Andrej Skerencak-Frech

Computing UV/vis spectra using a combined molecular dynamics and quantum chemistry approach: bis-triazin-pyridine (BTP) ligands studied in solution

We report a combined computational and experimental study to investigate the UV/vis spectra of 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine (BTP) ligands in solution. In order to study molecules in solution using theoretical methods, force-field parameters for the ligand-water interaction are adjusted to ab initio quantum chemical calculations. Based on these parameters, molecular dynamics (MD) simulations are carried out from which snapshots are extracted as input to quantum chemical excitation-energy calculations to obtain UV/vis spectra of BTP ligands in solution using time-dependent density functional theory (TDDFT) employing the Tamm-Dancoff approximation (TDA). The range-separated CAM-B3LYP functional is used to avoid large errors for charge-transfer states occurring in the electronic spectra. In order to study environment effects with theoretical methods, the frozen-density embedding scheme is applied. This computational procedure allows to obtain electronic spectra calculated at the (range-separated) DFT level of theory in solution, revealing solvatochromic shifts upon solvation of up to about 0.6 eV. Comparison to experimental data shows a significantly improved agreement compared to vacuum calculations and enables the analysis of relevant excitations for the line shape in solution.

The Complexation of Cm(III) with Oxalate in Aqueous Solution at T = 20–90 °C: A Combined TRLFS and Quantum Chemical Study

The complexation of Cm(III) with oxalate is studied in aqueous solution as a function of the ligand concentration, the ionic strength (NaCl), and the temperature (T = 20–90 °C) by time-resolved laser fluorescence spectroscopy (TRLFS) and quantum chemical calculations. Four complex species ([Cm(Ox)n](3–2n), n = 1, 2, 3, 4) are identified, and their molar fractions are determined by peak deconvolution of the emission spectra. The conditional log K′n(T) values of the first three complexes are calculated and extrapolated to zero ionic strength with the specific ion interaction theory approach. The [Cm(Ox)4](5–) complex forms only at high temperatures. Thus, the log K4(0)(T) value was determined at T > 60 °C. The log K1(0)(25 °C) = 6.86 ± 0.02 decreases by 0.1 logarithmic units in the studied temperature range. The log K2(0)(25 °C) = 4.68 ± 0.09 increases by 0.35, and log K3(0)(25 °C) = 2.11 ± 0.05 increases by 0.37 orders of magnitude. The log Kn(0)(T) (n = 1, 2, 3) values are linearly correlated with the reciprocal temperature. Thus, their temperature dependencies are fitted with the linear Van’t Hoff equation yielding the standard reaction enthalpy (ΔrHm(0)) and standard reaction entropy (ΔrSm(0)) of the stepwise formation of the [Cm(Ox)n](3–2n) species (n = 1, 2, 3). Furthermore, the binary ion–ion interaction coefficients of the four Cm(III) oxalate species with Cl(–)/Na(+) are determined. The binding energies, bond lengths, and bond angles of the different Cm(III) oxalate complexes are calculated in the gas phase as well as in a box containing 1000 H2O molecules by ab inito calculations and molecular dynamics simulations, respectively.

The pH dependence of Am(III) complexation with acetate: an EXAFS study

The complexation of acetate with Am(III) is studied as a function of the pH (1-6) by extended X-ray absorption fine-structure (EXAFS) spectroscopy. The molecular structure of the Am(III)-acetate complexes (coordination numbers, oxygen and carbon distances) is determined from the raw k(3)-weighted Am LIII-edge EXAFS spectra. The results show a continuous shift of Am(III) speciation with increasing pH value towards the complexed species. Furthermore, it is verified that acetate coordinates in a bidentate coordination mode to Am(III) (Am-C distance: 2.82 ± 0.03 Å). The EXAFS data are analyzed by iterative transformation factor analysis to further verify the chemical speciation, which is calculated on the basis of thermodynamic constants, and the used structural model. The experimental results are in very good agreement with the thermodynamic modelling.

2,6-Bis(5,6-diisopropyl-1,2,4-triazin-3-yl)pyridine: a highly selective N-donor ligand studied by TRLFS, liquid–liquid extraction and molecular dynamics

The complexation of Cm(III) and Eu(III) with 2,6-bis(5,6-di-i-propyl-1,2,4-triazin-3-yl)-pyridine (iPr-BTP) is studied in methanol:water (1:1, vol) by time-resolved laser-induced fluorescence spectroscopy (TRLFS). With increasing ligand concentration [Cm(iPr-BTP)3]3+ and [Eu(iPr-BTP)3]3+ are formed, respectively. Stability constants of logβ3′([Cm(iPr-BTP)3]3+) = 16.3 ± 0.3 and logβ3′([Eu(iPr-BTP)3]3+) = 14.9 ± 0.3 are determined. Thermodynamic data of the complexation reactions is obtained in a temperature range of 20–60 °C. The complexation of iPr-BTP with both metal ions is exothermic (Cm(III): ΔrH3′ = −(64.1 ± 3.0) kJ mol−1; Eu(III): ΔrH3′ = −(42.6 ± 2.0) kJ mol−1). The reaction entropy for the formation of [Eu(iPr-BTP)3]3+ is higher compared to the Cm(III) complex (Cm(III): ΔrS3′ = (96.5 ± 6.5) J mol−1 K−1; Eu(III): ΔrS3′ = (136.0 ± 15.2) J mol−1 K−1). Different complexation entropies for the formation of [Cm(iPr-BTP)3] and [Cm(nPr-BTP)3] are explained by molecular dynamics simulations. Results from liquid–liquid extraction tests confirm the ligands peculiar extraction kinetics observed in previous studies and link them to the thermodynamic data.

The Complexation of Cm(III) with Succinate Studied by Time-Resolved Laser Fluorescence Spectroscopy and Quantum Chemical Calculations

The complexation of Cm(III) with succinate in an aqueous NaCl solution was studied as a function of ionic strength, ligand concentration, and temperature using time-resolved laser fluorescence spectroscopy (TRLFS). After the Cm(III) speciation was determined by peak deconvolution, the temperature-dependent thermodynamic stability constants (log Kn0(T)) were determined for the stepwise formation of [CmSucn]3–2n (n = 1–3) in the temperature range 20–80 °C (n = 3 only when T ≥ 50 °C) using the specific ion interaction theory (SIT). The first and second complexation steps show an endothermic behavior, as the respective standard reaction enthalpies (ΔrHm0) and entropies (ΔrSm0) derived from the integrated van’t Hoff equation are positive. These TRLFS results are complemented by quantum chemical calculations to resolve the molecular structure of the formed Cm(III) complexes. The results show that the formation of a seven-membered chelate ring is the favored conformation of [CmSucn]3−2n (n = 1−3).

The complexation and thermodynamics of neptunium(V) with acetate in aqueous solution

The complexation of NpO2+ with acetate is studied in aqueous solution by absorption spectroscopy as a function of the total ligand concentration (NaAc), ionic strength (Im = 0.5–4.0 mol kg−1 Na+(Cl−/ClO4−)) and temperature (T = 20–85 °C). Three distinct Np(V) species (NpO2(Ac)n1−n; n = 0, 1, 2) are identified, and their molar fractions determined by peak deconvolution of the absorption spectra. With increasing temperature the molar fractions of the higher complex species increase. The conditional stability constants log βj′(T) are calculated for each temperature and extrapolated to IUPAC reference state conditions (Im = 0) using the specific ion interaction theory (SIT). The log β0n(T) values increase by approximately 0.4–0.5 logarithmic unit for NpO2(Ac) and by about 0.5–0.6 for NpO2(Ac)2−. Furthermore, the thermodynamic stability constants are linearly correlated with the reciprocal temperature. Thus, fitting the data according to the integrated Van’t Hoff equation yields the standard reaction enthalpy ΔrH0 and entropy ΔrS0 for the complexation reactions. Both complexation reactions are endothermic and driven by the entropy. In addition, the SIT specific binary ion–ion interaction coefficients of the complex species with Na+/ClO4− and Na+/Cl− (eT(i,k)) are determined as a function of temperature.

The influence of polarity in binary solvent mixtures on the conformation of bis-triazinyl-pyridine in solution

ABSTRACT We report a combined computational and experimental study to investigate the influence of the solvent on the electronic and molecular structure of the bis-triazinyl-pyridine (BTP) ligand. Experimental measurements and quantum-chemical calculations using geometries from molecular dynamics simulations in different solvent methanol/water mixtures reveal a change in the UV/vis absorption spectra for the investigated BTP compound. This change is investigated further using nuclear magnetic resonance (NMR) techniques, both experimental and computational, to gain insight to ligand conformation. Comparison of experimental and computational results enables the analysis of relevant BTP conformers, which cannot be accessed using experimental measurements alone. Based on this approach, we conclude that the BTP ligands change conformation with decreasing solvent polarity to maximise the lipophilic accessible surface – a concept which is transferrable to various classes of compounds.

Combined EXAFS Spectroscopic and Quantum Chemical Study on the Complex Formation of Am(III) with Formate

The complexation of Am(III) with formate in aqueous solution is studied as a function of the pH value using a combination of extended X-ray absorption fine structure (EXAFS) spectroscopy, iterative transformation factor analysis (ITFA), and quantum chemical calculations. The Am LIII-edge EXAFS spectra are analyzed to determine the molecular structure (coordination numbers; Am-O and Am-C distances) of the formed Am(III)-formate species and to track the shift of the Am(III) speciation with increasing pH. The experimental data are compared to predictions from density functional calculations. The results indicate that formate binds to Am(III) in a monodentate fashion, in agreement with crystal structures of lanthanide formates. Furthermore, the investigations are complemented by thermodynamic speciation calculations to verify further the results obtained.