Local spin density in the Cr₇Ni antiferromagnetic molecular ring and⁵³Cr-NMR

Abstract

We present (53)Cr-NMR spectra collected at low temperature in a single crystal of the heterometallic antiferromagnetic (AF) ring Cr(7)Ni in the S = 1/2 ground state with the aim of establishing the distribution of the local electronic moment in the ring. Due to the poor S/N we observed only one signal which is ascribed to three almost equivalent (53)Cr nuclei in the ring. The calculated spin density in Cr(7)Ni in the ground state, with the applied magnetic field both parallel and perpendicular to the plane of the ring, turns out to be AF staggered with the greatest component of the local spin <s> for the Cr(3+) ions next to the Ni(2+) ion. The (53)Cr-NMR frequency was found to be in good agreement with the local spin density calculated theoretically by assuming a core polarization field of H(cp) = - 11 T/μ(B) for both orientations, close to the value found previously in Cr(7)Cd. The observed orientation dependence of the local spin moments is well reproduced by the theoretical calculation and evidences the importance of single-ion and dipolar anisotropies.

Magnetic properties and hyperfine interactions in Cr8, Cr7Cd, and Cr7Ni molecular rings from 19F-NMR

“…This is justified by the fact that the previous comparison of the calculated spins density and the one determined directly by 53 Cr-NMR 19,20 and neutron scattering 21 experiments in the same rings lead to an excellent agreement. The low-temperature properties of Cr 7 M AFM wheels can be described by the following spin Hamiltonian:…”

Measuring Spin⋅⋅⋅Spin Interactions between Heterospins in a Hybrid [2]Rotaxane

“…Fourier transform gives the frequency–space spectrum which contains two peaks; the separation of these two peaks is a direct measure of 2  D , D being the magnetic interaction which is around 8 MHz (Figure 4 b). Fitting the data with DeerAnalysis17 using a point dipole model based on two localized S =$1/2$ species is wholly inadequate due to the spin density distribution in the ground state of the {Cr 7 Ni} ring 18, 19…”

“…The spin projection coefficients for the S =$1/2$ ground state of {Cr 7 Ni} were obtained using the “Irreducible Tensor Operator” (ITO) technique with the PHI21 program. This includes the full microscopic Hamiltonian,15 and reproduces the spin distribution measured by NMR spectroscopy 19. Using the structure and including random rotations of the TEMPO and ring about the thread (Figure 4 c), we directly fit the RIDME form factor by introducing the standard deviation for a Gaussian distribution of the ring–nitroxide distance, σ .…”

Measuring Spin⋅⋅⋅Spin Interactions between Heterospins in a Hybrid [2]Rotaxane

“…Fitting the data with DeerAnalysis [17] using ap oint dipole model based on two localized S = 1 = 2 species is wholly inadequate due to the spin density distribution in the ground state of the {Cr 7 Ni}r ing. [18,19] To go beyond this limitation we have simulated the data using an in-house code SSD ("Spatial Spin Density"), which calculates the form factor directly using aspatially distributed dipolar model [20] considering the perturbation from the flipping spin of the ring during the time interval T.T he spin projection coefficients for the S = 1 = 2 ground state of {Cr 7 Ni} were obtained using the "Irreducible Tensor Operator" (ITO) technique with the PHI [21] program. This includes the full microscopic Hamiltonian, [15] and reproduces the spin distribution measured by NMR spectroscopy.…”

“…This includes the full microscopic Hamiltonian, [15] and reproduces the spin distribution measured by NMR spectroscopy. [19] Using the structure and including random rotations of the TEMPO and ring about the thread (Figure 4c), we directly fit the RIDME form factor by introducing the standard deviation for aG aussian distribution of the ring-nitroxide distance, s.…”