Continuous wave terahertz spectroscopy based on photomixing offers the attractive feature of detecting both amplitude and phase of the terahertz radiation. Experimentally, it is challenging to achieve sufficient accuracy at a high data acquisition rate. We use two fiber stretchers as fast phase modulators in a symmetric setup. Compared to a mechanical delay stage, the fiber stretchers are rather fast (≈1 kHz), which enables us to record a spectrum up to 1.8 THz with a step size of 1 GHz in only 10 min. We achieve a stability of the optical path difference of around 10 μm and use low-doped Si as an example to demonstrate the performance of our spectrometer.

Enzymes with a dimetal-carboxylate cofactor catalyze reactions among the top challenges in chemistry such as methane and dioxygen (O) activation. Recently described proteins bind a manganese-iron cofactor (MnFe) instead of the classical diiron cofactor (FeFe). Determination of atomic-level differences of homo- versus hetero-bimetallic cofactors is crucial to understand their diverse redox reactions. We studied a ligand-binding oxidase from the bacterium Geobacillus kaustophilus (R2lox) loaded with a FeFe or MnFe cofactor, which catalyzes O reduction and an unusual tyrosine-valine ether cross-link formation, as revealed by X-ray crystallography. Advanced X-ray absorption, emission, and vibrational spectroscopy methods and quantum chemical and molecular mechanics calculations provided relative Mn/Fe contents, X-ray photoreduction kinetics, metal-ligand bond lengths, metal-metal distances, metal oxidation states, spin configurations, valence-level degeneracy, molecular orbital composition, nuclear quadrupole splitting energies, and vibrational normal modes for both cofactors. A protonation state with an axial water (HO) ligand at Mn or Fe in binding site 1 and a metal-bridging hydroxo group (μOH) in a hydrogen-bonded network is assigned. Our comprehensive picture of the molecular, electronic, and dynamic properties of the cofactors highlights reorientation of the unique axis along the Mn-OH bond for the Mn1(III) Jahn-Teller ion but along the Fe-μOH bond for the octahedral Fe1(III). This likely corresponds to a more positive redox potential of the Mn(III)Fe(III) cofactor and higher proton affinity of its μOH group. Refined model structures for the Mn(III)Fe(III) and Fe(III)Fe(III) cofactors are presented. Implications of our findings for the site-specific metalation of R2lox and performance of the O reduction and cross-link formation reactions are discussed.

Abstract:The nuclear inelastic scattering signatures of the low-spin centers of the methanosulphonate, tosylate, and perchlorate salts of the spin crossover polymer ([Fe(II)(4-amino-1,2,4-triazole) 3 ] 2+ ) n have been compared for the low-spin phase, for the mixed high-spin and low-spin phases, as well as for Zn(II) diluted samples. Within this series a change in the vibrational pattern in the 320-500 cm´1 region is observed. Significant shifts and decreasing intensity of bands at~320 cm´1 and bands over 400 cm´1 are observed as the molar fraction of the low-spin (LS) centers decrease. Density functional theory calculations using Gaussian09 (B3LYP/CEP-31G) for pentameric, heptameric, and nonameric model molecules yielded the normal modes of several spin isomers: these include the all high-spin (HS) and the all low-spin (LS) configuration but also mixtures of LS and HS centers, with a special focus on those with LS centers in a HS matrix and vice versa. The calculations reproduce the observed spectral changes and show that they are caused by strain extorted on a LS Fe(II) center by its HS neighbors due to the rigid character of the bridging aminotriazole ligand. Additionally, the normal mode analysis of several spin isomers points towards a coupling of the vibrations of the iron centers of the same spin: the metal-ligand stretching modes of the all LS and the all HS spin isomers reveal a collective character: all centers of the same spin are involved in characteristic normal modes. For the isomers containing both LS and HS centers, the vibrational behavior corresponds to two different subsets (sublattices) the vibrational modes of which are not coupled. Finally, the calculation of nuclear inelastic scattering data of spin isomers containing a ca. 1:1 mixture of HS and LS Fe(II) points towards the formation of blocks of the same spin during the spin transition, rather than to alternate structures with a HS-LS-HS-LS-HS motif.

Nuclear inelastic scattering (NIS) experiments have been performed in order to study the vibrational dynamics of the low- and high-spin states of the polynuclear 1D spin crossover compound [Fe(1,2,4-triazole)(1,2,4-triazolato)](BF) (1). Density functional theory (DFT) calculations using the functional B3LYP* and the basis set CEP-31G for heptameric and nonameric models of the compound yielded the normal vibrations and electronic energies for high-spin and low-spin isomers of three models differing in the distribution of anionic trz ligands and BF anions. On the basis of the obtained energies a structural model with a centrosymmetric Fe(trzH)(trz) coordination core of the mononuclear unit of the chain is proposed. The obtained distribution of the BF counteranions in the proposed structure is similar to that obtained on the basis of X-ray powder diffraction studies by Grossjean et al. (Eur. J. Inorg. Chem., 2013, 796). The NIS data of the system diluted to 10% Fe(ii) content in a 90% Zn(ii) matrix (compound (2)) show a characteristic change of the spectral pattern of the low-spin centres, compared to the low-spin phase of the parent Fe(ii) complex (1). DFT calculations reveal that this is caused by a change of the structure of the neighbours of the low-spin centres. The spectral pattern of the high-spin centres in (2) is within a good approximation identical to that of the high-spin Fe(ii) isomer of (1). The inspection of the molecular orbitals of the monomeric model systems of [Fe(trzH)(trz)] and [Fe(trzH)], together with calculations of spin transition energies, point towards the importance of an electrostatic effect caused by the negatively charged ligands. This results in the stabilisation of the low-spin state of the complex containing the anionic ligand and shortening of the Fe-N(trz) compared to the Fe-N(trzH) bond in high-spin, but not in low-spin [Fe(trzH)(trz)].

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