Temperature Dependence Of NQR Frequencies Research Articles

Temperature dependence of one-dimensional hydrogen bonding in morpholinium hydrogen chloranilate studied by 35Cl nuclear quadrupole resonance and multi-temperature X-ray diffraction

The temperature dependence of (35)Cl NQR frequencies and the spin-lattice relaxation times T(1) has been measured in the wide temperature range of 4.2-420 K for morpholinium hydrogen chloranilate in which a one-dimensional O-HO hydrogen-bonded molecular chain of hydrogen chloranilate ions is formed. An anomalous temperature dependence of the NQR frequencies was analyzed to deduce a drastic temperature variation of the electronic state of the hydrogen-bonded molecular chain. The hydrogen atom distribution in the OHO hydrogen bond is discussed from the results of NQR as well as multi-temperature X-ray diffraction. Above ca. 330 K, the T(1) showed a steep decrease with an activation energy of ca. 70 kJ mol(-1) and with an isotope ratio (37)Cl T(1)/(35)Cl T(1) = 0.97 ± 0.2. The orientational change of the z axis of electric field gradient tensor in conjunction with the hydrogen transfer between adjacent hydrogen chloranilate ions is suggested as a possible relaxation mechanism.

Anharmonic Librations and the Temperature Dependence of NQR Frequencies

Abstract The temperature dependence of NQR frequencies of nuclei in molecules undergoing quasi-har-monic rotational oscillations is calculated. Following the model proposed by Bayer, the instanta-neous orientation of the electric field gradient principal axes is assumed to change from its average orientation as a result of rotational oscillations, while the EFG magnitude remains constant. The eigenstates and energy eigenvalues of a rotator in a potential of the form V(θ)= V2θ2 - K4θ4 are calculated by standard perturbation methods, followed by the evaluation of statistical averages of square angular amplitudes. For molecular crystals with more than one molecule in a unit cell the angular displacements are expressed in terms of normal coordinates, and the same procedure is followed. The final expression for the change in the NQR frequency consists of two terms, the first one being related to the harmonic part of the potential and the second one to the quartic term. At the absolute zero it yields a nonzero contribution to ⟨θ2⟩, and for high temperatures it reproduces the nonlinear character of νQ(T) observed experimentally. The model has been tested with measure-ments on solid Cl2 performed by Nakamura and Chihara, combined with libration mode frequencies measured in Raman spectroscopy by Cahill and Leroi and analysed according to our anharmonic libration picture. The fitting of our equation to experimental data supports Cahill and Leroi's hypothesis about the existence of an undetected libration mode in Cl2 Raman spectra of frequency near 60 cm- 1

Anharmonic Lattice Vibrations and the Temperature Shift of Raman Spectral Lines

Abstract The temperature dependence of the average lattice mode frequency of a molecule undergoing librational motion in a quasi-harmonic potential has been calculated with the purpose of finding an explicit relationship between the observed shift of Raman spectral lines with temperature and the anharmonic term in the rotational potential. Calculations were carried out both for uncoupled and coupled oscillators. The equations obtained with this model provide good fittings for solid Cl2 and benzene data. This result can be applied to the analysis of the temperature dependence of NQR frequencies in molecular crystals.

Reorientational Motion of Hydrogen Bonded Octahedral Complex Anions in Hydrazinium Hexachlorostannate(IV), (N2H5)2SnCl6, as Studied by 35Cl NQR Spin-Lattice Relaxation Measurements

Abstract The temperature dependences of NQR frequencies and spin-lattice relaxation times, T1Q, of 35Cl in (N2H5 ) 2SnCl6 were measured. Four NQR signals distributed in a fairly wide frequency range were observed, the lowest-frequency line exhibiting an anomalous positive temperature coefficient. The highest-frequency line showed a steep temperature dependence. These results could be interpreted by considering the intermolecular interaction between CI and - NH2 in N2H5 + . Below ca. 250 K, T1Q of the upper three signals exhibited a gradual decrease upon heating, explainable by lattice vibrations, while a shallow T1Q minimum, ascribable to the modulation of electric quadrupole interaction, was observed at ca. 110K for the lowest-frequency signal. The reorientation of [SnCl6]2- was elucidated to be highly anisotropic, in that it reorients about one of the CI-Sn-Cl axes much more frequently than about the other two axes. The activation energies were determined to be 62 and 94 kj mol -1 for the respective reorientations.

NQR and X-ray Studies of [N(CH3)4]3M2X9 and (CH3NH3)3M2X9 (M = Sb,Bi; X = Cl,Br)

Abstract 35Cl, 81Br, 121Sb, and 209Bi NQR of the title compounds was observed. According to the results of the temperature dependences of NQR frequencies and the DTA measurements, phase transitions take place in [N(CH3)4]3Bi2Br9 (Ttr=183K), [N(CH3)4]3Bi2Cl9 (Ttr = 155K), and (CH3NH3)3Bi2Cl3 (Ttr = 200 and 249 K). 2D NMR spectra for partially deuterated (CH3ND3)3Bi,Br9 showed that the phase transitions in this compound are related to the motion of the methylammonium cations. Single-crystal X-ray work at room temperature shows that the space group for [N(CH3)4]3Sb2Cl9 is P63/mmc with a = 925.1 pm, c = 2173.4 pm, Z = 2. For (CH3NH3 ) 3Sb2Br9 the space group is P3ml with a = 818.8 pm, c = 992.7 pm, Z = 1; in both cases the cations show dynamical disorder. The Rietveld analysis of the powder X-ray diffraction for (CH3NH3)3Bi2Br9 reveals the space group P3ml with a = 821.0, c - 1000.4 pm, Z = 1 at room temperature; the compound is isomorphous with (CH3NH3 )3Sb2Br9 . The crystal symmetries of the low-temperature phases of (CH3NH3)3Bi2Br9 and [N(CH3)4]3Bi2Br9 were deduced from the results of the NQR spectroscopy

Temperature Dependence of the NQR Frequency and Linewidth of the High-J c Superconductor YBa 2 Cu 3 0 7 _ 5

Abstract For YBa2Cu3O7-δ a powder samples of different average grain sizes the temperature dependences of 63Cu NQR frequencies and signal intensities are reported for the Cu (1) site in a temperature range of 4 300 K. Both quantities show a non-monotonic behaviour in the vicinity of the superconduction transition temperature Tc . In the superconducting state the line shape of the NQR signal in zero-field depends on the strength of the dc magnetic field applied during the cooling-procedure. The resulting linewidth yields information about the internal trapped magnetic field.

Temperature Dependences of NQR Frequencies and Nuclear Quadrupole Relaxation Times of Chlorine in 2,6-Lutidinium Hexachlorotellurate (IV) as Studied by Pulsed NQR Techniques

Abstract The temperature dependences of the 35Cl NQR frequencies vQ and the nuclear quadrupole spin-lattice relaxation times T1Q in 2,6-lutidinium hexachlorotellurate (IV) was observed at various temperatures between 80 and 343 K. This crystal undergoes a phase transition at Tc = 229 K. A single and three pairs of 35Cl NQR frequencies were observed above and below Tc , respectively. The hysteresis of the phase transition and a discontinuity in the temperature dependence of the resonance frequencies at Tc indicate that this phase transition is of first order. Although the resonance frequencies of the pairs in the low temperature phase are very close to one another, T1Q and below Tc could be accurately determined by measuring the Fourier transform spectra of each line. Above ca. 250 K, T1Q showed an exponential decrease which is attributable to the overall reorientational motion of [TeCl6]2- with an activation energy of 82 kJ mol-1

NQR Investigations of La3-x Ba3+xCu6O3 , (x = 0, 0.75, 1)

Abstract NQR spectra due to 63Cu and 139La have been recorded for La3-xBa3+xCu6O (x = 0, 0.75, 1). Two groups of 63Cu NQR lines were observed (around 20-23 MHz and 29-36 MHz). The former is assigned to the Cu atoms in the CuO4 square planar groups, and the latter to that in the CuO s square pyramidal group. For the 63Cu NQR lines in La3Ba3Cu6O , the time dependent NQR spectra and the temperature dependence of NQR frequencies were observed.

Anomalous Temperature Dependence of NQR Frequencies

Abstract The anomalous temperature dependence of NQR frequencies which is often observed in crystals in which there is reorientational motion of ions or molecules is considered. Two theories of the phenomena, the jump rotation theory and the pseudo-spin theory, are described, and their successes and failures are discussed. The importance of considering other properties such as pressure dependence of NQR, heat capacity, and elastic constants is emphasized, and it is shown that neither theory is entirely satisfactory at present.

Temperature dependence studies of NQR frequencies of halogens and also of quadrupolar relaxation times of chlorine nuclei in ammonium hexahalotellurates(IV)

AbstractThe temperature dependence of NQR frequencies of halogens in ammonium hexahalotellurates(IV) has been reinvestigated in a wide range of temperature. A second‐order phase transition was definitely located at 89 K for (NH4)2TeCl6, below which temperature the existence of multiplet structure was suggested The temperature of a first‐order phase transition was determined for (NH4)2TeBr6 at 194 K in addition to the phase transition previously reported, which was located accurately at 212 K. The lowest‐frequency line among the three 81Br resonance lines observed below 194 K exhibited a positive temperature coefficient whereas the remaining two lines showed normal negative temperature coefficients. Except for the highest‐frequency v1 and v2 lines, the all four 127I lines observed for (NH4)2TeI6 showed anomalous temperature dependences. These temperature dependences could be interpreted by considering that the Te‐I…H type hydrogen bonds partially break due to the thermal activation of the motion of ammonium ions in crystals. A phase transition was located at 449 K by means of differential thermal analysis.

NQR investigation on ThBr 4 and instrumentation

The results of a recent NQR study of ThBr 4 and a brief survey of earlier work are presented. The temperature dependence of NQR frequencies of the α and β polymorphs is described. The line shape in the incommensurate phase, determination of the transition temperature, nuclear relaxation in the normal phase, far from and near the I transition, and strong phason relaxation in the I phase are also discussed.

Nitrogen-14 NQR in s-Triazine Derivatives

Within a study of the temperature dependence of NQR frequencies in some s-triazine derivatives [1], resonance frequencies at 4.2 K were determined in melamine, cyanuric acid, and cyanuric chloride. New lines, which were not reported so far, were detected in two of these compounds. In melamine, nine more lines were found as already described in [4], In cyanuric chloride, two more lines were found showing a second nitrogen site, in agreement with the existence of two chlorine NQR sites in this compound.

Effect of ammonium ion reorientation on the anomalous temperature dependence of nuclear quadrupole resonance frequencies

An anomalous temperature dependence of NQR frequencies in some ammonium salts has been successfully interpreted in terms of a model where the reorientational jumping time of NH + 4 was taken into account. Examples of the analysis are given for αNH 4HgCl 3 and NH 4ReO 4, leading to a jumping time of (2—3) x 10 -12s.

Temperature dependence of NQR frequencies in CuBr 2 and MgBr 2·2Et 2O

The temperature dependence of NQR frequencies is studied in CuBr2 and MgBr2·Et2O. In CuBr2 the resonance disappears as the temperature is decreased between 77 and 105 K. Since this may be related to a paramagnetic-antiferromagnetic phase transition, the corresponding Neel temperature is much lower than the value 193 K reported in the literature. In MgBr2·Et2O a phase transition associated with a change in the conformation of the ether molecule is clearly seen in the NQR frequency vs temperature diagram.

Some comments on the temperature dependence of NQR frequencies of solid nitrogen and chlorine

Some comments on the temperature dependence of NQR frequencies of solid nitrogen and chlorine

The calculation of the temperature dependence of NQR frequencies in chlorine and α-nitrogen crystals

The temperature dependence of the NQR frequency was investigated in terms of the anharmonic libration of a molecule in a crystal. The statistical average value of the electric field gradient at the nuclear site was evaluated by using the eigenvalues and the eigenfunctions which were obtained by solving a spheroidal wave equation as an equation of motion of the molecular libration in a potential of V o sin 2 θ. The present theoretical treatments not only improve the conventional theory in harmonic approximation but also lead to a good agreement between the calculated results and the experimental ones in chlorine and α-nitrogen crystals.

14N Nuclear Quadrupole Resonance in Ferroelectric Sodium Nitrite NaNO2

Nuclear quadrupole resonance has been studied in ferroelectric sodium nitrite (NaNO 2 ) from 77 K to its phase transition point 437 K. The three rotational frequencies ω a =190 cm -1 , ω b =120 cm -1 and ω c =227 cm -1 and their temperature variation when fitted in the Bayer-Kushida theory predict the temperature dependence of nqr frequencies reasonably well. A second order phase transition is found to occur at 180 K which is in confirmity with the one found earlier from thermal expansion and dielectric studies. The shift in resonance frequencies is seen to occur mainly by rotation around the ` c ' axis and hence it is inferred that the mechanism of polarization reversal is intimately connected with orientational motion about ` c ' axis.

Anharmonic effects on the temperature dependence of NQR frequencies

The temperature dependence of the nuclear quadrupole resonance frequency in a molecule undergoing torsional oscillations in a pure quartic potential well has been calculated. From a comparison with published data, it is concluded that there is no evidence that anharmonicity of this type influences NQR frequencies.

Temperature dependence of the NQR frequencies and relaxation times in Menshutkin's complexes on the basis of arsenic trichloride and tribromide

AbstractThe temperature dependence of NQR frequencies and spin‐lattice relaxation times in Menshutkin's complexes, prepared on the basis of AsCl3 and AsBr3 have been investigated.The rotational oscillation frequencies νt and the average life times τa of rotational oscillation quanta have been calculated by both the Bayer and Woessner‐Gutowsky theories.