Ab initio calculation of 14N nuclear quadrupole coupling constants

The use of ab initio Hartree-Fock electric field gradient calculations to predict nitrogen nuclear quadrupole coupling constants is examined using basis sets of split valence to triple zeta plus polarization size. From results on 20 to 35 molecules for each basis, such calculations are shown to be of predictive value if systematic errors are eliminated by using regression-derived scaling factors, and the reliability of each basis is assessed. For field gradient calculations on larger molecules, a significantly better alternative to a minimal basis which adds little extra computational cost, is proposed.     

The quadrupole moments of Zn and Cd isotopes—an update

The nuclear quadrupole moments of ¹¹¹*Cd (245 keV state) and ⁶⁷Zn were determined from the quadrupole coupling constants and lattice parameters of the metals at low temperature. The required electric field gradients were obtained with density functional calculations employing the WIEN2k code. The resultant numbers, 0.765(15) and 0.151(4) b, are in line with the previously used values but considerably more precise. Calculations for various other solids confirm the results, with less accuracy, however.     

Molecular relativistic electric field gradient calculations suggest revision of the value of the nuclear electric quadrupole moment of 127I

Relativistic ab initio methods are used to compute the electric field gradient at the iodine nucleus in nine different closed-shell diatomic molecules. Combining these theoretical electric field gradients with experimental nuclear quadrupole coupling constants gives a consistent value of the nuclear quadrupole moment of ¹²⁷I of—696(12)millibarn. We argue that this value is more precise than the current standard value of the nuclear quadrupole moment of ¹²⁷I and recommend adjusting the reference value accordingly. The precision of this determination is still determined by technical limitations in the theoretical work, in particular the neglect of the two-electron Gaunt interaction in the Hamiltonian and correlation contributions beyond those described at the CCSD(T) level of theory, but the errors are reduced relative to the theoretical work that underlies the current standard value of this nuclear quadrupole moment. As a secondary study we also considered the calculation of the small electric field gradient at the gold nucleus in the AuI molecule and conclude that this computation remains a challenge for theoreticians.     

Sternheimer shielding in molecules

The effect of perturbing electric fields and electric-field gradients on the nuclear quadrupole coupling in molecules is investigated. The theory for the first-order effects is developed and it is shown that not only a perturbing field gradient but also a perturbing field, and higher terms, induce a finite field gradient at a nucleus in a molecule. The so-called Sternheimer (anti)shielding is thus considerably more complex in molecular than in atomic systems. The different shielding components have been calculated for HCl, HCN and NH₄ ⁺ using a finite perturbation ab initio molecular orbital method. The calculated values are then used to rationalize experimentally determined ¹⁴N quadrupole coupling constants in condensed media for HCN and NH₄ ⁺.     

14N Pulsed nuclear quadrupole resonance. 2. Effect of a single radio-frequency pulse in the general case

A novel theory, based on density operator calculations, is provided for assessing the nuclear quadrupole resonance behaviour of a spin 1 (¹⁴N) subjected to a single radio-frequency pulse. It is for a powder sample in zero magnetic field for an electric field gradient tensor without symmetry. A complete set of equations is obtained for the quantities of interest. It is derived from the general Liouville–von Neumann equation and from a proper basis on which the density operator is expanded. Theoretical results, in terms of signal evolution as a function of the pulse length (nutation experiments), show that the same nutation curve is expected for the three different transitions which exist when the electric field gradient tensor is without symmetry. This latter nutation curve is, however, different from that which prevails in the case of an axially symmetric tensor, this apparent discrepancy being easily resolved on theoretical grounds. Experimental data (for NaNO₂, electric field gradient tensor without symmetry) are checked against values of the radio-frequency field amplitude provided by NMR measurements performed with the same equipment. Good agreement between theory and experiment is obtained.     

Nuclear quadrupole interaction in metals: Experiments, calculations, models

The results obtained from the investigation of the nuclear quadrupole interaction in metals, mostly at impurities, are reviewed. Examples of the experimental work are primarily taken from studies at ISOLDE/CERN. For the determination of the nuclear quadrupole moments Q, necessary to extract the electric field gradients from the measured coupling constants, various methods have been employed. Band structure calculations both for pure systems and also for impurities have been performed with the FLAPW density functional method. The theoretical understanding achieved for sp-impurities in simple noncubic metals is described. This revised version was published online in July 2006 with corrections to the Cover Date.     

14N pulsed nuclear quadrupole resonance. 1. Nutation experiments in the case of an axially symmetric electric field gradient tensor

An exact theory based on density matrix calculations is provided for assessing the nuclear quadrupole resonance (NQR) behaviour of a spin 1 (¹⁴N) subjected to a single radio-frequency pulse. It concerns a powder sample in zero magnetic field with the assumption of an axially symmetric electric field gradient tensor. Theoretical results, in terms of signal evolution as a function of the pulse length (nutation experiments), are checked against values of the radio-frequency field amplitude provided by NMR measurements performed with the same equipment. Good agreement between theory and experiment is obtained.     

Electric quadrupole interaction of210Po(6+) in Bi single crystal

The electric quadrupole coupling constant of the²¹⁰Po(6⁺) isomer in a Bi single crystal was measured as a function of the temperature using the time-differential perturbed angular correlations technique. The electric field gradient (EFG) for Po in Bi is found to be temperature independent in the range 77–477 K. Using a calculated value of the nuclear quadrupole moment, the magnitude of the EFG is derived, and in turn serves to re-examine the systematics of quadrupole moments of λh ₉ ^2/2 configurations in the lead region.     

The argon nuclear quadrupole moments

New standard values -116(2) mb and 76(3) mb are suggested for the nuclear quadrupole moments (Q) of the ³⁹Ar and ³⁷Ar nuclei, respectively. The Q values were obtained by combining optical measurements of the quadrupole coupling constant (B or eqQ/h) of the 3s²3p⁵4s[3/2]₂ (³P^o) and 3s²3p⁵4p[5/2]₃ (³D^e) states of argon with large scale numerical complete active space self-consistent field and restricted active space self-consistent field calculations of the electric field gradient at the nucleus (q) using the LUCAS code, which is a finite-element based multiconfiguration Hartree–Fock program for atomic structure calculations.     

The origin of non-planarity in ketyls

A series of one-electron properties, i.e. dipole, quadrupole, octupole moments, diamagnetic susceptibility, second moments and second moment anisotropies of charge distribution, electric field, electric field gradient, diamagnetic shielding, nuclear quadrupole coupling constant, have been obtained by ab initio S.C.F. calculations for the molecules COF₂, SO₂F₂ and SOF₂. The effect of the size of the gaussian basis sets, including d-type polarization functions, as well as the contraction schemes on the latter properties have been studied for the COF₂ molecule. This study enables us to determine the basis giving the best results as compared to experiment. With this basis the calculations have been extended to the properties of the molecules SO₂F₂ and SOF₂. Good overall agreement with experiment is obtained.     

Structure and quadrupole coupling measurements on the NO dimer

Rotational transition frequencies for ¹⁴NO-¹⁴NO, ¹⁴NO-¹⁵NO, and ¹⁵NO-¹⁵NO were measured using a pulsed-nozzle Fourier transform microwave spectrometer. Rotational constants for the different isotopic combinations allowed an unambiguous structure determination. The molecule is in a cis planar structure with a bond between the nitrogen atoms and an NNO angle θ = 99.6(2)°. The NN bond length is 2.236(1) Å and the NO bond length is 1.161(4) Å. Hyperfine structure due to nitrogen quadrupole coupling and spin-rotation interactions was observed and analyzed. Rotation constants, quadrupole coupling tensor, and spin-rotation tensor elements are given.     

Nuclear spectroscopic quantities under influences of covalency

On the basis of Mössbauer isomer shift data, nuclear quadrupole interactions and NMR¹J(¹³C?¹¹⁹Sn) spin-spin coupling constants, the problem of the nonadditivity of the isomer shifts and unexpected quadrupole interactions in tin organic compounds SnX_4?nR_n (X-halide, R-org. lig.) with the degree of substitution n has been treated by means of pseudopotential HF SCF calculations supplemented by efficient methods for comprehending core reorganizations and nucleus near electronic charge distributions. The nonadditivity of the isomer shifts and the progression of the nuclear quadrupole interactions has been explained by the evident momentum separated electronic reorganization. The clearly proved relation between s electron densities and higher orbital imbalances brings the isomer shift in relation to the valence part of the electric field gradient (EFG). An interrelation between isomer shifts and NMR spin-spin coupling constants has been deduced.     

Nuclear quadrupole coupling in the microwave spectrum of 1,2,3-triazole

The hyperfine structure in the microwave spectra of 1,2,3-triazole and N-deutero 1,2,3-triazole has been analyzed. The coupling constants derived from the analysis of each isotopic species have been combined to give the principal nuclear quadrupole coupling constants at the sites of the three inequivalent ¹⁴N nuclei.     

Electric quadrupole orientation in zinc. The variation of electric field gradient with atomic number through the 4sp series

Using the top-loading PrNi₅ nuclear demagnetisation stage /1/ to reach temperatures close to 1mK, electric quadrupole orientation of trace 4sp series impurities in zinc has been measured. Results establish the variation of the electric field gradient at impurity nuclei in this series and also yield the ratio of the electric quadrupole moments of⁷¹As and⁷²As.     

Quadrupole coupling constants and isomeric Mössbauer shifts for halogen-containing gold, platinum, niobium, tantalum and antimony compounds

We have analyzed by means of Density functional theory calculations the nuclear quadrupole coupling constants of a range of gold, antimony, platinum, niobium and tantalum compounds. The geometrical parameters and halogen nuclear quadrupole coupling constants obtained by these calculations substantially corresponded to the data of microwave and nuclear quadrupole resonance spectroscopy. An analysis of the quality of the calculations that employ pseudo-potentials and all-electron basis sets for the halogen compounds was carried out. The zero order regular approximation (ZORA) method is shown to be a viable alternative for the calculation of halogen coupling constants in molecules. In addition, the ZORA model, in contrast to the pseudo-potential model, leads to realistic values of all metal nuclear quadrupole coupling constants. From Klopman’s approach, it follows that the relationship between the electrostatic bonding and covalent depends on the nature of the central atom. The results on Mössbauer chemical shifts are also in a good agreement with the coordination number of the central atom.     

The temperature dependence of 14N pure quadrupole resonance in 4- and 3-cyanopyridines

In this paper it is shown that the temperature dependence of the ¹⁴N pure quadrupole resonance for both cyanide and pyridine nitrogen atoms in cyanopryidines may be used to relate the orientation of the electric field gradient tensor at the cyanide site to that at the pyridine. This provides a means to discuss the population of the two π orbitals on the cyanide nitrogen atom.     

14N Nuclear quadrupole coupling and methyl internal rotation in the microwave spectrum of 2-methylpyrrole

Using two molecular jet Fourier transform microwave spectrometers, the rotational spectrum of 2-methylpyrrole was recorded in the frequency range from 2 to 40?GHz. From the torsional splittings due to the internal rotation of the methyl group a barrier height of 279.7183(26) cm^?1 was deduced. Because of the ¹⁴N nucleus, all lines show a quadrupole hyperfine structure. The microwave spectra were analysed using the XIAM and BELGI-C_s-hyperfine codes. The XIAM code enabled us to reproduce the whole data set with a root-mean-square deviation of 5.6 kHz while the BELGI-C_s-hyperfine code could provide a better root-mean-square almost by a factor of 2 compared to that of XIAM. The experimental results were complemented by quantum chemical calculations. The values of the methyl torsional barrier and the ¹⁴N nuclear quadrupole coupling constants are discussed and compared with other methyl substituted pyrroles as well as other aromatic five-membered rings.     

Ratio of the electric quadrupole moments of122,124Sb

Low temperature nuclear orientation experiments have been performed with^122,124Sb in Sb and Zn single crystals. The ratio of the quadrupole moments of¹²⁴Sb and¹²²Sb is derived from the data asQ ¹²⁴/Q ¹²²= +2.17 (11). The sign of the effective electric field gradient at the nuclear site of Sb was found to be negative for host Sb and positive for host Zn.     

High-resolution spectroscopy near the continuum limit: the microwave spectrum of trans-3-bromo-1,1,1,2,2-pentafluoropropane

ABSTRACT

The microwave spectrum of 3-bromo-1,1,1,2,2-pentafluoropropane has been observed using CP-FTMW spectroscopy. Potential energy scans have been performed and confirm the existence of two conformers – trans and gauche – for which further structural optimisations and electric field gradient calculations have been performed in order to get highly accurate nuclear quadrupole coupling constants for assignment purposes. The combination of multiple conformers and large nuclear quadrupole coupling constants produce a very dense spectrum at an estimated 1?transition/MHz, near the continuum limit. This spectral density makes it necessary to have very sophisticated computational approaches in order to get geometric and electronic structures that are very close to experimental observation. Analysis of the spectrum allowed for the assignment of the trans conformer, but the gauche proved to be prohibitive, although it is believed to be present in the current spectrum. Full analysis of the rotational spectroscopic parameters of two isotopologues – the⁷⁹Br and⁸¹Br – have been observed and are reported. Geometric analysis of the experimentally observed conformer is also reported using Kraitchman coordinate and second moments arguments. Further analysis of the spectrum reveals the occurrence of dipole-forbidden, nuclear quadrupole allowed transitions with one forbidden transition possessing the first known x-type forbidden transition linkage pathway.     

Nuclear quadrupole coupling in the microwave spectrum of imidazole

The quadrupole hyperfine structure in the rotational spectrum of two isotopic forms of imidazole has been analyzed to obtain principal nuclear quadrupole coupling constants for the two nonequivalent nitrogen atoms. The results have been compared with field gradient tensors calculated using INDO molecular orbital wavefunctions and some implications for the electronic structure of imidazole are mentioned. Comparisons with pyrazole and the NQR results for imidazole are also made.