Effect of magnetic fields on the methyl rotation in a paramagnetic cobalt(ii) complex. Quasielastic neutron scattering studies

Abstract

Molecular dynamics is a fundamental property of metal complexes. These dynamic processes, especially for paramagnetic complexes under external magnetic fields, are in general not well understood. Quasielastic neutron scattering (QENS) in 0-4 T magnetic fields has been used to study the dynamics of Co(acac)2(D2O)2 (1-d4, acac = acetylacetonate). At 80-100 K, rotation of the methyl groups on the acac ligands is the dominant dynamical process. This rotation is slowed down by the magnetic field increase. Rotation times at 80 K are 5.6(3) × 10-10 s at 0 T and 2.04(10) × 10-9 s at 4 T. The QENS studies suggest that methyl groups in these paramagnetic Co(ii) molecules do not behave as isolated units, which is consistent with results from earlier magnetic susceptibility studies indicating the presence of intermolecular interactions. DFT calculations show that unpaired electron spin density in 1 is dispersed to the atoms of both acac and H2O ligands. Methyl torsions in 1-d4 have also been observed at 5-100 K in inelastic neutron spectroscopy (INS). The QENS and INS results here help understand the dynamics of the compound in the solid state.

Assessing Diffusion Relaxation of Interlayer Water in Clay Minerals Using a Minimalist Three-Parameter Model

“…The reduced χ 2 for S(Q, ω) varies between 0.8 and 4.81, is reported in the Supporting Information, and is in good agreement with values reported for QENS data analysis using the analytical approach. 34,35 Additionally, it is well accepted that the maximum likelihood estimation based on the Poisson statistic is an excellent evaluation method to estimate parameters when analyzing counting experiments, particularly for small total numbers of counts. 36 Thus, we can infer that the proposed set of parameters is reliable.…”

Applying Unconventional Spectroscopies to the Single‐Molecule Magnets, Co(PPh3)2X2(X=Cl, Br, I): Unveiling Magnetic Transitions and Spin‐Phonon Coupling

“…It is interesting to note that the slight decrease of spin density on the Co(II) ion from Co‐Cl (2.585), Co‐Br (2.570), to Co‐I (2.553) parallels the drop of D ′ from 14.89(2), to 13.83(2), and to 13.63(10) cm −1 (HFEPR results) for the three complexes, as shown in Figure S29 in Supporting Information. For Co(acac) 2 (H 2 O) 2 , also a SMM, [1r,13b, c] but with only light atom (Period 2) donor ligands, the calculated spin density of 2.81 (by the same VASP program) on its Co(II) ion and D ′ of 57 cm −1 are both larger than those of Co‐X . At least among these four Co(II) complexes, the larger spin density on a metal ion, the larger ZFS in the complex.…”

“…The DFT calculations using VASP also give spin densities, ρ s , in Co‐X , as summarized in Table 3 with detailed lists for each atom in Tables S13, S14 in Supporting Information. Although the VASP calculations here are not of high‐level, they provide a quantitative scale, revealing how the spin is dispersed onto X, P, C and peripheral H atoms in the molecules [1r] . VASP partitions electrons according to the Wigner‐Seitz radius a e in Eq.…”

“…Calculations using the VASP and phonopy programs [26] give phonon energies, their symmetries and calculated INS spectra to compare with the experimental INS spectra [13b,d,17d] . In addition, the calculations yield spin densities on all atoms of a metal complex [1r] …”

Neutron Instruments for Research in Coordination Chemistry

“…High‐resolution spectrometers, such as Backscattering Spectrometer (BASIS) at ORNL and High Flux Backscattering Spectrometer (HFBS) at NIST, are typically required for QENS, but any direct‐geometry TOF instrument such as CNCS and DCS can also be used. Recently QENS spectra of Co(acac) 2 (D 2 O) 2 , another SMM, at BASIS with a 4‐T magnet reveals that the rotation of the methyl groups on the acac ligands is slowed down by the magnetic field increase …”