Nuclear electric quadrupole moments of Rb from the hyperfine spectrum of RbF

The molecular beam electric resonance technique has been used to examine the hyperfine spectrum of RbF. The Rb nuclear electric quadrupole interaction, the spin-rotation interactions, and tensor and scalar spin-spin interactions have been measured for both Rb isotopes, including their dependence on vibrational and rotational states. Transition frequencies have been determined to a precision of better than 1 Hz in many cases. The magnetic interactions in the two isotopomers are consistent with what is expected from the known masses and magnetic dipole moments. In the case of the Rb nuclear electric quadrupole interaction, adjustments have been made for a small isotopomer shift, and for the ratio of the effective nuclear electric quadrupole moments, Q(87Rb)Q(85Rb) = 0.483 830 1+/-0.000 001 8. The effective quadrupole interaction includes a pseudoquadrupole interaction that may be significant at this level of precision, but cannot be distinguished experimentally.     

An anomaly in the isotopomer shift of the hyperfine spectrum of LiI

A high-precision examination of the hyperfine spectrum of 6LiI in comparison with 7LiI shows a shift in the iodine nuclear electric quadrupole moment that cannot be accounted for by a model in which the electric field gradient at the iodine site is assumed to depend only upon the internuclear distance between Li and I. The other hyperfine interactions are consistent between the two isotopomers, including the previously reported electric hexadecapole interaction of the iodine nucleus.     

Sternheimer free determination of the47Ti nuclear quadrupole moment from hyperfine structure measurements

The hyperfine structure (hfs) of 12 metastable states of⁴⁷Ti has been measured by laser induced fluorescence. 11 of these states have been measured additionally very precisely by a laser-rf double resonance method. Taking into account results of earlier hfs measurements, the hfs of altogether 17 fine structure states has been analyzed by the simultaneous parametrization of the one- and two-body interactions in the atomic hfs for the model space (3d+4s) ^N+2 (N=2). This gives 16 parameters for the magnetic dipole interaction and 12 parameters for the electric quadrupole interaction. From the model space parameters, obtained from the hfs fit, the nuclear quadrupole momentQ(⁴⁷Ti)=0.303(24b), has been evaluated at the first time; it is free of Sternheimer corrections up to second order.     

2D EPR investigation of endohedral La@C82 in solution

The analysis of a two-dimensional EPR spectrum gives direct evidence for a dominant nuclear quadrupole interaction at the site of the encapsulated La³⁺ ion. From the observed nuclear spin transition rates we deduce eQV_zz/[I(2I − 1)h] ≈ 0.5(3) MHz, assuming that the correlation time of the electric field gradient axes is determined by molecular reorientation of the molecule in solution. The non-vanishing value of the quadrupole interaction is further proof for an off-center position of the metal ion.     

Nuclear electric quadrupole moment of gold

The nuclear quadrupole moment for (197)Au has been determined on the base of the state-of-art relativistic molecular calculations. The experimental shifts in the nuclear coupling constants in the series of molecules AuF, XeAuF, KrAuF, ArAuF, (OC)AuF, and AuH have been combined with highly accurate determinations of the electric field gradient (EFG) at the gold nucleus, obtained by molecular relativistic Dirac-Coulomb-Gaunt Hartree-Fock calculations. The electronic correlation contribution to the EFG is included with the CCSD(T) and CCSD-T approaches, also in the four-component framework, using a finite-difference method. In order to estimate the accuracy of their approach the authors have thoroughly investigated the convergence of the results with respect to the basis set employed and the size of the correlated orbital space. The effect of the full Breit electron-electron interaction on the nuclear quadrupole moment of gold has also been considered explicitly for the AuF molecule. They obtain for (197)Au a nuclear quadrupole moment of 510+/-15 mb, which deviates by about 7% from the currently accepted muonic value.     

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

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.     

The application of radioactive isotope99Mo to the reconstituted MoFe protein

By substututing⁹⁹Mo for the Mo in the reconstituted MoFe protein, the nuclear quadrupole interactions (NQI) of⁹⁹Mo have been measured using the perturbed angular correlations (PAC). Two well-defined electric quadrupole interaction parameters have been observed. The configuration of the M-Center of the MoFe protein is identified by the quadrupole couplign constant _Q1(412(9)MHz) and the asymmetry parameter ₁(0.49(5)). Other parameters, V_Q2(1939(13)MHz) and ₁(0.90(1)), may correspond to a deformation M—Center of MoFe protein.     

Sternheimer free determination of the51V nuclear quadrupole moment from hyperfine structure measurements

The hyperfine structure (hfs) of 18 metastable states of⁵¹V has been measured by laser induced fluorescence. 15 of these states have been measured additionally very precisely by the ABMR-LIRF method. Using results of earlier hfs measurements, the hfs of altogether 33 fine structure states is analyzed using a method of simultaneous parametrization of one- and two-body interactions in the atomic hfs of the model space 3d ^{3 +M} 4s ^{2 ?M} (M = 0, 1, 2). The hfs of these states is described by 16 parameters for the magnetic dipole interaction and 12 parameters for the electric quadrupole interaction. From these model space parameters corresponding configuration dependent parameters for the three configurations were determined. These parameters allow a prediction of the hfs constants of all states of the modelspace within an accuracy of 5 to 10%. The evaluation of the nuclear quadrupole moment of⁵¹V, free of Sternheimer corrections up to second order, yielded the value of -0.043(5) barn.     

Molecular solutes in nematic liquid crystals: Orientational order and electric field gradients

The difference between liquid-crystal and gas-phase values for the nuclear quadrupole coupling constant in D₂ and HD is used to obtain the mean electric field gradient in various liquid crystals. Order parameters for small molecules dissolved in liquid crystals are calculated assuming that the orientational order arises from the interaction of the molecular quadrupole moment with the average field gradient. The results obtained are in good agreement with experimental values for hydrogen and several other solutes.     

Application of the sum peak method to determine electric field gradient in different environments of133Cs nucleus

The sum peak method based on the phenomenon of perturbed angular correlation has been applied to study the nuclear quadrupole interaction frequencies at¹³³Cs which is followed by electron-capture decay of¹³³Ba. These values are used to determine the z-component, which is the largest one of the electric field gradient. The electric field gradient /EFG/ is found to vary with different compounds of barium. The method is found to be useful in describing the chemical influences on the attenuation of angular correlations and hence in the study of EFG.     

Microwave spectra of O(2)-HF and O(2)-DF: hyperfine interactions and global fitting with infrared data

Spectra of the open shell complexes O(2)-HF and O(2)-DF were recorded using Fourier transform microwave spectroscopy. A complete analysis of the hyperfine structure and a global fit including microwave and infrared frequencies [W. M. Fawzy, C. M. Lovejoy, D. J. Nesbitt, and J. T. Hougen, J. Chem. Phys. 117, 693 (2002)] are reported. The Fermi contact interaction between the electron and nuclear spins, the electron spin-nuclear spin dipolar interaction, the nuclear spin-nuclear spin dipolar interaction, and the nuclear electric quadrupole interaction (for O(2)-DF) were considered in the analysis. The correspondence between the magnetic hyperfine constants and the two nuclei of the H(D)F is unambiguously established. In both O(2)-HF and O(2)-DF, the Fermi contact parameter is larger for the fluorine than for the hydrogen, while for the nuclear spin-electron spin dipolar hyperfine constants, the reverse is true. The effective angle between the HF bond and the a axis of the complex, determined from the nuclear spin-nuclear spin interaction constant, is 38(4) degrees. The same angle for the DF complex, derived from the deuterium nuclear quadrupole coupling constant, is 31(4) degrees.     

Theoretical study of N quadrupole coupling constants in some NO-containing complexes: N2O3 and FNO

The nuclear quadrupole coupling constants at nitrogen centers have been computed for N₂O₃ and FNO by employing the complete-active-space self-consistent field, internally contracted multireference configuration interaction and single-configuration coupled-cluster methods with correlation-consistent basis sets at the levels of attainable accuracy. To examine the overall quality of the wave functions used in our calculations, also electric dipole moments and potential energy characteristics were calculated and compared with available experimental and recent theoretical data. The effects of the choice of the basis set and reference configuration space were investigated. The robust changes in the electric field gradients occurring in the course of complex formation from isolated subunits were interpreted in terms of wave function composition. Our calculations confirm the assignment of the ¹⁴N nuclear quadrupole coupling constants to nuclear centers in N₂O₃ provided by the microwave measurements of Cox et al. [A.P. Cox, J. Randell, A.C. Legon, Chem. Phys. Lett. 126 (1986) 481.].     

14N NQR study of polymorphism and hydrogen bonding in molecular complex isonicotinamide-oxalic acid (2:1)

The complete (14)N nuclear quadrupole resonance (NQR) spectra have been measured in the two polymorphic crystalline phases of the molecular complex isonicotinamide-oxalic acid (2:1) by nuclear quadrupole double resonance. The observed NQR frequencies, quadrupole coupling constants, and asymmetry parameters (η) have been assigned to the two nitrogen positions (ring and amide) in a molecule on the basis of the intensity and multiplicity of the double resonance signals. The NQR data for the ring nitrogen in both polymorphic phases deviate from the correlation relations observed in substituted pyridines. This deviation is analyzed in a model, where it is assumed that an additional electric charge on the nitrogen atom changes the NQR parameters. The model suggests that this additional electric charge is negative so that the N···H-O hydrogen bond seem to be partially ionic, of the type N(-)···H-O.     

Direct demonstration of the presence of coordinated sulfate in the reaction pathway of Arabidopsis thaliana sulfite oxidase using 33S labeling and ESEEM spectroscopy

Sulfite oxidase from Arabidopsis thaliana has been reduced at pH = 6 with sulfite labeled with 33S (nuclear spin I = 3/2), followed by reoxidation by ferricyanide to generate the Mo(V) state of the active center. To obtain information about the hyperfine interaction (hfi) of 33S with Mo(V), continuous-wave electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) experiments have been performed. The interpretation of the EPR and ESEEM spectra was facilitated by a theoretical analysis of the nuclear transition frequencies expected for the situation of the nuclear quadrupole interaction being much stronger than the Zeeman and hyperfine interactions. The isotropic hfi constant of 33S determined in these experiments was about 3 MHz, which demonstrates the presence of coordinated sulfate in the sulfite-reduced low-pH form of the plant enzyme.     

Nuclear quadrupole hyperfine structure in the microwave spectrum of HCl-N2O: electric field gradient perturbation of N2O by HCl

The microwave spectra of six isotopomers of HCl-N(2)O have been obtained in the 7-19 GHz region with a pulsed molecular beam, Fourier transform microwave spectrometer. The nuclear quadrupole hyperfine structure due to all quadrupolar nuclei is resolved and the spectra are analyzed using the Watson S-reduced Hamiltonian with the inclusion of nuclear quadrupole coupling interactions. The spectroscopic constants determined include rotational constants, quartic and sextic centrifugal distortion constants, and nuclear quadrupole coupling constants for each quadrupolar nucleus. Due to correlations of the structural parameters, the effective structure of the complex cannot be obtained by fitting to the spectroscopic constants of the six isotopomers. Instead, the parameters for each isotopomer are calculated from the A and C rotational constants and the chlorine nuclear quadrupole coupling constant along the a-axis, chi(aa). There are two possible structures; the one in which hydrogen of HCl interacts with the more electronegative oxygen of N(2)O is taken to represent the complex. The two subunits are approximately slipped parallel. For H (35)Cl-(14)N(2)O, the distance between the central nitrogen and chlorine is 3.5153 A and the N(2)O and HCl subunits form angles of 72.30 degrees and 119.44 degrees with this N-Cl axis, respectively. The chlorine and oxygen atoms occupy the opposite, obtuse vertices of the quadrilateral formed by O, central N, Cl, and H. Nuclear quadrupole coupling constants show that while the electric field gradient of the HCl subunit remains essentially unchanged upon complexation, there is electronic rearrangement about the two nitrogen nuclei in N(2)O.     

An explicit relationship between the dielectric anisotropy and the average electric field gradient in nematic solvents

Nowadays, the interaction between the electric quadrupole moment of a probe-solute and the so-called average electric field gradient (EFG) of the nematic medium is commonly suggested as an important long-range mechanism contributing to the ordering of small molecules dissolved in nematic solvents. Anyway, an explicit relationship between the solvent's EFG and some macroscopic property of the medium has never been established explicitly. In this work, a derivation is carried out leading to a simple formula that shows that the EFG of a nematic solvent is directly related to the dielectric permittivity of the medium (in particular, to its dielectric anisotropy, Deltaepsilon) and to the quadrupole moment of the solute. The obtained expression (a) allows for immediate considerations about the sign and the value of EFG (and could be very useful in designing proper nematic mixtures with null EFG) and (b) reconciles two existing conceptual ways of describing long-range orientational interactions, revealing they are only seemingly different.     

Exploring hyperpolarized 83Kr by remotely detected NMR relaxometry

For the first time, a hyperpolarized (hp) noble gas with a nuclear electric quadrupole moment is available for high-field nuclear-magnetic-resonance (NMR) spectroscopy and magnetic-resonance imaging. Hp (83)Kr (I=92) is generated by spin-exchange optical pumping and separated from the rubidium vapor used in the pumping process. Optical pumping occurs under the previously unstudied condition of high krypton gas densities. Signal enhancements of more than three orders of magnitude compared to the thermal equilibrium (83)Kr signal at 9.4 T magnetic-field strength are obtained. The spin-lattice relaxation of (83)Kr is caused primarily by quadrupolar couplings during the brief adsorption periods of the krypton atoms on the surrounding container walls and significantly limits the currently obtained spin polarization. Measurements in macroscopic glass containers and in desiccated canine lung tissue at field strengths between 0.05 and 3 T using remotely detected hp (83)Kr NMR spectroscopy reveal that the longitudinal relaxation dramatically accelerates as the magnetic-field strength decreases.     

Nuclear quadrupole hyperfine structure in HC14N/H14NC and DC15N/D15NC isomerization: a diagnostic tool for characterizing vibrational localization

Large-amplitude molecular motions which occur during isomerization can cause significant changes in electronic structure. These variations in electronic properties can be used to identify vibrationally-excited eigenstates which are localized along the potential energy surface. This work demonstrates that nuclear quadrupole hyperfine interactions can be used as a diagnostic marker of progress along the isomerization path in both the HC14N/H14NC and DC15N/D15NC chemical systems. Ab initio calculations at the CCSD(T)/cc-pCVQZ level indicate that the hyperfine interaction is extremely sensitive to the chemical bonding of the quadrupolar 14N nucleus and can therefore be used to determine in which potential well the vibrational wavefunction is localized. A natural bonding orbital analysis along the isomerization path further demonstrates that hyperfine interactions arise from the asphericity of the electron density at the quadrupolar nucleus. Using the CCSD(T) potential surface, the quadrupole coupling constants of highly-excited vibrational states are computed from a one-dimensional internal coordinate path Hamiltonian. The excellent agreement between ab initio calculations and recent measurements demonstrates that nuclear quadrupole hyperfine structure can be used as a diagnostic tool for characterizing localized HCN and HNC vibrational states.     

Definitive solid-state 185/187Re NMR spectral evidence for and analysis of the origin of high-order quadrupole-induced effects for I=5/2

Rhenium-185/187 solid-state nuclear magnetic resonance (SSNMR) experiments using NaReO(4) and NH(4)ReO(4) powders provide unambiguous evidence for the existence of high-order quadrupole-induced effects (HOQIE) in SSNMR spectra. Fine structure, not predicted by second-order perturbation theory, has been observed in the (185/187)Re SSNMR spectrum of NaReO(4) at 11.75 T, where the ratio of the Larmor frequency (ν(0)) to the quadrupole frequency (ν(Q)) is ～2.6. This is the first experimental observation that under static conditions, HOQIE can directly manifest in SSNMR powder patterns as additional fine structure. Using NMR simulation software which includes the quadrupole interaction (QI) exactly, extremely large (185/187)Re nuclear quadrupole coupling constants (C(Q)) are accurately determined. QI parameters are confirmed independently using solid-state (185/187)Re nuclear quadrupole resonance (NQR). We explain the spectral origin of the HOQIE and provide general guidelines that may be used to assess when HOQIE may impact the interpretation of the SSNMR powder pattern of any spin-5/2 nucleus in a large, axially symmetric electric field gradient (EFG). We also quantify the errors incurred when modeling SSNMR spectra for any spin-5/2 nucleus within an axial EFG using second-order perturbation theory. Lastly, we measure rhenium chemical shifts in the solid state for the first time.     

Solid-State Nuclear Magnetic Resonance Techniques for the Structural Characterization of Geminal Alane-Phosphane Frustrated Lewis Pairs and Secondary Adducts

The first comprehensive solid-state nuclear magnetic resonance (NMR) characterization of geminal alane-phosphane frustrated Lewis pairs (Al/P FLPs) is reported. Their relevant NMR parameters (isotropic chemical shifts, direct and indirect ²⁷Al-³¹P spin-spin coupling constants, and ²⁷Al nuclear electric quadrupole coupling tensor components) have been determined by numerical analysis of the experimental NMR line shapes and compared with values computed from the known crystal structures by using density functional theory (DFT) methods. Our work demonstrates that the ³¹P NMR chemical shifts for the studied Al/P FLPs are very sensitive to slight structural inequivalences. The ²⁷Al NMR central transition signals are spread out over a broad frequency range (>200 kHz), owing to the presence of strong nuclear electric quadrupolar interactions that can be well-reproduced by the static ²⁷Al wideband uniform rate smooth truncation (WURST) Carr-Purcell-Meiboom-Gill (WCPMG) NMR experiment. ²⁷Al chemical shifts and quadrupole tensor components offer a facile and clear distinction between three- and four-coordinate aluminum environments. For measuring internuclear Al⋅⋅⋅P distances a new resonance-echo saturation-pulse double-resonance (RESPDOR) experiment was developed by using efficient saturation via frequency-swept WURST pulses. The successful implementation of this widely applicable technique indicates that internuclear Al⋅⋅⋅P distances in these compounds can be measured within a precision of ±0.1 Å.