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

The dance of compliance: performance management in Australian universities

This qualitative study identified the formal and informal performance management (PM) practices in use in Australian public universities for academic staff Levels A, B and C. It asked the following research questions. • What PM practices are currently in use in these universities? • What are the similarities in approach and what issues does PM raise? • How do academic staff who take part in these practices (as either staff or management) experience them? • What cultural and contextual factors (if any) contribute to this experience? • What are the perceived effects of these practices on the performance of individuals, teams and the organisation? • Which system elements do academic staff and academic managers perceive to be most effective in academic cultures and why? The context of substantive change within Australian universities was outlined and literature pertaining to the field of PM in general, and in educational organisations in particular, was explored. The existence, structure, purposes and other factual details of formal PM systems were identified, although the study focused on the opinions, perceptions and attitudes of the respondents. Findings suggested that current PM practice in Australian public universities did little to meet the needs of any of the key stakeholders and remained fundamentally unsatisfying to all concerned. Furthermore, the failure to clearly articulate the purposes and to consider the implementation and ongoing costs of a formal PM system typically resulted in widespread cynicism and a ritual dance of compliance that demonstrated palpably low engagement with systems. Formal PM systems helped to clarify objectives and workload allocation for some staff, but were found to be poorly linked to organisational planning processes, poor at differentiating levels of performance, not valued by academic staff as a vehicle for meaningful feedback, failing to follow through on development outcomes and thus did little to build team, individual or organisational capability. Study recommendations suggested that developmental models of PM were more appropriate and acceptable in academia and that considerable work would be required to incorporate evaluative links such as performance-related pay successfully. More rigorous evaluation, consultation processes regarding user preferences, piloting of PM systems prior to full implementation, and dedicated resources for the PM function and its outcomes (such as staff development), would be required as a part of a comprehensive change management strategy to overcome historical resistance. A thorough capability analysis of the people management skills for Heads of School and above was seen as a priority, given that feedback skill and the management of under-performance were consistently identified as problematic. The costs of under-performance warranted this expenditure. A national evaluation study of PM practice in higher education was recommended to assess the real outcomes, costs and benefits and determine whether continued investment in PM systems was actually merited. Alternative models and approaches such as modular PM systems for the different stages of an academic career, promotion portfolios, reflective practice or peer learning groups were suggested as potentially more successful in enhancing the accountability and performance of academic staff than mandated hierarchical PM

Comprehensive Studies of Magnetic Transitions and Spin–Phonon Couplings in the Tetrahedral Cobalt Complex Co(AsPh3)2I2

A combination of inelastic neutron scattering (INS), far-IR magneto-spectroscopy (FIRMS), and Raman magneto-spectroscopy (RaMS) has been used to comprehensively probe magnetic excitations in Co(AsPh3)2I2 (1), a reported single-molecule magnet (SMM). With applied field, the magnetic zero-field splitting (ZFS) peak (2D′) shifts to higher energies in each spectroscopy. INS placed the ZFS peak at 54 cm-1, as revealed by both variable-temperature (VT) and variable-magnetic-field data, giving results that agree well with those from both far-IR and Raman studies. Both FIRMS and RaMS also reveal the presence of multiple spin-phonon couplings as avoided crossings with neighboring phonons. Here, phonons refer to both intramolecular and lattice vibrations. The results constitute a rare case in which the spin-phonon couplings are observed with both Raman-active (g modes) and far-IR-active phonons (u modes; space group P21/c, no. 14, Z = 4 for 1). These couplings are fit using a simple avoided crossing model with coupling constants of ca. 1-2 cm-1. The combined spectroscopies accurately determine the magnetic excited level and the interaction of the magnetic excitation with phonon modes. Density functional theory (DFT) phonon calculations compare well with INS, allowing for the assignment of the modes and their symmetries. Electronic calculations elucidate the nature of ZFS in the complex. Features of different techniques to determine ZFS and other spin-Hamiltonian parameters in transition-metal complexes are summarized. © 2022 American Chemical Society

Probing Magnetic Excitations in Co^II Single-Molecule Magnets by Inelastic Neutron Scattering

Co(acac)2(H2O)2 (1, acac = acetylacetonate), a transition metal complex (S = 3/2), displays field‐induced slow magnetic relaxation as a single‐molecule magnet. For 1 and its isotopologues Co(acac)2(D2O)2 (1‐d4) and Co(acac‐d7)2(D2O)2 (1‐d18) in approximately D4h symmetry, zero‐field splitting of the ground electronic state leads to two Kramers doublets (KDs): lower energy MS = ±1/2 (ϕ1,2) and higher energy MS = ±3/2 (ϕ3,4) states. This work employs inelastic neutron scattering (INS), a unique method to probe magnetic transitions, to probe different magnetic excitations in 1‐d4 and 1‐d18. Direct‐geometry, time‐of‐flight Disk‐Chopper Spectrometer (DCS), with applied magnetic fields up to 10 T, has been used to study the intra‐KD transition as a result of Zeeman splitting, MS = –1/2 (ϕ1) → MS = +1/2 (ϕ2), in 1‐d18. This is a rare study of the MS = –1/2 → MS = +1/2 excitation in transition metal complexes by INS. Indirect‐geometry INS spectrometer VISION has been used to probe the inter‐KD, ZFS transition, MS = ±1/2 (ϕ1,2) → MS = ±3/2 (ϕ3,4) in both 1‐d4 and 1‐d18, by variable‐temperature (VT) properties of this excitation. The INS spectra measured on VISION also give phonon features of the complexes that are well described by periodic DFT phonon calculations

Magnetic Transitions in Iron Porphyrin Halides by Inelastic Neutron Scattering and Ab Initio Studies of Zero-Field Splittings

Zero-field splitting (ZFS) parameters of nondeuterated metalloporphyrins [Fe­(TPP)­X] (X = F, Br, I; H₂TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are <i>D</i> = 4.49(9) cm^–1 for tetragonal polycrystalline [Fe­(TPP)­F], and <i>D</i> = 8.8(2) cm^–1, <i>E</i> = 0.1(2) cm^–1 and <i>D</i> = 13.4(6) cm^–1, <i>E</i> = 0.3(6) cm^–1 for monoclinic polycrystalline [Fe­(TPP)­Br] and [Fe­(TPP)­I], respectively. Along with our recent report of the ZFS value of <i>D</i> = 6.33(8) cm^–1 for tetragonal polycrystalline [Fe­(TPP)­Cl], these data provide a rare, complete determination of ZFS parameters in a metalloporphyrin halide series. The electronic structure of [Fe­(TPP)­X] (X = F, Cl, Br, I) has been studied by multireference ab initio methods: the complete active space self-consistent field (CASSCF) and the N-electron valence perturbation theory (NEVPT2) with the aim of exploring the origin of the large and positive zero-field splitting <i>D</i> of the ⁶A₁ ground state. <i>D</i> was calculated from wave functions of the electronic multiplets spanned by the d⁵ configuration of Fe­(III) along with spin–orbit coupling accounted for by quasi degenerate perturbation theory. Results reproduce trends of <i>D</i> from inelastic neutron scattering data increasing in the order from F, Cl, Br, to I. A mapping of energy eigenvalues and eigenfunctions of the <i>S</i> = 3/2 excited states on ligand field theory was used to characterize the σ- and π-antibonding effects decreasing from F to I. This is in agreement with similar results deduced from ab initio calculations on CrX₆^3– complexes and also with the spectrochemical series showing a decrease of the ligand field in the same directions. A correlation is found between the increase of <i>D</i> and decrease of the π- and σ-antibonding energies <i>e</i>_λ^X (λ = σ, π) in the series from X = F to I. Analysis of this correlation using second-order perturbation theory expressions in terms of angular overlap parameters rationalizes the experimentally deduced trend. <i>D</i> parameters from CASSCF and NEVPT2 results have been calibrated against those from the INS data, yielding a predictive power of these approaches. Methods to improve the quantitative agreement between ab initio calculated and experimental <i>D</i> and spectroscopic transitions for high-spin Fe­(III) complexes are proposed

Advanced Magnetic Resonance Studies of Tetraphenylporphyrinatoiron(III) Halides

High-Frequency and -Field EPR (HFEPR) studies of Fe(TPP)X (X = F, Cl, Br; I, TPP2−= meso-tetraphenylporphyrinate dianion) and far-IR magnetic spectroscopic (FIRMS) studies of Fe(TPP)Br and Fe(TPP)I have been conducted to probe magnetic intra- and inter-Kramers doublet transitions in these S = 5/2 metalloporphyrin complexes, yielding zero-field splitting (ZFS) and g parameters for the complexes: Fe(TPP)F, D =  +4.67(1) cm−1, E = 0.00(1) cm−1, g⊥ = 1.97(1), g|| = 2.000(5) by HFEPR; Fe(TPP)Cl, D =  +6.458(2) cm−1, E =  +0.015(5) cm−1, E/D = 0.002, g⊥ = 2.004(3), g|| = 2.02(1) by HFEPR; Fe(TPP)Br, D = +9.03(5) cm−1, E =  +0.047(5) cm−1, E/D = 0.005, giso = 1.99(1) by HFEPR and D = +9.05 cm−1, giso = 2.0 by FIRMS; Fe(TPP)I, D =  +13.84 cm−1, E =  +0.07 cm−1, E/D = 0.005, giso = 2.0 by HFEPR and D = +13.95 cm−1, giso = 2.0 by FIRMS (the sign of E was in each case arbitrarily assigned as that of D). These results demonstrate the complementary nature of field- and frequency-domain magnetic resonance experiments in extracting with high accuracy and precision spin Hamiltonian parameters of metal complexes with S > 1/2. The spin Hamiltonian parameters obtained from these experiments have been compared with those obtained from other physical methods such as magnetic susceptibility, magnetic Mössbauer spectroscopy, inelastic neutron scattering (INS), and variable-temperature and -field magnetic circular dichroism (VT-VH MCD) experiments. INS, Mössbauer and MCD give good agreement with the results of HFEPR/FIRMS; the others not as much. The electronic structure of Fe(TPP)X (X = F, Cl, Br, I) was studied earlier by multi-reference ab initio methods to explore the origin of the large and positive D-values, reproducing the trends of D from the experiments. In the current work, a simpler model based on Ligand Field Theory (LFT) is used to explain qualitatively the trend of increasing ZFS from X = F to Cl to Br and to I as the axial ligand. Tetragonally elongated high-spin d5 systems such as Fe(TPP)X exhibit D > 0, but X plays a key role. Spin delocalization onto X means that there is a spin–orbit coupling (SOC) contribution to D from X•, as opposed to none from closed-shell X−. Over the range X = F, Cl, Br, I, X• character increases as does the intrinsic SOC of X• so that D increases correspondingly over this range

Comprehensive Studies of Magnetic Transitions and Spin–Phonon Couplings in the Tetrahedral Cobalt Complex Co(AsPh₃)₂I₂

A combination of inelastic neutron scattering (INS), far-IR magneto-spectroscopy (FIRMS), and Raman magneto-spectroscopy (RaMS) has been used to comprehensively probe magnetic excitations in Co(AsPh3)2I2 (1), a reported single-molecule magnet (SMM). With applied field, the magnetic zero-field splitting (ZFS) peak (2D′) shifts to higher energies in each spectroscopy. INS placed the ZFS peak at 54 cm–1, as revealed by both variable-temperature (VT) and variable-magnetic-field data, giving results that agree well with those from both far-IR and Raman studies. Both FIRMS and RaMS also reveal the presence of multiple spin–phonon couplings as avoided crossings with neighboring phonons. Here, phonons refer to both intramolecular and lattice vibrations. The results constitute a rare case in which the spin–phonon couplings are observed with both Raman-active (g modes) and far-IR-active phonons (u modes; space group P21/c, no. 14, Z = 4 for 1). These couplings are fit using a simple avoided crossing model with coupling constants of ca. 1–2 cm–1. The combined spectroscopies accurately determine the magnetic excited level and the interaction of the magnetic excitation with phonon modes. Density functional theory (DFT) phonon calculations compare well with INS, allowing for the assignment of the modes and their symmetries. Electronic calculations elucidate the nature of ZFS in the complex. Features of different techniques to determine ZFS and other spin-Hamiltonian parameters in transition-metal complexes are summarized