Chemical Research Center of the Hungarian Academy of Sciences

About: Chemical Research Center of the Hungarian Academy of Sciences is a facility organization based out in Budapest, Hungary. It is known for research contribution in the topics: Catalysis & Chemistry. The organization has 254 authors who have published 277 publications receiving 6778 citations. The organization is also known as: Magyar Tudományos Akadémia Kémiai Kutatóközpont & Research Centre for Natural Sciences of HAS.

Theoretical Studies on the Bifunctionality of Chiral Thiourea-Based Organocatalysts: Competing Routes to C−C Bond Formation

Abstract: The mechanism of enantioselective Michael addition of acetylacetone to a nitroolefin catalyzed by a thiourea-based chiral bifunctional organocatalyst is investigated using density functional theory calculations. A systematic conformational analysis is presented for the catalyst, and it is shown that both substrates coordinate preferentially via bidentate hydrogen bonds. The deprotonation of the enol form of acetylacetone by the amine of the catalyst is found to occur easily, leading to an ion pair characterized by multiple H-bonds involving the thiourea unit as well. Two distinct reaction pathways are explored toward the formation of the Michael product that differ in the mode of electrophile activation. Both reaction channels are shown to be consistent with the notion of noncovalent organocatalysis in that the transition states leading to the Michael adduct are stabilized by extensive H-bonded networks. The comparison of the obtained energetics for the two pathways allows us to propose an alternative mec...

Rationalizing the reactivity of frustrated Lewis pairs: Thermodynamics of H2 activation and the role of acid-base properties

Abstract: The acid-base strengths of recently reported frustrated Lewis pairs and their relation with the thermodynamic feasibility of heterolytic hydrogen splitting reactions are analyzed in terms of quantum chemical calculations. Reaction free energies of hydrogenation processes are computed, and an energy partitioning scheme is introduced, which involves quantitative measures of the acidity and basicity of the reacting Lewis centers. Additional terms are also included that account for possible dative bond formation between the active sites and for stabilizing electrostatic interactions occurring in the product species. For intermolecular combinations of donor-acceptor components, the calculated reaction free energies are found to correlate well with the cumulative acid-base strengths. Product stabilization for these systems represents a notable contribution to the overall energetics; however, it generally shows only a slight variation for the investigated series. The reactivity of linked donor-acceptor pairs is primarily governed by acid-base properties as well, but the magnitude of stabilizing effects arising from acid-base cooperativity of active sites is also of significant importance in determining the thermodynamic feasibility of the reactions.

Stability and Controlled Composition of Hexagonal WO3

Abstract: This paper discusses the formation of nanosized hexagonal tungsten oxide (h-WO3) during the annealing of hexagonal ammonium tungsten bronze (HATB), (NH4)0.33−xWO3−y. This process was investigated by TG/DTA-MS, XRD, SEM, Raman, XPS, and 1H-MAS NMR analyses. Through adjusting the temperature and atmosphere of annealing HATB, the composition (W oxidation state, residual NH4+ and NH3 content) of h-WO3 could be controlled. The effect of composition on the conductivity and gas sensitivity of h-WO3 was studied. New structural information was obtained about both HATB and h-WO3. It was found that NH4+ and NH3 could be situated at three different positions in HATB. Residual NH4+ and NH3 in the hexagonal channels seemed to be vital for stabilizing h-WO3: when they were completely released, the hexagonal framework collapsed. We propose that the structure of h-WO3 cannot be maintained without some stabilizing ions or molecules in the hexagonal channels.

Volumetric measure of isostructurality

Abstract: The numerical descriptors of isostructurality are critically reviewed and to overcome their limitations a new volumetric measure is proposed. It is defined as the percentage ratio of the overlapping volume of molecules in the analyzed structures to the average of the corresponding molecular volumes. For the calculation of this index a numerical approach is presented, which is also capable of treating disordered structures. The use of isostructurality calculations is demonstrated using a series of examples. Homologous 9-alkylthiophenanthrenes are used as simple illustrations of the necessity and applicability of the new descriptor. The structural changes in the inclusion compounds of 5-methoxysulfadiazine with chloroform, dioxane and tetrahydrofuran are analyzed and rationalized with the aid of isostructurality indices. The diverse relationships among a series of helical tubuland diols cocrystallized with simple phenols are also characterized. The modifications in molecular and crystal structures are correlated with the calculated degree of isostructurality.