Theoretical interpretation of hyperfine interactions was pioneered in the 1950s-1960s by the seminal works of McConnell, Karplus, and others for organic radicals and by Watson and Freeman for transition-metal (TM) complexes. In this work, we investigate a series of octahedral Ru(III) complexes with aromatic ligands to understand the mechanism of transmission of the spin density from the d-orbital of the metal to the s-orbitals of the ligand atoms. Spin densities and spin populations underlying ligand hyperfine couplings are analyzed in terms of π-conjugative or σ-hyperconjugative delocalization vs spin polarization based on symmetry considerations and restricted open-shell vs unrestricted wave function analysis. The transmission of spin density is shown to be most efficient in the case of symmetry-allowed π-conjugative delocalization, but when the π-conjugation is partially or fully symmetry-forbidden, it can be surpassed by σ-hyperconjugative delocalization. Despite a lower spin population of the ligand in σ-hyperconjugative transmission, the hyperfine couplings can be larger because of the direct involvement of the ligand s-orbitals in this delocalization pathway. We demonstrate a quantitative correlation between the hyperfine couplings of aromatic ligand atoms and the characteristics of the metal-ligand bond modulated by the trans substituent, a hyperfine trans effect.

• Publication type
• Journal Article MeSH

In recent decades, transition-metal coordination compounds have been extensively studied for their antitumor and antimetastatic activities. In this work, we synthesized a set of symmetric and asymmetric Ru(III) and Rh(III) coordination compounds of the general structure (Na+/K+/PPh4+/LH+) [trans-MIIIL(eq)nL(ax)2]- (M = RuIII or RhIII; L(eq) = Cl, n = 4; L(eq) = ox, n = 2; L(ax) = 4-R-pyridine, R = CH3, H, C6H5, COOH, CF3, CN; L(ax) = DMSO-S) and systematically investigated their structure, stability, and NMR properties. 1H and 13C NMR spectra measured at various temperatures were used to break down the total NMR shifts into the orbital (temperature-independent) and hyperfine (temperature-dependent) contributions. The hyperfine NMR shifts for paramagnetic Ru(III) compounds were analyzed in detail using relativistic density functional theory (DFT). The effects of (i) the 4-R substituent of pyridine, (ii) the axial trans ligand L(ax), and (iii) the equatorial ligands L(eq) on the distribution of spin density reflected in the "through-bond" (contact) and the "through-space" (pseudocontact) contributions to the hyperfine NMR shifts of the individual atoms of the pyridine ligands are rationalized. Further, we demonstrate the large effects of the solvent on the hyperfine NMR shifts and discuss our observations in the general context of the paramagnetic NMR spectroscopy of transition-metal complexes.

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• Journal Article MeSH

X-ray absorption spectroscopy (XAS) has gained popularity in recent years as it probes matter with high spatial and elemental sensitivities. However, the theoretical modeling of XAS is a challenging task since XAS spectra feature a fine structure due to scalar (SC) and spin-orbit (SO) relativistic effects, in particular near L and M absorption edges. While full four-component (4c) calculations of XAS are nowadays feasible, there is still interest in developing approximate relativistic methods that enable XAS calculations at the two-component (2c) level while maintaining the accuracy of the parent 4c approach. In this article we present theoretical and numerical insights into two simple yet accurate 2c approaches based on an (extended) atomic mean-field exact two-component Hamiltonian framework, (e)amfX2C, for the calculation of XAS using linear eigenvalue and damped response time-dependent density functional theory (TDDFT). In contrast to the commonly used one-electron X2C (1eX2C) Hamiltonian, both amfX2C and eamfX2C account for the SC and SO two-electron and exchange-correlation picture-change (PC) effects that arise from the X2C transformation. As we demonstrate on L- and M-edge XAS spectra of transition metal and actinide compounds, the absence of PC corrections in the 1eX2C approximation results in a substantial overestimation of SO splittings, whereas (e)amfX2C Hamiltonians reproduce all essential spectral features such as shape, position, and SO splitting of the 4c references in excellent agreement, while offering significant computational savings. Therefore, the (e)amfX2C PC correction models presented here constitute reliable relativistic 2c quantum-chemical approaches for modeling XAS.

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• Journal Article MeSH

Gold(II) complexes are rare, and their application to the catalysis of chemical transformations is underexplored. The reason is their easy oxidation or reduction to more stable gold(III) or gold(I) complexes, respectively. We explored the thermodynamics of the formation of [AuII (L)(X)]+ complexes (L=ligand, X=halogen) from the corresponding gold(III) precursors and investigated their stability and spectral properties in the IR and visible range in the gas phase. The results show that the best ancillary ligands L for stabilizing gaseous [AuII (L)(X)]+ complexes are bidentate and tridentate ligands with nitrogen donor atoms. The electronic structure and spectral properties of the investigated gold(II) complexes were correlated with quantum chemical calculations. The results show that the molecular and electronic structure of the gold(II) complexes as well as their spectroscopic properties are very similar to those of analogous stable copper(II) complexes.

• Keywords
• density functional calculations, electronic spectroscopy, gold, mass spectrometry, vibrational spectroscopy,
• MeSH
• Nitrogen MeSH
• Halogens MeSH
• Cations MeSH
• Crystallography, X-Ray MeSH
• Ligands MeSH
• Copper * chemistry   MeSH
• Models, Theoretical MeSH
• Gold * chemistry   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Nitrogen MeSH
• Halogens MeSH
• Cations MeSH
• Ligands MeSH
• Copper * MeSH
• Gold * MeSH

The simulation of X-ray absorption spectra requires both scalar and spin-orbit (SO) relativistic effects to be taken into account, particularly near L- and M-edges where the SO splitting of core p and d orbitals dominates. Four-component Dirac-Coulomb Hamiltonian-based linear damped response time-dependent density functional theory (4c-DR-TDDFT) calculates spectra directly for a selected frequency region while including the relativistic effects variationally, making the method well suited for X-ray applications. In this work, we show that accurate X-ray absorption spectra near L2,3- and M4,5-edges of closed-shell transition metal and actinide compounds with different central atoms, ligands, and oxidation states can be obtained by means of 4c-DR-TDDFT. While the main absorption lines do not change noticeably with the basis set and geometry, the exchange-correlation functional has a strong influence with hybrid functionals performing the best. The energy shift compared to the experiment is shown to depend linearly on the amount of Hartee-Fock exchange with the optimal value being 60% for spectral regions above 1000 eV, providing relative errors below 0.2% and 2% for edge energies and SO splittings, respectively. Finally, the methodology calibrated in this work is used to reproduce the experimental L2,3-edge X-ray absorption spectra of [RuCl2(DMSO)2(Im)2] and [WCl4(PMePh2)2], and resolve the broad bands into separated lines, allowing an interpretation based on ligand field theory and double point groups. These results support 4c-DR-TDDFT as a reliable method for calculating and analyzing X-ray absorption spectra of chemically interesting systems, advance the accuracy of state-of-the art relativistic DFT approaches, and provide a reference for benchmarking more approximate techniques.

• Publication type
• Journal Article MeSH

Nuclear magnetic resonance (NMR) spectroscopy of paramagnetic molecules provides detailed information about their molecular and electron-spin structure. The paramagnetic NMR spectrum is a very rich source of information about the hyperfine interaction between the atomic nuclei and the unpaired electron density. The Fermi-contact contribution to ligand hyperfine NMR shifts is particularly informative about the nature of the metal-ligand bonding and the structural arrangements of the ligands coordinated to the metal center. In this account, we provide a detailed experimental and theoretical NMR study of compounds of Cr(III) and Cu(II) coordinated with substituted acetylacetonate (acac) ligands in the solid state. For the first time, we report the experimental observation of extremely paramagnetically deshielded 13C NMR resonances for these compounds in the range of 900-1200 ppm. We demonstrate an excellent agreement between the experimental NMR shifts and those calculated using relativistic density-functional theory. Crystal packing is shown to significantly influence the NMR shifts in the solid state, as demonstrated by theoretical calculations of various supramolecular clusters. The resonances are assigned to individual atoms in octahedral Cr(acac)3 and square-planar Cu(acac)2 compounds and interpreted by different electron configurations and magnetizations at the central metal atoms resulting in different spin delocalizations and polarizations of the ligand atoms. Further, effects of substituents on the 13C NMR resonance of the ipso carbon atom reaching almost 700 ppm for Cr(acac)3 compounds are interpreted based on the analysis of Fermi-contact hyperfine contributions.

• Publication type
• Journal Article MeSH

This work aims to find the most suitable method that is practically applicable for the calculation of 31P NMR chemical shifts of Pt(II) complexes. The influence of various all-electron and ECP basis sets, DFT functionals, and solvent effects on the optimized geometry was tested. A variety of combinations of DFT functionals BP86, B3LYP, PBE0, TPSSh, CAM-B3LYP, and ωB97XD with all-electron basis sets 6-31G, 6-31G(d), 6-31G(d,p), 6-311G(d,p), and TZVP and ECP basis sets SDD, LanL2DZ, and CEP-31G were used. Chemical shielding constants were then calculated using BP86, PBE0, and B3LYP functionals in combination with the TZ2P basis. The magnitude of spin-orbit interactions was also evaluated.

• Keywords
• 31P NMR, DFT calculations, Pt complexes, Spin-orbit interactions,
• Publication type
• Journal Article MeSH

Despite substantial evidence of short Au⋅⋅⋅H-X contacts derived from a number of X-ray structures of AuI compounds, the nature of AuI ⋅⋅⋅H bonding in these systems has not been clearly understood. Herein, we present the first spectroscopic evidence for an intramolecular AuI ⋅⋅⋅H+ -N hydrogen bond in a [Cl-Au-L]+ complex, where L is a protonated N-heterocyclic carbene. The complex was isolated in the gas phase and characterized with helium-tagging infrared photodissociation (IRPD) spectra, in which H+ -N-mode-derived bands evidence the intramolecular AuI ⋅⋅⋅H+ -N bond. Quantum chemical calculations reproduce the experimental IRPD spectra and allow to characterize the intramolecular Au⋅⋅⋅H+ -N bonding with a short rAu⋅⋅⋅H distance of 2.17 Å and an interaction energy of approximately -10 kcal mol-1 . Various theoretical descriptors of chemical bonding calculated for the Au⋅⋅⋅H+ -N interaction provide strong evidence for a hydrogen bond of moderate strength.

• Keywords
• anharmonic spectra, gold(I) carbenes, hydrogen bonding to gold, infrared photodissociation spectroscopy, quantum chemical calculations,
• Publication type
• Journal Article MeSH