Nuclear Electric Quadrupole Interaction Research Articles

High-resolution calcium-43 NMR in solids

The resolution afforded by alkaline earth ion NMR in solution is often insufficient to obtain individual lines for chemically distinct sites, particularly in macromolecular environments (Z-4). The problem inherent in a small chemical shift range of about 70 ppm is aggravated further by broadening associated with the often complex spin relaxation driven by the nuclear electric quadrupole interactions with fluctuating electric field gradients (5). The dynamical part of the broadening may be defeated if the nucleus to be observed is immobilized. The solid-state experiment has been successful on a number of nonintegral spin quadrupole nuclei where the central (+f c-) -1) transition is exploited and the residual second-order quadrupole broadenings are minimized by rapid sample rotation at the magic angle (6-9). However, the possible gain in resolution is made at the cost of decreased sensitivity because only the central transition in a ; or $ manifold is observed (IO). In principle, the loss may be regained using magnetization-transfer methods such as those exploited by Pines, Gibby, and Waugh (II). Double-resonance approaches have been discussed for calcium fluoride (12,13), and a direct detection of calcium-43 reported using adiabatic demagnetization methods at low temperature (14). We report here observation of the high-resolution calcium-43 NMR spectrum using CP-MASS methods which demonstrates the feasibility of the more routine experiment in a noncubic environment and defines several of the critical experimental parameters. Figure 1 shows the CP-MASS calcium-43 NMR spectrum of calcium acetate obtained at 4.7 T using a spectrometer constructed in this laboratory. The relevant experimental parameters are summarized in the caption. The proton-calcium doubleresonance probe employed a Doty Scientific stator and rotor coupled to the proton tuning elements using a half wavelength coaxial line tapped at the proton null point for injection and detection of the calcium-43 resonance frequency. The spectrometer used Amplifier Research (AR-200L) and Henry Radio power amplifiers, Nicolet-GE 1280 data system, and a quadrature receiver built in the laboratory. The phase-shifting circuitry was designed after a very similar version by Robert MacKay, Monsanto Chemical Company. The match for the magnetization transfer was found by changing the power level on the calcium side while keeping the proton field fixed at 46 kHz; the optimum cross-polarization rate was found when the strength of the calcium rf field was approximately the same as that which gave w1 for calcium of 45 kHz in a saturated aqueous solution of calcium chloride. For calcium acetate, optimum contact times were long, in the tens of milliseconds range as shown in Fig. 2.

Nuclear Quadrupole Resonance: The Present State and Further Development

The importance of nuclear electric quadrupole interactions in chemistry, both present and future, depends very much on their power to resolve problems in electronic structure and molecular dynamics. Fortunately, the subject, by its very nature, is “multinuclear”; even if the ground state of a given nucleus is non-quadrupolar, there often exist excited nuclear states which are, an example being 19F, for which quadrupole coupling constants are now being published from angular correlation measurements. Other new techniques are constantly extending the range of the experiments, recent examples being the use of SQUID magnetometers to detect acoustic 121Sb and 123Sb quadrupole resonance in antimony metal and Fourier transform quadrupole resonance spectroscopy based on fast field cycling to measure 2H quadrupole interactions in powders. Recently, much work on quadrupole interactions in solids of half-integral spin nuclei such as 17O or 27Al has been pursued in two different ways; by quadrupole double resonance in natural abundance, and nuclear magnetic resonance in very high magnetic fields, for which enrichment of low-abundance nuclei such as 170 is often required. In the liquid phase, measurements are now sufficiently reliable for comparisons of changes in the nuclear electric quadrupole tensor from gas to liquid and solid phases to be made. The new methods of partial alignment of polar molecules in the liquid phase in strong electric fields, or magnetically anisotropic molecules in high magnetic fields, seem certain to contribute to these developments.

NMR study of electric quadrupole interactions in GdCo 2

Quadrupole oscillations have been observed with 59Co pulsed NMR in the intermetallic compound GdCo 2. From these oscillations the nuclear electric quadrupole interaction (EQI) has been studied as a function of temperature in the range 4−312 K. The value measured at 4 K, ν Q=672±3 kHz, is the largest so far reported for the cobalt EQI in the RCo 2 intermetallics. The EQI decreases with increasing temperature, reaching 432±10 kHz at 312 K. The amplitude of the oscillations tends to decrease with temperature, being also dependent on the easy direction of magnetization of the compound. Thus, above 200 K, as the direction of magnetization changes, large oscillations are again visible in the satellite line; the main line shows no oscillations in this range. The observed temperature dependence of the EQI is roughly linear, as found in othr transition metal systems.

High-resolution, laser-induced fluorescence spectroscopy of nascent IF: Determination of X- and B-state molecular constants

Nascent IF molecules are prepared in a crossed molecular beam experiment in different vibrational-rotational manifolds of the X 1 Σ + electronic ground state using the reactions F + CF 3I → IF + CF 3 and F + ICI → IF + Cl. High-resolution, laser-induced fluorescence spectroscopy is applied for precision measurements of the bandhead structure of 13 transitions of the IF B → X system (0 ≦ v′ ≦ 4, 3 ≦ v″ ≦ 9). A splitting of the resolved ro-vibronic lines of the order of 500 MHz is observed which is due to the nuclear electric quadrupole interaction. Band origins are derived from the excitation spectra in the vicinity of the band heads. Dunham coefficients are presented for the X 1Σ + and the B 3Π(O +) state, based on a simultaneous fit of the laser-induced fluorescence data obtained in this work, as well as of the microwave spectroscopy data of Tiemann et al. ( Z. Naturforsch. a 28, 1405–1407) and of original flame emission spectra of Durie ( Canad. J. Phys. 44, 337–352) .

Microwave Spectroscopy of Boron Chloride (BCl). The Chlorine Nuclear Quadrupole Coupling Constant

Abstract The pure rotational spectrum of an unstable diatomic molecule, boron chloride (BCl), has been observed in the millimeter wave region. The BCl molecule was directly generated in a 3.7-m long free space absorp-tion cell by a d.c. glow discharge in BCl3. Rotational transitions of 11B35Cl and 11B37Cl in the ground vibrational state, which are split into a few components by the nuclear electric quadrupole interactions, have been observed and analyzed to yield molecular constants including the chlorine nuclear quadrupole coupling constant with high accuracy. The eQq constant thus determined is discussed along with those of other diatomic chlorides, to obtain information on the electronic structure of the molecules.

Nuclear electric quadrupole interaction in LnVO 4

The nuclear electric quadrupole interaction parameter ∣ P/ h∣ for 51V ( I = 7/2) has been determined from NMR measurements for the tetragonal lanthanide vanadates of general formula LnVO 4. A point charge calculation for the electric field gradient V zz gives general agreement with experiment and the Sternheimer parameter γ∞ may be deduced from the data.

Nuclear magnetic resonance of51V (I=7/2) in lanthanide vanadates: I. The paramagnetic shifts

Nuclear magnetic resonance of 51V (I=7/2) has been studied at temperatures down to 1.5K in single crystals of lanthanide vanadates LnVO4 where Ln3+ ranges from Pr3+, 4f2 to Yb3+, 4f13. The crystals are tetragonal, four molecules per unit cell, but all V5+ sites and all Ln3+ sites are magnetically equivalent. Measurements are made with applied fields parallel and perpendicular to the tetragonal axis, and fitted to the spin-Hamiltonian H=- gamma //h(cross)BzIz- gamma perpendicular to h(cross)(BxIx+ByIy)+P(Iz2-1/3I(I+1)). Values for the paramagnetic shifts are used to deduce the local fields at both the V5+ and Ln3+ sites. These fields are compared with the dipolar values, from which the magnitude of the transferred hyperfine interaction at the V5+ sites is deduced, together with estimates of the exchange interaction between the Ln3+ ions. The nuclear electric quadrupole interaction for 51V is discussed in paper II, and measurements in the antiferromagnetic phases of GdVO4 and DyVO4 in paper III.

Magnetic resonance in a cubic Ґ 3 (E) doublet system IV. Improved theory of the nuclear interactions

In previous papers of this series, a study has been made of the theory of magnetic resonance in a Ґ 3 doublet (paper I), together with experimental studies of Cs 2 NaHoCl 6 by n. m. r. (paper II) and by e. s. r. (paper III). In this substance the Ho 3+ ion, 4f 10 , 5 I 8 , is in an octahedral crystal field which gives a Ґ 3 doublet as the ground electronic state. The results, both from enhanced n. m. r. of the 165 Ho nucleus ( I = 7/2) and from e. s. r., provide evidence of fast motional averaging, ascribed to interactions between the ions. The measurements are in substantial agreement with the theory of I; however, though the theory is correct for the electronic levels and transitions, and for the nuclear electric quadrupole interaction in zero magnetic field, it needs improvement in the treatment of the nuclear terms when a field B is applied. In this paper a general theory is developed for the case when B is in an arbitrary direction, together with some special cases that are analytically simpler. These include B along [111], the spectrum of an isolated ion, and the spectrum of a concentrated compound where there are fast flip-flops between the electronic states.

Electric field gradient and its temperature variation atCd111probe nuclei in hafnium metal

The nuclear electric quadrupole interaction at $^{111}\mathrm{Cd}$ nuclei in hafnium-metal host is measured in temperature range 4.2-878 K with the use of time-differential perturbed ($\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$) angular correlations. The observed quadrupole-interaction frequency ${\ensuremath{\nu}}_{Q}=+28.8$ MHz at 295 K.

Enhanced n.m.r. of Cs 2 NaPrCl 6 , Cs 2 NaTbCl 6 and Cs 2 NaTmCl 6

Elpasolite compounds with the formula Cs 2 NaLnCl 6 , where Ln is a lantha­nide ion, have the cubic structure Fm3m (O 5 h ) at room temperature, but some compounds (La, Ce, Pr, Nd) are known to have phase transitions at temperatures between 100 and 160 K to a lower symmetry (Anistratov et al . 1978, Nevald et al . 1979). On the other hand, Cs 2 NaHoCl 6 remains cubic at least down to 0.6 K (Bleaney et al . 1981 a ), although its ground state is a Γ 3 doublet. The compounds of the non-Kramers ions Pr 3+ , Tb 3+ , Tm 3+ have singlet states Γ 1 and we have investigated the enhanced n. m. r. spectra at low temperatures of 141 Pr ( I = 5/2), 159 Tb ( I = 3/2) and 169 Tm ( I = 1/2). The results are interpreted in terms of a predominantly cubic field with a small tetragonal distortion which splits the first excited state, the triplet Γ 4 . The magnitude of this splitting and its sign are consistent with a small elongation along the tetragonal axis for Cs 2 NaPrCl 6 . A novel feature is that the sign of the nuclear electric quadrupole interaction for Pr can be deduced from variations in the line width, ascribed to small, linked variations in the values of P and γ .

The configuration of the hydrogen atoms in HfV 2H x compounds at low temperatures: A NMR study

The 51V nuclear electric quadrupole interaction and the shape of the proton resonance line in the low temperature phase of the HfV 2H x (0≤ x≤4) compounds has been studied by NMR. The data yield information about the hydrogen arrangement in the low temperature phase. They confirm the results of recent neutron diffraction studies that for x=4 and ordered super lattice of the hydrogen atoms exists, with the hydrogen atoms occupying only tetrahedral interstitial sites formed by two Hf and two V atoms. Our results show that in the lower concentration compound the hydrogen atoms most probably also occupy the same type of interstices, but without crystallographic order.

25Mg-NMR investigations of the magnesium ion-DNA interaction.

Abstract25Mg‐nmr data are reported that address the nature of the magnesium ion–DNA interaction. It is found that competitor ions such as calcium, mercury, zinc, and cobalt ions are not effective in competing for all of the magnesium ion–DNA interaction that is reported by the 25Mg‐nmr spectrum. The temperature dependence of the 25Mg‐nmr spectrum in DNA solution studied at high concentrations of competitor ion indicates that the chemical‐exchange lifetime of the magnesium ions at DNA binding sites makes a major contribution to the 25Mg‐nmr line width. However, the activation parameters are not consistent with the temperature dependence of either transport properties or chemical exchange with phosphate groups alone, but are consistent with a sum of at least two processes that provide opposing contributions to the 25Mg‐nmr relaxation. It is also shown that the non‐Lorentzian character of the 25Mg‐nmr line previously reported is consistent with the effect of an incompletely averaged static nuclear electric quadrupole interaction and/or an exchange process that is slow with respect to the magnitude of this interaction. Because the concentrations employed in these experiments are high, the present data do not provide a direct or critical test of the electrostatic theories of ion–polyelectrolyte interaction. The present data do demonstrate, however, that such theories alone are insufficient as a basis for understanding the 25Mg‐nmr data.

Nuclear-acoustic-resonance determination of the strain electric-field-gradient tensorSand its temperature dependence in Ta single crystals

The first quantitative nuclear-acoustic-resonance study of the temperature dependence of the tensor $\mathit{S}$ is presented for a cubic metal. The fourth-rank tensor $\mathit{S}$ relates the electric field gradient (EFG) at a nuclear site to the elastic strain field. For cubic metals, its temperature dependence allows a conclusive test of actual theoretical EFG models. A comprehensive discussion of the tensor $\mathit{S}$ is given and, by applying the screened potential approach of Nishiyama et al. [Phys. Rev. Lett. 37, 357 (1976)] to a lattice with both thermal and ultrasonic vibrations, simple expressions are derived for $\mathit{S}$ and its temperature dependence. In addition, the rotational contribution to the ultrasound-induced EFG is discussed in an appendix. With an internal calibration by the Alpher-Rubin effect the following experimental results were obtained for Ta at 300 K: $|{S}_{44}|=(6.33\ifmmode\pm\else\textpm\fi{}0.29)\ifmmode\times\else\texttimes\fi{}{10}^{22}$ V/${\mathrm{m}}^{2}$ and $\frac{({S}_{11}\ensuremath{-}{S}_{12})}{2{S}_{44}}=\ensuremath{-}0.65\ifmmode\pm\else\textpm\fi{}0.03$. For both components of $\mathit{S}$ a decrease with temperature is observed. With regard to the empirical ${T}^{1.5}$ law of the static EFG the decrease can be described by $S(T)=S(0)(1\ensuremath{-}B{T}^{1.5})$ with $B=(7\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ ${\mathrm{K}}^{\ensuremath{-}1.5}$. This result strongly supports the "phonon model" of Nishiyama et al. which attributes the temperature dependence of the nuclear electric quadrupole interaction in metals to the effect of lattice vibrations.

Exact analytic expressions for NQR parameters in terms of the transition frequencies

Exact solutions of the nuclear electric quadrupole interaction Hamiltonian are obtained for spins 5 2 , 7 2 , and 9 2 . These solutions express the conventional nuclear quadrupole interaction parameters (coupling constant and asymmetry parameter) as closed-form functions of the measured transition frequencies.

Properties of HoVO 4 below 1 K I. Predictions from nuclear magnetic resonance

The size of the internal field in HoVO 4 has been determined by n. m. r. measurements (Bleaney et al . 1978); the interactions between the Ho 3+ ions are mainly dipolar, with a small contribution from antiferromagnetic exchange. It is shown that these measurements can be used directly to calculate the interactions between the enhanced 165 Ho nuclear moments. These interactions are used to predict the nature of the nuclear spin arrangement in the ordered state, and it is shown that a whole range of ‘puckered’ four sub-lattice arrangements all have the same energy. The results are similar to those for a purely dipolar antiferromagnet, with a Néel temperature of 4.8 mK. The sign of the nuclear electric quadrupole interaction is discussed, and the lattice contribution is compared with that deduced by Cook & Cashion (1979) for GdVO 4 .

The electric field gradient in noncubic metals

Electric field gradients present at the atomic nuclei in solids may be studied through the nuclear electric quadrupole interaction by several methods. In noncubic metallic systems, a large amount of data has become available and is critically testing our theoretical understanding of the sources of field gradients in these systems. After summarizing the relevant measurement techniques, we discuss the development of the existing experimental data base and the various theoretical approaches to explain it. Comprehensive coverage of the published literature extends through 1977.

Determination of Nuclear Quadrupole Coupling by Simulation of EPR spectra of frozen solutions

Nuclear electric quadrupole interactions can influence EPR spectra very strongly at certain magnetic field orientations through the positions and intensities of double-spin-flip transitions. Previously, we analyzed orientation-dependent EPR spectra of magnetically dilute single crystals having 63Cu in a variety of coordination sites to obtain principal components of the quadrupole coupling tensor. The values showed a striking correlation with site geometry and electronic structure. High-symmetry, large-coordination-number sites have the largest quadrupole coupling constants. This potentially useful structural tool has been limited by its restriction to dilute single crystals. Accordingly, we report now on the importance of the quadrupole-sensitive double-spin-flip transitions in EPR spectra of frozen solutions. The X-band and Q-band spectra of some frozen solutions of copper /f-ketoenolates (with the copper isotopes in natural abundance) in more-or-less coordinating solvents had rather broad, poorly resolved features. Nevertheless, inclusion of the double-spin-flip transitions proved essential to successful stimulation of the X-band spectra. By matching computed simulations with the experimental spectra we show that these extra transitions contribute sufficiently to permit one to extract nuclear quadrupole coupling information. The resulting quadrupole coupling constants follow the expected order of solvent ligand strength and lie in between those previously found for sites of 5- and 6-coordination. Specific axial solvent coordination is implied.

Hyperfine structure in the electronic spectrum of 127I2 by saturated absorption spectroscopy at 633 nm

The full hyperfine components of the R (127) line in the 11-5 band and the 10 components of the P (33) line in the 6-3 band of the B←X electronic transition of 127I2 were observed by the saturated absorption spectroscopy using a complex resonator He-Ne laser. The frequency of the laser was locked to each component and the frequency separations of the components were precisely measured. The hyperfine structure of the R (127) line was compared with a model which included both a nuclear electric quadrupole and magnetic hyperfine interactions.

ESR of Mn2+diffused into sodium β-alumina

The location of Mn2+ diffused into single crystals of sodium beta -alumina has been studied. ESR measurements confirm that the only site in the spinel block occupied by these ions is the tetrahedral site closest to the conduction plane. The spin Hamiltonian parameters obtained for Mn2+ in this site are g/sub ///=2.0006(2), gperpendicular to =2.0019(2), A/sub ///=-8.20(3) mT, Aperpendicular to =-8.28(4)mT, D=-27.11(6) mT, (a-F)=-0.10(6) mT. The site allocation is the same as that of Antoine et al. but the spin Hamiltonian parameters differ slightly from the values they obtained. From the forbidden line splittings a value of P=14+or-5 mu T is found for the nuclear electric quadrupole interaction.

Sign of nuclear electric quadrupole coupling constants in solids from?-ray anisotropy measurements after capture of polarized neutrons

An experimental method is presented which allows the determination of the sign of nuclear electric quadrupole interactions in solids. Activated target nuclei with a purely dipolar spin polarization are produced by capture of polarized thermal neutrons. The quadrupole coupling of the nuclei to the electric field gradient tensor in the target crystal converts this (dipolar) polarization partly into a (quadrupolar) alignment, which can be measured by the anisotropy of theγ-ray emission in a succeeding nuclear transition. The sign of the alignment created in this reorientation process depends on the sign of the electric quadrupole interaction. The reorientation effect can be enhanced by selective induction of nuclear magnetic resonance transitions. The method has been applied to measure the sign of the crystal electric field gradient (efg) in tetragonal MgF2. Further, the sign of an efg in cubic CaF2 originating from a19F interstitial adjacent to the activated20F probe nucleus has been determined. The method is in principle applicable to a considerable number of nuclides.