Nuclear Quadrupole Resonance Frequencies Research Articles

Nuclear magnetic relaxation induced by exchange-mediated orientational randomization: Longitudinal relaxation dispersion for spin I = 1

The frequency dependence of the longitudinal relaxation rate, known as the magnetic relaxation dispersion (MRD), can provide a frequency-resolved characterization of molecular motions in complex biological and colloidal systems on time scales ranging from 1 ns to 100 μs. The conformational dynamics of immobilized proteins and other biopolymers can thus be probed in vitro or in vivo by exploiting internal water molecules or labile hydrogens that exchange with a dominant bulk water pool. Numerous water (1)H and (2)H MRD studies of such systems have been reported, but the widely different theoretical models currently used to analyze the MRD data have resulted in divergent views of the underlying molecular motions. We have argued that the essential mechanism responsible for the main dispersion is the exchange-mediated orientational randomization (EMOR) of anisotropic nuclear (electric quadrupole or magnetic dipole) couplings when internal water molecules or labile hydrogens escape from orientationally confining macromolecular sites. In the EMOR model, the exchange process is thus not just a means of mixing spin populations but it is also the direct cause of spin relaxation. Although the EMOR theory has been used in several studies to analyze water (2)H MRD data from immobilized biopolymers, the fully developed theory has not been described. Here, we present a comprehensive account of a generalized version of the EMOR theory for spin I = 1 nuclides like (2)H. As compared to a previously described version of the EMOR theory, the present version incorporates three generalizations that are all essential in applications to experimental data: (i) a biaxial (residual) electric field gradient tensor, (ii) direct and indirect effects of internal motions, and (iii) multiple sites with different exchange rates. In addition, we describe and assess different approximations to the exact EMOR theory that are useful in various regimes. In particular, we consider the experimentally important dilute regime, for which approximate analytical results are derived. As shown by the analytical expressions, and confirmed by exact numerical calculations, the dispersion is governed by the pure nuclear quadrupole resonance frequencies in the ultraslow-motion regime, where the relaxation rate also exhibits a much stronger dependence on the electric field gradient asymmetry than in the motional-narrowing regime.

Stereoelectronic structure of MCl4-benzoyl chloride (M=Si, Ge, Sn) systems resulting from ab initio calculations and NQR 35Cl

Results of quantum-chemical calculations of MCl4–C6H5COCl (M=Si, Ge, Sn) systems of 1 : 1 composition using RHF/3-21 G* and MP2/3-21 G* levels as well as those of 1 : 2 composition using the RHF/3-21 G* level have been represented. MCl4 ← C6H5COCl complexes of 1 : 1 composition are energetically more advantageous. They are formed in solid state provided that the M···O distance in individual systems is considerably less than the sum of van der Waals radii of M and O and their total energies are appreciably less than the sum of total energies of components. These conditions are realized only for M=Sn. In systems of 1 : 2 compositions, calculated M···O distances are practically equal to the sum of covalent radii of M and O. Nonetheless, complexes with such composition are not formed in solid state. Total energy of the system which is lower than the sum of its components’ energies is not an indispensable condition for complex formation. The 35Cl nuclear quadrupole resonance (NQR) frequencies and asymmetry parameters of the electric field gradient at the 35Cl nuclei have been evaluated using the results of ab initio calculations.

75As NMR-NQR study in superconducting LiFeAs

We report results of 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments as well as 7Li NMR on different samples of self flux grown LiFeAs and 5 % Co doped LiFeAs single crystals, and a polycrystalline LiFeAs sample. We were able to distinguish the samples by their slightly different quadrupole frequencies, $\nu_Q$, which is a direct measure of the electric field gradient (EFG) at the As site. Interestingly, samples with a large quadrupole frequency appear to show a different Knight shift and spin lattice relaxation in the superconducting state from those with a lower $\nu_Q$, yet all the samples are clearly superconducting. For sample S1 which has the largest $\nu_Q$, we find constant Knight shift K across Tc for a certain direction of the magnetic field and a peculiar upturn of the NQR spin lattice relaxation rate 1/T1 below Tc. In contrast, samples with a lower $\nu_Q$ exhibit the expected behavior for a singlet superconductor: a drop of K and 1/T1 for both NMR and NQR below Tc. Our results show that already tiny changes in stoichiometry uncovered by slightly different NQR frequencies lead to very different behavior of the NMR parameters in the superconducting state of LiFeAs. Different possibilities will be discussed which may explain the contrasting behavior.

PEANUT experiment in NQR spectroscopy for I=3/2

The experiment with phase inversion and phase-inverted echo-amplitude detected nutation (PEANUT) was introduced in the nuclear quadrupole resonance (NQR). Formulas were obtained describing the NQR (I=3/2) experiment. Exemplary experiments are provided confirming the predicted particularities of the PEANUT spectra in NQR Cl-35. It is proposed to apply the method for the purpose of determination the asymmetry parameter of the electric field gradient (EFG) tensor in powders with the help of the analysis of PEANUT interferograms. Application of two-dimensional PEANUT experiments, in which the nutation frequencies correlate with the resonance NQR frequencies, can substantially simplify the interpretation of complex spectra.

NQR parameters of complexes and polarizability effect

The literature data on substituent influence on the nuclear quadrupole resonance frequencies (ν), quadrupole coupling constants (e(2) Qq ⋅ h(- 1) ), and asymmetry parameters (η) for 36 series of the H-complexes, charge-transfer complexes, transition metal complexes and other donor-acceptor complexes have been considered, using the correlation analysis. Generally the ν, e(2) Qq ⋅ h(- 1) , and η values were first established to depend on the inductive, resonance, polarizability, and steric effects of substituents. The presence or otherwise of certain effects as well as relation between their contributions are determined by the type of series. The polarizability effect owes its existence to the appearance of an excess charge on the indicator centre as a result of the complexation. The contribution of this effect ranges up to 75%.

New Methods for Detection of 14N NQR Frequencies

Nitrogen atoms are present in a number of solid explosives and illicit substances. The nuclear quadrupole resonance (NQR) spectra and spin–lattice relaxation of the nitrogen atomic nucleus 14N can be used to characterize these compounds and to distinguish between possible crystal polymorphs. After the characteristic 14N NQR frequencies and spin–lattice relaxation rates in a compound are determined, NQR can be used to detect this compounds and, in case of crystal polymorphs, also to determine the method of preparation. The 14N NQR frequencies and spin–lattice relaxation rates are measured either by pulse NQR or by nuclear quadrupole double resonance (NQDR) based on magnetic field cycling. Here, we discuss several 1H–14N NQDR techniques which can be used to measure the 14N NQR frequencies and spin–lattice relaxation rates under various experimental conditions. Some characteristic applications of these techniques are presented and discussed in details.

Protonic motion in one-dimensional hydrogen-bonded molecular crystal [4,6-dmpH][Hca] as revealed by 35Cl NQR

A protonic motion in the 1-D hydrogen-bonded zigzag chain in 4,6-dimethylpyrimidinium hydrogen chloranilate, [4,6-dmpH][Hca], was studied from the measurements of 35Cl nuclear quadrupole resonance (NQR) spin-lattice relaxation time (T1). The four NQR frequencies were observed at 36.846, 36.767, 35.945, and 35.905MHz at 77K. The temperature dependences of these frequencies were smooth and continuous in 77–360K. The absence of solid–solid phase transition and the metastable phase was confirmed by heat capacity measurements. Temperature dependence of the NQR T1 suggests an excitation of a motion above ca. 200K with the activation energy of 11.5kJmol−1. In the stable phase too, the excitation of proton transfer is expected to occur as suggested for the metastable phase of as-grown crystals. Such transfer motion may be possible at the kink structure of the 1-D ferroelectric zigzag chain, and the kink can migrate in the chain accordingly.

Stereoelectronic structure and 35Cl NQR parameters of 4-(trichlorgermyl)butan-2-one using ab initio calculations

Abstract The results of ab initio calculations at the RHF/6-31G(d) and MP2/6-31G(d) levels of two stable structures of the 4-(trichlorgermyl)butan-2-one molecule with total optimization of their geometry have been represented. The structure with pentacoordinated Ge atom is energetically more advantageous as compared with that with tetracoordinated one. Using these results, the 35 Cl nuclear quadrupole resonance (NQR) frequencies and asymmetry parameters of the electric field gradient (EFG) at the 35 Cl nuclei in molecule with pentacoordinated Ge atom have been assessed, the frequencies satisfactorily agreeing with experimental data. Calculations at the RHF/6-31G(d) level have been performed also at various Ge⋯O distances. It has been demonstrated that convergence of the Ge and O coordination centers leads to the increase of positive charge at the Ge coordination center and of negative charge at the O coordination center, at that, electron density from the Ge atom shifts mainly to the axial Cl atom and from the C atom of carbonyl group – to its O atom. The electron density transfer from the O to Ge atom does not occur.

Nuclear quadrupole resonance of norephedrine

Toward searching for illegal drugs, we investigated the pulsed nuclear quadrupole resonance (NQR) response of 14N in (1R,2S)-(-)-norephedrine, based on the predictions of quantum chemical calculations. Two pairs of spectral lines ( ν +=3.089, 3.093 MHz and ν −=2.594, 2.608 MHz) were observed despite its molecule structure having only a single nitrogen atom. This indicates that the molecular crystal has two nonequivalent nitrogen atoms in the unit cell. The temperature dependence of the NQR frequencies and relaxation properties were investigated for the purpose of accurate remote sensing of the drugs. The NQR frequency shift was approximately 0.23 kHz/K around room temperature. The spin-lattice relaxation and spin-phase memory times were 5.2–10.2 ms and 0.6–1.5 ms, respectively.

Composite pairing in a mixed-valent two-channel Anderson model

Using a two-channel Anderson model, we develop a theory of composite pairing in the 115 family of heavy fermion superconductors that incorporates the effects of f-electron valence fluctuations. Our calculations introduce "symplectic Hubbard operators": an extension of the slave boson Hubbard operators that preserves both spin rotation and time-reversal symmetry in a large N expansion, permitting a unified treatment of anisotropic singlet pairing and valence fluctuations. We find that the development of composite pairing in the presence of valence fluctuations manifests itself as a phase-coherent mixing of the empty and doubly occupied configurations of the mixed valent ion. This effect redistributes the f-electron charge within the unit cell. Our theory predicts a sharp superconducting shift in the nuclear quadrupole resonance frequency associated with this redistribution. We calculate the magnitude and sign of the predicted shift expected in CeCoIn_5.

Spectroscopic Study of75As and139La NMR on Layered Structure Ferromagnet LaCoAsO

75 As and 139 La field-swept NMR spectra were obtained for the novel weakly itinerant ferromagnet LaCoAsO with a two-dimensional layered structure above the Curie temperature of 55 K. By analyzing NMR spectra, temperature dependences of Knight shift K and nuclear quadrupole resonance frequency ν Q were obtained successfully for each nucleus. We confirmed from the so-called K –χ 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 –χ plots and compared them with that of an iron-arsenide superconductor.

Correlation between proton transfer and 35Cl NQR frequency as well as molecular geometry of chloranilic acid in co‐crystals with some organic bases

Proton transfer in hydrogen-bonded organic co-crystals of chloranilic acid with some organic bases was investigated by nuclear quadrupole resonance (NQR) spectroscopy. The (35)Cl NQR frequencies of chloranilic acid molecule as well as (14)N NQR frequencies of the organic base molecule were measured with the conventional pulse methods as well as double-resonance methods, respectively. The extent of proton transfer in the O...H...N hydrogen bond was estimated from Townes-Dailey analysis of the (14)N NQR parameters. The (35)Cl NQR frequency and molecular geometry of chloranilic acid are correlated to the extent of proton transfer in the protonation process of the organic base molecule. It is shown that the hydrogen bond affects the pi-electron system of chloranilic acid. Geometry dependence of the O...H...N hydrogen bond, i.e. the H-N valence bond order versus the hydrogen-bond geometry correlation is also discussed.

NQR study of pressure induced superconductivity in CeCu2Si2

AbstractWe have measured nuclear quadrupole resonance (NQR) for 63Cu nuclei under high pressure up to 4.8 GPa. A linear increase in NQR frequency $^{63} \nu _{\rm Q}$ of 63Cu nuclei was observed below 3 GPa. However, $^{63} \nu _{\rm Q}$ deviates gradually from the linear relation above 3.5 GPa. The downward deviation is associated with a change of valence and consistent with the results of band calculations with the structural parameters under pressure. Antiferromagnetic (AFM) spin fluctuations are drastically suppressed with increasing pressure, whereas Tc increases from 0.7 K at ambient pressure to 1.64 K at 4.2 GPa. We have found that bulk superconductivity suddenly disappears at 4.8 GPa accompanied with a rapid decrease in the density of states at the Fermi level. magnified image These are indenter pressure cells for NQR measurements.

Short-range magnetic order and temperature-dependent properties of cupric oxide

The temperature dependence of the optical and magnetic properties of CuO were examinedby means of hybrid density functional theory calculations. Our work shows that thespin exchange interactions in CuO are neither fully one-dimensional nor fullythree-dimensional. The temperature dependence of the optical band gap and the 63Cu nuclear quadrupole resonance frequency of CuO originate from the combined effect of astrong coupling between the spin order and the electronic structure and the progressiveappearance of short-range order with temperature.

SVD for enhanced explosives detection using NQR

Nuclear Quadrupole Resonance (NQR) is a solid-state radio frequency (RF) spectroscopic technique, allowing the detection of many high explosives and narcotics. Unlike metal detectors and ground penetrating radar (GPR), NQR detectors measure signals that are unique to the explosives, thus can provide a lower false alarm rate. The practical use of NQR is restricted by the inherently low signal-to-noise ratio (SNR) of the observed signals, a problem that is further exacerbated by the presence of strong RF interference (RFI), so a robust detection method is required. Singular value decomposition (SVD) based on Hankel matrix is proposed to detect the NQR signal in this paper. The original signal can be decomposed into the linear superposition of a series of component signals by SVD using Hankel matrix, and the process of constructing the Hankel matrix shows that a component T signal can be produced by averaging the elements on the inverse diagonal of the Ai=σiuiviT. Then an eigendecomposition based spectrum estimator called “Multiple Signal Characterization” (MUSIC) is applied to estimate power spectrum of each component signal, so the RFI and the noise are canceled by using the priori knowledge of the NQR frequency. Finally, an average power detector is applied to detect NQR signal. The result using measured data shows that the proposed method can provide robust NQR detection performance.

Direct Evidence for Nitrogen Ligation to the High Stability Semiquinone Intermediate in Escherichia coli Nitrate Reductase A

The membrane-bound heterotrimeric nitrate reductase A (NarGHI) catalyzes the oxidation of quinols in the cytoplasmic membrane of Escherichia coli and reduces nitrate to nitrite in the cytoplasm. The enzyme strongly stabilizes a menasemiquinone intermediate at a quinol oxidation site (Q(D)) located in the vicinity of the distal heme b(D). Here molecular details of the interaction between the semiquinone radical and the protein environment have been provided using advanced multifrequency pulsed EPR methods. (14)N and (15)N ESEEM and HYSCORE measurements carried out at X-band ( approximately 9.7 GHz) on the wild-type enzyme or the enzyme uniformly labeled with (15)N nuclei reveal an interaction between the semiquinone and a single nitrogen nucleus. The isotropic hyperfine coupling constant A(iso)((14)N) approximately 0.8 MHz shows that it occurs via an H-bond to one of the quinone carbonyl group. Using (14)N ESEEM and HYSCORE spectroscopies at a lower frequency (S-band, approximately 3.4 GHz), the (14)N nuclear quadrupolar parameters of the interacting nitrogen nucleus (kappa = 0.49, eta = 0.50) were determined and correspond to those of a histidine N(delta), assigned to the heme b(D) ligand His-66 residue. Moreover S-band (15)N ESEEM spectra enabled us to directly measure the anisotropic part of the nitrogen hyperfine interaction (T((15)N) = 0.16 MHz). A distance of approximately 2.2 Abetween the carbonyl oxygen and the nitrogen could then be calculated. Mechanistic implications of these results are discussed in the context of the peculiar properties of the menasemiquinone intermediate stabilized at the Q(D) site of NarGHI.

Nuclear Quadrupole Resonance Frequency in Multilayered Cuprates

The 63 Cu nuclear quadrupole resonance (NQR) frequency ν Q in the multilayered cuprates is calculated in the cluster model by the exact diagonalization method. The charge imbalance between the outer CuO 2 plane (OP) with apical oxygen O A and the inner plane (IP) without O A in three-layered Tl 2 Ba 2 Ca 2 Cu 3 O 10 is estimated by comparing our results with the experimental ν Q . In Tl-based cuprates with more than three layers, we predict a large enhancement of the splitting of ν Q between OP and IP by taking into account the reduction in bond length between Cu and O A and the resulting increase in charge transfer energy. Our results show that the NQR frequency is a useful quantity for estimating the charge imbalance in the multilayered cuprates.

Temperature Dependence of Internal Torsional Frequencies in 3,4-Dichloro Nitrobenzene and 4-Chloro 2 Nitrophenol Using NQR Data

The temperature variation of the nuclear quadrupole resonance frequency in the two compounds 3,4-dichloro nitrobenzene and 4-chloro 2 nitrophenol were analyzed in the light of the Kushida and Brown models. The internal torsional frequencies at each temperature was calculated using Bayer’s model with Tatsuzaki correction under a two-mode approximation to obtain the temperature variation of the torsional frequencies in both compounds. The torsional frequencies are seen to lie between 25 cm i1 and 85 cm i1 in the first compound and between 30 cm i1 and 42 cm i1 in the case of the second compound. The temperature dependence of the torsional frequencies is linear above 150 K in the first compound and is linear throughout the range 77 K to 350 K in the second. The smaller temperature dependence in the second compound may be the eect of hydrogen bonding in this compound. PACS numbers: 76.60 Gv

F-doping dependence of 75As nuclear quadrupole resonance frequency in LaFeAs(O1−xFx)

We report the F-doping dependence of 75As nuclear quadruple resonance frequency (75νQ) in the iron-oxypnictide LaFeAs(O1−xFx) (x=0, 0.07, 0.08, 0.11 and 0.14). It was revealed that 75νQ becomes larger with increasing x up to 0.14. From point charge calculations, we found that experimental F-doping dependence of 75νQ cannot be accounted for by the effect of surrounding ions. Therefore, the on-site charge distribution at the 75As site modified by the hybridization between As-4p and Fe-3d orbitals gives dominant contribution to 75νQ in LaFeAs(O1−xFx).

AB INITIO CALCULATIONS OF 14N NQR PARAMETERS AND 13C, 1H, AND 15N CHEMICAL SHIFTS INCLUDING A COMPARISON WITH EXPERIMENTAL NMR DATA FOR CYCLOTRISAZOBENZENE

Nuclear quadrupole coupling constant, χ, and asymmetry parameter, η, of 14 N nucleus and 13 C , 1 H , and 15 N chemical shifts for cyclotrisazobenzene at the level of B3LYP and MP2 methods have been studied using the Gaussian 98 suite of programs. Also, nuclear quadrupole resonance (NQR) frequencies (ν0, ν+, ν-) for 14 N have been calculated, thoroughly. The optimized structure of the compound was very similar to that given by the X-ray crystallographic data. The electric field gradient (EFG) calculation verified that the N = N bonds are highly localized in this molecule; therefore, electron-pairs of the nitrogen atoms remain nonbonding. The comparison of the calculated chemical shifts with the experimental values for cyclotrisazobenzene shows no significant structural changes in solution.