Quadrupole Resonance Frequencies Research Articles

Temperature dependence of nonequivalent potential wells for pyridinium ion reorientation in pyridinium tetrachloroiodate(III), PyHICl4, studied by 1H14N nuclear quadrupole double resonance

The reorientation of a pyridinium ion in the paraelectric and antiferroelectric phase of PyHICl(4) is investigated using (1)H-(14)N nuclear quadrupole double resonance (NQDR). The (14)N nuclear quadrupole resonance frequencies are measured. The temperature variations of the principal values of the time-averaged electric-field-gradient (EFG) tensor at the nitrogen position are used to determine the occupation probabilities of the six orientations of a pyridinium ion in both crystallographic phases. The energy difference between various orientations is determined. The molar transition entropy associated with the reorientation of the pyridinium ions is calculated and compared to the experimental value.

Measurement of the 14N nuclear quadrupole resonance frequencies by the solid effect

1H– 14N nuclear quadrupole double resonance using magnetic field cycling between high and low magnetic field and solid effect in the low magnetic field is analyzed in details. The transition probabilities per unit time for the solid-effect transitions are calculated. The double resonance spectra are calculated in the limiting cases of fast and slow nitrogen spin–lattice relaxation. The double resonance spectra are measured in histamine and quinolinic acid. The experimental spectra are analyzed and the 14N NQR frequencies are determined.

Pressure and temperature dependence of the chlorine NQR in caesium and sodium chlorates

The (35)Cl nuclear quadrupole resonance (NQR) frequencies (nu(Q)) in caesium and sodium chlorates were measured as a function of temperature, from 77 to 300 K at different pressures up to 5.1 kbar, and the data were analysed to estimate the volume dependence of the electric field gradient (EFG), torsional frequency and also the contributions to the NQR frequency from static and dynamic effects. The variation of spin-lattice relaxation time with pressure at different temperatures was studied in the case of sodium chlorate and at room temperature in case of caesium chlorate. The pressure dependence of the spin-lattice relaxation time (T(1)) suggests that the relaxation is mainly due to the torsional motions.

Anomalous H/D isotope effect on 35Cl NQR frequencies in piperidinium p-chlorobenzoate

Anomalous isotope effects were detected in the 35Cl nuclear quadrupole resonance (NQR) frequency of piperidinium p-chlrobenzoate (C5H10NH· ClC6H4COOH) by deuteration of hydrogen atoms. The atoms were determined to form two kinds of N–H···O type H-bonds in the crystal structure. Large frequency shifts of the 35Cl resonance lines reaching 288 kHz at 77 K and 278 kHz at room temperature were caused upon deuteration, in spite of the fact that the Cl atoms in the molecule do not form hydrogen bonds in the crystal. Results of single crystal X-ray diffraction measurements and density-functional-theorem calculations suggest that a dihedral-angle change of 1.8° between benzene and the piperidine ring contributes to 35Cl NQR anomalous frequency shifts.

14N NQR study of nicotinamide and related compounds

14N nuclear quadrupole resonance (NQR) frequencies have been measured in picolinamide, nicotinamide, isonicotinamide, 2,6-pyridine dicarboxamide, and acetamide by double resonance. The 14N NQR spectra in picolinamide, nicotinamide, isonicotinamide, and 2,6-pyridine dicarboxamide show the presence of two distinct nitrogen positions: the ring position with the quadrupole coupling constant about 4,5 MHz and the amide position with the quadrupole coupling constant about 2.6 MHz. The NQR data are related to the structure of the investigated compounds and to the N--H...O hydrogen bonds.

Unconventional superconductivity near quantum critical point revealed by Co-NQR measurements on [formula omitted

The nuclear quadrupole resonance (NQR) frequency of Co nuclei and the nuclear spin–lattice relaxation rate 1 / T 1 were measured on several bilayered hydrate (BLH) Na x ( H 3 O ) z CoO 2 · y H 2 O ( y ∼ 1.3 ) with variety of superconducting (SC) and magnetic transition temperatures, T c and T M , together with non-SC mono-layered hydrate (MLH) Na x ( H 3 O ) z CoO 2 · y H 2 O ( y ∼ 0.7 ). In the high temperature region above 70 K, 1 / T 1 T in all the samples follows the same temperature dependence which is interpreted as the pseudogap behavior. In the BLH compounds, 1 / T 1 T increases with decreasing temperature below 70 K, and the values of 1 / T 1 T at T c are large in high- T c samples. The magnetic ordering is ascertained from the observation of the prominent divergence of 1 / T 1 T at T M in the samples whose NQR frequency is higher than 12.5 MHz. The temperature dependence of 1 / T 1 T is found to be consistently expressed by a unique function with two fitting parameters. We analyze the temperature dependence of 1 / T 1 T on the basis of this function, and investigate the relationship between the magnetic fluctuations and superconductivity in the BLH compounds.

Dependence of 35Cl NQR on hydrogen bonding and temperature in dichlorophenol–aniline charge transfer complexes

The hydrogen-bonded charge transfer complexes of aniline with pi-acceptors (or proton donors) such as 2,5-, 2,6-, 3,4- and 3,5-dichlorophenol were prepared. The (35)Cl nuclear quadrupole resonance (NQR) frequencies of these charge transfer complexes in the temperature range 77-300 K were measured to ascertain the existence or otherwise of a phase transition upon complex formation. Further, the NQR frequency and asymmetry parameter of the electric field gradient at the site of quadrupole nucleus were used to estimate the chemical bond parameters, namely ionic bond, double bond character of the carbon-chlorine(C--Cl) bond and the percentage charge transfer between the donor-acceptor components in charge transfer complexes. The effect of hydrogen bonding and temperature on the charge transfer process is analysed.

Hydrogen Bonds between Nitrogen Donors and the Semiquinone in the Qi-site of the bc1 Complex

The ubisemiquinone stabilized at the Qi-site of the bc1 complex of Rhodobacter sphaeroides forms a hydrogen bond with a nitrogen from the local protein environment, tentatively identified as ring N from His-217. The interactions of 14N and 15N have been studied by X-band (approximately 9.7 GHz) and S-band (3.4 GHz) pulsed EPR spectroscopy. The application of S-band spectroscopy has allowed us to determine the complete nuclear quadrupole tensor of the 14N involved in H-bond formation and to assign it unambiguously to the Nepsilon of His-217. This tensor has distinct characteristics in comparison with H-bonds between semiquinones and Ndelta in other quinone-processing sites. The experiments with 15N showed that the Nepsilon of His-217 was the only nitrogen carrying any considerable unpaired spin density in the ubiquinone environment, and allowed calculation of the isotropic and anisotropic couplings with the Nepsilon of His-217. From these data, we could estimate the unpaired spin density transferred onto 2s and 2p orbitals of nitrogen and the distance from the nitrogen to the carbonyl oxygen of 2.38+/-0.13A. The hyperfine coupling of other protein nitrogens with semiquinone is <0.1 MHz. This did not exclude the nitrogen of the Asn-221 as a possible hydrogen bond donor to the methoxy oxygen of the semiquinone. A mechanistic role for this residue is supported by kinetic experiments with mutant strains N221T, N221H, N221I, N221S, N221P, and N221D, all of which showed some inhibition but retained partial turnover.

Low-field approach to double resonance in nuclear quadrupole resonance of spin-1 nuclei

Using double-resonance conditions, in which the Larmor frequency of a spin-1/2 nucleus is matched to one of the nuclear quadrupole resonance frequencies of a spin-1 nucleus, the authors demonstrate increased cross relaxation between the two nuclear spin species. They calculate the cross-relaxation rate using the motionally averaged heterogeneous dipole Hamiltonian as a perturbation to the combined quadrupole and Zeeman Hamiltonians. Using this cross-relaxation rate, in addition to hydrogen and nitrogen autorelaxation rates, expressions governing spin-1/2 and spin-1 spin-lattice relaxation are determined. With ammonium nitrate, containing nitrogen (spin-1) and hydrogen (spin-1/2), increased nitrogen signal and spin-lattice relaxation are demonstrated, using fields less than 120 G. The cross-relaxation rate is also measured and an overall signal/noise improvement by a factor of 2.3+/-0.1 is attained.

Temperature Dependence of Torsional Frequencies in Some Dichloro-Anilines Using Nuclear Quadrupole Resonance Data

The temperature variation of the nuclear quadrupole resonance frequencies in 3,4-dichloro-aniline and 2,6-dichloro-aniline were fitted to three different models available in literature, namely the Kushida model, the Brown model, and a fourth order polynomial. Brown's model with T0 = 205 K gave the best fit in the case of both compounds indicating that the temperature dependence of the torsional frequencies cannot be neglected in these compounds. The internal torsional frequencies were evaluated from the nuclear quadrupole resonance data by using a two-mode approximation in a temperature range of 77-300 K for the above two compounds. The torsional frequencies lie in a range of 30-60 cm -1 in both compounds and agree closely with the Raman lines obtained in 2,6-dichloro-aniline. The lower frequency mode around 37 cm-1 may be associated with the motion around an axis perpendicular to the benzene ring which is present in both compounds.

Spin dynamics in the pulsed spin locking of nuclear quadrupole resonance

We theoretically examine the perturbative effects of a series of radio-frequency (rf) pulses, electric field gradient inhomogeneity, and dipole-dipole coupling on the spin dynamics of spin-1 nuclei dominated by the quadrupole interaction. The dipole-dipole coupling is between neighboring spin-1 nuclei with identical nuclear quadrupole resonance frequencies, but the principal axes frames of the electric field gradient at each nucleus are not aligned. Such a comprehensive treatment is necessary to determine the optimal sequence of rf pulses which maximizes the echoes in the detection of the substance of interest, for example, an explosive or other contraband material. We confirm our theoretical model using the nitrogen in powder samples of $p$-chloroaniline and sodium nitrite.

In-plane hole density in(Ca0.1La0.9)(Ba1.65La0.35)Cu3Oy: Nuclear resonance study over the full doping range

We report in-plane $^{63}\mathrm{Cu}$ nuclear magnetic resonance measurements for a series of fully enriched $({\mathrm{Ca}}_{0.1}{\mathrm{La}}_{0.9})({\mathrm{Ba}}_{1.65}{\mathrm{La}}_{0.35}){\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ powder samples, which belong to the $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ (YBCO) family, but the doping could vary from very underdoped to extremely overdoped. From these measurements, we determine the average nuclear quadrupole resonance frequency ${\ensuremath{\nu}}_{Q}$ and its second moment $\ensuremath{\Delta}{\ensuremath{\nu}}_{Q}$, both set by the in-plane hole density $n$, as a function of oxygen level $y$. We find that in the overdoped side $n$ is saturated, but $\ensuremath{\Delta}{\ensuremath{\nu}}_{Q}$ rapidly increases with increasing $y$. The relevance of these results to the increasing penetration depth in overdoped cuprates is discussed.

Isoelectronically substituted heavy-fermion compoundsCeCu6−xAux, studied by local probe and electronic band structure calculation

For $\mathrm{Ce}{\mathrm{Cu}}_{6\ensuremath{-}x}{\mathrm{Au}}_{x}$ $(x=0,0.1,1)$, we measured the temperature dependence of $^{63,65}\mathrm{Cu}$ nuclear magnetic resonance spectra of oriented single-crystal powder at $7\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ in the $5--100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ range. ${\mathrm{Ce}}^{3+}$ crystal field and molecular field parameters are derived from single-crystal magnetic susceptibility data. Calculated electric field gradients are used to correctly assign nuclear quadrupole resonance frequencies to lattice sites $(n)$. Bare Knight shifts and transferred hyperfine interaction coupling constants $\ensuremath{\alpha}(n)$ as well as electric field gradient information are extracted for the various Cu sites $(n)$. Ab initio self-consistent energy-band calculations explain the electric field gradients and the alternation of the sign of $\ensuremath{\alpha}(n)$. The anisotropy of the hyperfine coupling parameter $\ensuremath{\alpha}$ is proved for the $a\text{\ensuremath{-}}c$ plane for the Cu(5) site.

On the estimation of the field constant of 35Cl nuclear quadrupole resonance frequency in the series of [R1R2R3AsCl]+M− compounds

The field constant of the NQR frequency of a chlorine atom (35Cl) in a series of arsenic derivatives [R1R2R3AsCl]+M− was estimated from correlations. The field frequency is ∼41.5±3.5 Hz cm kV−1, which is nearly twice as much as that in analogous phosphorus compounds.

Menshutkin's {(Arene)·nSbX3} Molecular CompoundsExploring the Correlation between Crystal Structures and Nuclear Quadrupole Resonance Spectra

By use of data from the literature to assign 35Cl, 81Br, 121Sb, and 123Sb nuclear quadrupole resonance (NQR) frequencies to particular atoms, an attempt is made to correlate crystal structures of a number of {(arene)·nSbX3} (X = Cl, Br) Menshutkin molecular compounds with their NQR spectra. Comparing NQR spectra of crystalline neat SbX3 (X = Cl, Br) with those of the molecular compounds shows well-established effects due to the perturbations introduced by changes in the detailed structures of the networks of SbX3 moieties and in the interactions of Sb atoms with the arene moieties. NQR spectra are much more sensitive in this regard than the changes in interatomic distances that are among the main quantitative outcomes of crystal structure analysis. Although a qualitative pattern can be discerned that associates the frequency of the 35Cl NQR spectra with the different structural roles of the crystallographically independent chlorines, it has not been possible to identify particular interactions with the differences found in the NQR frequencies. Generalizations based on NQR spectra do not appear currently achievable, and it seems likely that the spectra of the various molecular compounds will require explanation on an individual basis.

Antiferromagnetic Ordering Coupled with Phonon Mode Anomalies in Rare-Earth Cuprate NdCu2O4, Probed by Nuclear Quadrupole Resonance and Raman Spectroscopy

We found evidence that two successive antiferromagnetic orderings of a rare earth cuprate NdCu 2 O 4 (nonstandard monoclinic structure with magnetic Cu(1) and nonmagnetic Cu(2) sites) are strongly coupled with phonon mode anomalies. The nuclear quadrupole resonance frequency of 63,65 Cu on the Cu(2) site shows a step decrease below each of the ordering temperatures T N1 ∼20 K and T N2 ∼10 K, that originates from changes in the symmetry of the charge distribution on the surrounding lattice. In the Raman spectrum, the shift of a phonon mode near 300 cm -1 displays an anomalous temperature dependence, which suggests a hardening of the lattice below T N1 and a softening below T N2 . The Cu 2+ spins on the Cu(1) site order antiferromagnetically below T N1 , and the complex magnetic behavior is ascribed to the variations in the exchange interactions between the Cu(1) spins and in the paramagnetic spin polarization of Nd 3+ below T N2 .

11B NMR study of calcium-hexaborides

We have performed 11B nuclear magnetic resonance (NMR) measurements to look for microscopic evidence of the ferromagnetic state in several CaB 6 single crystals. A number of 11B NMR resonance peaks are observed with the frequency and intensity of those peaks distinctively changing depending on the angle between the crystalline axis and a magnetic field. Analyzing this behavior, we find that the electric field gradient tensor at the boron has its principal axis perpendicular to the six cubic faces with a quadrupole resonance frequency ν Q ≈ 600 kHz . However, the satellite resonances are found to be made of two peaks. Detailed analysis of the four composite satellite peaks confirms that there are two different boron sites with slightly different ν Q 's. This suggests that the boron octahedron cages are locally distorted. However, this distortion is not directly related to ferromagnetism. Even though the magnetization data highlight the ferromagnetic hysteresis, 11B NMR linewidth and shift data show no clear microscopic evidence of the ferromagnetic state in several different compositions of CaB 6 single crystals.

Carrier concentration independent antiferromagnetic spin fluctuations in the electron-doped high-temperature superconducting cupratePr2−xCexCuO4

We have performed ${}^{63}\mathrm{Cu}$ NMR measurements on the electron-doped high-temperature superconducting cuprate (HTSC) ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ $(x=0.10,$ 0.15, and 0.20), at 9 T, which is sufficient to suppress ${T}_{c}$ to zero. The Cu spin-lattice relaxation rate and the Cu spin-lattice relaxation rate anisotropy can be consistently interpreted in terms of coupling to antiferromagnetic spin fluctuations for temperatures as low as 6 K. We find that the spin-fluctuation spectrum probed by the Cu spin-lattice relaxation rate does not change with increasing electronic concentration, contrary to a recent theoretical predication. There is no evidence in the Cu spin-lattice relaxation rate data for a temperature-independent spin gap that is as large as theoretically predicted or as large as the normal-state pseudogap energy reported from infrared reflectance measurements on the electron-doped HTSC ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}.$ The Cu nuclear quadrupole resonance frequency is significantly smaller than that observed in the hole doped HTSC, which implies a nearly complete cancellation of the Cu $3d,$ O $2p,$ and nuclei contributions to the electric-field gradient at the Cu nucleus. The orbital shift anisotropy is similar to that observed in the hole doped HTSC, implying a similar relative splitting of the Cu $3d$ orbitals.

DTA and NQR studies of polymorphism in p-chlorofluorobenzene

Differential thermal analysis, and the 35Cl Nuclear Quadrupole Resonance frequency ( ν Q) and spin-lattice relaxation time ( T 1) were measured as a function of temperature in p-chlorofluorobenzene. Three different phases were found according to the thermal history of the sample and only one was stable up to the melting point. In one of the phases, the presence of molecular reorientations can be inferred from T 1 data with activation energy of 3.7 kcal/mol. A value of 13.9 kJ/mol for the fusion heat of this compound has been obtained.

The unusual solid state structure of heroin hydrochloride monohydrate and its selective detection using NQR spectroscopy

Heroin hydrochloride monohydrate, the normal crystalline form of this material, has been studied by X-ray crystallography, nuclear quadrupole resonance spectroscopy, and theoretical calculations. The X-ray data at 120 K could be refined in either of the two space groups P41 or P41212 with the latter higher symmetry form being the more likely, but for the NQR spectra at 4.2 K the doublet structure suggests that a slight symmetry breaking has ocurrred, which lowers the symmetry to P41. The crystals have the unusual unit cell dimensions of a = b = 7.5335(2) Å, c = 71.976(4) Å, with Z = 8 (and Z′ = 2 in P41); the molecules are arranged on a four-fold helix of pitch 72.0 Å with its axis parallel to the c axis, requiring a total of eight molecules in order to complete one turn. As in the free base the nitrogen ring system of the N-protonated heroin molecule has a chair conformation; in P41 the N–H hydrogen is bonded to a Cl− with N–H⋯Cl distances of 3.056(15) and 3.095(14) Å and each Cl− ion is also hydrogen bonded to two water molecules. 14N and 35Cl quadrupole resonance frequencies have been observed in both the pure material and material from a drugs seizure by two double resonance techniques; in cross-relaxation spectra, a broad 35,37Cl resonance is found near 1.95 MHz at room temperature and an analysis of the line shape predicts an average quadrupole coupling constant for the two ions of 3.817 MHz and an asymmetry parameter of 0.4. In double resonance by level crossing experiments, 14N doublets are observed near 0.965 and 1.035 MHz at 4.2 K, predicting a mean quadrupole coupling constant of 1.333 MHz and an asymmetry parameter of 0.158. A doublet 35Cl signal is also observed at 1.965 MHz. The doublet patterns observed are consistent with a P41 space group at 4.2 K. The similarity of the NQR data from the pure sample and the seizure confirms that the illicit sample is of the helical monohydrate, suggesting that these methods can be used to detect the narcotic in real situations. Theoretical calculations with Gaussian at the HF/6-31+G* level for two molecules at the configurations they adopt in the solid state are in reasonable agreement with these conclusions for 14N and one of the 35Cl ions.