Field Magnetic Circular Dichroism Research Articles

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 Mossbauer spectroscopy, inelastic neutron scattering (INS), and variable-temperature and -field magnetic circular dichroism (VT-VH MCD) experiments. INS, Mossbauer 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.

Structure and low field magnetic properties in phosphate-tellurite glasses

Phosphate-tellurite glasses exhibit magnetic properties, due to the presence of the small metallic Te colloids which were revealed in low field magnetic circular dichroism. These metallic colloids induce the red coloring of these glasses together with the absorbance in the visible region. The temperature dependence of the absorption spectrum and the A-term in magnetic circular dichroism are specific for Te metallic nanoparticles, which results during the melting procedure over 10000C due to the conversion of Te4+ in Te0 atoms. The X-ray photoelectron spectroscopy supports this fact due to the presence of small peaks as satellites in the region of Te 3d core-level spectrum. Quantification of these satellites compared with Te 3d3/2 and 3d5/2 peaks suggests a 14% concentration of Te metallic nanoparticles in these phosphate-tellurite glasses. The presence of metallic particles induces the crystallization effects of Te micrograins upon thermal treatments at higher temperatures.

Orbital and spin moments in the ferromagnetic superconductor URhGe by x-ray magnetic circular dichroism

The ferromagnetic superconductor URhGe has been investigated by high field magnetic circular dichroism (XMCD) at the U ${M}_{4,5}$, Rh ${L}_{2,3}$, and Ge $K$ edges at 2.1 K and at applied fields up to 17 T. The XMCD performed at the ${M}_{4,5}$ absorption edges allows us to determine the spectroscopic branching ratio and the $5f$ electron contribution to the valence spin-orbit interaction. Combination with polarized neutron diffraction results allows us to derive the individual $\mathrm{U}$ orbital and spin moments and the magnetic-dipole contribution $\ensuremath{\langle}{T}_{z}\ensuremath{\rangle}$. There is no evidence for any change of the orbital-to-spin moment ratios across the spin reorientation transition at ${H}_{R}=12\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, when the field is applied along the initial hard $b$ axis. We also confirm that the magnetism of URhGe is dominated by $\mathrm{U}$, with the contribution of Rh representing only about $10%$ of the macroscopic moment. The orbital and spin moments at the Rh site are found to be parallel to each other and parallel to the macroscopic magnetization, but an unexpectedly large orbital-to-spin moment ratio is observed. The XMCD at the Ge $K$ edge reveals the presence of a small induced Ge $4p$ orbital moment, parallel to the macroscopic magnetization. The results are discussed against predictions of the electronic band structure calculations by the density functional theory plus Coulomb $U$, including spin-orbit coupling $(\mathrm{DFT}+U+\mathrm{SOC})$.

Effects of Fluorination on Iridium(III) Complex Phosphorescence: Magnetic Circular Dichroism and Relativistic Time-Dependent Density Functional Theory

We use a combination of low temperature, high field magnetic circular dichroism, absorption, and emission spectroscopy with relativistic time-dependent density functional calculations to reveal a subtle interplay between the effects of chemical substitution and spin-orbit coupling (SOC) in a family of iridium(III) complexes. Fluorination at the ortho and para positions of the phenyl group of fac-tris(1-methyl-5-phenyl-3-n-propyl-[1,2,4]triazolyl)iridium(III) cause changes that are independent of whether the other position is fluorinated or protonated. This is demonstrated by a simple linear relationship found for a range of measured and calculated properties of these complexes. Further, we show that the phosphorescent radiative rate, k(r), is determined by the degree to which SOC is able to hybridize T(1) to S(3) and that k(r) is proportional to the inverse fourth power of the energy gap between these excitations. We show that fluorination in the para position leads to a much larger increase of the energy gap than fluorination at the ortho position. Theory is used to trace this back to the fact that fluorination at the para position increases the difference in electron density between the phenyl and triazolyl groups, which distorts the complex further from octahedral symmetry, and increases the energy separation between the highest occupied molecular orbital (HOMO) and the HOMO-1. This provides a new design criterion for phosphorescent iridium(III) complexes for organic optoelectronic applications. In contrast, the nonradiative rate is greatly enhanced by fluorination at the ortho position. This may be connected to a significant redistribution of spectral weight. We also show that the lowest energy excitation, 1A, has almost no oscillator strength; therefore, the second lowest excitation, 2E, is the dominant emissive state at room temperature. Nevertheless the mirror image rule between absorption and emission is obeyed, as 2E is responsible for both absorption and emission at all but very low (<10 K) temperatures.

Hybrid Genetic Algorithm with an Adaptive Penalty Function for Fitting Multimodal Experimental Data: Application to Exchange-Coupled Non-Kramers Binuclear Iron Active Sites

A Genetic Algorithm (GA) is a stochastic optimization technique based on the mechanisms of biological evolution. These algorithms have been successfully applied in many fields to solve a variety of complex nonlinear problems. While they have been used with some success in chemical problems such as fitting spectroscopic and kinetic data, many have avoided their use due to the unconstrained nature of the fitting process. In engineering, this problem is now being addressed through incorporation of adaptive penalty functions, but their transfer to other fields has been slow. This study updates the Nanakorrn Adaptive Penalty function theory, expanding its validity beyond maximization problems to minimization as well. The expanded theory, using a hybrid genetic algorithm with an adaptive penalty function, was applied to analyze variable temperature variable field magnetic circular dichroism (VTVH MCD) spectroscopic data collected on exchange coupled Fe(II)Fe(II) enzyme active sites. The data obtained are described by a complex nonlinear multimodal solution space with at least 6 to 13 interdependent variables and are costly to search efficiently. The use of the hybrid GA is shown to improve the probability of detecting the global optimum. It also provides large gains in computational and user efficiency. This method allows a full search of a multimodal solution space, greatly improving the quality and confidence in the final solution obtained, and can be applied to other complex systems such as fitting of other spectroscopic or kinetics data.

Theoretical Analysis of the Spin Hamiltonian Parameters in Co(II)S4 Complexes, Using Density Functional Theory and Correlated ab initio Methods

A systematic Density Functional Theory (DFT) and multiconfigurational ab initio computational analysis of the Spin Hamiltonian (SH) parameters of tetracoordinate S = 3/2 Co((II))S(4)-containing complexes has been performed. The complexes under study bear either arylthiolato, ArS(-), or dithioimidodiphosphinato, [R(2)P(S)NP(S)R'(2)](-) ligands. These complexes were chosen because accurate structural and spectroscopic data are available, including extensive Electron Paramagnetic Resonance (EPR)/Electron Nuclear Double Resonance (ENDOR) studies. For comparison purposes, the [Co(PPh(3))(2)Cl(2)] complex, which was thoroughly studied in the past by High-Field and Frequency EPR and Variable Temperature, Variable Field Magnetic Circular Dichroism (MCD) spectroscopies, was included in the studied set. The magnitude of the computed axial zero-field splitting parameter D (ZFS), of the Co((II))S(4) systems, was found to be within ~10% of the experimental values, provided that the property calculation is taken beyond the accuracy obtained with a second-order treatment of the spin-orbit coupling interaction. This is achieved by quasi degenerate perturbation theory (QDPT), in conjunction with complete active space configuration interaction (CAS-CI). The accuracy was increased upon recovering dynamic correlation with multiconfigurational ab initio methods. Specifically, spectroscopy oriented configuration interaction (SORCI), and difference dedicated configuration interaction (DDCI) were employed for the calculation of the D-tensor. The sign and magnitude of parameter D was analyzed in the framework of Ligand Field Theory, to reveal the differences in the electronic structures of the investigated Co((II))S(4) systems. For the axial complexes, accurate effective g'-tensors were obtained in the QDPT studies. These provide a diagnostic tool for the adopted ground state configuration (±3/2 or ±1/2) and are hence indicative of the sign of D. On the other hand, for the rhombic complexes, the determination of the sign of D required the SH parameters to be derived along suitably constructed symmetry interconversion pathways. This procedure, which introduces a dynamic perspective into the theoretical investigation, helped to shed some light on unresolved issues of the corresponding experimental studies. The metal hyperfine and ligand super-hyperfine A-tensors of the C(2) [Co{(SPPh(2))(SP(i)Pr(2))N}(2)] complex were estimated by DFT calculations. The theoretical data were shown to be in good agreement with the available experimental data. Decomposition of the metal A-tensor into individual contributions revealed that, despite the large ZFS, the observed significant anisotropy should be largely attributed to spin-dipolar contributions. The analysis of both, metal and ligand A-tensors, is consistent with a highly covalent character of the Co-S bonds.

Relativistic effects in a phosphorescent Ir(III) complex

We compare high field magnetic circular dichroism, absorption, and photoluminescence spectra for Ir(ptz)(3) with time-dependent density functional theory. By gradually turning on the relativistic effects we identify several distinct relativistic effects in the spectra of this complex. We show that relativistic effects must be included to accurately predict the low-temperature spectra. This leads to new insights into the low-lying excitations responsible for the observed phosphorescence, and suggests new avenues to improve the performance of organic light emitting diodes.

Electronic Structure Analysis of the Dinuclear Metal Center in the Bioremediator Glycerophosphodiesterase (GpdQ) from Enterobacter aerogenes

The glycerophosphodiesterase (GpdQ) from Enterobacter aerogenes is a promiscuous, dinuclear metallohydrolase that has potential application in the remediation of organophosphate nerve agents and pesticides. GpdQ employs an unusual reaction mechanism in which the enzyme is predominantly mononuclear in the resting state, and substrate binding induces the formation of the catalytically competent dinuclear center (Hadler et al. J. Am. Chem. Soc. 2008, 130, 14129). Reactivity is further modulated by the coordination flexibility of Asn80, a ligand that binds to the second, loosely bound metal ion (Hadler et al. J. Am. Chem. Soc. 2009, 131, 11900). It is proposed that hydrolysis is initiated by a terminal, metal-bound hydroxide molecule which is activated at unusually low pH by electrostatic/hydrogen bonding interactions with a bridging hydroxide species. In this study, electronic structure analysis of the dinuclear center is employed to study the coordination environment of the dinuclear center at the resting and product-bound stage of catalysis. This is achieved through the use of variable temperature, variable field magnetic circular dichroism experiments involving the Co(II)-substituted wild type enzyme and its Asn80Asp variant. The data support the above model for the catalytic mechanism whereby the metal ion-bridging hydroxide molecule activates a terminally bound hydroxide nucleophile. Replacement of Asn80 by an aspartate residue does prevent coordination flexibility but also leads to cleavage of the mu-hydroxide bridge and reduced reactivity. This is the first study to investigate the electronic structure of an enzyme with a mu-1,1-carboxylate bridged dicobalt(II) center.

The Heme of Cystathionine β-synthase Likely Undergoes a Thermally Induced Redox-Mediated Ligand Switch

Cystathionine beta-synthase (CBS) is a pyridoxal-5'-dependent enzyme that catalyzes the condensation of homocysteine and serine to form cystathionine. Human CBS is unique in that heme is also required for maximal activity, although the function of heme in this enzyme is presently unclear. The study presented herein reveals that the heme of human CBS undergoes a coordination change upon reduction at elevated temperatures. We have termed this new species "CBS424" and demonstrate that its formation is likely irreversible when pH 9 Fe(III) CBS is reduced at moderately elevated temperatures (approximately 40 degrees C and higher) or when pH 9 Fe(II) CBS is heated to similar temperatures. Spectroscopic techniques, including resonance Raman, electronic absorption, and variable temperature/variable field magnetic circular dichroism spectroscopy, provide strong evidence that CBS424 is coordinated by two neutral donor ligands. It appears likely that the native cysteine(thiolate) heme ligand is displaced by an endogenous neutral donor upon conversion to CBS424. This behavior is consistent with other six-coordinate, cysteine(thiolate)-ligated heme centers, which seek to avoid this coordination structure in the Fe(II) state. Functional assays show that CBS424 is inactive and suggest that the ligand switch is responsible for eliminating enzyme activity. When this investigation is taken together with other functional studies of CBS, it provides strong evidence that coordination of Cys52 to the heme iron is crucial for full activity in this enzyme. We hypothesize that cysteine displacement may serve as a mechanism for CBS inactivation and that second-sphere interactions of the Cys52 thiolate with surrounding residues are responsible for communicating the heme ligand displacement to the CBS active site.

Optical detection of spin polarization in single-molecule magnets [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)

A magneto-optical study has been undertaken of the mixed-valence single-molecule magnet [Mn(IV)(4)Mn(III)(8)O(12)L(16)] in which the ligands, L, are acetate (Mn(12)Ac) or the long-chain carboxylic acid, C(14)H(29)COOH (Mn(12)C(15)), that confers better solubility in organic solvents. Thin polymer films of these compounds in poly(methyl methacrylate) (PMM) have been cast by solvent evaporation to provide samples suitable for variable-temperature and field magnetic circular dichroism (MCD) studies. The absorption spectra in isotropic light are featureless, whereas the low-temperature MCD spectra contain resolved peaks, both positive and negative. MCD magnetization curves measured at temperatures above 4.2 K have established a ground-state spin of S = 10 and an axial zero-field parameter, D, of -0.61 K, similar to that determined for single crystals of Mn(12)Ac. By studying at a variety of optical wavelengths, the polarization ratios of the optical transitions relative to the unique axis of the zero-field distortion have been determined. The MCD magnetization curves measured at 4.2 K between 0 and 5 T for the case of Mn(12)C(15) in the PMM film can be fitted only on the assumption of nonrandom distribution of molecular z-axes arising from stresses in the polymer film during the process of casting. MCD-detected hysteresis curves measured in both frozen solution and PMM films, below the blocking temperature of approximately 3 K, show a high retention of spin polarization after reduction to zero of a polarizing magnetic field. This generates intense zero-field circular dichroism (CD) with maximum intensity for xy-polarized optical transitions whose sign depends on the direction of the original polarizing field. The optical polarization and the selection rules for MCD select a subset of molecular orientations with respect to the direction of field. Thus, the magnetically induced CD provides a highly sensitive and rapid optical method of reading the spin polarization of molecular magnets.

Invited award contribution for ACS Award in Inorganic Chemistry. Geometric and electronic structure contributions to function in bioinorganic chemistry: active sites in non-heme iron enzymes.

Spectroscopy has played a major role in the definition of structure/function correlations in bioinorganic chemistry. The importance of spectroscopy combined with electronic structure calculations is clearly demonstrated by the non-heme iron enzymes. Many members of this large class of enzymes activate dioxygen using a ferrous active site that has generally been difficult to study with most spectroscopic methods. A new spectroscopic methodology has been developed utilizing variable temperature, variable field magnetic circular dichroism, which enables one to obtain detailed insight into the geometric and electronic structure of the non-heme ferrous active site and probe its reaction mechanism on a molecular level. This spectroscopic methodology is presented and applied to a number of key mononuclear non-heme iron enzymes leading to a general mechanistic strategy for O2 activation. These studies are then extended to consider the new features present in the binuclear non-heme iron enzymes and applied to understand (1) the mechanism of the two electron/coupled proton transfer to dioxygen binding to a single iron center in hemerythrin and (2) structure/function correlations over the oxygen-activating enzymes stearoyl-ACP Delta9-desaturase, ribonucleotide reductase, and methane monooxygenase. Electronic structure/reactivity correlations for O2 activation by non-heme relative to heme iron enzymes will also be developed.