NQR Parameters Research Articles

9Be-NMR studies of the heavy-Fermion superconductor [formula omitted

Field-angle dependence of 9Be-NMR measurements has been carried out for a single crystal UBe 13 in the temperature range of 0.35–1.5 K. For H ∥ [ 0 0 1 ] , we observed that 9Be-NMR spectra consisted of 10 lines; one line comes from a Be(I) site located in the cubic symmetry, while nine lines come from Be(II) sites located in the lower symmetry. From the field angle dependence of the NMR spectra, we determined NQR parameters such as the nuclear quadrupole frequency ( ν Q ≈ 84 kHz ) and the asymmetry parameter ( η ≈ 0.21 ) . In the superconducting state, we found that 9Be-NMR Knight shift for H ∥ [ 0 0 1 ] does not change at all. This result is consistent with the spin-triplet pairing state in UBe 13 .

An ab initio quantum chemical investigation of solvent-induced effect on 14N-NQR parameters of alanine, glycine, valine, and serine using a polarizable continuum model

In this paper ab initio quantum mechanical calculations of the 14N-nuclear quadrupole resonance (NQR) parameters of alanine, glycine, valine, and serine obtained from an electric field gradient (EFG) tensor, such as quadrupole frequencies and asymmetry parameters in gas phase and different solvents, have been carried out with the Gaussian 98 software package and the solvent-induced effects on these parameters have been evaluated using density functional theory (DFT). Furthermore, direct and indirect solvent effects on asymmetry parameters have been also calculated. We determined that the NQR parameters of the nitrogen atoms of amino acids are highly sensitive to environmental effects and that the observed solvent-induced shielding variation is strongly related to the values of the dielectric constants of the solvent and whether it is protic or aprotic. For more investigation of the solvent effect, the relative energies of each amino acid in various solvents have been calculated and the graphs of the relative energies versus dielectric constants have been analyzed.

Correlation between NQR parameters and residue size of aliphatic amino acids and their dimers

Nuclear quadrupole coupling constants (NQCC), χ, and asymmetry parameters, η, of 2D, 14N and 17O nuclei have been calculated for aliphatic amino acids and their dimers using MP2/6-311++G** method to shed some light on the differences between the structural parameters in the aliphatic amino acids and their dimers. For this purpose, electric field gradient (EFG) at the sites of quadrupolar nuclei have been calculated and evaluated for each compound. A correlation is observed between the calculated NQCC parameters and the conformational structures of the compounds, showing that extraction of structural data from the NQR spectra might be promising. Our results showed that 17O NQCC of terminal carboxylic acid and 14N NQCC of the terminal amino groups are, respectively, the least and the most sensitive parameters to the variation of the size of the residue. It is found also that conformation of R (i.e. values of the dihedral angles) plays a very effective role in the determination of the values of the calculated NQCC parameters. Sensitivity of the NQR parameters to the changes in the conformational structure is significantly greater (nearly 20-fold) than that to the changes in the other structural parameters such as bond lengths.

Ab initio NQR study of piperidine umbrella inversions

Two conformers of piperidine interconvert through nitrogen inversion. In this equilibrium, conformers with equatorial N–H appear dominant over those with axial N–H. In this study, substituent (X) effects are investigated on nitrogen inversions, for axially substituted 3-X-piperidines, using ab initio calculated NQR parameters, where X=H, F, Cl, Br, CH 3, CF 3, OH, CHO, COOH, CN, NH 2 and NO 2. These 12 species are in three forms. They may have an axial sp 3 N–H ( 1 X), a planar sp 2 N–H ( 2 X), and/or an equatorial sp 3 N–H ( 3 X). The axial minima, 1 X, appear to go through transition states (TS), 2 X, and convert to equatorial minima, 3 X. Ab initio and DFT methods are employed to study these equilibria ( 1 X (min)⇌ 2 X(TS)⇌ 3 X(min)). Energies, structural parameters as well as 2H ( 2H–N) and 14N quadrupole coupling constants ( χ 2H and χ 14N, respectively) are calculated for 1 X, 2 X and 3 X. The order of χ 2H magnitude appears to be: 2 X> 3 X> 1 X. The same order is obtained for χ 14N when X=F, Cl, Br, CH 3, CF 3, COOH, CN, NH 2 and NO 2. For X=H, OH and CHO, χ 14N trend changes to: 2 X> 1 X> 3 X. According to our B3LYP/6-31 g (d,p) and MP2/6-31g(d,p) data, the equilibrium shifts in favor of 1 X over 3 X form is demonstrated, when X is F, Cl, Br, COOH, NO 2, CN or CF 3. On the other hand, equatorial 3 X dominate over axial 1 X when X=H, CH 3, OH and NH 2.

A correlation study of quinoline derivatives and their pharmaceutical behavior by ab initio calculated NQR parameters.

In this paper, ab initio calculated NQR parameters for some quinoline-containing derivatives are presented. The calculations are carried out in a search for the relationships between the charge distribution of these compounds and their ability to interact with haematin. On the basis of NQR parameters, pi-electron density on the nitrogen atom of the quinoline ring plays a dominant role in determining the ability of quinolines to interact with haematin. This point was confirmed with investigation of Fe+3 cation-pi quinoline ring interactions in 2- and 4-aminoquinoline. However, our results do not show any preference for those carbon atoms of the quinoline ring which previous reports have noted. In order to calculate the NQR parameters, the electric field gradient (EFG) should be evaluated at the site of a quadrupolar nucleus in each compound. EFGs are calculated by the Gaussian 98 program using the B3LYP/6-31 G* level of theory.

The tautomeric equilibria of cytosine studied by NQR spectroscopy and HF, MP2 and DFT calculations

A study was undertaken to check if such parameters as NQR frequencies, quadrupole coupling constants and the asymmetry parameters determined from the 14N NQR spectra can provide sufficient information to infer about the tautomeric form of a compound studied. The influence of the choice of functional on the 14N NQR parameters calculated by ab initio and DFT methods was analysed. Moreover, the influence of the neighbouring molecules on the NQR parameters was checked for the assumed symmetric planar environment of the cytosine molecule in which it is hydrogen-bonded to four other cytosine molecules. Since the use of a finite basis set is always a source of uncertainties in the electric field gradient tensor components, the final calculations for a layer were performed in two different basis sets: middle and extended.

NQR parameters: Electric field gradient tensor and asymmetry parameter studied in terms of density functional theory

AbstractThe calculations of the electric field gradients were performed in the different basis sets at the selected ab initio, density functional theory (DFT), and ECP levels of the theory. The most reliable QCC and η values were obtained within the DFT and with the use of B3LYP functional and middle 6‐31G* or extended 6‐311G* basis sets. The universality of the B3LYP/6‐31G* method was checked by comparing the experimental and calculated Up values as only such a comparison enables assessment of the quality of the results obtained for different (heavy and light) nuclei. The NQR parameters obtained with the use of all‐electrons (AE) and pseudopotential (ECP) methods for a group of halogens or nitrogen‐containing compounds were compared and the former were found to be close to the experimental values. The explanation of so extremely small values of EFG calculated with the use of pseudopotential was proposed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Nuclear quadrupole resonance in 2,4-dinitrochlorobenzene

A study of the spin lattice relaxation ( T 1) and the nuclear quadrupole resonance frequency ( ν Q) gives an important information about the dynamics of molecular groups in molecular solids. In the present paper, we analyze the contributions of the reorientational motion of nitro groups of 2,4-dinitrochlorobenzene to the NQR parameters of the chlorine nucleus in the molecule. We found two contributions to T 1 and ν Q due to the onset of the reorientation of nitro groups in the molecule; one of these contributions is mostly due to intermolecular effects in the crystal. For the chlorine nuclei, the efficiency of the modulation mechanism is usually provided by the change of the electric field gradient due to the moving molecular group; this gives us a way of how to assign each contribution to T 1 from the ortho and para positions of the NO 2 groups in the molecule. It is observed that there are two different potential barriers depending on the position of the nitro groups in the molecule. The behavior in the temperature dependence of the line width shows a thermal history dependence of the molecular crystal.

Ab Initio Calculation of the Structure of 5-Chloro-1,2,4-triazole

Three tautomeric forms of 5-chloro-1,2,4-triazole were calculated using the Hartree-Fock (ab initio) and Meller-Plesset methods in the 6-31G(d) basis. The 35Cl NQR parameters were calculated using the occupancies of the 3p-components of the valence p-orbitals of the chlorine atom. The structure of this triazole was derived from the data obtained.

Correlation between the 35CI NQR Parameters of Chloro-containing Organic and Organometallic Compounds and the Results of ab initio Calculations

Abstract The results of comparison of experimental 35C1 NQR parameters for a great number of organic and organometallic compounds and estimated ones using ab initio calculations at the RHF/6-31G(d) level were systematized. The NQR frequency changes on going from one compound to another depend, in general, on the changes of populations of the CI atom po-orbitals in these compounds and, first of all, of populations of their less diffuse parts

Solid State Effects in 4,6-Dichloropyrimidine Studied by 35Cl-NQR Spectroscopy and ab initio Calculations

Quantum chemistry methods -ab initio -have been used for the assignment of the 35Cl-NQR resonance frequencies to particular chlorine atoms substituted in positions 4 and 6 of pyrimidine. The so-called solid state effect, that is the influence of intermolecular interactions, in particular hydrogen bond formation, on the NQR parameters has also been studied.

Analysis of the NQR parameters in 2-nitro-5-methylimidazole derivatives by quantum chemical calculations

The present study involves nitrogen NMR-NQR measurements and semiempirical as well as ab initio calculations at the different levels in three 2-nitro-5-methylimidazole derivatives. The NMR-NQR data were compared with the results of the quantum chemical calculations with the geometry optimization by seven semiempirical and a few ab initio methods. Since the use of a finite basis set is always a source of uncertainties in the electric field gradient (EFG) tensor components, the calculations were performed in different basis sets, and—regarding the effect of the functional—also at different levels of the theory. The results closest to experimental ones were obtained at the BLYP level of theory.

Features of atomic interaction in the molecules of furan, pyrrole, and their 2-chlorine-substituted derivatives according to results ofab initio calculations

The results of abinitio studies by the RHF/6-31G(d) method on the molecules of furan, pyrrole, and their 2-chlorine-substituted derivatives with full optimization of the geometry show thatunshared electron pairs of their heteroatoms are not delocalized solely in the π-system of the heterocycle. The participation of these electron pairs in formation of bonding and other MOs does not make it possible to speak of substantial role for p,π-conjugation in the distribution of electron density in the molecules and its effect on the characteristics of thecompounds. The results of the ab initio calculations were used to determine the35Cl NQR parameters of the 2-chlorine-substituted molecules.

Temperature-induced orientational disorder inNaClO3

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

Zeeman effect of 35Cl and 127INQR of Form-I of [SCl 3][ICl 4

35Cl and 127INQR frequencies in Form-I of [SCl 3][ICl 4] were observed as a function of temperature. Only the lowest resonance line due to 35Cl near 15 MHz gave a positive temperature coefficient. The 127I resonance line was observed around 460 MHz. The zeeman effect on each resonance line was examined at room temperature by using a single crystal. The asymmetry parameter (η) of the chlorine bonded to sulfur was about 0.05, whereas those of the chlorines bonded to iodine were in the range of 0.12 to 0.36 and that of the iodine was negligibly small. For the chlorine atoms in SCl + 3 a linear relationship was found between the coupling constant and the length of the secondary bond. The orbital populations on the chlorine atoms were evaluated from the observed NQR parameters.

Nonempirical calculations of NQR spectrum parameters of azetidine

The components of the 14N electric field gradient (EFG) tensor, the corresponding nuclear quadrupole coupling constant (NQCC) X, and the asymmetry parameter q of azetidine were calculated using the restricted Hartree--Fock--Roothaan method. The geometry of azetidine was optimized with the 4-31G basis set, and the values of the ring puckering angle (0) and the angle between the N--H bond and the CNC plane ([3) were refined with the 6-31G* basis set. The effect of choice of geometry on calculated NQR parameters was studied. To clarify the origin of EFG at the nitrogen atom nucleus, the contributions from individual bond orbitals and lone electron pairs to the EFG tensor components eqi i were calculated in the framework of the LMO approach. It was demonstrated that the 4-31G + 6-31G*//6-31G* level calculations give NQCC and q values of azetidine that are in good agreement with the results of MW spectroscopy.

121,123Sb NQR investigation of chlorofluoroantimonates(III) M2SbCl3F2 (M = Rb. Cs, and NH4)

Antimony(m) chlorofluoride complexes M2SbCl3F2 (M = Rb, Cs, or NH4) were studied by the121,123Sb NQR method. A temperature range (77–285 K) with anomalous change in the NQR parameters and a second-order phase transition at 250–280 K for (NH4)2SbCl3F2 were found.

The Townes and Dailey approach to the interpretation of NQR parameters in the light of non‐empirical calculations. Three‐membered rings

AbstractThe nuclear and electronic contributions to the EFG tensor components eqii at 14N, 17O, 33S and 35Cl nuclei in aziridine, methylaziridine, chloroaziridine, oxirane and thiirane were determined. The nuclear contributions were calculated within classical representations, while electronic contributions were calculated non‐empirically using a 6–31G* basis set within the framework of the LMO approach. On the basis of the analysis of the contributions to the EFG tensor components from definite bond and lone‐pair orbitals, the main concepts of the Townes and Dailey approach were tested.

Nonempirical calculations of NQR parameters in some three-membered rings

Ab initio calculations (RHF/6–31G∗) of the electric field gradients (EFGs) at 14N, 17O, 2H and 33S and the corresponding nuclear quadrupole coupling constants (NQCCs) are reported for a number of three-membered rings containing atoms of the first and the second rows. The results of the calculations are in good agreement with the available experimental data. The orientations of the principal axes of the EFG tensor at the quadrupole nuclei are determined. The origin of the EFG is analyzed within the framework of the LMO approach. To compare the charge density distributions in aziridine and Cl-aziridine, the charge density maps are obtained using the canonical molecular orbitals.

Correlation of NQR and Chemical Bond Parameters

Abstract Nuclear quadrupole resonance is an effective spectroscopic method to investigate the charge density distribution in solids. The range of compounds which may be investigated is wide, reaching from a crystal lattice built up by an ordered arrangement of neutral molecules in the crystalline material to ionic crystals and to metallic solids. First the correlations between NQR quantities and bond parameters which become apparent from the molecular orbital picture will be discussed, e.g. Hammett a, x-parameter, ionic character i, s-character, ionicity, electronegativity in their connection to NQR. Then, the correlations of NQR frequencies, respectively nuclear quadrupole coupling constants, with bond lengths and bond angles are considered. Cross correlations, e.g. of NQR frequencies with bond distances, bond distances with vibrational frequencies, and vibrational frequencies with NQR frequencies are discussed as are pK A dependencies of NQR parameters. The connection of e