Anisotropic Spin Interactions Research Articles

Electric field N.M.R.

Application of a strong de electric field to solutions of apolar molecules in an apolar solvent leads to partial orientation of the molecules. This orientation could be detected by N.M.R. spectroscopy through the anisotropic spin interactions that show up in the spectra. Subsequent analysis of the spectrum leads to information about the static electric polarizability tensor (a). The electric field N.M.R. (EFNMR) method was applied to measure, from the 2H EFNMR spectrum of naphthalene-d8, the anisotropy of a (i.e. Δα), and asymmetry (δα). The result is: 7·1 ± 0·3 and -(4·4 ± 0·5) × 10-24 cm3 for Δα and δα respectively. For triphenylene-d12 the value of Δα is measured to be -(14·4 ± 1·4) × 10-24 cm3. The results from the EFNMR method and from other methods are compared and discussed

NMR study of the low temperature structure of HfV2·4

Abstract The NMR powder spectrum of 51 V in the low temperature tetragonal phase of HfV 2 · 4 has been studied in fields up to 4 T. A detailed lineshape analysis proves that only one V-site is present, with strongly anisotropic nuclear spin interaction parameters. The results are in full agreement with neutron diffraction data and exclude the recently proposed existence of two inequivalent V-sites.

Solid echoes in the slow-motion region

NMR lineshapes obtained by Fourier transform of the solid-echo decay are compared with those from the free-induction decay in the slow-motion region where the motional correlation times are of the same order as the reciprocal anisotropic spin interactions. The theory has been worked out explicitly for the example of two-site exchange in an I = 1 spin system that applies, e.g., to 1H or 2D NMR of water molecules flipping by 180° about the twofold axis. Large differences are found in the region where the time between the two 90° pulses of the solid-echo sequence is comparable with the motional correlation time. The intensity of the solid echo is reduced by a factor R > 0.5 relative to the free-induction decay for the example of anisotropic motion considered. Much smaller reduction factors are obtained for isotropic reorientation by small- or large-angle rotational jumps.

PMR studies on the single crystal of a pyrazine complex with copper(II) nitrate

Proton magnetic resonance spectra were recorded on the single crystal of Cu(NO 3) 2(pyrazine) in a magnetic field applied perpendicularly to -Cu-(pyrazine)-Cu-chains. The spectra were interpreted in terms of three effects: (1) the dipolar interaction between neighboring protons, (2) the dipole sum from copper(II) ions, and (3) the hyperfine interaction arising from the combined effects of the isotropic contact interaction and the anisotropic nuclear spin-electron spin interaction. From the dipolar interaction between protons, the CH bond distance of pyrazine was determined to be equal to 1.03 Å. Interpretation of the hyperfine interaction provides the following information about spin distribution: (1) a negative spin density amounting to −0.07 is distributed essentially in the pπ orbitals of carbon atoms, and (2) the spin distribution function is deformed by electrostatic interaction due to the π-electron clouds of the adjacent atoms from the otherwise pure carbon p orbital.

Investigation of very slowly tumbling spin labels by nonlinear spin response techniques: theory and experiment for stationary electron electron double resonance.

The investigation of very slowly tumbling spin labels by nonlinear electron spin response techniques is discussed. Such techniques permit characterization of rotational processes with correlation times from 10(-3) to 10(-7) sec even though the linear spin response (ESR) technique is insensitive to motion in this region. Nonlinear techniques fall into two categories: (a) Techniques (referred to as passage techniques) in which the distribution of saturation throughout the spin system is determined both by the applied magnetic field modulation of the resonance condition and by the modulation of the resonance frequency induced by the molecular motion. The time dependence of this distribution produces phase and amplitude changes in the observed signals. (b) Techniques that measure the integral of the distribution function of the time required for saturated spin packets to move between pumped and observed portions of the spectrum [stationary and pulsed electron electron double resonance (ELDOR) techniques]. Quantitative analysis of passage ESR and stationary ELDOR techniques can be accomplished employing a density matrix treatment that explicitly includes the interaction of the spins with applied radiation and modulation fields. The effect of molecular motion inducing a random modulation of the anisotropic spin interactions can be calculated by describing the motion by the diffusion equation appropriate to the motional model assumed. For infinitesimal steps the eigen-functions of the diffusion operator are known analytically, while for random motion of arbitrary step size they are determined by diagonalizing the transition matrix appropriate for the step model used. The present communication reports investigation of the rotational diffusion of the spin label probes 2,2,6,6-tetramethyl-4-piperidinol-1-oxyl and 17beta-hydroxy-4',4'-dimethylspiro-[5alpha-androstane-3,2'-oxazolidin]-3'-oxyl in sec-butylbenzene. Experimental spectra are compared with computer simulations of spectra carried out for isotropic Brownian (limit of infinitesimal step size) and free diffusion (arbitrary step size) models.

Electron Spin Resonance ofNd3+Pairs in LaCl3and LaBr3

We have examined the electron-spin-resonance spectra due to ${\mathrm{Nd}}^{3+}$ pairs in La${\mathrm{Cl}}_{3}$ and La${\mathrm{Br}}_{3}$. These measurements were made at 4.2\ifmmode^\circ\else\textdegree\fi{}K and 9 kMc/sec using 0.1-2.0 at.% doping of enriched even-even isotopes of neodymium. A general anisotropic exchange interaction between two real spins is transformed into an effective-spin Hamiltonian formalism containing a Zeeman term and a traceless coupling tensor ${A}_{\mathrm{ij}}$ between two effective spins each of magnitude \textonehalf{}. This latter term represents an anisotropic effective spin interaction within the triplet levels of the system. About 5000 resonances due to allowed and forbidden transitions within these triplet levels were observed and fitted to this Hamiltonian for various angles $\ensuremath{\theta}$, between the magnetic field H and the crystal axis, and $\ensuremath{\varphi}$, between H and the plane containing a second-neighbor pair. Some resonances due to first-neighbor pairs were shifted by 2500 Oe from the unpaired-ion resonance and require analysis techniques other than perturbation theory. The observed symmetry of the second-neighbor interaction tensors in these two salts suggests that the halide ions play an important role. By removing the dipolar contributions to ${A}_{\mathrm{ij}}$, quantitative results regarding the nature of the nondipolar spin-spin interaction are obtained. No resonances attributable to pairs were detected in La${\mathrm{Cl}}_{3}$ doped with either ${\mathrm{Ce}}^{3+}$ or ${\mathrm{Er}}^{3+}$.