Showing papers on "Nuclear quadrupole resonance published in 2009"

Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe2As2 Studied by 75As Nuclear Quadrupole Resonance and Specific Heat

Abstract: We report the 75 As nuclear quadrupole resonance (NQR) and specific heat measurements of the heavily hole-doped superconductor KFe 2 As 2 (superconducting transition temperature T c ≃3.5 K). The spin-lattice relaxation rate 1/ T 1 in the superconducting state exhibits a gradual temperature dependence with no coherence peak below T c . The quasiparticle specific heat C QP / T shows a small jump, which is about 30% of the electronic specific heat coefficient just below T c . The C QP / T suggests the existence of low-energy quasiparticle excitation at the lowest measurement temperature T =0.4 K≃ T c /10. The T dependences of 1/ T 1 and C QP / T can be explained by a multiple nodal superconducting gap scenario rather than by a multiple fully gapped s ± -wave scenario determined using simple gap analysis.

Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe2As2 Studied by 75 As-NQR and Specific Heat

Abstract: We report the 75 As nuclear quadrupole resonance (NQR) and specific heat measurements of the heavily hole-doped superconductor KFe2As2 (superconducting transition temperature Tc ≃ 3.5 K). The spin-lattice relaxation rate 1/T1 in the superconducting state exhibits quite gradual temperature dependence with no coherence peak below Tc. The quasi-particle specific heat CQP/T shows small specific heat jump which is about 30% of electronic specific heat coefficient just below Tc. In addition, it suggests the existence of low-energy quasi-particle excitation at the lowest measurement temperature T = 0.4 K≃ Tc/10. These temperature dependence of 1/T1 and CQP/T can be explained by multiple nodal superconducting gap scenario rather than multiple fully-gapped s±-wave one within simple gap analysis.

Electronic phase diagram of the layered cobalt oxide system LixCoO2 (0.0≤x≤1.0)

Abstract: Here we report the magnetic properties of the layered cobalt oxide system, LixCoO2, in the whole range of Li composition, 0x1. Based on dc-magnetic-susceptibility data, combined with results of 59 Co nuclear magnetic resonance NMR and nuclear quadrupole resonance NQR observations, the electronic phase diagram of LixCoO2 has been established. As in the related material NaxCoO2, a magnetic critical point is found to exist between x=0.35 and 0.40, which separates the Pauli-paramagnetic and Curie-Weiss metals. In the Pauli-paramagnetic regime x0.35, the antiferromagnetic spin correlations systematically increase with decreasing x. Nevertheless, CoO2, the x=0 end member is a noncorrelated metal in the whole temperature range studied. In the Curie-Weiss regime x0.40, on the other hand, various phase transitions are observed. For x=0.40, a susceptibility hump is seen at 30 K, suggesting the onset of static antiferromagnetic order. A magnetic jump, which is likely to be triggered by charge ordering, is clearly observed at Tt175 K in samples with x=0.50 =1/2 and 0.67 =2/3, while only a tiny kink appears at T210 K in the sample with an intermediate Li composition, x=0.60. Thus, the phase diagram of the LixCoO2 system is complex and the electronic properties are sensitively influenced by the Li content x.

Novel Phase Transition in CeRu2Al10 Probed by 27Al-NQR/NMR: No Evidence of Magnetic Ordering

Abstract: The 27 Al-NQR/NMR measurements of CeRu 2 Al 10 were carried out to clarify the unusual phase transition at T 0 =27 K Distinct NQR peaks associated with five nonequivalent Al sites have been observed at T > T 0 , and each peak is successfully assigned to their respective Al sites Below the transition temperature T < T 0 , each peak simply splits into two peaks except for one site This indicates that the phase transition is not accompanied by magnetic order, but is presumably associated with the onset of structural transition with lowering symmetry The nuclear spin–lattice relaxation rate 1/ T 1 suggests a local moment picture above T * ∼60 K, and the development of Kondo coherence toward T 0 Below T 0 , 1/ T 1 shows a gaplike decrease with a gap magnitude of 106 K, being consistent with the macroscopic measurements The Korringa term below 10 K after the gaplike decrease suggests a gap opening over a portion of the Fermi surface

Explosives Detection Using Magnetic and Nuclear Resonance Techniques

Abstract: Preface.- Fundamentals of Pulsed Nitrogen-14 Quadrupole Resonance D. Canet, M. Ferrari.- 14N NQR Detection of Explosives with Hybrid Sensors M. Pannetier-Lecoeur et al.- Polarization Enhanced NQR Detection at Low Frequencies J. Luznik et al.- Efficient Excitation and Ringing Suppression in Nuclear Quadrupole Resonance J.B. Miller et al.- Detection of Concealed Liquid Explosives and Illicit Drugs in Unopened Bottles S. Kumar, P.J. Prado.- Prospectives and Limitations of NQR Signal Enhancement by Polarisation Transfer A.F. Privalov et al.- Modeling of QR Sensors for Optimized Explosives Detection H. Robert et al.- Detection of Explosives by NQR Method: Main Aspects for Transport Security T.N. Rudakov.- Double Resonance Detection of (Mainly Nitrogen) NQR Frequencies in Explosives and Drugs J. Seliger, V. Zagar.- Signal Processing Methods in NQR V.S. Grechishkin et al.- 14N Nuclear Quadrupole Resonance Signals in Paranitrotoluene and Trinitrotoluene. Spin-Lock Spin-Echo off-resonance Effects A. Gregorovic et al.- Identification of Liquids Encountered in Carry-on-Luggage by Mobile NMR J. Mauler et al.- The Two-Frequency Multipulse Sequence in Nuclear Quadrupole Resonance of N-14 Nuclei G.V. Mozzhukhin et al.- The Detection of Industrial Explosives by the Quadrupole Resonance Method: Some Aspects of the Detection of Ammonium Nitrate and Trinitrotoluene G.V. Mozzhukhin et al.- Development of Electric Field NMR Signal Acquisition System R.J. Prance et al.- Berry's Phase in NQR of Powders N. Sinyavsky et al.- Contribution of Copper NQR Spectroscopy to the Geological Studies of Complex Sulfides and Oxides R.R. Gainov et al.- Index.-

Nuclear quadrupole resonances in compact vapor cells: The crossover between the NMR and the nuclear quadrupole resonance interaction regimes

Abstract: We present an experimental study that maps the transformation of nuclear quadrupole resonances (NQRs) from the pure nuclear quadrupole regime to the quadrupole-perturbed Zeeman regime. The transformation presents an interesting quantum-mechanical problem since the quantization axis changes from being aligned along the axis of the electric-field gradient tensor to being aligned along the magnetic field. The large nuclear quadrupole shifts present in our system enable us to study this regime with relatively high resolution. We achieve large nuclear quadrupole shifts for $I=3∕2$ $^{131}\mathrm{Xe}$ by using a cube-shaped $1\phantom{\rule{0.3em}{0ex}}{\mathrm{mm}}^{3}$ vapor cell with walls of different materials. The enhancement of the NQR shift from the cell wall materials is an observation that opens up an additional adjustable parameter to tune and enhance the nuclear quadrupole interactions in vapor cells. As a confirmation that the interesting and complex spectra that we observe are indeed expected, we compare our data to numerical calculations and find excellent agreement.

Nuclear quadrupole resonance and x-ray investigation of the structure of Na 2 / 3 CoO 2

Abstract: We have synthesized various samples of the $x=2/3$ phase of sodium cobaltate ${\text{Na}}_{x}{\text{CoO}}_{2}$ and performed x-ray powder diffractions spectra to compare the diffraction with the structure proposed previously from NMR and nuclear quadrupole resonance (NQR) experiments [H. Alloul, I. R. Mukhamedshin, T. A. Platova, and A. V. Dooglav, EPL 85, 47006 (2009)]. Rietveld analyses of the data are found in perfect agreement with those and confirm the concentration $x=2/3$ obtained in the synthesis procedure. They even give indications on the atomic displacements of Na inside the unit cell. The detailed NQR data allow us to identify the NQR transitions and electric field gradient parameters for four cobalt sites and three Na sites. The spin-lattice and spin-spin relaxation rates are found much smaller for the nonmagnetic ${\text{Co}}^{3+}$ sites than for the magnetic sites on which the holes are delocalized. The atomic ordering of the Na layers is therefore at the source of this ordered distribution of cobalt charges. The method used here to resolve the Na ordering and the subsequent Co charge order can be used valuably for similar structural determinations for various phases with $xg0.45$ for which Na ordering has been established.

Phase transition and anomalous electronic behavior in the layered superconductor CuS probed by NQR

Abstract: Nuclear quadrupole resonance (NQR) on copper nuclei has been applied for studies of the electronic properties of quasi-two-dimensional (2D) low-temperature superconductor CuS (covellite) in the temperature range of 1.47--290 K. Two NQR signals corresponding to two structural nonequivalent sites of copper, Cu(1) and Cu(2), have been found. The temperature dependences of copper quadrupole frequencies, linewidths, and spin-lattice relaxation rates altogether demonstrate the structural phase transition near 55 K, which is accompanied by transformations of the electronic spectrum not typical for simple metals. The analysis of NQR results and their comparison with literature data show that the valence of copper ions at both sites is intermediate between monovalent and divalent states with the dominance of the former. It has been found that there is a strong hybridization of the Cu(1) and Cu(2) conduction bands at low temperatures, indicating that the charge delocalization between these ions takes place even in 2D regime. On the basis of our data, the occurrence of an energy gap, charge fluctuations, and charge-density waves, as well as the nature of the phase transition in CuS, are discussed. It is concluded that some physical properties of CuS are similar to those of high-temperature superconductors in the normal state.

Doping Dependence of Normal-State Properties in Iron-Based Oxypnictide Superconductor LaFeAsO1-y Probed by 57Fe-NMR and 75As-NMR/NQR

Abstract: We report systematic 57 Fe-NMR and 75 As-NMR/NQR studies on an underdoped sample ( T c =20 K), an optimally doped sample ( T c =28 K), and an overdoped sample ( T c =22 K) of oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO 1- y . A microscopic phase separation between superconducting domains and magnetic domains is shown to take place in the underdoped sample, indicating a local inhomogeneity in association with the density distribution of oxygen deficiencies. As a result, 1/ T 1 T in the normal state of the superconducting domain decreases significantly upon cooling at both the Fe and As sites regardless of the electron-doping level in LaFeAsO 1- y . On the basis of this result, we claim that 1/ T 1 T is not always enhanced by antiferromagnetic fluctuations close to an antiferromagnetic phase in the underdoped superconducting sample. This contrasts with the behavior in hole-doped Ba 0.6 K 0.4 Fe 2 As 2 ( T c =38 K), which exhibits a significant increase in 1/ T 1 T upon cooling. We re...

Countering Radio Frequency Interference in Single-Sensor Quadrupole Resonance

Abstract: Nuclear quadrupole resonance (NQR) is a solid-state radio frequency (RF) spectroscopic technique that allows for the detection of many narcotics and highly explosive substances. Unfortunately, the practical use of NQR is often restricted by the presence of strong RF interference (RFI). In this letter, extending our recent work on stochastic NQR (sNQR), we propose acquiring signal-of-interest free samples, containing only corrupting signals, and exploiting them to reduce the effects of RFI on conventional NQR (cNQR) measurements. Similar to the sNQR case, the presented detectors are able to substantially outperform previous cNQR detectors when RFI is present.

Electronic properties of LaO1?xFxFeAs in the normal state probed by NMR/NQR

Abstract: We report 139La, 57Fe and 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on powders of the new LaO1?xFxFeAs superconductor for x=0 and 0.1 at temperatures up to 480?K, and compare our measured NQR spectra with local density approximation (LDA) calculations. For all three nuclei in the x=0.1 material, it is found that the local Knight shift increases monotonically with an increase in temperature, and scales with the macroscopic susceptibility, suggesting a single magnetic degree of freedom. Surprisingly, the spin lattice relaxation rates for all nuclei also scale with one another, despite the fact that the form factors for each site sample different regions of q-space. This result suggests a lack of any q-space structure in the dynamical spin susceptibility that might be expected in the presence of antiferromagnetic correlations. Rather, our results are more compatible with simple quasi-particle scattering. Furthermore, we find that the increase in the electric field gradient at the As cannot be accounted for by LDA calculations, suggesting that structural changes, in particular the position of the As in the unit cell, dominate the NQR response.

NQR and NMR study of hydrogen bonding interactions in anhydrous and monohydrated guanine cluster model: A computational study

Abstract: In this paper extensive systematic computational study has been carried out to justify hydrogen bonding interactions and their influence on the oxygen, nitrogen and hydrogen NQR and NMR parameters of the anhydrous and monohydrated guanine crystal structures at two different levels, B3LYP and MP2, using 6-311++G** and D95** basis sets. These theoretical data have been compared with experimental NMR and NQR measurements. For further investigation, results of cluster calculation have been compared with that of a single molecule. Our theoretical NQR and NMR parameters of 17O, 15N and 2H atoms of anhydrous and monohydrated guanine exhibited extreme sensitivity to electron distribution around mentioned nuclei caused by cooperative influences of various types of hydrogen bonding interactions. Fortunately, our calculated isotropic shielding values and CS tensors for the 17O and 15N nuclei as well as obtained 14N-NQR parameters are in excellent agreement with experimental data. Therefore, we can undoubtedly conclude that for anhydrous and monohydrated guanine tetrameric clusters including intermolecular interactions, our theoretical estimates are in better agreement with observed experimental values than those in which these interactions have been ignored.

Measurement of electron correlations in LixCoO2 (x=0.0-0.35) using 59Co nuclear magnetic resonance and nuclear quadrupole resonance techniques

Abstract: CoO2 is the parent compound for the superconductor NaxCoO2\cdot1.3H2O and was widely believed to be a Mott insulator. We performed 59Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on LixCoO2 (x = 0.35, 0.25, 0.12, and 0.0) to uncover the electronic state and spin correlations in this series of compounds which was recently obtained through electrochemical de-intercalation of Li from pristine LiCoO2. We find that although the antiferromagnetic spin correlations systematically increase with decreasing Li-content (x), the end member, CoO2 is a non-correlated metal that well satisfies the Korringa relation for a Fermi liquid. Thus, CoO2 is not simply located at the limit of x->0 for AxCoO2 (A = Li, Na) compounds. The disappearance of the electron correlations in CoO2 is due to the three dimensionality of the compound which is in contrast to the highly two dimensional structure of AxCoO2.

B24N24 nanocages: a GIAO density functional theory study of 14N and 11B nuclear magnetic shielding and electric field gradient tensors

Abstract: Density functional theory (DFT) calculations were performed to determine boron-11 and nitrogen-14 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spectroscopy parameters in the three most stable B24N24 fullerenes for the first time. The considered samples were first allowed to relax entirely, and then the NMR and NQR calculations were performed on the geometrically optimized models. The calculations of the 11B and 14N nuclear magnetic shielding tensors and electric field gradient tensors employed the Gaussian 98 software implementation of the gauge-including atomic orbital (GIAO) method using the Becke3, Lee-Yang-Parr (B3LYP) DFT level and 6-311G** and 6-311++G** standard basis sets in each of the three optimized forms, and converted the results to experimentally measurable NMR parameters.The calculated NMR chemical shieldings of the three cages show significant differences, providing a way to identify these clusters. The evaluated NQR parameters of the 11B and 14N nuclei in the clusters are also reported and discussed.

The Detection of Industrial Explosives by the Quadrupole Resonance Method: Some Aspects of the Detection of Ammonium Nitrate and Trinitrotoluene

Abstract: This paper is devoted to some aspects of the detection of Indus- trial explosives. The main components of these explosives are trinitrotoluene C6H2(NO2)3CH3 (TNT) and ammonium nitrate NH4NO3 (AN). A study of the behaviour of the nuclear quadrupole resonance (NQR) signals of AN and TNT is reported. A method for attenuating outside interference signals under conditions of an industrial environment are proposed and discussed.

NQR-based explosives detection—an overview

Abstract: Nuclear quadrupole resonance (NQR) is a radio frequency spectroscopic technique that can be used to detect solid-state compounds containing quadrupolar nuclei, a requirement fulfilled by most high explosives (and narcotics). In this paper, we present an overview of recent research in the detection of explosives using this technique. We also present mathematical models for the data for different acquistion techniques and discuss different state of the art detection algorithms. Finally, we evaluate various algorithms on measured and simulated NQR data.

Temperature-Induced Change in the Magnitude of the Effective Density of States: A NQR/NMR Study of the A-Site-Ordered Perovskite System CaCu3Ru4O12

Abstract: NQR/NMR measurements, including spin–lattice relaxation rate (1/ T 1 ) and Knight shift ( K ) evaluations, have been carried out for Cu and Ru nuclear spins in the A -site ordered perovskite system CaCu 3 Ru 4 O 12 , which exhibits a mass-enhancement-like behavior although it possesses no f electrons. The temperature evolution of T 1 for Cu, which was measured up to 700 K, exhibits 1/ T 1 T = constant behavior at high enough temperatures. This clearly denies the existence of a Cu localized moment, which was previously suggested as the origin of the mass enhancement in a dense Kondo scenario. Furthermore, K and (1/ T 1 T ) for Cu exhibit significant temperature dependences with two characteristic temperatures T F =20 K and T X =180 K, suggesting a temperature-induced transition of microscopic electronic character at T F and T X . We propose a two–band model, where the effective density of states for one of the bands shows a significant temperature dependence in T F < T < T X . Possible origins are discussed.

Nutation versus angular-dependent NQR spectroscopy and impact of underdoping on charge inhomogeneities in YBa 2 Cu 3 O y

Abstract: We describe two different nuclear quadrupole resonance (NQR) based techniques, designed to measure the local asymmetry of the internal electric field gradient, and the tilt angle of the main NQR principal axis z from the crystallographic axis c. These techniques use the dependence of the NQR signal on the duration of the radio frequency (rf) pulse and on the direction of the rf field H1 with respect to the crystal axis. The techniques are applied to oriented powder of YBa$_{2}$Cu$%_{3}$O$_{y}$ fully enriched with 63Cu. Measurements were performed at different frequencies, corresponding to different in-plane copper sites with respect to the dopant. Combining the results from both techniques, we conclude that oxygen deficiency in the chain layer lead to a rotation of the NQR main principal axis at the nearby Cu on the CuO2 planes by 20+-degrees. This occurs with no change to the asymmetry. The axis rotation associated with oxygen deficiency means that there must be electric field inhomogeneities in the CuO2 planes only in the vicinity of the missing oxygen.

Spectroscopic Study of $^{75}$As and $^{139}$La NMR on Layered Structure Ferromagnet LaCoAsO

Abstract: $^{75}$As and $^{139}$La field-swept NMR spectra were obtained for the novel weakly itinerant ferromagnet LaCoAsO with 2D layered structure above the Curie temperature of 55 K. By analyzing NMR spectra, temperature dependences of Knight shift $K$ and nuclear quadrupole resonance frequency $ u_Q$ were obtained successfully for each nucleus. We confirmed from the so-called $K$-$\chi$ plots that the macroscopic magnetization of our {LaCoAsO} powder sample is intrinsic and does not contain the contribution from impurity phases. We estimated hyperfine coupling constants from the slope of $K$-$\chi$ plots and compared to that of iron-arsenide superconductor.

Quantitative 35Cl nuclear quadrupole resonance in tablets of the antidiabetic medicine Diabinese.

Abstract: Pulsed 35Cl nuclear quadrupole resonance (NQR) experiments have been performed on 250-mg tablets of the antidiabetic medicine Diabinese to establish the conditions needed for noninvasive quantitati

NMR and NQR study of pressure-induced superconductivity and the origin of critical-temperature enhancement in the spin-ladder cuprate Sr2Ca12Cu24O41

Abstract: Pressure-induced superconductivity was studied for a spin-ladder cuprate Sr$_2$Ca$_{12}$Cu$_{24}$O$_{41}$ using nuclear magnetic resonance (NMR) under pressures up to the optimal pressure 3.8 GPa. Pressure application leads to a transitional change from a spin-gapped state to a Fermi-liquid state at temperatures higher than $T_c$. The relaxation rate $1/T_1$ shows activated-type behavior at an onset pressure, whereas Korringa-like behavior becomes predominant at the optimal pressure, suggesting that an increase in the density of states (DOS) at the Fermi energy leads to enhancement of $T_c$. Nuclear quadrupole resonance (NQR) spectra suggest that pressure application causes transfer of holes from the chain to the ladder sites. The transfer of holes increases DOS below the optimal pressure. A dome-shaped $T_c$ versus pressure curve arises from naive balance between the transfer of holes and broadening of the band width.

Conventional s-Wave Superconductivity in Noncentrosymmetric Ir2Ga9: 71Ga-NQR Evidence

Abstract: Superconductivity without spatial inversion symmetry has begun to attract much attention since the discovery of the superconductor CePt3Si. 1) It is presumed that the absence of spatial inversion symmetry causes an admixture between even and odd parities of superconducting pairing due to antisymmetric spin–orbit coupling (ASOC). However, there has been no clear evidence of the admixture thus far, and hence the spatial noncentrosymmetric effect on superconductivity remains experimentally controversial. Recently, ASOC has also been discussed in transition-metal superconductors. In this system, we can consider ASOC without strongly correlated electron systems, which would be appropriate for tracing the effect of ASOC. Interestingly, in Li2Pt3B, it was reported that unconventional superconductivity with a line node and a spin-triplet pairing state emerge, which are ascribed to ASOC. These reports suggest that ASOC can induce a novel superconducting state even though there is no strong electron correlation. The intermetallic binary Ir2Ga9 does not possess spatial inversion symmetry and becomes superconducting below Tc 1⁄4 2:2K. Specific heat and resistivity measurements showed that the compound is a weak-coupling BCS superconductor with an isotropic gap, and located near the boundary between type-I and type-II superconductivities with an upper critical field Hc2 150Oe. In this letter, we report on the characteristics of superconductivity in noncentrosymmetric Ir2Ga9 probed by Ga-nuclear-quadrupole-resonance (NQR) measurement at a zero field (H 1⁄4 0). The spatial noncentrosymmetric effect on the superconducting state is discussed. A single crystal of Ir2Ga9 was grown by the Ga flux method. Powder X-ray diffraction indicated that the compound forms in the primitive monoclinic Rh2Ga9 type structure. The sample of Ir2Ga9 was crushed into coarse powder for NQR measurement to allow the penetration of the rf field. The NQR measurement was performed by the conventional spin-echo method in the temperature (T) range of 1–280K. T1 was measured at f 26:45MHz, which was the 1 Qð 1=2 $ 3=2Þ transition of Ga (I 1⁄4 3=2), as shown in Fig. 1. The NQR spectrum ensures the quality of the sample, because the full width at half maximum (FWHM) is as small as 54 kHz, as shown in Fig. 1. The NQR signal of the isotope Ga (I 1⁄4 3=2) was confirmed at f 42:08MHz, which was consistent with the ratio 69Q=Q 1:59 (Q is the quadrupole moment) because NQR frequency is proportional to Q. Figure 2 shows 1=T1T as a function of T in the temperature range of 1 T 280K along with the ac-susceptibility measured using an in-situ NQR coil. In the normal state above Tc, a T1T 1⁄4 const. relation is valid up to T 50K, which is usually seen in conventional metals. Above T 50K, although 1=T1T gradually deviates from the constant, it is probably ascribed to the local structural distortion of the crystal because NQR frequency also shifts in the same T range, as seen in the inset of Fig. 2. This structural change may bring about an increase in the density of states at the Fermi level, leading to an increase of the 1=T1T constant value in the high T region. It is important to investigate the precise crystal structure of Ir2Ga9 at a low T where the superconductivity emerges. In the superconducting state, NQR intensity is largely suppressed below Tc 1⁄4 2:2K, as shown in the inset of Fig. 1. This is consistent with the previous reports that 26.2 26.3 26.4 26.5 26.6 26.7 26.8 N Q R in te ns ity ( ar b. u ni t)

Study of hydrogen bonds in crystalline 5-nitrouracil. Density functional theory calculations of the O-17, N-14, and H-2 nuclear quadrupole resonance parameters

Abstract: Hydrogen bond interaction properties of backbone uracil was studied in crystalline structure of 5-nitrouracil. To this aim the electric field gradient tensors were calculated at the level of density functional theory in two single (non-hydrogen bonded) and cluster (hydrogen-bonded four-molecule) models of 5-nitrouracil. The electric field gradient tensors at the sites of O-17, N-14, and H-2 nuclei were converted to the experimentally measurable nuclear quadrupole resonance spectroscopy parameters, quadrupole coupling constant and asymmetry parameter. The results indicated different hydrogen bond interaction properties at the sites of various nuclei and also the protective role of −NO2 group for contribution of O1 to hydrogen bond interactions in comparison with uracil. The density functional theory calculations were performed using GAUSSIAN 98 package employing B3LYP method and 6-311G** and 6-311++G** basis sets.

Structure and Physical–Chemical Properties of Nitroazoles

Abstract: The critical evaluation of a large body of the information on five-membered nitroazoles and nitrobenzazoles study by physical chemical methods – nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), electron spin resonance (ESR), ion-cyclotron resonance, UV and IR- spectroscopy, X-ray analysis, mass spectrometry, polarography, dipole moments, chromatography, luminescence, photolysis, etc. is presented. The extensive investigations of structure, tautomerism, and properties of nitroazoles by multinuclear 1H, 13C, 15N, 19F, 31P, 29Si and two-dimensional NMR spectroscopy are reviewed. A great emphasis is given to tautomerism studies of nitroazoles by multinuclear dynamic NMR because prototropic transformations of almost all azoles in solutions proceed so quickly. The mechanisms of electrochemical reactions and vicarious nucleophilic C-amination of nitroazoles are discussed. Quantum-chemical investigations of nitroazoles are covered in detail.