Surface-layer effect in MnTiO3 ceramics at low temperatures

MnTiO₃ ceramics were prepared via the traditional solid-state reaction route. The low-temperature (100–330 K) dielectric properties of MnTiO₃ have been systematically investigated in the frequency range from 100 Hz to 5 MHz. Our results showed that MnTiO₃ exhibits intrinsic dilectric response in a wide temperature range up to 200 K. A relaxation appears near room temperature. This relaxation can be enhanced by annealing treatment in oxygen and weakened in nitrogen. Reducing the sample thickness gradually leads to the disappearance of the relaxation. Based on these experimental facts, the relaxation was ascribed to be a Maxwell–Wagner relaxation due to surface-layer effect.     

Relaxation times and line widths of isotopically-substituted nitroxides in aqueous solution at X-band

Optimization of nitroxides as probes for EPR imaging requires detailed understanding of spectral properties. Spin lattice relaxation times, spin packet line widths, nuclear hyperfine splitting, and overall lineshapes were characterized for six low molecular weight nitroxides in dilute deoxygenated aqueous solution at X-band. The nitroxides included 6-member, unsaturated 5-member, or saturated 5-member rings, most of which were isotopically labeled. The spectra are near the fast tumbling limit with T₁∼T₂ in the range of 0.50–1.1 μs at ambient temperature. Both spin–lattice relaxation T₁ and spin–spin relaxation T₂ are longer for ¹⁵N- than for ¹⁴N-nitroxides. The dominant contributions to T₁ are modulation of nitrogen hyperfine anisotropy and spin rotation. Dependence of T₁ on nitrogen nuclear spin state m_I was observed for both ¹⁴N and ¹⁵N. Unresolved hydrogen/deuterium hyperfine couplings dominate overall line widths. Lineshapes were simulated by including all nuclear hyperfine couplings and spin packet line widths that agreed with values obtained by electron spin echo. Line widths and relaxation times are predicted to be about the same at 250 MHz as at X-band.     

Studies of molecular dynamics of thiazides by<Superscript>35</Superscript>Cl NQR spectroscopy and DFT calculation

Molecular dynamics of three derivatives of 1,2,4-benzothiadiazine-1,1-dioxide, hydrochlorothiazide (HCTZ), althiazide (ATZ) and chlorothiazide (CTZ), was studied by³⁵Cl nuclear quadrupole resonance (NQR) spectroscopy. The temperature dependence of the resonance frequency was analyzed within the 6 known standard models. The activation energies estimated from the temperature dependence of the³⁵Cl NQR frequency assuming the Bayer model were 1.07, 2.35 and 2.76 kJ/mol for HCTZ, ATZ and CTZ respectively, which confirms that HCTZ is less rigid than CTZ and ATZ is much more rigid than HCTZ, and suggests that the mechanism of relaxation is based on small amplitude librations. The characteristic temperatures estimated from the Bayer model, with that for CTZ (332.5 K) being much higher than for HCTZ (132.1 K), mean that the intermolecular interactions in CTZ are much stronger than in HCTZ, as suggested by the melting point of CTZ being higher than that for HCTZ. For ATZ the characteristic temperature (288 K) takes an intermediate value, which suggests that the intermolecular interactions in this compound are stronger than in HCTZ and weaker than in CTZ. A significant narrowing of the resonance³⁵Cl NQR line observed for all these compounds at room temperature, relative to that at the liquid nitrogen temperature, suggests an averaging of dipolar interactions as a result of fast rotation of nonquadrupole nuclei in the vicinity of the quadrupole nuclei, when 2πν_Qτ_c ≫ 1 (a rotation of the −NH₂ group in the direct neighborhood of the chlorine nuclei) or a change in the gradient orientation with its value preserved (which is equivalent to rotation of the quadrupole nucleus Cl). The influence of the rotations of the −NH₂ and −CH₂SCH₂CH=CH₂ groups (ATZ) or −CHCl₂ group (TCTZ) on the³⁵Cl NQR frequency was modelled by the B3LYP/6-31G^* method. The frequencies of the libration vibrations calculated from the temperature dependence of the NQR resonance frequency were compared with experimental ones and those implied by the density functional theory, infrared and Raman spectra. For HCTZ the anomalies in the temperature dependence of the³⁵Cl NQR frequency, the lack of hysteresis and small but notable changes in the slope and the jump in the frequency observed at 253 K which does not exceed 0.05 MHz suggest a second-order phase transition at 253 K.     

Transitions and Spin Dynamics at Very Low Temperature in the Pyrochlores Yb2Ti2O7 and Gd2Sn2O7

An orientational disorder of the cation in [(PyO)D][AuCl₄] crystal was investigated by the ³⁵Cl NQR and ¹H NMR measurements. A structural phase transition was found at ca. 70 K from the temperature dependence of the NQR frequencies both in [(PyO)D][AuCl₄] and [(PyO)H][AuCl₄]. Temperature dependence of the spin-lattice relaxation time T ₁ of the NQR of [AuCl₄]⁻ could be interpreted by an electric field gradient modulation due to the motion of the cation. Characteristics of T ₁ of ³⁵Cl NQR as well as that of ¹H NMR suggest a dynamic orientational disorder of the cation.     

Infrared spectra of acetylene dimers and acetylene–nitrogen: (DCCD)2, H-bonded DCCD–HCCH, and DCCD–NN in the 4.1 μm region

Infrared spectra of the weakly-bound T-shaped acetylene dimers DCCD–DCCD and DCCD–HCCH are studied in the region of the DCCD ν₃ fundamental (∼2440 cm⁻¹) using a pulsed supersonic slit-jet expansion and a tunable diode laser probe. The K_a = 0 ← 1 and 1 ← 0 subbands, corresponding to the vibration of the DCCD monomer at the “top” of the T, are analyzed. Compared to the analogous spectrum of HCCH–HCCH, the present results are much less perturbed. The tunneling splitting for (DCCD)₂ in the excited state is determined to be 141 MHz, a big reduction from the previously determined ground state value of 424 MHz. The dimer A rotational constants show a large apparent increase upon vibrational excitation, and we discuss whether this increase is real. The linear DCCD–NN complex is also observed as an impurity in the spectrum, and it too is found to be unperturbed, in contrast with HCCH–NN.     

ABSORPTION INTENSITIES, PRESSURE-BROADENING AND LINE MIXING PARAMETERS OF SOME LINES OF NH3 IN THE ν4 BAND

The intensities and foreign gas broadening coefficients of 57 selected lines of the ν₄ band of NH₃ have been measured in the region of 1550 cm^-1 using a high resolution Brucker Fourier transform spectrometer. The line intensities were obtained by using the methods of absorbance at the line center and by fitting Voigt profiles to the measured shapes of the lines. The latter method also provides the collisional widths of the lines. In addition, collision cross relaxation coefficients of O₂ and air foreign gases were measured for 9 doublets of NH₃ in the ν₄ band. The J and K quantum numbers dependencies of pressure-broadening coefficients and line intensities are discussed. The observed air broadening and cross relaxation coefficients were found to be in reasonable quantitative agreement with the concentration-weighted average of the N₂ and O₂ broadening coefficients. The comparison of our present and previous results obtained for the NH₃–H₂, NH₃–air, NH₃–N₂ and NH₃–CO₂ collisions shows an increase of the pressure broadening and cross relaxation coefficients with quadrupole moment of the foreign gas. The analysis of the line intensities was based on the third-order theory of line strengths and yields effective transition moments, vibrational band strengths and correction parameters of the symmetric and antisymmetric partial bands of the fundamental ν₄ band.     

The first high-resolution infrared study of diazirinone, N2CO: Analysis of the Fermi-coupled ν1 and 2ν5 bands

The first validated detection of the elusive diazirinone molecule (N₂CO) in the gas phase was performed using high-resolution infrared spectra recorded in the 1810–2100 cm⁻¹ region. The ν₁ and 2ν₅ bands were identified at 2043.8 and 1863.3 cm⁻¹ close to the ab initio prediction [X.Q. Zeng, H. Beckers, H. Willner, J.F. Stanton, Angew. Chem. Int. Ed. 50 (2011) 1720–1723]. For these two bands, the individual lines exhibit a (2:1) intensity alternation, confirming a three membered ring structure (C_2V symmetry) for N₂CO. The major output of the ν₁ and 2ν₅ bands analysis is the first experimental determination of the ground-state rotational constants of cyclic N₂CO. The observed intensity pattern of the ν₁ and 2ν₅ bands is in agreement with the existence of a strong Fermi resonance coupling the 1¹ and 5² energy levels.     

Molecular motions in chloramphenicol studied by35Cl nuclear quadrupole and1H nuclear magnetic resonance spectroscopy

The³⁵Cl nuclear quadrupole resonance (NQR) frequency (v_Q), nuclear quadrupole spinlattice relaxation time (T_1Q),¹H nuclear magnetic resonance second moment (M₂) and nuclear magnetic spin-lattice relaxation timeT ₁) were measured for polycrystalline chloramphenicol (drug) as a function of temperature. Hindered rotation of the CHC1₂ group and the phenyl ring was detected, the relevant activation energies were determined. The rotations are discussed in detail.     

Effect of <Superscript>14</Superscript>N Nucleus Capture in Nuclear Quadrupole Resonance at Small Relaxation Parameters

Experiments on detecting induction signals of nuclear quadrupole resonance (NQR) on ¹⁴N nuclei in hexahydro-1,3,5-trinitro-s-triazine C₃H₆N₆O₆ (RDX) with preliminary changed population densities of NQR energy levels upon exposure to a saturating pulse are described. It is demonstrated that this actually causes the induction signal amplitude to change; however, complete saturation of the transition cannot be reached because of the NQR relaxation time. It is also demonstrated that the results of calculations without regard for the relaxation processes in RDX lack experimental support. Experimental data demonstrate the feasibility of application of the methods of preliminary change of the population densities (by pumping) for multifrequency experiments in NQR and explosive detection.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 49–52, April, 2005.     

Nuclear quadrupole resonance studies of209Bi in Bi4(GeO4)3 and Bi4(SiO4)3

Nuclear quadrupole resonance (NQR) of²⁰⁹Bi has been studied in Bi₄ (GeO₄)₃ and Bi₄ (SiO₄)₃ using a wide band coherence-controlled superregenerative oscillator-detector. All the four allowed (ΔM _I=±1) transitions are observed. In both cases the electric field gradient (EFG) tensor is axially symmetric (η=0.0). The quadrupole coupling constante ² qQ is measured to be 490.8±1 MHz and 470.4±1 MHz respectively. It is pointed out that the purely ionic model is inadequate to understand these results. With the available experimental accuracy and the strength of the applied electric field (∼ 6 KV/cm), no field-induced effects on the NQR spectrum could be observed in the case of Bi₄ (SiO₄)₃.     

Application of the Wavelet Transform for Detecting Signals of Nuclear Quadrupole Resonance

In the present study, the wavelet transform is used to increase the signal-to-noise ratio of nuclear quadrupole resonance (NQR) signals in the direct pulse method. The efficiency of the wavelet and Fourier transforms used for detecting the NQR signal component is compared. The signal consists of noise, magnetoacoustic and piezoelectric signals, noise from external sources, and NQR signal. Signals from ¹⁴ N nuclei in hexahydro-1,3,5-trinitro-s-triazine C₃H₆N₆O₆ (RDX) are investigated at a temperature of 298 K.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 47–50, January, 2005.     

Synthesis,structural and complex impedance spectroscopy studies of Ni0.4Co0.4Mg0.2Fe2O4 spinel ferrite

Spinel ferrite having composition Ni_0.4Co_0.4Mg_0.2Fe₂O₄ was prepared by sol-gel method. X-ray diffraction result indicates that the ferrite sample has a cubic spinel type structure. FT-IR showed two absorption bands (ν₁ and ν₂) that are attributed to the stretching vibration of tetrahedral and octahedral sites. Complex impedance properties have been investigated in 200–420 K temperature range with varying frequency between 40 and 10⁷ Hz. Frequency and temperature dependency of imaginary part of permittivity (?″) and dielectric loss (tanδ) has been discussed in terms of hopping of charge carriers between Fe²⁺ and Fe³⁺ ions. Activation energy has been estimated from both temperature dependency of dc conductivity and relaxation time data, which indicates that the relaxation process and conductivity have the same origin. Nyquist plots of impedance show semicircle arcs for sample and an electrical equivalent circuit has been proposed to explain the impedance results.     

Fast magnetic relaxation, studied by microwave absorption in neutron-irradiated YBa2Cu3O7−x ceramic

A contactless microwave method is used to measure the fast magnetic relaxation of granular ceramic samples of YBa₂Cu₃O_7−x irradiated by neutrons with fluences of 10¹⁶–10¹⁹ cm⁻². An experimental study of the time dependence of the relaxation of high-frequency (rf) absorption (f=100 MHz) after the action of an external magnetic field pulse has shown the magnetic relaxation times τ ₀ to be in the time interval of 0.5–150 ms. The rf-absorption mechanism is discussed in terms of an intergranular system with a thermally activated flux of vortices and their diffusion in the granular medium. Fiz. Tverd. Tela (St. Petersburg) 39, 977–981 (June 1997)     

The C–N stretching band of methylamine

For the first time the C–N stretching band of methylamine has been assigned in a high resolution spectrum in the region from 960 to 1200 cm⁻¹. Over 3500 transitions with a resolution of 0.001 cm⁻¹ for K from 0 to 12 have been assigned. A global fit has been made and the band center was determined at 1044.8134(33) cm⁻¹. Several branches of the C–N band are strongly perturbed through Fermi and Coriolis type resonances. The sources of the perturbations have been identified – the fourth torsional state 4ν₁₅ and the combination state ν₉ + ν₁₅. An attempt to fit the observed transitions to a single state model based on the group theoretical formalism of Ohashi and Hougen [1] resulted in the standard deviation of 0.04 cm⁻¹.     

DNA Duplex Dynamics: NMR Relaxation Studies of a Decamer with UniformlyC-Labeled Purine Nucleotides

Dynamics in a DNA decamer duplex,d(CATTTGCATC) ·d(GATGCAAATG), were investigated via a detailed¹³C NMR relaxation study. Every 2′-deoxyadenosine and 2′-deoxyguanidine was chemically enriched with 15%¹³C and 98%¹⁵N isotopes. Six nuclear relaxation parameters [R(¹³C_z),R(¹H_z),R(2¹H_z¹³C_z),R(¹³C_x),R(2¹H_z¹³C_x) and steady-state¹³C{¹H} NOE] were measured at 600 MHz and three were measured at 500 MHz (¹H frequency) for the CH spin systems of sugar 1′, 3′, and 4′ as well as base 8 and 2 positions. A dependence of relaxation parameter values on chemical position was clearly observed; however, no sequence-specific variation was readily evident within our experimental error of ∼5–10%, except for 3′ and 5′ termini. It was demonstrated that the random 15%¹³C enrichment effectively suppressed both scalar and dipolar contributions of the neighboring carbons and protons on the relaxation parameters. To analyze dynamics via all observed relaxation parameters, full spectral density mapping (1992, J. W. Peng and G. Wagner,J. Magn. Reson.98, 308) and the “model-free” approach (1982, Lipari and Szabo,J. Am. Chem. Soc.104, 4546) were applied complementarily. A linear correlation between three spectral density values,J(ω_C),J(ω_H− ω_C), andJ(ω_H+ ω_C) was observed in plots containing all measured values, but not for the other spectral density terms includingJ(0). These linear correlations reflect the effect of overall motion and similar internal motions for each CH vector in the decamer. The correlations yielded two correlation times, 3–4 ns and 10–200 ps. One value, 3–4 ns, corresponds to the value of 3.3 ns obtained for the overall isotropic tumbling correlation time determined from analysis of¹³C T1/T2 ratios. The possibility of overall anisotropic tumbling was examined, but statistical analysis showed no advantage over the assumption of simple isotropic tumbling. Lack of correlations entailingJ(0) implies that a relatively slow chemical exchange contributes to yielding of effectiveJ_eff(0) values. Based on spectral density mapping and the T1/T2 ratio analysis, three basic assumptions were initially employed (and subsequently justified) for the model-free calculation: isotropic overall tumbling, one internal motion, and the presence of chemical exchange terms. Except for terminal residues, the order parameterS²and the corresponding fast internal motion correlation time were determined to be about 0.8 ± 0.1 and 20 ± 20 ps, respectively, for the various CH vectors. Only a few differences were observed between or within sugars and bases. The internal motion is very fast (ps–ns time scale) and its amplitude restricted; e.g., assuming a simple wobble-in-a-cone model, the internal motion is restricted to an angular amplitude of ±22.5° for each of the 1′, 3′, 4′, 2, and 8 positions in the purine nucleotides in the entire duplex.     

High-Resolution Infrared Spectrum of H3SiI in the ν1/ν4Region near 2200 cm

The Fourier transform infrared spectrum of H₃SiI has been recorded in the ν₁/ν₄region from 2075 to 2315 cm⁻¹at an optical resolution of 2.3 × 10⁻³cm⁻¹. The ν₁/ν₄fundamental bands and the (ν₁+ ν₃) − ν₃/(ν₄+ ν₃) − ν₃hot bands have been rotationally investigated. Numerous local perturbations have been observed in the ν₁and ν₄bands and in the hot bands. Without the lines involved in perturbations, more than 2900 transitions of the ν₁/ν₄bands were used to determine the band origins and the vibration–rotation parameters of the ν₁= 1 and νv₄= 1 states. A least-squares fit of 766 apparently unperturbed transitions of the hot bands gave the parameters of the ν₁= ν₃= 1 and ν₄= ν₃= 1 states. Thel(2, 2) resonance in ν₄and theA₁–E Coriolis coupling between ν₁and ν₄have been investigated. Most of the local perturbations have been studied individually using a simple model by which the main perturber for each resonance was identified.     

Broad-band homo-nuclear correlations assisted by 1H irradiation for bio-molecules in very high magnetic field at fast and ultra-fast MAS frequencies

We propose a new broadband second-order proton-assisted ¹³C–¹³C correlation experiment, SHANGHAI. The ¹³C–¹³C magnetization transfer is promoted by ¹H irradiation with interspersed four phases super-cycling. This through-space homo-nuclear sequence only irradiates on the proton channel during the mixing time. SHANGHAI benefits from a large number of modulation sidebands, hence leading to a large robustness with respect to chemical shift differences, which permits its use in a broad MAS frequency range. At ultra-fast MAS (ν_R ? 60 kHz), SHANGHAI is only efficient when the amplitude of ¹H recoupling rf-field is close to half the spinning speed (ν₁ ≈ ν_R/2). However, at moderate to fast MAS (ν_R = 20–35 kHz), SHANGHAI is efficient at any rf-power level larger than ν₁ ≈ 10 kHz, which simultaneously permits avoiding excessive heating of bio-molecules, and using large sample volumes. We show that SHANGHAI can be employed at the very high magnetic field of 23.5 T and then allows the observation of correlation between ¹³C nuclei, even if their resonance frequencies differ by more than 38 kHz.     

Fabrication, structure and luminescence properties of polycrystalline Tb-doped Lu2SiO5 films by Pechini sol–gel method

Tb³⁺-doped lutetium oxyorthosilicate (Tb:Lu₂SiO₅, LSO) films have been successfully fabricated on carefully cleaned silicon (1 1 1) substrates by Pechini sol–gel method combined with the spin-coating technique. X-ray diffraction (XRD), photoluminescence (PL) spectra and atomic force microscopy (AFM) were employed to characterize the resultant films. XRD patterns indicated that the films were crystallized into A-type LSO phase at 1000 °C, followed by a phase transition from A-type LSO to B-type LSO occurred at 1100 °C. The AFM observation revealed that the phosphor films were uniform and crack-free, consisting of closely packed grains with an average size of 200–300 nm. The PL spectra showed the characteristic emission ⁵D₄ → ⁷F_J (J = 3–6) for Tb³⁺, The lifetime of Tb³⁺ in Tb:LSO films was 2.33 ms. The effect of heat-treatment temperature on the luminescent properties was also investigated.     

Joint Optimization of the Number of Pulses and the Relaxation Pause in the Combined Multiple-Pulse Spin-Locking Sequence for Detecting Explosives by NQR

The problem has been solved for the joint optimization of the number N of pulses and the relaxation pause t _p in the combined multiple-pulse spin-locking sequence, which is successfully used for detecting some explosives by nuclear quadrupole resonance (NQR). Initially, the formula is derived for the signal-to-noise ratio provided by this sequence. Using the condition that an explosive detection time T _det is constant (it is typical in practice), from this expression the set of two interrelated equations is obtained for optimum values of N and t _p, which provide the maximum signal-to-noise ratio. These values are determined by both effective transverse T _2e and longitudinal T ₁ relaxation times and sequence parameters, and do not depend on the number m of sequence repetitions. The m value is calculated as the quotient of T _det by the duration of the onefold sequence with optimum values of N and t _p. This set of equations is shown to be mutually compatible at all values of sequence parameters and relaxation times T ₁ and T _2e, which occur in actual practice. The results of the experiments, which have been carried out on cyclotrimethylene trinitramine (RDX) at a ¹⁴N NQR frequency of 3.41 MHz and temperatures of 26 and −3°C, are in good agreement with the theoretical ones.     

Influence of relaxation on the two-dimensional nutational nuclear-quadrupole-resonance spectra

The influence of relaxation and frequency detuning on the nutational frequency and the damping factor of a nutational interference pattern is analyzed. It is established that relaxation leads to decrease in nutational frequency, while detuning increases the nutation frequency. A relation is obtained between the relaxation times and the frequency detuning at which the influence of both factors cancels out. Conditions such that singularities of the nutational nuclear-quadrupole-resonance (NQR) spectrum (I=3/2) may be resolved are determined, and a formula for the asymmetry parameter ν is obtained, taking account of relaxation. Having determined the region of existence of nutation and established the conditions under which the singularities of the NQR spectrum of the powder (I=3/2) may be resolved, the asymmetry parameter may be reliably determined from the experimental nutational spectrum of the powder. Experimental nutational³⁵Cl NQR spectra are given for TiCl₄ and 2,4,6-trichloro-1,3,5-triazine. Kaliningrad State University, Baltiisk State Academy of the Russian Federation, Kaliningrad. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 41, No. 12, pp 66–71, December, 1998.