.3 X lozo CO’s per gram of catalyst. Using a CO/Ru ratio of 1 for linear CO and a CO/Ru ratio of 2 for the dicarbonyl, we obtain a dispersion of 30%. Chemisorption measurements yielded a dispersion of 27%. Upon high-temperature exposure, a greater number of dicarbonyls are formed, indicating that dicarbonyl adsorption is activated. A bridged species is also formed, whereas the relative proportion of linear species decreases. We cannot determine whether the bridged species derives directly from the linear species, or whether the adsorption of the bridged species precludes the presence of a linear species a t a particular site. Since no change is evident in the metal surface area after heating, this change in CO distribution cannot be attributed to a change in metal particle size. We have shown that I3C magic angle spinning NMR can be used to identify and quantify CO adsorbates. This technique holds promise for studying differential reactivity of adsorbed species. In addition, since dicarbonyl species may only bond to isolated atoms, the degree of atomic dispersion may be detected by 13C NMR.

35 Cl nuclear quadrupole resonance studies of hydrogen bonding in solid complexes of chlorobenzoic acids with amines

Abstract: 35 Cl nuclear quadrupole resonance studies of hydrogen-bonded adducts containing o-, m- and p-chlorobenzoic acid and 2,6-dichlorobenzoic acid have been carried out. Average frequencies are correlated with ΔpKa values in terms of the proton-transfer model. The results obtained for various proton donors are compared and discussed taking into account both intra- and inter-molecular effects.

Bromine nuclear quadrupole resonance studies of some hexabromostannates: X-ray crystal structure of pyridinium hexabromostannate(IV), [Hpy]2[SnBr6]

Abstract: Bromine n.q.r. measurements have shown that the anion in [NHMe3]2[SnBr6] has a regular octahedral structure in the temperature range 77–300 K, and that [SMe3]2[SnBr6] may well have a similar structure at room temperature, whereas [NMe4]2[SnBr6] undergoes a phase transition to a less regular structure between 178 and 77 K. No signals were obtained from the hexa-bromostannates(IV) of the cations [NEt4]+, [NPrn4]+, or [IPh2]+. A variable-temperature study of the corresponding pyridinium compound in the range 77–335 K has shown that one of the three bromine signals (79Br or 81Br) is at a much lower frequency than the other two, probably due to hydrogen bonding. The only evidence of a possible phase transition was a cusp at 296 ± 1 K in the temperature-dependence plot for one of the 81Br resonances. The crystals of this compound are monoclinic at room temperature, with a= 13.02(1), b= 8.66(1), c= 8.25(1)A, β= 96(1)°, Z= 2, and space group C2. The structure has been refined to R= 0.041 for 926 diffractometer-measured intensities for which F > 4σ(F). Three significantly different Sn–Br distances of 2.624(5), 2.582(1), and 2.559(6)A were found, in agreement with the n.q.r. results where the three signals at 300 K are in a 3:2:1 intensity ratio. The relationship between Sn–Br bond lengths and bromine n.q.r. frequencies in this and other hexabromostannates is discussed.

Nuclear quadrupole resonance spectroscopy

Abstract: This report considers critically, new publications in pure nuclear quadrupole resonance (NQR) spectroscopy up to the end of 2007, including reviews, results for main group elements, and those for transition metals and lanthanides. Recent trends are highlighted, particularly those involving less common nuclei.

The chlorine-35 and antimony-121 and -123 nuclear quadrupole resonance spectra of five-membered ring chelate derivatives of antimony pentachloride

Abstract: The 35Cl and 121,123Sb quadrupole resonance spectra of several five-membered ring chelate derivatives of antimony pentachloride are reported. Four of these are of known structure: the singly chelated derivatives of tropolone, N-methyloxamide, and NN′-dimethyloxadiamide, and the doubly chelated derivative of oxalic acid. The doubly chelated derivative of NN′-dimethyloxadiamide, of unknown crystal structure, has also been measured. The 35Cl resonance frequencies of the axial and equatorial chlorine atoms are quite distinct and, for the tropolone complex, imply a 10% difference between the ionicity of the axial and equatorial groups, and are affected to a different extent by the nature of the chelate group. The large value of the 121Sb asymmetry parameter of the oxalate complex is satisfactorily explained by the observed small angular deformations of the geometry from that of a regular octahedron.

79Br and 127I Nuclear Quadrupole Resonance of Mixed Valency Compounds Ga2X4(X=Br, I)

Abstract: 79Br and 127I nuclear quadrupole resonances(NQR) were observed in α-Ga2Br4, β-Ga2Br4, and Ga2I4 at various temperatures. Resonance lines disappeared at 353, 335, and 448 K for α-, β-Ga2Br4, and Ga2I4 respectively. The wide separation of the 127I resonance lines in Ga2I4 may be attributed to the interaction between the Ga+ ions and the I atoms.

NQR and NMR study of pseudorotation in solids

Abstract: Nuclear quadrupole resonance and nuclear magnetic resonance methods have been used to study the pseudorotation process in solids. In crystalline dimeric (Hal3PNR)2 phosphazo compounds, where Hal = CI or F, and R contains protons, the temperature dependences of the 35CI, 19F and 1H spectra and the relaxation times indicate intramolecular exchange of the ‘trigonal twist’ type between one axial and two equatorial halogen atoms attached to the pentacoordinated phosphorus. The activation energy, Ea, for the exchange process is obtained from the T1 temperature dependence of the 35CI nuclei and from the T1e temperature dependence of the 19F and 1H nuclei. In the case of chlorinated phosphazo compounds, Ea, lies in the 59-105 kJ mol−1 range, whereas in solid (F3PNCH3) Ea, is about 38 kJ mol−1.

Chlorine nuclear quadrupole resonance lineshape in the cubic antifluorite K2OsCl6

Abstract: Chlorine nuclear quadrupole resonance lineshapes are reported for the cubic antifluorite K2OsCl6 in the temperature range 70–300 K. For temperatures above 172 K the spectrum consists of a single symmetric line; for temperatures below 172 K an asymmetric line is observed which can be represented as a sum of two symmetric lines. The symmetric lines are temperature dependent mixtures of Lorentzian and Gaussian profiles. The explanation is that the nuclear quadrupole resonance lines observed in K2OSCl6 are inhomogeneously broadened by temperature dependent local strains caused by point defects and dislocations.

14N Nuclear quadrupole resonance in substituted isatin compounds

Abstract: 14N nuclear quadrupole resonances have been found for eight substituted isatin compounds Variations in the electric field gradient (EFG) and asymmetry parameter, η, have been attributed to inductive rearrangement of the electron distribution at the nitrogen site A correlation between the inductive Taft parameter, σ*, of the substituent and the charge density difference between the hybridized nitrogen bonding orbitals, σNHσNc, has been established Implications of the results with respect to the available biological data on substituted isatins are discussed

Pressure dependence of 35Cl nuclear quadrupole resonance in 2,5-, 2,6- and 3,5-dichlorophenols

Abstract: Pressure dependence of the 35Cl Nuclear Quadrupole Resonances (N.Q.R.) in 2,5-, 2,6- and 3,5-dichlorophenols (DCP) has been studied up to a pressure of about 6·5 kbar at room temperature. While the pressure dependence of the two resonance lines in 2,6-DCP is essentially similar, the lower frequency line in 2,5-DCP is almost pressure independent and the higher frequency line shows a linear variation with pressure upto about 3·5 kbar but shows a negative pressure coefficient beyond this pressure. The two lines in 3,5-DCP have a non-linear pressure dependence with the curvature changing smoothly with pressure. The pressure coefficient for both lines becomes negative beyond a pressure of 5 kbar. The pressure dependence of the N.Q.R. frequencies is discussed in relation to intra- and inter-molecular contacts. Also, a thermodynamic analysis of the data is carried out to determine the constant volume temperature derivative of the N.Q.R. frequency.

The Iodine-127 Nuclear Quadrupole Resonance of Several Iodine Complexes with Sulfur-containing Organic Molecules

Abstract: The 127I NQR spectra of 1,4-dithiane–2I2, thiane–I2, 1,4-thioxane–I2, and 1,3,5-trithiane–I2 are reported. Two distinct iodine atoms are observed in the former three complexes, whereas only one iodine atom is present in the last-named complex. The charge densities on the donors and the iodine atoms are obtained according to the Townes-Dailey approximation. The order of donor strength toward the iodine molecule is: thiane>1,4-thioxane>1,4-dithiane>>1,3,5-trithiane. The thioethers can compete with amines in terms of the donor strength to iodine molecules. The no-bond ionic structure of the ground state is less important in the thioether–iodine complexes than in amine–iodine complexes. The positive temperature coefficients found previously in a few similar complexes of amines are observed for the terminal iodine atoms of 1,4-dithiane–2I2, thiane–I2, and 1,4-thioxane–I2. This phenomenon is ascribed to the increasing shift of the dative structure to the no-bond covalent structure with the rise in the temperature.

Dependence of nuclear quadrupole double resonance spectra on the level of the saturating radio-frequency field

Abstract: The dependence of the intensity of nuclear quadrupole double resonance (NQDR) lines on the level of the saturating rf field and the nature of its absorption has been studied. It has been shown that the intensity of the solid-state doublets depends on the square of the amplitude of the saturating field. Pure solid-state doublets have been recorded for the first time for amino groups in eight compounds. Picolinamide, nicotinamide, acetamide, and trichloroacetamide comprise a group of compounds with similar values for the quadrupole coupling constants and the asymmetry parameters. Urosulfan, guanidine chloride, urea, and thiourea make up another group. The interproton distances in the amino groups have been determined in all the compounds on the basis of the multiplet structure of the NQDR lines.

Cubic to tetragonal structural phase transition in K2OsCl6 as studied by chlorine nuclear quadrupole resonance: tetragonal phase lineshapes

Abstract: Chlorine nuclear quadrupole resonance lineshapes are reported for the antifluorite K2OsCl6 for a series of temperatures below the temperature of the cubic to tetragonal structural phase transition. The data serve to clarify the identification of the transition temperature Tc. A detailed analysis of the magnitude spectra shows that: (i) the data are consistent with the occurrence of a commensurate structure phase transition at Tc; (ii) additional strains are introduced into the crystal as a result of the structural alteration.

Powder Zeeman Study of the Nuclear Quadrupole Resonance Lower Transition Spectrum for I = 5/2; Application to Orthoperiodic Acid

Abstract: The Zeeman effect of the nuclear quadrupole resonance (NQR) lower transition (± 3/2 ↔ 1/2) spectrum for I=5/2 in crystalline powder has been studied and the frequency splittings of the ±3/2 ↔ ± 1/2 transition line have been plotted as a function of the asymmetry parameter η and the external magnetic field H. The experimental Zeeman frequency splittings of the 127I lower transition line in crystalline powder of H5IO6 have been compared with the theoretical values in order to evaluate η. The present value of η agrees with the earlier values reported from the two transition frequencies and also from a single crystal Zeeman study on the ± 3/2 ↔ ± 1/2 transition line.