Showing papers on "Nuclear quadrupole resonance published in 1983"

Cl 35 nuclear-quadrupole-resonance investigations of the lock-in phase and of the new low-temperature phase in K 2 Zn Cl 4

Abstract: The ferroelectric lock-in phase III and the new low-temperature phase IV of ${\mathrm{K}}_{2}$Zn${\mathrm{Cl}}_{4}$ are analyzed by means of $^{35}\mathrm{Cl}$ NQR measurements combined with group-theoretical considerations. It is shown that the eigenvector of the frozen-in soft mode in phase III, which consists mainly of rotations of the Zn${\mathrm{Cl}}_{4}$ tetrahedra around the pseudohexagonal $a$ axis, is very big compared to ${\mathrm{Rb}}_{2}$Zn${\mathrm{Cl}}_{4}$. The amplitude of the frozen-in rotation wave is measured to be 79\ifmmode^\circ\else\textdegree\fi{} at 200 K. In the new low-temperature phase, a quadruplication of the NQR lines is observed which can be explained by a zone-boundary transition leading to a monoclinic phase. A possible influence of the domain pattern upon the NQR-signal amplitude, originating from phase solitons which survived the lock-in transition, is discussed.

Measurement of the hyperfine structure of Pr 3: :YAG by quantum-beat free-induction decay, hole burning, and optically detected nuclear quadrupole resonance

Abstract: The hyperfine splitting of the ground state and the lowest electronic component of the 1D2 excited state have been measured for Pr3+ in YAG. We show that a particularly simple and powerful technique is the production of sublevel coherence by short-pulse (impact) excitation, which is detected by coherent forward scattering of a weak probe beam and Fourier transformation of the resulting quantum-beat signal. This yielded excited-state splittings of 6.49 and 8.29 MHz. The ground state shows large pseudoquadrupole splittings of 33.4 and 41.6 MHz, which were obtained by optically detected nuclear quadrupole resonance.

Nuclear quadrupole resonance and stereochemistry. Part 8. cis–tans isomerism in octahedral SnCl4L2 complexes

Abstract: The 35Cl n.q.r. spectra of several octahedral complexes of tin tetrachloride, SnCl4L2(L = donor ligand), have been measured as a function of temperature between 77 K and room temperature. The significance of the n.q.r. data to the question of the cis or trans relationship of the two ligands is discussed and it is shown that the n.q.r. results must be interpreted with considerable caution.

14N and 35Cl double resonance study of the phase transition in the intercalated bilayer compound C10H21NH3Cl

Abstract: The pure nuclear quadrupole (NQR) spectra of 14N, 35Cl, and 37Cl have been measured in C10H21NH3Cl as a function of temperature. The chlorine NQR frequency νQ (35Cl)=1215 kHz at 47 °C is determined by the N–H––Cl hydrogen bonds and is the lowest reported so far. The 14N quadrupole coupling data e2qQ/h=760 kHz. η=0 shows that the C–N–H––Cl backbone is practically rigid at this temperature. Above 53 °C the effects of the phase transition from an intercalated to a nonintercalated structure show up in the N–H––Cl backbone as a sharp drop of the NQR frequencies to ∼60% of their rigid lattice values. A model accounting for this effect has been proposed.

Vibrations of the atomic nucleus

Abstract: Recently a richly varied spectrum of vibrations has been discovered in the nucleus of an atom. The study of the nuclear vibrations is proving to be a major source of information on the structure of the nucleus and on the forces that hold it together. So far six giant vibrational modes have been observed. They fall in two classes. The first class consists of the modes that the nucleus shares with other spherical bodies. The spherical vibrational modes of the nucleus observed, in order of their discovery, are the giant dipole, the giant quadrupole, the giant monopole and the giant-octupole. The other class involves the spin orientation of the nucleons. Two vibrations of this kind have been observed: they are called the giant Gamov-Teller resonance and the giant magnetic-dipole resonance. (SC)

The C–Cl bonds in trichloroethylidene trichlorolactic ester, Cl3CCHOCOCHOCCl3. A 35Cl–NQR single crystal study

Abstract: By single crystal 35Cl–NQR with a 4π Zeeman spectrometer, the six independent 35Cl‐nuclear quadrupole coupling tensors in trichloroethylidene trichlorolactic ester (chloralide) Cl3CCHOCOCHOCC13 have been studied at T=21 °C. The directions of the electric field gradient (EFG) tensor components have been determined. The main axes φzz are within ±0.5° parallel to the C–Cl bound directions. Very small asymmetry parameters in the range 0.005≤η≤0.053 are characteristic for the EFG tensors of Cl bond to carbon in aliphatic systems, and the six nuclear quadrupole coupling constants are found in the range 76.053≤e2qQh−1/MHz≤78.790. The orientations of the principal axes φxx and φyy is explained as due to intramolecular interactions likely of through space origin.

Zeeman perturbed nuclear quadrupole spin echo envelope modulations for spin 3/2 nuclei in polycrystalline specimens

Abstract: The results of theoretical and experimental studies of Zeeman-perturbed nuclear quadrupole spin echo envelope modulations (ZSEEM) for spin 3/2 nuclei in polycrystalline specimens are presented. The response of the Zeeman-perturbed spin ensemble to resonant two pulse excitations has been calculated using the density matrix formalism. The theoretical calculation assumes a parallel orientation of the external r.f. and static Zeeman fields and an arbitrary orientation of these fields to the principal axes system of the electric field gradient. A numerical powder averaging procedure has been adopted to simulate the response of the polycrystalline specimens. Using a coherent pulsed nuclear quadrupole resonance spectrometer the ZSEEM patterns of the 35Cl nuclei have been recorded in polycrystalline specimens of potassium chlorate, barium chlorate, mercuric chloride (two sites) and antimony trichloride (two sites) using the π/2-τ-π/2 sequence. The theoretical and experimental ZSEEM patterns have been compared. In...

Structural changes in the antifluorite crystal (NH4)2SnBr6

Abstract: A study of structural phase transitions in the antifluorite crystal (NH4)2SnBr6 by nuclear quadrupole resonance and neutron powder diffraction techniques is presented. Interpretation of the bromine...

The Trimethylsulfonium Ion, [(CH3)3S]⊕, in Hexachlorometallate(IV) Complexes, [(CH3)3S]2MCl6 (M = Pt, Se, Te, Sn). 1H NMR, 35Cl NQR, and X-Ray Diffraction Studies

Abstract: The measurements of 35Cl NQR spectra and X-ray powder diffraction patterns were carried out in a series of the complexes of trimethylsulfonium hexachlorometallates(IV), (triMS)2MCl6 (M = Pt, Se, Te, Sn). For (triMS)2TeCl6 and (triMS)2SnCl6, 1H NMR second moments were determined at various temperatures between 77 K and 370 K. The temperature dependence of 1H NMR spin-lattice relaxation times was observed for (triMS)2PtCl6, (triMS)2TeCl6, and (triMS)3SnCl6. From X-ray powder patterns, it was found that the complexes studied are isomorphous with each other and form cubic crystals belonging to the space group T-Pa3. The unit cell dimensions were determined. A single 35Cl NQR line was observed for all of the complexes in agreement with the crystal symmetry. From 1H NMR studies, two kinds of motions of (triMS)⊕, namely, the intracationic CH3 group reorientation and the reorientation of the whole cation about its C3 axis, were observed separately in the above three complexes. The activation energies of these motions were evaluated and compared with those of trimethylammonium ions in hexahalometallate(IV) complexes previously reported.

Electric Quadrupole Interaction in Noncubic Metals

Abstract: The knowledge of the charge distribution is of basic importance for the detailed understanding of the properties of a solid. In noncubic solids this charge distribution creates an electric field gradient (EFG). The quadrupole hyperfine interaction (QI) between the nuclear quadrupole moment Q and the EFG at the site of the nucleus results in an energy-splitting of the nuclear levels. The investigation of this QI in noncubic metals and alloys can be carried out by a variety of experimental techniques. The application of the perturbed angular correlation methods has been especially fruitful in this field. Its advantages, essential for QI studies in metals, are the ease with which temperature and pressure dependencies can be measured and the applicability to pure metals as well as to systems with extremely diluted impurity probe atoms. Thus, most experimental data have been obtained by this technique during the last few years.

35Cl nuclear quadrupole resonance and infrared spectroscopic studies of 2,6-dichloro-4-nitrophenol–amine hydrogen-bonded adducts

Abstract: The charge distribution in hydrogen bridges of varying strengths in solid 2,6-dichloro-4-nitrophenol complexes has been studied using the n.q.r. technique. The dependence of the average 35Cl n.q.r. frequencies, text-decoration:overlinevn.q.r., on the pKa can be analysed in terms of the proton-transfer equilibrium. text-decoration:overlinevn.q.r. provides evidence for the existence of either a double-minimum potential function or one broad minimum. Infrared spectra of such complexes show very broad absorptions extending to the far-infrared, with numerous so-called ‘Evans holes’ and a weakening or complete disappearance of the protonic stretching vibration band in the usual range above 1800 cm–1.

NQR study of the structural phase transitions in a system with reduced effective dimensionality: K2ReCl6

Abstract: Chlorine 35 nuclear quadrupole resonance (NQR) frequency and lineshape data, obtained with a Fourier transform magnetic resonance spectrometer, are presented for the antifluorite crystal K2ReCl6. In the temperature range investigated, 85 to 130 K, two structural phase transitions occur which reduce the crystal symmetry from cubic to tetragonal to monoclinic. The variation of the NQR spectrum with temperature and with hydrostatic pressure up to 2.64 kbar is documented. The structure of the spectrum and the intensity distribution within it are unexpected from previous studies of antifluorite systems. An explanation of the results is given which is based on recent discussions of nonlinear phenomena at phase transitions in crystals exhibiting reduced effective dimensionality.

Structure and bonding in the mixed chalcogenide system As2SxSe3−x

Abstract: 75 As Nuclear Quadrupole Resonance (NQR) lineshape measurements for the amorphous mixed chalcogenide system As 2 S x Se 3−x are reported The line-shapes are asymmetric and approximately 8 MHz in width (full width at half maximum) The peak resonance frequency is observed to increase approximately linearly with x The NQR results indicate the presence of mixed As 2 (S, Se) 3 pyramidal structural units and are thus not consistent with models that predict the occurrence of anion subsite segregation NQR measurements performed on crystalline As 2 SSe 2 lend support to the structural model proposed for the glasses

The bond indium-halogen in isomorphous complexes A2InX5.H2O (A=K,NH4,Rb,Cs; X=Cl,Br). A 35Cl 79,81Br, and 115In NQR study

Abstract: The isomorphous series of complexes A2InX5 · H2O (A = K, NH4, Rb, Cs; X = Cl, Br) was investigated by 35Cl, 79,81Br, and 115In nuclear quadrupole resonance spectroscopy. For the bromine compounds the temperature dependence of the NQR spectra was studied in the range 77 ≦ T/K ≦ 380. Solid state phase transitions are found for K2InBr5 · H2O at T = 145.5 K and for (NH4)2InBr5 · H2O at T = 82 K. The 81Br NQR frequencies are observed between 65 and 80 MHz. At room temperature the 35CI NQR frequencies are found in the range 9.8 ≦ ν (35Cl)/MHz ≦ 11.6. The nuclear quadrupole coupling constants /e2 ϕzzQh−1 (115In)/in the compounds vary between 60 MHz and 115 MHz. There is a marked dependence of /e2 ϕzzQh−1 (115In)/on cation radius rA and temperature. /e2 ϕzzQh−1 (115In) /(increases with increasing rA, and such a dependence is observed for ν(81Br), too. As a function of temperature, n(115In) goes through 1. On the basis of the semiempirical MO model of Townes and Dailey the covalent character of the bond In-Halogen is estimated. From this the increase of /e2 ϕzzQh−1 (115In)/and of ν(81Br) is understood to be due to small changes in the bond character with rA and with temperature.

Experimental and theoretical investigations of the EFG parameters at the Br site in Cd(BrO3)2 ⋅ 2H2O

Abstract: In Cd(BrO3)2 ⋅ 2H2O there are two chemically inequivalent sites of bromine in the unit cell which give rise to two nuclear quadrupole resonances for each isotope of bromine both at RT as well as at LNT. Single crystal Zeeman experiments carried out on the 79Br resonances at RT have revealed the presence of four physically inequivalent sites belonging to each crystallographic site of bromine and the EFG parameters at these sites have been obtained. The conventional point‐charge model, when applied to this system for the evaluation of the field gradient at the Br sites, has yielded unsatisfactory results. In an attempt to include the effect of the covalent bonding in the bromate group, CNDO/2 MO calculations have been carried out on the two inequivalent bromate groups of the system and from the resulting population densities in the orbitals, intraionic EFG or qcov has been estimated. Later, interionic contribution qion, obtained from a point‐charge approximation to the rest of the lattice has been added to ...

A variable-temperature 79Br nuclear quadrupole resonance and X-ray crystallographic investigation of dimethylammonium hexabromostannate(IV), [NH2Me2]2[SnBr6]

Abstract: A variable-temperature 79Br n.q.r. Study of dimethylammonium hexabromostannate, [NH2Me2]2[SnBr6], in the range 77–300 K, has shown that a phase transition occurs at 253 ± 1 K. At room temperature the crystals of the compound are orthorhombic, with a= 14.88(1), b= 7.61(1), c= 7.60(1)A, Z= 2, and space group Pnnm. The structure has been refined to R= 0.076 for 846 diffractometer-measured intensities for which I > 3σ(I). Two Sn–Br bonds in the anion are slightly longer [2.609(3)A] than the remaining four [2.601(2)A], in agreement with the n.q.r. results for the higher temperature phase. The results are interpreted in terms of stronger H-bonding in the low temperature phase.

Nuclear quadrupole resonance and stereochemistry. Part 7. The axial and equatorial chlorine atoms in octahedral complexes of antimony pentachloride, SbCl5L (L = donor ligand)

Abstract: Chlorine-35 and 121,123Sb n.q.r. frequencies and their temperature dependencies for various octahedral complexes of antimony pentachloride, SbCl5L (L = donor ligand), are reported. Assignments of 35Cl resonances to axial or equatorial chlorine atoms of the SbCl5 fragment have been possible in some cases. There are no apparent systematic differences between the frequencies of the axial or equatorial chlorine atoms, such differences as there are being masked by crystal-field effects of similar magnitude.

Nuclear orientation study of 182 Ta in 2H-TaS 2

Abstract: Quadrupole-interaction nuclear orientation experiments have been performed on182Ta (jπ=3−1; T1/2=115d) in 2H-TaS2. γ-anisotropies up to ∼ 30 % have been observed at a temperature of ∼ 8 mK, which, however, is only about half of the value expected taking into account known results of TDPAC measurements on181Ta in 2H-TaS2. In principle, matrices of dichalcogenides could be well suited for nuclear quadrupole resonance on oriented nuclei (NQR-ON) experiments.

Collisionally activated dissociation spectra of protonated methyl N-alkanoates studied using a triple quadrupole

Abstract: The collisionally activated dissociation (CAD) spectra of protonated methyl n-alkanoates CH3 (CH2)nCOOCH3 with n = 1−9 were studied using a triple quadrupole. The influence of hydrocarbon chain length and collision energy on characteristic fragment ions such as those due to the loss of methanol was examined. It was found that most characteristic fragment ions became less important in the CAD spectra with increasing chain length of the esters and finally disappeared in the higher members of the series. Special care would have to be taken when trying to use such neutral losses in analysis for identifying the presence of a certain class of compounds.