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  1. Free, publicly-accessible full text available September 1, 2023
  2. In this work, we investigated the catalytic effects of a Sharpless dimeric titanium (IV)–tartrate–diester catalyst on the epoxidation of allylalcohol with methyl–hydroperoxide considering four different orientations of the reacting species coordinated at the titanium atom (reactions R1–R4) as well as a model for the non-catalyzed reaction (reaction R0). As major analysis tools, we applied the URVA (Unified Reaction Valley Approach) and LMA (Local Mode Analysis), both being based on vibrational spectroscopy and complemented by a QTAIM analysis of the electron density calculated at the DFT level of theory. The energetics of each reaction were recalculated at the DLPNO-CCSD(T) level of theory. The URVA curvature profiles identified the important chemical events of all five reactions as peroxide OO bond cleavage taking place before the TS (i.e., accounting for the energy barrier) and epoxide CO bond formation together with rehybridization of the carbon atoms of the targeted CC double bond after the TS. The energy decomposition into reaction phase contribution phases showed that the major effect of the catalyst is the weakening of the OO bond to be broken and replacement of OH bond breakage in the non-catalyzed reaction by an energetically more favorable TiO bond breakage. LMA performed at all stationarymore »points rounded up the investigation (i) quantifying OO bond weakening of the oxidizing peroxide upon coordination at the metal atom, (ii) showing that a more synchronous formation of the new CO epoxide bonds correlates with smaller bond strength differences between these bonds, and (iii) elucidating the different roles of the three TiO bonds formed between catalyst and reactants and their interplay as orchestrated by the Sharpless catalyst. We hope that this article will inspire the computational community to use URVA complemented with LMA in the future as an efficient mechanistic tool for the optimization and fine-tuning of current Sharpless catalysts and for the design new of catalysts for epoxidation reactions.« less
    Free, publicly-accessible full text available July 1, 2023
  3. In this work, we investigated bonding features of 15 ruthenium(II) nitrile complexes of the type [Ru(tpy)(L)-(CH 3 CN)] n+ , containing the tridentate tpy ligand (tpy = 2,2′:6′,2″-terpyridine) and various bidentate ancillary ligands L; 12 compounds originally synthesized by Loftus et al. [J. Phys. Chem. C 123, 10291–10299 (2019)] and three new complexes. We utilized local vibrational force constants derived from the local mode theory as a quantitative measure of bond strength complemented with the topological analysis of the electron density and the natural bond orbital analysis. Loftus et al. suggested that nitrile dissociation occurs after light induced singlet–triplet transition of the original complexes and they used as a measure of nitrile release efficiency quantum yields for ligand exchange in water. They observed larger quantum yields for complexes with smaller singlet–triplet energy gaps. The major goal of this work was to assess how the Ru–NC and Ru–L bond strengths in these 15 compounds relate to and explain the experimental data of Loftus et al., particularly focusing on the question whether there is a direct correlation between Ru–NC bond strength and measured quantum yield. Our study provides the interesting result that the compounds with the highest quantum yields also have themore »strongest Ru–NC bonds suggesting that breaking the Ru–NC bond is not the driving force for the delivery process rather than the change of the metal framework as revealed by first results of a unified reaction valley approach investigation of the mechanism. Compounds with the highest quantum yield show larger electronic structure changes upon singlet–triplet excitation, i.e., larger changes in bond strength, covalency, and difference between the singlet and triplet HOMOs, with exception of the compound 12. In summary, this work provides new insights into the interplay of local properties and experimental quantum yields forming in synergy a useful tool for fine tuning of existing and future design of new nitrile releasing ruthenium compounds. We hope that this work will bring theoretical and experimental studies closer together and serves as an incubator for future collaboration between computational chemists and their experimental colleagues.« less
    Free, publicly-accessible full text available July 7, 2023
  4. We present a computational study of a substrate isomerization catalyzed by Ketosteroid Isomerase based on QM/MM calculations, our Unified Reaction Valley Approach and Local Vibrational Mode Analysis. In summary, our study quantifies Talaly’s postulate that the major role of the enzyme pocket is to shield the migrating hydrogen atom from interactions with solvent molecules. Our analysis further confirms that there is no exceptional hydrogen bonding between the substrate and surrounding enzyme amino acids, which could account for lowering the activation barrier.
    Free, publicly-accessible full text available May 1, 2023
  5. This review summarizes the recent developments regarding the use of uranium as nuclear fuel, including recycling and health aspects, elucidated from a chemical point of view, i.e., emphasizing the rich uranium coordination chemistry, which has also raised interest in using uranium compounds in synthesis and catalysis. A number of novel uranium coordination features are addressed, such the emerging number of U(II) complexes and uranium nitride complexes as a promising class of materials for more efficient and safer nuclear fuels. The current discussion about uranium triple bonds is addressed by quantum chemical investigations using local vibrational mode force constants as quantitative bond strength descriptors based on vibrational spectroscopy. The local mode analysis of selected uranium nitrides, N≡U≡N, U≡N, N≡U=NH and N≡U=O, could confirm and quantify, for the first time, that these molecules exhibit a UN triple bond as hypothesized in the literature. We hope that this review will inspire the community interested in uranium chemistry and will serve as an incubator for fruitful collaborations between theory and experimentation in exploring the wealth of uranium chemistry.
    Free, publicly-accessible full text available May 1, 2023
  6. In this study we investigate the Diels–Alder reaction between methyl acrylate and butadiene, which is catalyzed by BF3 Lewis acid in explicit water solution, using URVA and Local Mode Analysis as major tools complemented with NBO, electron density and ring puckering analyses. We considered four different starting orientations of methyl acrylate and butadiene, which led to 16 DA reactions in total. In order to isolate the catalytic effects of the BF3 catalyst and those of the water environment and exploring how these effects are synchronized, we systematically compared the non-catalyzed reaction in gas phase and aqueous solution with the catalyzed reaction in gas phase and aqueous solution. Gas phase studies were performed at the B3LYP/6-311+G(2d,p) level of theory and studies in aqueous solution were performed utilizing a QM/MM approach at the B3LYP/6-311+G(2d,p)/AMBER level of theory. The URVA results revealed reaction path curvature profiles with an overall similar pattern for all 16 reactions showing the same sequence of CC single bond formation for all of them. In contrast to the parent DA reaction with symmetric substrates causing a synchronous bond formation process, here, first the new CC single bond on the CH2 side of methyl acrylate is formed followed by themore »CC bond at the ester side. As for the parent DA reaction, both bond formation events occur after the TS, i.e., they do not contribute to the energy barrier. What determines the barrier is the preparation process for CC bond formation, including the approach diene and dienophile, CC bond length changes and, in particular, rehybridization of the carbon atoms involved in the formation of the cyclohexene ring. This process is modified by both the BF3 catalyst and the water environment, where both work in a hand-in-hand fashion leading to the lowest energy barrier of 9.06 kcal/mol found for the catalyzed reaction R1 in aqueous solution compared to the highest energy barrier of 20.68 kcal/mol found for the non-catalyzed reaction R1 in the gas phase. The major effect of the BF3 catalyst is the increased mutual polarization and the increased charge transfer between methyl acrylate and butadiene, facilitating the approach of diene and dienophile and the pyramidalization of the CC atoms involved in the ring formation, which leads to a lowering of the activation energy. The catalytic effect of water solution is threefold. The polar environment leads also to increased polarization and charge transfer between the reacting species, similar as in the case of the BF3 catalyst, although to a smaller extend. More important is the formation of hydrogen bonds with the reaction complex, which are stronger for the TS than for the reactant, thus stabilizing the TS which leads to a further reduction of the activation energy. As shown by the ring puckering analysis, the third effect of water is space confinement of the reacting partners, conserving the boat form of the six-member ring from the entrance to the exit reaction channel. In summary, URVA combined with LMA has led to a clearer picture on how both BF3 catalyst and aqueous environment in a synchronized effort lower the reaction barrier. These new insights will serve to further fine-tune the DA reaction of methyl acrylate and butadiene and DA reactions in general.« less
    Free, publicly-accessible full text available April 1, 2023
  7. Free, publicly-accessible full text available March 8, 2023
  8. Free, publicly-accessible full text available January 11, 2023
  9. Previous microwave studies of naphthol monomers were supplemented by measuring spectra of all 13 C mono-substituted isotopologues of the cis - and trans -conformers of 1-naphthol and 2-naphthol in their natural abundances. The resulting data were utilized to determine substitution structures and so-called semi-experimental effective structures. Results from electronic structure calculations show that the OH group of cis -1-naphthol points ≈6° out of plane, which is consistent with the inertial defect data of cis - and trans -1-naphthol. The non-planarity of cis -1-naphthol is a result of a close-contact H-atom–H-atom interaction. This type of H–H interaction has been the subject of much controversy in the past and we provide here an in-depth theoretical analysis of it. The naphthol system is particularly well-suited for such analysis as it provides internal standards with its four different isomers. The methods used include quantum theory of atoms in molecules, non-covalent interactions, independent gradient model, local vibrational mode, charge model 5, and natural bond orbital analyses. We demonstrate that the close-contact H–H interaction is neither a purely attractive nor repulsive interaction, but rather a mixture of the two.
    Free, publicly-accessible full text available February 9, 2023
  10. Free, publicly-accessible full text available January 1, 2023