Showing papers on "Nuclear quadrupole resonance published in 1977"

Nuclear quadrupole resonance in the interaction representation

Abstract: Using the interaction representation, the transient pulsed response of an ensemble of nuclei with spin I=3/2 experiencing electric quadrupole couplings is calculated, both in zero field and in the presence of a weak, applied static field. It is found that, in this representation, the evolution of the density operator depends, to a good approximation, on a time-independent effective hamiltonian. For the response to a solitary, resonant 90° pulse, the prediction is made of the existence of two transient decay signals, 90° out of phase with each other, one being determined by the even moments of the absorption line (as in the ordinary N.M.R. case) and the other being determined by the odd moments. This second signal will be present in the N.Q.R. case, since here the effect of dipole-dipole couplings seems to be, in general, to broaden the resonance line at ω0 asymmetrically, and while the first moment can be proved to be always zero, this is not, in general, true of the higher odd moments. Expressions are de...

187Re, 14N, and 2H nuclear quadrupole couplings in NH4ReO4: Evidence for a possible phase transition

Abstract: The nuclear quadrupole couplings of the 187Re, 14N, and 2H nuclei in NH4ReO4 have been measured over a wide range of temperatures. The 187Re nuclear quadrupole resonance has been observed throughout the previously reported ’’wipeout’’ range of temperature, showing that disappearance of the resonance is due to gradients of temperature and strain in the sample. The pressure dependence of 187Re quadrupole resonance has been measured as a function of temperature between 77 and 362 K. The slope of the graph of (∂ν/∂P)T versus temperature is indicative of a continuous phase transition at this temperature. The nature of the phase transition and its relationship to other phenomena in NH4ReO4 is unclear.

Theory of nuclear quadrupole interaction in hexagonal close packed metals—cadmium and zinc

Abstract: The conduction-electron contribution to the electric field gradient has been calculated for zinc and cadmium. From a study of the conduction and core electron distributions relative to each other, it is concluded that different components of the field gradient due to the conduction electrons, such as from the local and plane-wave components of the conduction-electron densities, should be shielded differently. A value of about ..gamma../sub infinity/is estimated for the field gradient due to the plane-wave component of the density. Using the calculated field gradients in the two metals, the quadrupole moments of /sup 67/Zn and /sup 111/Cd are obtained as 0.50 and 0.76 b, the latter being in good agreement with the value derived earlier from ionic crystal measurements. The effect of the larger antishielding factor on the earlier calculated field gradients in beryllium and magnesium is discussed and it is concluded that the earlier good agreement between theory and experiment for these metals is not significantly affected. Finally, the bearing of the results of the present work on the empirical correlation obtained recently between the conduction-electron and lattice contributions is discussed.

N 14 NQR and relaxation at the ferroelectric transition in NaN O 2

Abstract: $^{14}\mathrm{N}$ nuclear quadrupole resonance and relaxation measurements in NaN${\mathrm{O}}_{2}$ around the ferroelectric transitions have been performed by pulse and Fourier-transform technique, with high-temperature resolution and accuracy In the narrow temperature range in which the antiferroelectric phase exists and where previous authors failed to detect the nuclear-quadrupole-resonance (NQR) signals, the splitting of the ${\ensuremath{ u}}_{+}$ and ${\ensuremath{ u}}_{\ensuremath{-}}$ lines in two components were observed A theory is developed to connect the NQR parameters to the torsional vibrations of the N${\mathrm{O}}_{2}^{\ensuremath{-}}$ ions as well as to the order-disorder reorientational fluctuations driving the transitions The critical fluctuations are described through a kinematical Ising model It is proven that the reorientational order-disorder variable is reversed by an angle which is not exactly $\ensuremath{\pi}$ By analyzing the experimental results in the light of the theoretical treatment interesting information on the critical dynamics is obtained

Crystal data and nuclear quadrupole resonance spectra of tetra-arsenic triselenide

Abstract: Tetra-arsenic triselenide, As4Se3, has been synthesized and isolated in two polymorphs of which α-As4Se3 is isostructural with α-As4Se3. It is orthorhombic, space group Pnma, with Z= 4, in a unit cell of dimensions a= 9.46(2), b= 7.97(2), c= 10.47 (2)A. The 75As n.q.r. spectrum at 77 K shows three resonances with intensities 1 : 2 : 1 at 60.78 and 61.07 MHz due to the basal As and at 68.97 due to the apical As of the As4Se3 molecule. The polymorph β-As4Se3 is monoclinic, space group C2/c or Cc, with Z= 16 in a unit cell of dimensions a= 25.62(5), b= 6.52(2), c= 23.01(5)A, β= 126.6 ± 0.5°. Its n.q.r. spectrum at 77 K shows a number of incompletely resolved lines between 57.5 and 61.0 MHz and between 69.5 and 71.5 MHz, associated with basal and apical As atoms of the As4Se3 molecule.

Nitrogen-14 nuclear quadrupole resonance study of several hydroxypyrimidines

Abstract: Nitrogen‐14 nuclear quadrupole resonance (NQR) spectra of several hydroxypyrimidines of biological and pharmacological importance are reported and analyzed in the framework of the semiempirical Townes and Dailey theory. Included among the compounds are thymine, uracil, uridine, barbituric acid, orotic acid, and cyanuric acid. The results obtained suggest that in all cases the molecules in their ground state exist predominantly in the dilactam tautomeric form. Significant differences have been observed in the NQR spectra of the two nitrogens in barbituric acid which cannot be accounted for by the differences in the hydrogen bond lengths alone.

Progress in nuclear quadrupole resonance investigations of halogen‐containing organic and organometallic compounds

Abstract: Recent nuclear quadrupole resonance (n.q.r.) investigations of halogen-containing organic and organometallic compounds, covering the period between 1970–1976, are reviewed. Special attention is paid to n.q.r. studies of the stereochemistry of organic molecules with particular emphasis on the interactions of geminal atoms, and the application of Stark effects to the study of the coordination and hybridization of atoms.

14N PNQR investigation of the effect of pressure on phase transition in malononitrile

Abstract: The 14N pure nuclear quadrupole resonance frequencies in the molecular crystal malononitrile were measured as a function of hydrostatic pressure up to 5.6 kbar. Above T=21.55°C, the second order phase transition previously studied by Zussman and Alexander was observed at high pressures. Above the transition pressure pc, each resonance line splits into two. The splittings Δν along an isotherm at 23°C were fitted to the equation Δν=B (p−pc)β and the critical exponent β was found to be close to 0.5. The variation of Tc with pressure was measured to 2 kbar and the line of transition could be described by the linear equation Tc(p) = 21.51+16.5734p. The present results for the variation of Δν along an isotherm were compared with previous results obtained along an isobar.

Nuclear quadrupole double resonance

Abstract: The basic theories of Nuclear Quadrupole Resonance are outlined, together with some basic NMR theory and its application to double resonance techniques. An improved design of spectrometer using double resonance is presented, including a description of a new flow cryostat which can operate down to 4.2°K with low (250 cc/hour) consumption of liquid He. The whole apparatus may be used to detect NQR signals in solids over the frequency range 100 kHz-5 MHz. The theory of double resonance with spin mixing by continuous coupling (D R CC) is developed and illustrated by experiments performed on HCOOD, HDO and several nitrogen-containing compounds, including Imidazole. Further experiments were then carried out on samples dilute in deuterium, including the detection of NQR in formic acid (HCOOH) due to naturally abundant deuterium. Other experiments on proton-deuteron interactions in amide groups and in water molecules are also described and compared with results previously published on fully-deuterated amides and D20 as a water of crystallisation in L-serine hydrate. The NQR spectrum of Lanthanum Nicotinate Dihydrate is obtained using F.M. techniques and related to its structure. The theory of Zeeman NQR in spin 3/2 nuclei is developed with reference to the double resonance technique, and the measured quadrupole coupling constants of 23Na and 2D in NaOH and some of its hydrates are discussed. Finally, some standard theories concerning proton relaxation times in solids at low temperatures (4°K-300°K) are outlined and related to some, measured relaxation times, illustrating their significance to the double resonance technique. The concluding chapter outlines a speculative proposal for a new spectrometer design.

XIX. Nuclear Quadrupole Resonance Spectroscopy in Solids

Abstract: In these lecture notes some basic aspects of Nuclear Quadrupole Resonance (NQR) spectroscopy are presented. The parameters of the Electric Field Gradient (EFG) tensor which are obtained from NQR experiments are discussed. The theory of Townes and Dailey is used for the derivation of the expressions for the occupation numbers of electronic wave function of spn hybridized atoms in terms of the asymmetry parameter η and the nuclear quadrupole coupling constant eq. The point-charge model is applied in order to obtain the possible sp2 and sp3 configurations for which η = 1.

Résonance quadripolaire nucléaire du brome dans des composés organomagnésiens

Abstract: Bromine nuclear quadrupole resonance signals corresponding to both isotopes 79Br and 81Br have been observed, at 77 °K, in some crystallized organomagnesium bromides of formula RMgBr•nR′2O, and in MgBr2•nEt2O The low value of the resonance frequencies, of about 33 MHz-to be compared to 220 MHz for a typical bond, bond, shows the ionic character of the Mg—Br bond The fact that the resonance frequencies are found within a restricted frequency range, between 30 and 42 MHz, shows that the Mg—Br bond is only slightly affected by the nature of the radicals R and R′ through the series studied; However the variation of the bromine resonance frequency in this series may be interpreted in terms of the nature of R and R′

Conformational studies of chlorotetrahydropyrans by nuclear quadrupole resonance spectroscopy

Abstract: A 35Cl nuclear quadrupole resonance study of 2,2,3,3-tetrachlorotetrahydropyran (I) at 77 K has revealed an anomeric shift of ca. 2·4 MHz between the chlorine atoms in the 2-position; in 2,3,3-trichlorotetrahydropyran (II), similar arguments show that the 2-chlorine takes up an axial position in the solid state.

Reaction of 5-substituted 4,6-dichloro-2-aminopyrimidines with oxalyl chloride

Abstract: The reaction of 5-substituted 4,6-dichloro-2-aminopyrimidines with oxalyl chloride gives 2-isocyanatopyrimidines, the structures of which were proved by chemical means and the IR, PMR, and 35Cl nuclear quadrupole resonance spectra. The intermediate pyrimidinyloxamic acid chlorides were not isolated; this is evidently explained by the low basicities of 2-amino-pyrimidines.

14 N nuclear quadrupole resonance spectra of Co-ordinated thiocyanate

Abstract: The 14N n.q.r. spectra have been measured for N- and S-bonded thiocyanates as ligands in several PdII complexes and a NiII complex; this technique sharply distinguishes between the two modes of bonding.

An NQR Study of TNT Characteristics.

Abstract: : The results of this research have established that the nuclear quadrupole resonance (NQR) spectrometric technique is ideally suited for the characterization of explosives such as TNT. NQR data yields information on chemical structure, crystalline states and morphology. For the explosive TNT it has been shown that: a. TNT exists in two morphological states, b. TNT undergoes a phase transition at approximately 200K. In addition a new mechanism was discovered for obtaining NQR signals at high data rates thus enhancing system efficiency by a factor of up to 100. Finally, initial research was performed using soft ferrite cores in place of the standard sample coils in an effort to reduce the amount of sample required for an NQR measurement. (Author)

The Nuclear Quadrupole Resonance of Dibromoiodate Ions

Abstract: The NQR sepctra due to 81Br and 127I in potassium dibromoiodate and its monohydrate were observed, and the Zeeman effect of 81Br NQR in the monohydrate was examined. The resonance lines indicate that both compounds have two kinds of dibromoiodate ions and that these ions in the anhydride are slightly asymmetric, while those in the monohydrate are symmetric. The difference in frequency between the two resonance lines due to each resonant nucleus in the monohydrate is explained in terms of the I···Br intermolecular interaction. The charge distributions in both compounds were evaluated to be about −0.6e and 0.2e on the terminal and central atoms respectively.

Hydrogen bonds in single crystals of para and meta-aminobenzonitrile as revealed by the Zeeman effect on the nuclear quadrupole resonance of 14N

Abstract: The |e 2 qQ/h| and η values for both the cyano and the amino-nitrogen of the title compounds are reported at room temperature as well as at 273 K and 77 K. From Zeeman effects on single crystals of both compounds the orientation of the principal axes of the electric field gradient (efg) on both nitrogen nuclei, as well as a number of data on the crystallographic and molecular structure, are obtained. A reliable interpretation is given of the experimental resonance frequencies, which are also safely assigned to the right nitrogen atoms. By studying the dependence of the resonance linewidth on the orientation, with respect to the efg principal axes, of the Zeeman field B 0, the spatial distribution of the hydrogen nuclei around the 14N nuclei is determined. In this way hydrogen bonds are shown to exist in both single crystals. An analysis based on the Townes and Dailey theory allows an estimate to be made of the possible variation of the population of the cyano-nitrogen lone-pair orbital when involved in hy...

Proton magnetic resonance study of molecular motion in (NH4)2HAsO4 and (NH4)2HPO4

Abstract: The temperature dependences of the second moments of the proton magnetic resonance absorption spectra and spin-lattice relaxation times are measured in (NH4)2HAsO4 and (NH4)2HPO4. Characteristic T1 minima and proton line width transitions are found demonstrating the existence of two chemically different NH4 groups in the unit cell. The anomalous increase in the As75 NQR frequency with increasing temperature in (NH4)2HAsO4 is explained as a result of thermally induced “acid” proton fluctuations in the HAsO4 groups. In (NH4)2HAsO4 and (NH4)2HPO4 werden die Temperaturabhangigkeiten der zweiten Momente der magnetischen Protonenresonanzabsorptionsspektren und Spin—Gitter-Relaxationszeiten gemessen. Charakteristische T1-Minima und Ubergange der Protonenlinienbreiten werden gefunden und zeigen die Existenz von zwei chemisch verschiedenen NH4-Gruppen in der Einheitszelle an. Der anomale Anstieg in der As75-NQR-Frequenz mit steigender Temperatur in (NH4)2HAsO4 wird als Ergebnis thermisch induzierter „azider” Protonenfluktuationen in den HAsO4-Gruppen interpretiert.