Nuclear quadrupole coupling tensors of 17O in forsterite,Mg2SiO4

The nuclear quadrupole coupling tensors (eigenvalues and eigenvectors) of ¹⁷O were determined in a single crystal of Mg₂SiO₄ using pulsed nuclear magnetic resonance. The tensors are of the same order of magnitude as in Al₂O₃ and TiO₂.     

Nuclear magnetic shielding tensors of207Pb2+ in Pb(NO3)2

The NMR signals of²⁰⁷Pb were observed in a single crystal of Pb(NO₃)₂ and could be assigned to the four different Pb²⁺ sites by the dependence of the linewidths on the orientation. Four different nuclear magnetic shielding tensors with equal principal values but with different characteristic vectors could be determined. The symmetry of the shielding tensors is in agreement with the symmetry at the Pb²⁺ sites. It is shown, that intermolecular contributions can not account for the anisotropy of the nuclear magnetic shielding, which is 3 of the isotropic absolute magnetic shielding.     

Microwave spectra of the Cl and Cl isotopomers of dichloromethylene: Nuclear quadrupole-, spin-rotation-, and nuclear shielding constants from the hyperfine structures of rotational lines

The rotational spectra of the isotopomers C³⁵Cl³⁷Cl and C³⁷Cl₂ of dichloromethylene in the ground vibronic state were recorded in the range 10-33 GHz using a molecular beam Fourier transform microwave spectrometer. CCl₂ was generated by flash pyrolysis using different precursors. The observed spectra were analyzed to yield rotational and centrifugal distortion constants, as well as the complete Cl nuclear quadrupole coupling tensors and the spin-rotation interaction constants from the hyperfine structure of the rotational lines. With inclusion of data from previous work on the most abundant species C³⁵Cl₂ [N. Hansen, H. Mäder, F. Temps, Phys. Chem. Chem. Phys. (3) (2001) 50-55.] a refined r₀ structure was determined. The spin-rotation interaction constants of all three isotopomers were used to derive ³⁵Cl and ³⁷Cl principal inertial axis nuclear magnetic shielding components which have not yet been determined by NMR spectroscopy.     

Hydrogen bond studies

The deuteron magnetic resonance spectra from partially deuterated single crystals of NaHC₂O₄. H₂O have been studied. The quadrupole coupling tensors for stationary deuterons are determined at about 25°C. The quadrupole coupling constants and the assymmetry parameters η of the tensors for the two deuterons in the water molecule are, 235·2 ± 1·9 and 228·5 ± 1·6 kHz, and 0·09 ± 0·01 and 0·13 ± 0·01, respectively. The principal axes corresponding to the largest components both deviate by 0·9 ± 0·5° from their respective O-H directions. For one of the tensors this angular displacement is in the plane of the H₂O molecule and towards the other O-H direction. The displacement for the other tensor is out of the plane of the H₂O molecule.     

NMR of 29Si and 25Mg In Mg2SiO4 with dynamic polarization technique

Nuclear magnetic resonance of ²⁹Si and ²⁵Mg in natural abundance has been observed in Cr³⁺ doped Mg₂SiO₄ after cooling the nuclear spin system by dynamic nuclear polarization. The quadrupole coupling tensors of ²⁵Mg at the 4a and 4c positions have been determined.     

1H ENDOR investigations on gamma-irradiated betaine calcium chloride dihydrate in the incommensurate phase

The electron nuclear double resonance (ENDOR) technique is applied to study the influence of the incommensurate modulation on the proton hyperfine structure (hfs) tensors of the N(CH₃)₃ radical in betaine calcium chloride dihydrate (BCCD). The temperature dependence of the edge singularities due to the increasing of the incommensurate (IC) modulation amplitude could be observed in the¹H ENDOR of the CH₃ groups for the first time. The rotation pattern of the edge singularities observed in the¹H spectra in the IC phase of BCCD can be explained by a libration around the C₁-N axis with a tilt angle of about ±7°.     

Nuclear magnetic resonance and transferred hyperfine interaction study of the crystallographically inequivalent Cs(I) and Cs(II) in a Cs2CuCl4 single crystal

¹³³Cs (I=7/2) nuclear magnetic resonance in a Cs₂CuCl₄ single crystal grown by using the slow evaporation method was measured in its three mutually perpendicular crystal planes. The ¹³³Cs resonances of two different groups with two crystallographically inequivalent cesium nuclei, Cs(I) and Cs(II), in the unit cell were recorded. The transferred hyperfine fields for Cs(I) and Cs(II) calculated from the paramagnetic shift and the molecular susceptibility measurements could be expressed by the linear equation H_hf=AT+B. The angular dependence of the ¹³³Cs nuclear magnetic resonance spectra showed that the Cs(I) and the Cs(II) nuclei had different values for the quadrupole coupling constant. The electric field gradient tensors of Cs(I) and Cs(II) were symmetric, and the orientations of their principal axes did not coincide. The Cs(I) ion surrounded by 11 chlorine ions had a small quadrupole parameter, a smaller charge distribution, and a small value for the transferred hyperfine field. However, the Cs(II) ion surrounded by nine chlorine ions had a larger quadrupole parameter, a larger charge distribution, and a larger value for the transferred hyperfine field.     

The local structure of Cs(I) and Cs(II) in a Cs2ZnCl4 single crystal studied by nuclear magnetic resonance

The rotation patterns of the ¹³³Cs (I=7/2) nuclear magnetic resonance (NMR) in a Cs₂ZnCl₄ single crystal grown by using the slow evaporation method were measured in two mutually perpendicular crystal planes. Two different groups of Cs resonances were recorded; this result points to the existence of two types of crystallographically inequivalent Cs(I) and Cs(II). The angular dependences of the NMR spectra led to different values for the quadrupole coupling constants and asymmetry parameters: e²qQ/h=148 kHz and η=0.11 for the Cs(I) ion, and e²qQ/h=274 kHz and η=0.66 for the Cs(II) ion. The EFG tensors of Cs(I) and Cs(II) are asymmetric, and the orientations of the principal axes of the EFG tensors do not coincide. Only, the principal Y-axes of the EFG tensors coincide for the Cs(I) and Cs(II) sites. The Cs(I) ion is surrounded by 11 chlorine ions, making it rather free and high in symmetry. The Cs(II) ion has only nine neighbors and seems to be more tight than the Cs(I) ion.     

Pulsed ESR investigations of anisotropic interactions in M@C82 (M=Sc,Y,La)

82 , Y@C₈₂, and La@C₈₂ in frozen solutions. We were able to determine the g tensors of these molecules by analysing magnetic field spectra at X-band (9.5 GHz) and W-band (94 GHz) frequencies. Moreover, in Y@C₈₂ we have investigated the hyperfine interaction of the ⁸⁹Y nuclear spin (I=1/2) with the electron spin on the C₈₂ cage. The principal values of the hyperfine tensor A and the relative orientation of g and A tensors were determined by applying three- and four-pulse electron spin echo envelope modulation techniques (ESEEM). Received: 3 September 1997/Accepted: 10 November 1997     

Molecular motions in chloramphenicol studied by35Cl nuclear quadrupole and1H nuclear magnetic resonance spectroscopy

The³⁵Cl nuclear quadrupole resonance (NQR) frequency (v_Q), nuclear quadrupole spinlattice relaxation time (T_1Q),¹H nuclear magnetic resonance second moment (M₂) and nuclear magnetic spin-lattice relaxation timeT ₁) were measured for polycrystalline chloramphenicol (drug) as a function of temperature. Hindered rotation of the CHC1₂ group and the phenyl ring was detected, the relevant activation energies were determined. The rotations are discussed in detail.     

Double resonance experiments in low magnetic field: Dynamic polarization of protons by N and measurement of low NQR frequencies

The possibilities of dynamically polarizing proton spin system via the quadrupole ¹⁴N spin system in low magnetic field are analyzed. The increase of the proton magnetization is calculated. The polarization rate of the proton spin system is related to the transition probabilities per unit time between the ¹⁴N quadrupole energy levels and proton energy levels. The experiments performed in 1,3,5-triazine confirm the results of the theoretical analysis. A new double resonance technique is proposed for the measurement of nuclear quadrupole resonance frequencies ν_Q of the order of 100 kHz and lower. The technique is based on magnetic field cycling between a high and a low static magnetic field and observation of the proton NMR signal in the high magnetic field. In the low magnetic field the quadrupole nuclei and protons resonantly interact at the proton Larmor frequency ν_H = ν_Q/2. The quadrupole nuclei are simultaneously excited by a resonant rf magnetic field oriented along the direction of the low static magnetic field. The experimental procedure is described and the sensitivity of the new technique is estimated. Some examples of the measurement of low ¹⁴N and ²H nuclear quadrupole resonance frequencies are presented.     

Magnetoacoustic resonance on nuclear spin waves in the cubic antiferromagnet RbMnF3

Magnetoacoustic resonance on nuclear spin waves is measured in the cubic antiferromagnet RbMnF₃. A resonance change with respect to a constant magnetic field H ₀ with maximum damping at H ₀≈4×10³ Oe is observed in the amplitude of an acoustic pulse passing through a sample owing to excitation of nuclear spin waves under nuclear magnetoacoustic resonance conditions. A study of the angular dependence of the damping revealed a 90° periodicity consistent with the fact that the [001] direction, around which the rotation takes place, is a four-fold axis of the crystal. An analysis of the dispersion law for nuclear spin waves shows that longitudinal ultrasound propagating along the [001] axis perpendicular to H ₀ excites a branch of nuclear spin waves whose frequency depends on the magnitude of the constant magnetic field. Fiz. Tverd. Tela (St. Petersburg) 41, 297–300 (February 1999)     

A determination of zeeman parameters for NO2 in its ground state

Extensive measurements of magnetic dipole transitions in the gas phase E.P.R. spectrum of NO₂ in its [Xtilde] ² A ₁ state are reported. This type of transition was first identified by Burch, Tanttila and Mizushima. The data have been fitted simultaneously with measurements from the far infra-red laser magnetic resonance spectrum of NO₂ to determine the principal components of the electron spin g-tensor. The results are consistent with Curl's relationship between the components of the electron spin and spin-rotation tensors. It is necessary to include the effects of centrifugal distortion of the spin-rotation interaction and two parameters describing this interaction are determined in the course of the analysis. Some discussion of the origins of these parameters is included.     

The E.S.R. spectrum of irradiated ammonium hypophosphite

Electron spin resonance spectra of irradiated crystals of ammonium hypophosphite (NH₄)⁺(H₂PO₂)^- indicate that the ionic radical H[Pdot]O₂ ^- is initially formed. The hyperfine interaction tensor of the phosphorus nucleus has the principal values 1698, 1228 and 1228 Mc/s. Interaction with the proton gives rise to an almost isotropic hyperfine coupling of 230 Mc/s. These results indicate that the species H[Pdot]O₂ ^- is irregular tetrahedral in shape, the unpaired electron occupying a hybrid σ orbital centred on the phosphorus nucleus.

There is evidence that the radical H[Pdot]O₂ ^- reacts with an adjacent H₂PO₂ ^- ion forming the species O₂[Pdot]-PHO₂ ⁼. The principal values of the hyperfine interaction tensors of the two phosphorus nuclei are 1365, 944, 921 Mc/s and 527, 362, 359 Mc/s respectively. The proton has an isotropic hyperfine coupling of 90 Mc/s. The spectra of irradiated deuterated crystals confirmed this analysis.     

Estimation of single-ion anisotropies, crystal-field and exchange interactions in Gd-based frustrated pyrochlore anti-ferromagnets Gd2M2O7 (M=Ti, Sn, Hf, Zr)

The temperature dependence of the experimental results of dc (macroscopic) magnetic susceptibility and nuclear hyperfine properties of frustrated magnetic Gd-based pyrochlore compounds, Gd₂Ti₂O₇, Gd₂Sn₂O₇, Gd₂Hf₂O₇ and Gd₂Zr₂O₇, are analyzed within the frame work of appropriate crystal-field theory and a mean field approximation by introducing effective anisotropic molecular field tensors, and formulating an exact relation between single-ion susceptibility tensors and site susceptibility tensors. Components of the calculated susceptibility along and perpendicular to the local 〈1 1 1〉 axis of the tetrahedral sublattice of pyrochlore structure show that these pyrochlores are easy-planar anisotropic magnetic systems. The crystal-field parameters and anisotropic exchange coupling have been determined and their systematic variations over the Gd-based pyrochlores studied here are discussed.     

NMR Line Shape for a Rectangular Configuration of Nuclei

We present an approach to describe nuclear magnetic resonance absorption of dipolar-coupled rectangular configuration of nuclear spins associated with reorientation of ethylene bridge (–CH₂–CH₂–) in the crystal. The approach was applied to investigate reorientational mobility of dabco-linkers (1,4-diazobycyclo[2,2,2]octane) in metal–organic framework [Zn₂(bdc)₂(dabco)] (bdc—benzenedicarboxylate). Dabco rotation around the N–N axis is shown to remain down to 80 K and the activation energy of the molecule reorientation does not exceed 12 kJ/mol.     

Intermediate state perturbation in low temperature nuclear orientation

This paper deals with the perturbation of low temperature nuclear orientation in an intermediate state by an interaction not axially symmetric around the orientation axis. The directional distribution of the emitted radiation is factorized into initial orientation tensors and perturbation factors known from perturbed angular correlation theory. Low temperature nuclear orientation coefficients have been calculated numerically for¹¹¹Cd in a polycrystalline sample with axially symmetric electric field gradient. Nuclear orientation experiment on¹¹¹In in indium host confirms theoretical perturbation predictions and demonstrates the sensitivity of the method on different parameters. Moreover this orientation allows the determination, in magnitude and sign of¹¹¹In (ground state) nuclear magnetic moment: = +5.48(10)_n.     

Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds

Two hexanuclear niobium halide cluster compounds with a [Nb₆X₁₂]²⁺ (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable B_o field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of ⁹⁵Mo nuclei in structurally related compounds containing [Mo₆Br₈]⁴⁺ cluster cores. Experimentally determined δ_iso(⁹³Nb) values are in the range from 2400 to 3000 ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (V_zz and δ₃₃) coinciding with the molecular four-fold axis of the [Nb₆X₁₂]²⁺ unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (V_zz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (V_yy) and the largest CS principal axis (δ₁₁) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by ^79/81Br NQR and ³⁵Cl solid-state NMR measurements.     

Nuclear Magnetic Resonance Spectra of Polyhydroxylated Fullerene C60(OH)n

Polyhydroxylated fullerene C₆₀(OH)_n (with an estimated number of hydroxyl groups n = 38–44) synthesized from pure fullerene by mixing a benzene solution of C₆₀ with a NaOH aqueous solution in the presence of a catalyst was studied with ¹H and ¹³C solid-state nuclear magnetic resonance. Possible features of the structure of a molecule shell were revealed from ¹H NMR data. The ¹³C spectrum showed a peak splitting with an increase in temperature, which is probably due to fullerenol isomers.