Probing Magnetic and Vibrational Properties of Molecular Compounds by Neutron Scattering

The primary focus of this dissertation is using inelastic neutron scattering (INS) to probe magnetic excitations in paramagnetic complexes including single-molecule magnets (SMMs). Other related studies include the following: (1) Simulating vibrational frequencies to understand spin-phonon coupling (SPC) in a single-molecule magnet; (2) Using quasi-elastic neutron scattering (QENS) to study molecular dynamics of a paramagnet. Zero-field splitting (ZFS) parameters (axial: D and rhombic: E) of metalloporphryins Fe(TPP)X [X = F, Br, I; H₂TPP = tetraphenylporphyrin] have been directly determined by INS. These studies provide a complete determination of ZFS parameters for a metalloporphryin halide series demonstrating that D increases from F to I complexes. Ab initio methods were led to the understanding of the origin of the halide trend. INS has also been used to probe several Co(II) and an Er(III) SMMs. The magnetic excitations were determined by a variety of methods demonstrating that INS is a unique technique to determine the magnitude of these excitations. Most prominently, INS conducted under variable magnetic fields, reveals magnetic excitations in single crystals and powder samples in the energy region above 40 cm^-1. In addition, this work shows a unique strength of INS to show the origin of spin-phonon entangled peaks at 0 T. Vibrational frequencies and simulation of atomic displacements in Co(II) SMMs have been calculated via ab intio methods to study SPC. Raman spectroscopy of Co(acac)₂(H₂O)₂ (acac = acetylacetonate), Co(acac)₂(D₂O)₂ and Co(acac-d₇)₂(D₂O)₂ gives experimental SPC constants of different magnitudes. By probing the displacements in atoms in the SMMs, a correlation between the largest bond angle change in the first coordination sphere and largest SPC constant has been discovered. This work leads to understanding of how the electron spins in the Co(II) complexes interact with phonons in the energy region near the magnetic excitation. QENS has been used to study methyl rotation in Co(acac)₂(D₂O)₂, which behaves as a paramagnet in the temperature range probed (80–100 K). The use of external magnetic fields leads to the observation of field-dependent methyl rotation. This field-dependent behavior sheds light on intermolecular interactions in the solid state.