Nuclear Quadrupole Spin-lattice Relaxation Research Articles

Thermoactivated molecular motions in solids and determination of the energy of their activation by NQR spectroscopy methods

The activation energy of thermoactivated molecular motions in solids is determined by examining the influence of these motions on the temperature dependence of the nuclear quadrupole spin-lattice relaxation rate and quadrupole resonance (NQR) signal intensity for chlorine-35. In the latter case, the heating of the crystals is accompanied by the fading of resonance signals, which is analyzed using a linear correlation between the activation energy of this motion and the fade-out temperature. The correlation parameters are demonstrated to be dependant on the type of molecular motion. The relaxation method is shown to be more effective in studying molecular motion and evaluating its activation energy as compared to the NQR signal fading technique.

Nuclear quadrupole spin-lattice relaxation in Bi4Ge3O12 single crystals doped with atoms of d or f elements. Crystal field effects in compounds exhibiting anomalous magnetic properties

The nuclear quadrupole spin-lattice relaxation was studied in the range 4.2–300 K for single crystals of Bi4Ge3O12 doped with minor amounts (the tenth fractions of mol%) of paramagnetic atoms of Cr, Nd, and Gd. Unusual spin dynamic features were recently found for these crystals at room temperature: a dramatic (up to 8-fold) increase in the effective nuclear quadrupole spin-spin relaxation time T2* occurred upon doping the pure Bi4Ge3O12 sample. Unlike T2*, the effective spin-lattice relaxation time T1* at room temperature differs insignificantly for both doped and pure samples. But at lower temperatures, the samples exhibit considerably different behavior of the spin-lattice relaxation with temperature, which is caused by different contributions to the relaxation process of the dopant paramagnetic atoms. The distinctive maximum in the temperature dependence of the spin-lattice relaxation time for the Nd-doped crystal is shown to result from the crystal electric field effects.

NQR data on the activation energy of orientational defects in crystal lattice

The nuclear quadrupole spin-lattice relaxation related to the formation of orientational defects in crystals has been analyzed. The defects arise because of thermally activated reorientational motion of molecules and their fragments between the nonequivalent potential wells. These molecular reorientations are considered for double-and triple-well potentials, both periodic and bounded by external side walls. It was shown that, for such potentials, the temperature dependence of the relaxation time provides the data on the activation energy for transitions of molecules in the crystal lattice from the equilibrium ground-state positions to those corresponding to metastable states. We systematized the known NQR data on the activation energies for orientational defects arising in the intracrystal rotations of molecules and atomic groups possessing the imperfect two-, three-, and sixfold axial symmetries.

Temperature-induced orientational disorder inNaClO3

A detailed study of the temperature dependence of ${}^{35}\mathrm{Cl}$ nuclear quadrupole spin-lattice relaxation time and resonance frequency, in a wide temperature range (77--450 K), is used to discuss old controversial conclusions about the experimental behavior of low frequency dielectric permittivity, Raman spectroscopy, and nuclear quadrupole resonance, in sodium chlorate. Compatibility among experimental data coming from all these techniques is achieved by assuming a continuous transition of the ${\mathrm{ClO}}_{3}^{\ensuremath{-}}$ ion dynamics from vibrating around an equilibrium position for $T<250$ K, to exchanging between two orientations at higher temperatures. That is, the pseudoharmonic single well potential, that determines the chlorate ion dynamics at lower temperatures, mutates continuously with increasing temperature, to reach a symmetric two-well potential for temperatures higher than 380 K. In the intermediate-temperature range the behavior of NQR parameters are explained in terms of scattering of phonons by a distribution of disordered chlorate ions. We use the Green function formalism for calculating the corrections to the spin-lattice relaxation time and the resonance frequency due to disorder.

35-Chlorine NQR spectra and molecular structure of solid chlorophosphoric compounds in the series of 1,3,2 λ5-Benzodioxaphosphole

Nuclear quadrupole resonance (NQR) is a high-resolution spectroscopic method of molecular structure determination for solids due to its capability to give signals at specific frequencies for each resonant nucleus in a molecular structure. 35-Chlorine NQR is used here to establish the molecular structure of a number of crystalline chlorine-containing compounds of pentavalent phosphorus and to study their coordination isomerism, which is related to the chlorotropy effect. The 35C1 NQR spectra of solid 4,5-CI2C6H202PCI 3 (I), C6H402PCI2N=CCICCI 3 (II), and C6H402PCI= NCC12CF 3 (HI) (Table 1) were recorded at 77 K on a pulse NQR spectrometer (resonance frequency measurement error v up to _.+5 kHz, nuclear quadrupole spin-lattice relaxation time T 1 about ___10%). The results of measurements in Table i show how 35C1 NQR lines are grouped in the compounds under study. Figure 1 demonstrates the spectrograms of these compounds and compares them with the literature data for "reference" compounds, making it possible to assign the resonance frequencies to molecular fragments in crystals I-III and thus to identify the molecular structure of the corresponding compounds. It is natural to compare the 35-chlorine resonance frequencies of P-CI bonds in sample I with the 35C1 NQR spectrum of the phosphorane C6H402PC13 (spectrumA, Fig. 1). In the latter spectrum, the two high-frequency resonance signals belong to the equatorial chlorines at phosphorus, and the low-frequency signal corresponds to the axial chlorine [1]. The doubled number of 35C1 NQR lines with respect to the number of chlorine atoms in molecule I for both P-CI and C--C1 bonds indicates the presence of two nonequivalent molecular sites in the unit cell of the crystal. The 35C1 NQR lines of P-C1 bonds for the nonequivalent molecules in this double set may be grouped as follows (Table 1): lines 5, 6, and 9 one molecule; lines 7, 8, and 10 the other (according to characteristic ranges of 35-chlorine resonance frequencies for CI e (equatorial) and CI a (axial) and taking into account the splittings and the ratios of the times T 1 in the NQR spectra of the nonequivalent molecules). As to 35C1 NQR lines of C-CI bonds, their assignment to a position in the benzene ring is quite clear, since the 35C1 NQR spectrum of 1,2,4,5-tetrachlorobenzene [2] (spectrum B, Fig. 1) is observed in the same frequency range. Compounds II and III can exist as forms with pentaand tetracoordinated phosphorus atom (chiorotropic tautomerism in the PNC triad):

Nuclear quadrupole spin–lattice relaxation due to molecular reorientations in crystals with orientational disorder

p -chloronitrobenzene (PCNB) and p-chlorobromobenzene (PCBB) crystallize in the centrosymmetric space group P21/c with two molecules per unit cell. The space lattice will have an equal number of points with molecules facing in opposite directions. As a consequence, these compounds exhibit an orientational rigid disorder. In this work, we have measured the temperature dependence of the chlorine nuclear quadrupole spin–lattice relaxation time (T1), linewidth, and resonance frequency for both compounds for temperatures higher than 80 K. Both compounds exhibit an inhomogeneously broadened line shape and a “normal” Bayer-type temperature dependence of the resonance frequency. The analysis focuses on the identification of the dominant relaxation process at high temperatures (T>240 K in PCNB and T>260 K in PCBB). It is shown that T1(T) reflects the existence of 180° molecular reorientations through a modulation of the crystalline contribution to the electric field gradient.

Nuclear Spin-Lattice Relaxation of a Spin System with Strong Heteronuclear Magnetic Interaction

Abstract Results are presented of a theroretical consideration of the nuclear quadrupole resonance and spin-lattice relaxation for a sample containing nuclei of two different types coupled by strong heteronuclear dipole-dipole interactions and influenced by an external continuous or pulsed ra-diofrequency magnetic field acting only on the nuclei of one sort with spin I > 1/2. A kinetic equation is obtained from which the time dependence of the magnetization of the sample is derived. The kinetic coefficients are calculated as a function of the concentration and distribution of the nuclei of both sorts.

Pressure dependence of nuclear quadrupole spin-lattice relaxation

Pressure dependence of nuclear quadrupole spin-lattice relaxation

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

Quadrupole relaxation study of molecular dynamics in ferroelectric phase of tetramethylammonium hydrogen bis-trichloroacetate

Abstract The effect of temperature on the 35Cl nuclear quadrupole spin-lattice relaxation time T 1 in ferroelectric phase of tetramethylammonium hydrogen bis-trichloroacetate has been studied in the temperature range 7 to 160 K. The chlorine relaxation data indicates that the dominant relaxation mechanism for T 1 below 130 K can be attributed to the fluctuations of the electric field gradient due to the Raman process. For temperatures below ca. 18 K the relaxation rate T −1 1 is proportional to T 5; for higher temperatures (between ca. 25 K and 130 K) the T −1 1 data obey a quadratic relation: T −1 1 ∼ T 2. Above ca. 130 K the relaxation rate T −1 1 increases exponentially with increasing temperature suggesting the onset of the C 3 reorientation of the CCl3-groups with the activation energy of 41 kJ mol−1. This mechanism is responsible for the observed fade-out phenomenon of the NQR signal near 160 K, i.e. far below Tc .

A Study on the 79Br Nuclear Quadrupole Spin‐Lattice Relaxation Times Affected by the Soft Acoustic Mode in Anilinium Bromide Crystals

AbstractThe nuclear quadrupole spin‐lattice relaxation time T1Q, spin‐spin relaxation time T2Q, and NQR linewidth of 79Br in anilinium bromide were determined at various temperatures between 100 and 445 K. An unusual temperature variation of T1Q was observed over a wide range of temperature far below and also above the phase transition already reported, in addition to the normal temperature dependence (T−11Q α T2) due to lattice vibrations activated in crystals. The mechanism of the occurrence of the T1Q anomaly observed is explained in terms of the soft acoustic mode already reported to exist.

The temperature dependence of 35Cl NQR spectrum and study of spin—lattice relaxation times in chloral hydrate

The temperature investigation of nuclear quadrupole spin—lattice relaxation times in chloral hydrate above 290 K revealed the presence of an additional mechanism of relaxation. Its activation energy is 14.5 kcal mol −1 which is almost 6 kcal mol −1 greater than the energy corresponding to the hindered rotation of the CCl 3 group. We propose this additional mechanism to be the breaking of hydrogen bonds and translational diffusion of water molecules.

A study of the nuclear quadrupole resonance of halogens in chloro-substituted pyridinium bromides and iodides

The temperature dependences of 35Cl, 79Br and 127I NQR frequencies were observed for 2-chloropyridinium bromide and iodide, while 35Cl NQR frequencies were determined at various temperatures for 3-chloropyridinium bromide and 3,5-dichloropyridinium iodide. 3,5-Dichloropyridinium iodide yielded two 35Cl NQR lines at all temperatures studied, indicating the existence of two crystallographically nonequivalent chlorines in the crystal. The nuclear quadrupole spin-lattice relaxation time T 1Q of 35Cl as well as 127I nuclei for 2-chloropyridinium iodide and that of 35Cl for the remaining salts were determined as a function of temperature. Below 200 K, an almost linear relation with a gradient of about −2 was obtained for the 35Cl log T 1Q vs. log T plot of 2-chloropyridinium iodide, indicating that the relaxation mechanism might be governed by the librational modes. Above 200 K, the plot deviated from the linear relation suggesting the presence of considerable anharmonicity in the librational modes. Similar results of 35Cl T 1Q were obtained for the other salts studied. The 127I T 1Q of 2-chloropyridinium iodide decreased with increasing temperature as a function of T −2.2 and T −2.1 for ν 1 and ν 2 lines, respectively, in the whole temperature range (80–350 K) studied. This can be well interpreted by the ionic model proposed by Van Kranendonk.

Nuclear quadrupole resonance and relaxation in incommensurate and commensurate ferroelectrics

Abstract The 35Cl, 14N nuclear quadrupole resonance and nuclear quadrupole double resonance spectra have been measured in a number of systems exhibiting structural, incommensurate and commensurate ferroelectric phase transitions. In this contribution we illustrate the power of NQR spectroscopy for the study of the incommensurate and commensurate ferroelectric phases as well as for the study of structural phase transitions in models of biological membranes and related systems. We specifically studied the sequence of structural phase transitions in the Octylammonium N-Octylammoniumchloride layer structure compound and the Paraelectric to Ferroelectric transition in (CH3NH3)HgCl3. The dynamical properties of the above transitions are reflected in the temperature dependence of the NQR frequencies as well as in the time anomalous behaviour of the nuclear quadrupole spin-lattice relaxation times. From the above experimental data one can extract detailed information about the microscopic nature of the above transition.

Orientation dependence of the nuclear quadrupole spin-lattice relaxation time of 23Na in NaNO 2 and NaNO 3

The spin-lattice relaxation of 23Na was studied in single crystals of NaNO 2 and NaNO 3 at room temperature using a pulsed NMR spectrometer operating at 14.5 MHz. Measurements were made at crystal orientations at which the quadrupole-split resonances approximately overlap. In NaNO 3 good overlap was achieved, and the relaxation behavior was found to be exponential, with a relaxation rate T 1 q −1 which was independent of the crystal orientation, as predicted theoretically. The measured T 1 q −1 agrees well with previously reported data. In NaNO 2 good overlap could not be achieved because of the large second-order quadrupole shifts. The relaxation was therefore studied with two types of overlap, first, with two overlapping satellites, and second, with one satellite located midway between the other two resonances. The relaxation was exponential, with the same T 1 q −1 in both cases. However, T 1 q −1 was found to depend upon the relative contribution of the center line and satellites to the amplitude of the FID. It was deduced that T 1 q −1 is larger for the satellites than the center line, the largest difference being about 10% when the magnetic field is perpendicular to the c axis of the crystal. The orientation dependence of T 1 q −1 follows the theoretical form, but is not in good agreement with recently measured values of the M-tensor components.

Orientation dependence of the23Na nuclear quadrupole spin-lattice relaxation in sodium nitrite

Orientation dependence of the²³Na nuclear quadrupole spin-lattice relaxation in sodium nitrite

On spin-spin reservoir relaxation in NQR

An attempt is made to discuss in a preliminary manner the question of the possible influence of absence of the inner equilibrium in the quadrupole spin system on the nuclear quadrupole spin-lattice relaxation. A general approach is proposed and special situations considered: small percentage of the quadrupole nuclei on the one hand and the paramagnetic impurity relaxation mechanism on the other. In the case of spin 5 2 and zero asymmetry parameter the spin-lattice relaxation time of the “quadrupole spin-spin reservoir” is calculated and compared with the “usual” quadrupole spin-lattice relaxation time.

Chlorine-35 qttadrupole relaxation and molecular motion in (Cl 3PNR) 2 crystals

The temperature dependence of the cnlorine-35 nuclear quadrupole spin-lattice relaxation time, obtained for the (CI 3PNCH 3) 2 crystal, shows the existance of a thermal-activated process with the energy barrier 25 kcal/mole. This process seems to be connected with the intramolecular rearrangement of the chlorine nuclei, the most probable mechanism being a trigonal twist which is one of several kinds of ligand exchange taking place in pentacoordinated phosphorus molecules. The effects of such a motion in NCR are discussed.

Coordination bonds in KCu(CN) 2 as studied by 14N and 63Cu NQR

The 63Cu nuclear quadrupole spin-lattice relaxation time, T 1, was measured as a function of temperature between 77 and 260 K for potassium dicyanocuprate(I), KCu(CN) 2 and found to depend on the temperature in T −2. This shows that the quadrupole relaxation is governed by the vibration of the anion and that possibilities of reorientation on self-diffusion of ionic species are excluded below 260 K. Our proposed bond model was thus indirectly reinforced. The Zeeman effect experiment on a single crystal for the 63Cu nuclear quadrupole resonance line at 200 K was not conclusive as to the temperature dependence of the asymmetry parameter.

Nuclear quadrupolar spin-lattice relaxation in the amorphous state

It is proposed that the nuclear quadrupole spin-lattice relaxation in glasses is due to interaction of solitons with the nuclear quadrupole moment. The magnitude of the interaction is estimated and the resulting temperature dependence of the spin-lattice relaxation time is compared to experiment.