Showing papers on "Nuclear quadrupole resonance published in 1978"

Radio-frequency spectroscopy inside a laser cavity; "pure" nuclear quadrupole resonance of gaseous C H 3 I

Abstract: The highly sensitive method of infrared-radio-frequency double resonance inside a laser cavity has been applied to the observations of "pure" nuclear quadrupole resonances of gaseous C${\mathrm{H}}_{3}$I corresponding to the transitions $\ensuremath{\Delta}J=0$ and $\ensuremath{\Delta}F=\ifmmode\pm\else\textpm\fi{}1$. Radio-frequency signals with large signal-to-noise ratios (\ensuremath{\sim} ${10}^{4}$) have been observed for many C${\mathrm{C}}_{2}$ and ${\mathrm{N}}_{2}$O laser lines. The characteristic pattern of the radio-frequency spectrum specific to each rovibrational level has enabled us to assign the rotational levels, the infrared coincidences, and the associated far-infrared laser transitions unambiguously. From the radio-frequency spectrum the accurate vibrational and rotational dependences of the nuclear-quadrupole coupling constant and the values of the spin-rotation coupling constants have been determined. Because of the high sensitivity of the method, it was possible to observe collision-induced radio-frequency resonances as satellites to main line. This effect not only increases the number of observable quadrupole resonances, but also provides information on the detailed mechanism of rotational energy transfer. This method of observing "pure" quadrupole spectra of gases is applicable to any polar symmetric-top molecule with hyperfine structure which has rotational levels with double parity.

NMR of Sodium-23 and Potassium-39 in Biological Systems

Abstract: The aim of the introduction is to give the general reader some perspective concerning the nature of the biological problems to which NMR techniques have been applied. In addition, we give a brief description of other major techniques that have been used, and point out how NMR can be helpful in obtaining different complementary information concerning Na+ and K+ in biological systems.

135 Ba and 137 Ba Fourier transform NMR and NQR studies

Abstract: 135Ba and137Ba Fourier transform nuclear magnetic resonance and nuclear quadrupole resonance investigations are reported in liquids resp. solids. From these measurements ratios of g1-factors, hyperfine structure anomalies, magnetic moments, atomic shielding constants and the ratio of the quadrupole moments are evaluated using also data from literature.

14N PNQR study of the effect of pressure on the phase transition and molecular reorientation in s‐triazine

Abstract: 14N pure nuclear quadrupole resonance frequencies and inverse linewidth in s‐triazine were measured as functions of hydrostatic pressure to 6.2 kbar along two isotherms at 23.3 and −0.4 °C. The second order phase transition, previously studied at atmospheric pressure, was observed at high pressures. Above the transition pressure pc each resonance line splits into two. The line splittings were fitted to the equation Δν=B (p−pc)β, and the critical exponent β was found to be close to 0.5. The pressure dependence of the transition temperature Tc is described by the (linear) equation Tc(p) =198.8+18.2p, [Tc(°K);p (kbar)]. The present results along isotherms were compared with previous results along an isobar. In the low pressure region the inverse linewidth T2* is dominated by hindered rotations of the s‐triazine molecule. The activation volume for this motion is ΔV*=26 cm3/mole.

Biaxial Order Parameters in Liquid-Crystals - Their Meaning and Determination With Nuclear-Quadrupole Resonance

Abstract: A simple formulation is presented for the analysis of nuclear quadrupole spectra in biaxial liquid crystals based on orientational order parameters. The analysis contains no tilt angle or other model-dependent parameters not directly measured in NMR experiments. The observability and effect of various order parameters on the asymmetry of the electric field gradient q is discussed. It is shown that q is insensitive to rotational freeze-out of the variety expected to b'e observed-in some biaxial phases. Nuclear quadrupole resonance experiments are described which are expected to yield rotational freeze-out parameters.

Bromine-81 Nuclear Quadrupole Resonance and Aluminum-27 Nuclear Magnetic Resonance in AlBr3 Complexes

Abstract: The Zeeman effect on 81Br NQR and the quadrupole effect on 27Al NMR in L·AlBr3 complexes (L=KBr, POBr3, and H2S) were studied at room temperature. It was found that the 27Al quadrupole-coupling constant for these complexes decreases with decreasing coupling constant of the 81Br atom in –AlBr3 and that the atomic arrangement about the Al atom has C3V symmetry in POBr3·AlBr3 and H2S·AlBr3. In the latter complex, H2S molecule is reorientated at room temperature. The amount of charge transfer in these complexes was estimated on the basis of the Townes-Dailey theory and the bond angle ∠Br–Al–Br was found to depend on the amount of the charge transfer. The temperature dependence of the 81Br NQR frequencies was observed in order to obtain information about molecular motion in H2S·AlBr3 and POBr3·AlBr3.

Nuclear quadrupole resonances of phosphorus iodides and some phosphorus trihalide-boron trihalide complexes.

Abstract: Nuclear quadrupole resonance (NQR) frequencies due to 79Br and 127I are examined in phosphorus iodides, PI3 and P2I4, and the complexes PBr3·BBr3, PBr3·BI3, and PI3·BBr3. The quadrupole coupling constants and asymmetry parameters are also derived for 127I in PI3, P2I4, and PBr3·BI3. The resonance lines of PI3 fade out above 257 K, at which temperature a structural phase transition is suggested. The number of resonance lines in P2I4 shows that the molecular symmetry of P2I4 is \bar1 instead of 2/m in its crystal. The similarity of the resonance patterns of the three complexes indicates that they are isomorphous with one another. The electronic structures of the halogens in the compounds mentioned above have been discussed by using the Townes-Dailey theory. On complexing, the electric charge of the halogen atoms in the donor PX3 decreases due to the polarization of pσ bonds, whereas that of the halogen atoms in the acceptor BY3 increases owing to the backdonation through pπ bonds.

Perturbed angular correlation studies of chemically generated electric field gradients in Hf compounds

Abstract: The time‐differential‐perturbed‐angular‐correlation (TDPAC) technique has been applied to the study of electric quadrupole interactions in HfO2, HfO(H2PO4) ⋅H2O, and HfP2O7. Strong perturbations were seen in each case, with time evolution characteristic of the static electric field gradients generated by the electron distributions in chemical bonds. Analysis of the experimental data within a theoretical framework has permitted derivation of the electric field gradient parameters associated with the Hf–O bonds in the oxide and the two phosphate compounds. A simple model calculation involving orbital populations appears capable of reproducing the experimental results and provides insight into the bonding features of these relatively complex, but structurally known, systems. The TDPAC signature of each compound appears sufficiently unique so as to offer the possibility of its adaptation as a microscopic probe of more complicated or unknown crystalline and liquid systems.

Magnetic Resonance Spectroscopy at High Pressure

Abstract: The field of magnetic resonance spectroscopy at high pressure is discussed with the main emphasis on nuclear magnetic resonance. The lectures include the following main sections: 1. Introduction; 2. Experimental high pressure techniques in NMR; 3. Applications of NMR at high pressure; 4. Pure nuclear quadrupole resonance and electron spin resonance spectroscopy at high pressure.

On the Phase Transition of Chloranil (2,3,5,6-Tetrachloro-p-Benzoquinone)

Abstract: Experiments of Zeeman effect of 35 Cl nuclear quadrupole resonance on single crystals of chloranil below and above the second order phase transition point (95 K) determined the electric field gradient tensors and suggested a coupling of an acoustic shear strain wave to the soft librational mode associated with the phase change. The previously proposed dynamical model of the transition was reexamined and its mechanism was discussed on the basis of the symmetry consideration with respect to the soft librational and the shear strain modes. Different behaviors of crystals with different growth habits are reported.

The charge transfer in some alkaline salts of tetracyanoquinodimethane (TCNQ): A Zeeman perturbation study of the 14N nuclear quadrupole resonance spectrum of polycrystalline Rb–TCNQ

Abstract: Two 14N nuclear quadrupole resonance absorption lines (2904.4 and 2967.7 kHz) detected in Rb–tetracyanoquinodimethane (TCNQ) at room temperature were studied by the Zeeman perturbation effect and the results interpreted in terms of the theory of Pissanetzky [J. Chem. Phys. 59, 4197 (1973)]. The two lines were identified as ν+ with η of 0.40±0.04 and 0.57±0.05 and coupling constants of 3490 and 3254 kHz, respectively. The observed spectrum is compatible with a crystal structure with two inequivalent nitrogen sites such as the monoclinic phase of Rb–TCNQ II. Using the Townes and Dailey theory and an estimation of the effects of the crystal field, and by comparison of the NQR data of K, Rb, and neutral TCNQ, we found an increase of about 10% to 14% in the πx population and a decrease of about 5% in the σ population at the nitrogen. A two‐way charge transfer within the CN group seems to take place upon the molecular charge transfer as predicted by Clementi and Klint [J. Chem. Phys. 50, 4899 (1969)].

Raman and nuclear quadrupole resonance studies of the phase transition in pentachlorophenol

Abstract: Raman and nqr spectra of pentachlorophenol (PCP) have been recorded at temperatures above and below the transition at 62 + 2 °C. The complete loss of structure in the Raman external mode region and the disappearance of the 35Cl nuclear quadrupole resonances above the transition temperature indicate a disordered structure for the high temperature phase. Super-cooling is observed in nqr and Raman spectra when the sample is cooled through the transition temperature. No evidence for any new crystal phase is observed on cooling the sample to liquid nitrogen temperature. Eleven of the 12 external modes predicted from the reported crystal structure of PCP have been observed in the Raman spectrum at 77 K.

14N Nuclear Quadrupole Resonance of Some Heterocyclic Compounds via Nuclear Double Resonance

Abstract: Using proton-nitrogen double resonance in the laboratory frame, the 14N nuclear quadrupole resonances (NQR) of 2,4(1H,3H)-pyrimidmedione[uracil], 2,3-dihydro-2-thioxo-4(1H)-pyrimidinone[2-thiouraci...

Temperature dependence of chlorine nuclear quadrupole resonance and internal motions in o-chloro-, m-chloro-, 2-chloro-5-nitro- and 4-chloro-3-nitro-benzoic acids

Abstract: Based on crystal structure data, the torsional molecular motions (in solids) in four substituted benzoic acids are discussed in the light of nuclear (35Cl) quadrupole resonance results. The torsional frequencies show anonlinear variation with temperature and compare reasonably well with the Raman data on the parent compound. The average temperature coefficient of these motions has been evaluated by Brown's method as well as using Bayer's model. The torsional frequencies in the ortho-chloro substituted compounds do not vary smoothly with temperature.

Order-disorder phase transition and electrical polarization features of pyrargyrite

Abstract: It is concluded that the low temperature phase transition (PT) in pyrargyrite (Ag3SbS3) is the order-disorder PT using investigations of temperature dependences of the Sb121, Sb123 nuclear quadrupole resonance (NQR) frequencies and its electrical field shifts and spin-lattice relaxation (SLR) times as well.The temperature effect on the electrical shift of the Sb121 NQR frequency is discovered near PT in Ag3SbS3. This effect is caused by the temperature anomaly of dielectric permittivity near PT.The electret polarization effect is found by applying an external electric field in Ag3SbS3.