Direct Dipole-dipole Interaction Research Articles

Transverse Relaxation of Cu Nuclear Spins in YBa2Cu3O6.98

We have measured the transverse relaxation of the planar Cu(2) nuclear spins in an oriented powder sample of YBa 2 Cu 3 O 6.98 ( T c =92 K) by using the nuclear quadrupole resonance (NQR) technique. Above T c , after subtraction of the T 1 process, the spin echo envelope decay follows a Gaussian form, and its time constant is almost independent of temperature. In the vicinity of T c , however, the Gaussian line shape is gradually narrowed. The calculated value ( T G =149 µsec) for the Gaussian time constant based on the direct nuclear dipole-dipole interaction is comparable to the experimental one ( T G =131±2 µsec) at 297 K. Hence, the nuclear indirect coupling proposed previously may not be needed. Below T c , the Gaussian line shape was found to be narrowed depending on H 1 , and especially, it is sharply narrowed at 35 K and 87 K.

Mechanism of 3He-mediated nuclear spin-lattice relaxation at surfaces

We measured the nuclear spin-lattice relaxation time T 1, of several surface-bound nuclei, 1H, 19F, 11B, 13C, 29Si, and 2H, immersed in liquid 3He over the temperature range 0.01 K ⩽ T < 1 K. The Larmor frequencies of these nuclei in a 3.39 T field extended from 22 to 144 MHz. All T 1 values were temperature-independent and ranged from a few seconds to several hours, depending on the particular nucleus and the surface geometry of the sample. The results indicate that the coupled relaxation of surface spins is a phenomenon occurring in all solids immersed in 3He and thus provides a general mechanism for obtaining high nuclear polarization in solids, that the relaxation is controlled by direct dipole-dipole interactions between the surface spins and 3He in the first surface layer, that the 3He motion dynamics do not change appreciably from one surface to another, and that measurements of T 1 may thus be useful for determining the structure of surfaces.

Assignment of symmetry-related cadmium shielding tensors to lattice positions in cadmium acetate dihydrate

An oriented single-crystal NMR experiment was carried out for a cadmium acetate dihydrate crystal having 66% 13C enrichment of the acetate carboxylate groups. The resulting 113Cd- 13C direct dipole-dipole interactions give rise to an orientation-dependent modulation of 113Cd NMR linewidths for each of fdur magnetically distinguishable shielding tensors in the cadmium acetate crystal. Comparison of observed cadmium linewidths with those computed as a function of crystal orientation allows for unambiguous assignment of each of the four symmetry-related 113Cd shielding tensors to specific lattice positions in the cadmium acetate crystal. These assignments are in agreement with those made based upon empirical rules, and serve to confirm by means independent of previous assumptions the reliability of the assignment rules used previously. Confirmation of the validity and reliability of such rules is essential for the accurate interpretation of cadmium single-crystal NMR data in terms of shielding tensor-structure correlations to cadmium chemical shifts.

Buildup of the acceptor emission as a result of energy transfer from Tb 3+ to Sm 3+ in barium borate glass

Intensity and lifetime data indicate that the self-quenching of Sm 3+ fluorescence in barium borate glass matrix is by direct dipole-quadrupole interactions. The quenching of Tb 3+ fluorescence by Sm 3+ has been found due to direct dipole-dipole interactions. However, at relatively low Sm 3+ concentrations, energy transfer through migration among Tb 3+ ions also occurs. Further at low Sm 3+ concentrations, there is an enhancement of Sm 3+ emission due to energy transfer to an Sm 3+ that is not coupled to a second Tb 3+ or Sm 3+ ion. At high Sm 3+ concentrations, no enhancement of Sm 3+ emission occurs and is attributed to transfer to Sm 3+Sm 3+ or Sm 3+Tb 3+ couples that take up the energy by simultaneous transitions that lie well below the 4G 5 2 manifold of Sm 3+.

Effects of paramagnetics on proton spin relaxation in poly(p-phenylene)

Measurements of T 1 e for protons in poly( p-phenylene) indicate that relaxation is dominated by direct interaction with paramagnetic centers. Processes to remove paramagnetic centers decrease the number of centers, but the process cannot be carried to completion. Temperature-dependent relaxation measurements give an activation energy of 4.2 kJ mol −1 for the process which modulates the direct electron-nuclear dipole-dipole interaction.

Investigation of molecular order in liquid-crystalline solutions of cellulose triacetate using PMR spectroscopy

Liquid-crystalline solutions of cellulose triacetate (CTA) in trifluoroacetic acid (TFA)–CH2Cl2, TFA–1.2-dichloroethane (1,2-DCE) solvent mixtures were examined by means of PMR spectroscopy. CTA forms both cholesteric and nematic phases in these solvents depending on the CTA concentration. In cholesteric solutions the CH2Cl2 signal is initially a singlet and then splits into a doublet. The time dependence of the splitting and the effect of CTA concentration are reported. The results suggest that the cholesteric phase slowly changes into a nematic phase in the magnetic field. The splitting of the CH2Cl2 proton signal into a doublet and the 1,2-DCE signal into a quartet are due to direct magnetic dipole-dipole interactions. Rotation of the sample in the magnetic field results in the disappearance of the doublet or quartet and suggests that the solvent molecules are originally oriented in the direction of the magnetic field. In the biphasic region, immediate splitting of the CH2Cl2 proton signal suggests that the anisotropic phase is nematic.

Static dielectric properties of the Stockmayer fluid from computer simulation

A number of summation methods for simulating polar liquids are compared using the Stockmayer potential fluid. The nearest image convention is found to have thermodynamic properties dependent on the cell shape. Spherical truncation and a summation proposed by Smith and Perram give reasonable agreement with the thermodynamic properties found from other methods but are not suitable for determining dielectric properties. The reaction field and Ewald-Kornfeld summation methods are in close agreement on the thermodynamic properties and have some measure of agreement on dielectric properties. However, the correlation function h Δ(r) clearly shows the effect of the truncation of the direct dipole-dipole interaction. It is concluded that an Ewald summation is the most appropriate method for simulating polar liquids. Thermodynamic and static dielectric properties of the Stockmayer fluid, determined from Monte Carlo and molecular dynamics calculations using the Ewald-Kornfeld summation, are presented. Calculations with large applied fields have been made to study dielectric saturation. The local field is examined in some detail and a distinction is made between Onsager's analysis of the local field into two components, which is supported by the present results, and Onsager's model for polar liquids which provides values for these components that do not agree well with the present results.

125Te NMR studies of indirect and direct dipole-dipole coupling in polycrystalline CdTe, HgTe, and PbTe

The NMR signals of 125Te in CdTe, HgTe, and PbTe powders show a fine structure due to direct and indirect dipole-dipole interactions. A theoretical powder pattern lineshape is given, with which the isotropic and anisotropic contributions of the direct and indirect dipole-dipole interaction can be separated. Experimental results for J ∥ and J ⊥ in CdTe, HgTe, and PbTe are presented. A linear dependence of the reduced coupling on the atomic number predicts a vanishingly small coupling for ZnTe and this has been confirmed.

On a Modification of the Dicke Model

The modification of the Dicke model for the case of long-wave photons, describing the direct dipole-dipole interaction and dipole-phonon interaction, is constructed. It is shown that the phonon contribution can lead to a change in phase transition order and plays the role of a supplementary ordering factor.

Interaction energy between two identical atoms chemisorbed on a normal metal

The interaction energy between two identical monovalent adatoms is studied and its dependence on the interatomic bond length R is investigated. This interaction essentially involves three processes (direct coupling, indirect coupling and dipole-dipole interaction). When R is large enough some approximations allow us to evaluate their contribution in an algebraic way. In the short distance region this algebraical study is completed by a computation carried out in the case of the chemisorption of Na on Cu. In the range of R values in which the hopping parameter between the atomic states on the adsorbate is large compared with 1 (2πR) the chemisorption weakens the interatomic bond. In the neutral chemisorption case, this weakening arises from the filling of the antibonding virtual level of the adsorbate to the detriment of the bonding one. In the ionic chemisorption case, since the valence orbitals are nearly empty (cathionic chemisorption) or nearly filled (anionic chemisorption), the bond between the adatoms is broken. Then the interaction between each dipole consisted of an adatom and its screening charge laying inside the substrate subsists alone. Conversely, when the hopping parameter between the adsorbate valence states is small compared with 1 (2πR) , the interaction energy between the adatoms reduces to the sum of the indirect energy and the dipole-dipole one. The relative importance of these energies depends on the more or less ionic character of the chemisorption.

Phase transition of superparamagnetic system in spontaneous coherent state

In the system of magnetic particles in superparamagnetic that interacted with resonance radiation field, phase transition in the spontaneous coherent state takes place. There are brought critical value for temperature and density of the system. Direct magnetic dipole-dipole interactions take into account in the molecular field approximation and it influence on the conditions of the phase transition is investigated.

NMR studies of spherical molecules dissolved in smectic liquid crystal solvents

The proton NMR spectra of the nearly tetrahedral molecules CH 3CCl 3 and CH 3SiCl 3 and the tetrahedral molecules (CH 3) 4Si and (CH 3) 4C in smectic liquid crystal solvents are discussed. The relatively large magnitudes observed for the orientation parameter S zz in the smectic phase enable the 1H chemical shielding anisotropies of CH 3CCl 3 and CH 3SiCl 3 to be obtained with good accuracy. In addition, it was also observed that the intramolecular direct dipole-dipole interaction did not vanish for the spherical (CH 3) 4Si and (CH 3) 4C systems. A splitting is observed in the 1H spectra for these molecules which is an order of magnitude larger than previous values obtained using nematic liquid crystal solvents. Values of the direct dipole-dipole coupling constant D HH, and orientation parameter S zz for the two molecules are reported, as well as D 13 CH for (CH 3) 4Si.

ESR study of exchange coupled pairs in copper diethyldithiocarbamate

ESR investigations on exchange coupled pairs of Cu ions in single crystals of Cu(dtc)2, isomorphously diluted with the corresponding diamagnetic zinc salt, are reported. The spin Hamiltonian parameters for the coupled species (S=1) are:g ‖=2.1025,g +=2.031,A=75.1×10−4 cm−1,B=14.8×10−4,D=276.0×10−4 cm−1 andE=46.7×10−4 cm−1. While theg andA tensors show tetragonal symmetry, the zeor-field splitting tensor is rhombic and has principal axes different from those of theg andA tensors. Intensity measurements made down to 4.2 K indicate that the exchange is ferromagnetic with |FFF| ∼ 10 cm−1. Direct dipole-dipole interaction appears to be the major contribution to the zero-field splitting. A calculation on the distributed point dipole model shows that dipolar interaction is considerably modified by the high covalency of the Cu-S bond and accounts for the rhombic nature of the tensor. The possible exchange mechanisms in Cu(dtc)2—direct exchange and superexchange through the bridging sulphurs—are discussed.

Electron spin relaxation through63CuT1in dilute CuMn alloys: dipolar relaxation

High field (up to 50 kG) nuclear spin lattice relaxation data taken on 63Cu in CuMn alloys allow an unambiguous determination of the mechanism responsible for the impurity contribution to T1, the direct dipole-dipole interaction between Mn electron spins and 63Cu nuclear spins. The analysis of the data allows a determination of the temperature dependence of the electron spin relaxation time tau 1 which fits a Korringa like relationship tau 1 varies as T-1. A discussion of these results compared to direct EPR measurements is then attempted.

NMR in liquid crystalline solvents. Vibrational corrections on nuclear dipole-dipole interactions in the methylhalides

Evidence of large anisotropic indirect spin-spin couplings has been obtained by Krugh and Bemheim from NMR measurements of methyl fluoride in a liquid crystal. In the present study the effects of vibrational motions on the anisotropic couplings in the methyl halides are investigated. The corrections due to vibrational motions are based on a Urey-Bradley force field, the anharmonicity of the force field being estimated from semiempirical intramolecular potential functions. In the case of methyl fluoride two additional calculations based on a general force field but constrained by assumptions of the Urey-Bradley model, have been made. The direct dipole-dipole interactions in methyl fluoride, calculated while making allowance for the vibrational corrections, show a large discrepancy with respect to the total anisotropic couplings measured by Krugh and Bernheim. This discrepancy, which is interpreted by these authors in terms of anisotropic indirect couplings, is even larger than that calculated with neglect of vibrational effects. It is concluded that solvent effects on isotropic J couplings seem to be the most obvious cause of the observed discrepancy.

Proton Magnetic Resonance Study of Mercury Dimethyl in a Nematic Liquid Crystal

Abstract The P.M.R.-spectrum of mercury dimethyl partially oriented in a nematic liquid crystal has been studied. From the ratio of the direct magnetic dipole-dipole interactions of the protons the ratio ξ0/ρ is calculated. ξ0 being the distance of the two parallel planes containing the methyl protons, and ρ being the distance of the methyl protons from the molecular symmetry axis. From this ratio the splitting due to the direct coupling of the protons with 199Hg of spin 1/2, present in about 17% natural abundance, is calculated and compared with the experimentally observed one. Exact agreement is obtained under the assumption that the methyl groups are rapidly rotating and that the direct and indirect 199Hg-H-couplings have like signs. Since the preferred orientation of the long axis of mercury dimethyl is probably parallel to the direction of the magnetic field, the absolute sign of J199Hg, H is found to be negative. Correspondingly it is shown that the sign of JCH3, CH3 is positive. No evidence is found for a possible contribution of the anisotropic part of the indirect couplings.

Superhyperfine interactions in the EPR and UPR spectra of CaF 2: U 4+

Satellite components have been observed (at 9.0 GHz) in the EPR and UPR spectra of CaF 2: U 4+ in sites of C 3 v symmetry, and interpreted on the basis of a theoretical model as superhyperfine structure. This structure is due to the interaction between the substitutional U 4+ ion and its eight nearest neighbor F − nuclei. Interpretation of the experimental results on the basis of the model indicates that the shf interaction is anisotropic and arises predominantly from the direct dipole-dipole interaction. The 2 p σ-overlap contribution δA is within the range 0 $ ̌ (δAg ∥β) ⩽ 0.4 G .

Direct Observation of Coherent Exchange Scattering by Low-Energy Electron Diffraction from Antiferromagnetic NiO

Coherent exchange scattering has been observed in low-energy electron diffraction from antiferromagnetic NiO. Half-order diffraction beams which correspond to the translational symmetry of the magnetic cell occur below the N\'eel temperature, ${T}_{\mathrm{N}}=525$\ifmmode^\circ\else\textdegree\fi{}K, and disappear at ${T}_{\mathrm{N}}$. These beams are attributed entirely to exchange scattering because of their intensity, which is much too great to be accounted for by direct dipole-dipole and spin-orbit interactions.

Nuclear Radio-Frequency Spectra of a Series of Tetrahedral Molecules

The molecular-beam magnetic-resonance method has been applied to a series of tetrahedral molecules. In each case, the radio-frequency spectrum was observed which corresponds to the reorientation of the total nuclear spin with respect to a large external magnetic field. In order to analyze these spectra, the energy levels of the complete spinrotation interaction and the direct dipole-dipole interaction have been calculated to first order in high-field perturbation theory by applying the recent group-theoretical treatment of Yi, Ozier, and Anderson. On the basis of these energy levels, the theoretical spectrum for each molecule has been constructed on a high-speed digital computer. By fitting this theoretical spectrum to the data, it has been found that: for where ${c}_{a}$ is the average spin-rotation constant and ${c}_{d}$ is the anisotropy in the spin-rotation matrix. Only the relative sign of these two constants was determined. Several earlier approximate methods of treating spherical rotor problems which involve the tensor part of the spin-rotation interaction are examined in the light of the present results.

Nuclear magnetic resonance of molecules oriented by liquid crystals

The nuclear magnetic resonance spectra of molecules dissolved in nematic liquid crystal solvents display direct dipole-dipole interactions, chemical shift anisotropies, and nuclear electric quadrupole interactions (if the resonant nucleus possesses an electric quadrupole moment), in addition to the spin-spin interactions and average chemical shifts normally observed in isotropic liquids. An analysis of the spectra yields details concerning the interaction of small solute molecules with the large liquid crystal species. An investigation of the interaction of the different fluoromethanes with the liquid crystal p, p′-di- n-hexyloxyazoxybenzene shows that geometrical factors are more important than electric dipolar interactions in determining the orientation of the fluoromethane molecule in the liquid crystal. Thus, CH 3F orients with the CF bond direction (the longest geometrical direction) parallel to the long axis of the nematic liquid crystal. On the other hand, CHF 3 orients with the CH bond direction (the shortest geometrical direction) perpendicular to the long axis of the nematic liquid crystal. Such investigations of semioriented molecules may elucidate some of the observations of nuclear magnetic resonance of molecules that have been oriented by being adsorbed on a surface.