Showing papers on "Nuclear quadrupole resonance published in 1986"

Zero-field NMR and NQR spectrometer

Abstract: In comparison to high‐field NMR, zero‐field techniques offer advantages in terms of spectral interpretability in studies of polycrystalline or amorphous solids. This article describes a technique and apparatus for time‐domain measurements of nuclear magnetism in the absence of applied fields (Fourier transform zero‐field NMR and NQR). Magnetic field cycling and high field detection are employed to enhance sensitivity. The field cycling is accomplished with an air‐driven shuttle system which moves the sample between regions of high and low magnetic field, in combination with switchable electromagnets in the low‐field region. Sudden field steps or pulses are used to initiate coherent nuclear spin evolution in zero field and to monitor such evolution as a function of time. Experimental results are shown and analyzed. Possible variations on the basic method are described and their relative advantages are discussed.

Quasistationary state and its decay to equilibrium in the pulsed spin locking of a nuclear quadrupole resonance.

Abstract: Pulsed spin locking in nuclear quadrupole resonance (NQR) is analyzed within the framework of Floquet theory. This produces an effective Hamiltonian to which the techniques of statistical mechanics can be applied and the equilibrium properties of the system explored. Under the Ostroff-Waugh pulse sequence, the magnetization rapidly evolves to a quasistationary state. This state subsequently decays slowly to equilibrium. The behavior of the magnetization with respect to pulse angle, pulse spacing, and resonance offset is analogous to the NMR case. The major differences are the dependence of the results on the orientation of the quadrupole principal axes with respect to the laboratory frame and the additional dipole interaction terms that arise in the three-level system. The results are compared both to NQR experiments and the analogous NMR experiments.

Nuclear electric quadrupole induction of atomic polarization.

Abstract: Precessing nuclear quadrupole moments are observed to induce oscillating electric dipole moments in neighboring atoms. The $^{35}\mathrm{Cl}$ nuclei of a single crystal of NaCl${\mathrm{O}}_{3}$ placed between the plates of a capacitor are excited into precession by a rf pulse. After the pulse, the electric polarization induced by the chlorine nuclear quadrupole moments produces a voltage across the capacitor, which is amplified with a dc SQUID. This technique provides information on the location and polarizability of atoms near nuclear quadrupole moments in noncentrosymmetric crystals.

Nuclear Magnetic Resonance and Nuclear Quadrupole Resonance Study of 181Ta in the Commensurate Charge Density Wave State of 1T-TaS2

Abstract: The nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) of the 181 Ta nuclei have been made on 1 T -TaS 2 at 4.2 K in order to get detailed information on the superlattice structure in the commensurate charge density wave state of this material from a microscopic viewpoint. The NMR ane NQR spectra are consistent with the star-shaped cluster model inferred from diffraction experiments. Furthermore the spectra indicate that the commensurate phase of 1 T -TaS 2 has a complicated and somewhat disordered stacking of CDW layers in contrast with that of isostructural 1 T -TaSe 2 (the ordered \(\bm{c}_{0}+\bm{2a}_{0}\) stacking). On the basis of these results, we propose a new picture of the semiconductive origin of 1 T -TaS 2 , including an explanation of the quite different behaviors of the low temperature conduction between 1 T -TaS 2 and 1 T -TaSe 2 .

17O and 39K quadrupole resonance study of the ferroelastic phase transition in KH3(SeO3)2

Abstract: The 17O‐proton nuclear double‐resonance spectra of KH3(SeO3)2 have been measured above and below the ferroelastic transition and the quadrupole coupling has been determined for all three chemically nonequivalent oxygen sites in the unit cell. The 17O‐proton magnetic dipolar coupling as well as the 17O quadrupole coupling data clearly show the presence of proton motion between two equilibrium sites in the O(2)–H‐ ‐ ‐O(2) bond which freezes out below Tc. The temperature dependence of 30K quadrupole frequency has been as well determined.

Nuclear Quadrupole Resonance in Chloro- and Bromoantimonate(III) Complexes

Abstract: C1, 3581Br, and 121Sb N Q R spectra were observed in com plexes o f the type RxSbyX z with anionstoichiometries SbCl4 , SbClg- , SbBr4 , SbBr^~, SbBr^- , and Sb2Br^~. Several complexes showwidely spaced halogen resonance lines and unusual temperature dependences o f the NQ Rfrequencies. These findings are accounted for 35Cl, 81Br, and 121Sb NQR spectra were observed in complexes of the type RxSbyXz with anion stoichiometries SbCl4- , SbCl63- , SbBr4- , SbBr52-, SbBr63- , and Sb2Br93-. Several complexes show widely spaced halogen resonance lines and unusual temperature dependences of the NQR frequencies. These findings are accounted for in terms of a three-center four-electron bond of the X-Sb-X group and cation-anion interaction.

Zero field nmr of small amplitude motions in a polycrystalline solid

Abstract: The librational motions of the water molecules in polycrystalline barium chlorate monohydrate have been studied by using proton and deuterium zero-field NMR. In contrast to high-field NMR, subtle molecular motions produce readily observable changes in the zero-field spectrum. Computer simulations and application of a novel-pulsed zero-field technique confirm that the splitting observed in the zero-field spectrum of the hydrate results from the motionally induced asymmetry of the magnetic dipole-dipole coupling tensor.

Temperature dependence of 35Cl nuclear quadrupole resonance frequencies and hydrogen bonding in some metal(II) hexachlorostannate(IV) hexahydrates.

Abstract: The temperature dependences of 35Cl NQR frequencies were observed over a fairly wide range of temperature for MSnCl6·6H2O [M(II)=Mg, Ca, Mn, Co, Ni] forming isomorphous crystals (R\bar3) at room temperature. These complexes yielded unusual 35Cl NQR temperature dependence curves showing a positive temperature coefficient. This can be attributed to the existence of Cl···H–O type weak hydrogen bonds.

Deuterium and sodium quadrupole interactions in sodium hydroxide. II. The monoclinic phase

Abstract: The temperature dependence of the 23NaOH pure quadrupole resonance (NQR) frequency has been studied from 77 to 550 K and shows a change in slope and a discontinuous change in frequency of 6±2 kHz at the orthorhombic to monoclinic phase transition at 505 K. At 292 K the 23Na NQR frequency is 1.778±0.001 MHz, η<0.04. The temperature dependence of the deuterium quadrupole coupling was fit to a librating molecule model between 293 and 570 K (QCC=245±2 kHz, η=0.05±0.01 at 293 K) and confirms that the frequency of the OD− libration decreases in the high temperature monoclinic phase.

Boron nuclear quadrupole coupling in o‐ and m‐carborane (B10C2H12)

Abstract: The 11B nuclear quadrupole coupling in the title compounds is reported. The experimental technique applied was double resonance with level crossing. The values for (e2qQ/h)(1+η2/3)1/2 are (in MHz, errors of last digits and assignments in brackets): o‐carborane: 1.10(6) (B8,B10) (tentative), 1.22(6) (B9,B12), 1.60(4) (B4,B5,B7,B11), 1.86(4) (B3,B6); m‐carborane: 1.22(4) (B9,B10), 1.56(12) (all other borons, unresolved).

Nqr study of the floating of the incommensurate modulation wave in Rb2ZnCl4

Abstract: Floating of the incommensurate modulation wave partially averages out dispersion of NQR signals in Rb2ZnCl4. Theoretical analysis is based on a realistic model which considers linear and quadratic term in the relation betwen the frequency and nuclear displacement. Measurable effects are limited to a relatively narrow interval (∼ 5 K) just below the transition (T 1) from the paraelectric into the incommensurate state. The root mean square phase fluctuations decrease from ≃ 30–50° at T 1 - T = 1 K to less than σ ≈ 20° at T 1 - T = 4 K.

Cluster distribution in paraelectric KH 2 AsO 4 . I. Dispersion of the proton spin-lattice relaxation time

Abstract: The anomalous frequency dependence of the proton spin-lattice relaxation time in paraelectric ${\mathrm{KH}}_{2}$${\mathrm{AsO}}_{4}$ is the result of proton${\mathrm{\ensuremath{-}}}^{75}$As level crossing in partially polarized regions where the As nuclear quadrupole resonance frequency matches the proton Larmor frequency. The experiment allows the determination of the short-range-order polarization distribution function even for strongly polarized regions which, in view of their small abundance, are hard to detect otherwise.

Tunneling rotation of two inequivalent methyl groups in orthorhombic acetamide

Abstract: The proton spin-lattice relaxation in orthorhombic acetamide haS been investigated between 250 K and 20 K at 25.0 MHz and 11.4 MHz. The temperature dependence of the spin-lattice relaxation was explained from the consideration of the rotational tunneling motions of two inequivalent methyl groups having different tunneling frequencies, ω=119±20 MHz and another one estimated to be a few hundreds MHz. Nonexponential magnetization recovery observed in the temperature region above 40 K was explained in terms of rapidly reorientating three spin systems. The spin-lattice relaxation rate of 1 H NMR was also compared with that of 14 N NQR in the same acetamide.

Cubic-to-tetragonal structural phase transition in K 2 OsCl 6 as studied by chlorine nuclear quadrupole resonance: spin-lattice relaxation

Abstract: Measurements of the chlorine nuclear spin-lattice relaxation time are reported for the antifluorite K2OsCl6 in the temperature range 17 to 300K. This substance undergoes a cubic-to-tetragonal structural phase transition at a temperature Tc of about 40K. Except at low temperatures, where impurity effects become important, the temperature dependence of the relaxation rate obeys an equation of the form T1-1=CT2/(T-Tc). This result is consistent with a Raman two-phonon relaxation mechanism for an undamped soft mode described by a dispersion curve of the form omega (q)= omega 0+ alpha q2. As Tc is approached from above the recovery of the nuclear magnetisation becomes non-exponential, and can be represented as a double exponential characterised by two time constants T1L and T1S. This result is accounted for by assuming that the fluctuations responsible for the relaxation have the symmetry of the rotary lattice mode and that these fluctuations occur on two very different scales in spatially localised regions in the specimen. Below Tc the relaxation time T1Z and T1XY associated with the two resonance lines in the tetragonal phase are different and T1Z>T1XY. This observation is explained as a consequence of the two-dimensional nature of the correlations in this compounds.

Chlorine-35 nuclear quadrupole resonance studies in 2,3- and 3,5-dichloroanisoles

Abstract: The temperature variation of the 35Cl n.q.r. frequencies in 3,5- and 2,3- dichloroanisoles has been reported here. Both compounds show two lines each, and these have been assigned to the two chlorines in the same molecule with the help of the additive model for the substituent effect. The temperature dependence has been analysed in terms of Bayer–Kushida–Brown model. The torsional frequencies and their temperature dependence have been calculated numerically under a two-mode approximation. On comparing the results in 3,5-dichloroanisole with those in 3,5-dichlorophenol it can be seen that they show similar behaviour owing to the absence of hydrogen bonding in both.

14N nuclear quadrupole resonance study of phase transitions of 1M and 2Or polytypes of potassium hexacyanoferrate (III)

Abstract: 14N nuclear quadrupole resonance (NQR) frequencies were measured between 77–328 K for simple monoclinic (1M) and orthorhombic (2Or) polytypes of potassium hexacyanoferrate (III) (KFR) A reversible first order transition was found at 133 K for 1M and at 138 K for 2Or polytype The transition was interpreted in terms of the coupling of a quasisoft rotatory mode with a temperature dependent tilt motion of the complex ion An irreversible interpolytype transition produced by internal strains was observed in a sample formed by 1M and 2Or polytypes and repeatedly cycled between around 80–140 K and room temperature Selective broadening of lines observed in only a pair of N atoms of each hexacyanoferrate ion was attributed to the occurrence of one dimensional disorder of neighboring K ions The disorder disappeared after sample recrystallization It was found that low crystallization rates produced only the 1M structure while high rates increased the occurrence of the 2Or over 1M polytype and even produced a th

Nuclear Quadrupole Resonance: The Present State and Further Development

Abstract: The importance of nuclear electric quadrupole interactions in chemistry, both present and future, depends very much on their power to resolve problems in electronic structure and molecular dynamics. Fortunately, the subject, by its very nature, is “multinuclear”; even if the ground state of a given nucleus is non-quadrupolar, there often exist excited nuclear states which are, an example being 19F, for which quadrupole coupling constants are now being published from angular correlation measurements. Other new techniques are constantly extending the range of the experiments, recent examples being the use of SQUID magnetometers to detect acoustic 121Sb and 123Sb quadrupole resonance in antimony metal and Fourier transform quadrupole resonance spectroscopy based on fast field cycling to measure 2H quadrupole interactions in powders. Recently, much work on quadrupole interactions in solids of half-integral spin nuclei such as 17O or 27Al has been pursued in two different ways; by quadrupole double resonance in natural abundance, and nuclear magnetic resonance in very high magnetic fields, for which enrichment of low-abundance nuclei such as 170 is often required. In the liquid phase, measurements are now sufficiently reliable for comparisons of changes in the nuclear electric quadrupole tensor from gas to liquid and solid phases to be made. The new methods of partial alignment of polar molecules in the liquid phase in strong electric fields, or magnetically anisotropic molecules in high magnetic fields, seem certain to contribute to these developments.

Nuclear quadrupole resonance studies of chelated antimony complexes. Part 2. The crystal structure and the 35Cl and 121,123Sb resonance frequencies of tetrachloro(acetylacetonato)antimony(V) and related compounds

Abstract: 35 Cl and 121,123Sb nuclear quadrupole resonance (n.q.r.) spectra of tetrachloro (acetylacetonato)antimony(v), (1), and ten related compounds have been observed and measured in the range 77–300 K. The crystal structures of (1) and of tetrachloro(benzoylbenzophenato)antimony (5) are also reported. The n.q.r. spectrum of the latter compound indicates the occurrence of a second-order phase-change in the neighbourhood of 135 K; this is confirmed by thermal analysis. Comparison of the crystal structure and n.q.r. spectrum of (1) indicates that a similar phase-change takes place here below 77 K. The distinction between the 35Cl resonance frequencies of the axial and equatorial chlorine atoms of these compounds, already suggested in Part 1, is confirmed.

Molecular Motion in GeCl4 as Studied by 35Cl Nuclear Quadrupole Resonance

Abstract: Temperature dependence of 35Cl nuclear quadrupole resonance frequencies and quadrupole relaxation time was measured of GeCl4 in the metastable phase. Above 160 K, molecular reorientation is seen ac...

35Cl NQR investigations on crystalline α,α′‐dichloro‐o‐xylene

Abstract: The Zeeman spectrum of the 35Cl NQR line in a cylindrical single crystal of α,α′-dichloro-o-xylene was recorded at room temperature and analysed to obtain information regarding the nature of the crystalline unit cell. The crystal system is determined from the analysis. The two observed loci are due to the two chlorines in a single molecule, and all the benzene rings are parallel to one another whereas the two CCl bonds are inclined to one another at an angle of 102°. The absence of a cleavage plane may be due to the out-of-plane CCl and CH bonds in the chloromethyl groups. The ionic, single bond and double bond characters are in the ratio 28.54:70.64:0.82.