Investigation of the Flavonoids in Croatian Propolis by Thin-Layer Chromatography

JPC – Journal of Planar Chromatography – Modern TLC - Flavonoids and phenolic acids with a variety of biological activity are considered to be the main compounds in propolis–a...     

A computational foray into the mechanism and catalysis of the adduct formation reaction of guanine with crotonaldehyde

Crotonaldehyde, a common environmental pollutant and product of endogenous lipid peroxidation, reacts with guanine to form DNA adducts with pronounced genotoxicity and mutagenicity. Here, we explore the molecular mechanism of this adduct formation using double-hybrid density functional theory methods. The reaction can be envisaged to occur in a two-step fashion via an aza-Michael addition leading to an intermediate ring-open adduct followed by a cyclization reaction giving the mutagenic ring-closed adduct. We find that (i) a 1,2-type addition is favored over a 1,4-type addition for the aza-Michael addition, and (ii) an initial tautomerization of the guanine moiety in the resulting ring-open adduct significantly reduces the barrier toward cyclization compared to the direct cyclization of the ring-open adduct in its keto-form. Overall, the aza-Michael addition is found to be rate-determining. We further find that participation of a catalytic water molecule significantly reduces the energy barriers of both the addition and cyclization reaction. © 2018 Wiley Periodicals, Inc.     

Postsynthetic Metal and Ligand Exchange in MFU‐4l: A Screening Approach toward Functional Metal–Organic Frameworks Comprising Single‐Site Active Centers

The isomorphous partial substitution of Zn²⁺ ions in the secondary building unit (SBU) of MFU‐4l leads to frameworks with the general formula [M_xZn_(5–x)Cl₄(BTDD)₃], in which x≈2, M=Mn^II, Fe^II, Co^II, Ni^II, or Cu^II, and BTDD=bis(1,2,3‐triazolato‐[4,5‐b],[4′,5′‐i])dibenzo‐[1,4]‐dioxin. Subsequent exchange of chloride ligands by nitrite, nitrate, triflate, azide, isocyanate, formate, acetate, or fluoride leads to a variety of MFU‐4l derivatives, which have been characterized by using XRPD, EDX, IR, UV/Vis‐NIR, TGA, and gas sorption measurements. Several MFU‐4l derivatives show high catalytic activity in a liquid‐phase oxidation of ethylbenzene to acetophenone with air under mild conditions, among which Co‐ and Cu derivatives with chloride side‐ligands are the most active catalysts. Upon thermal treatment, several side‐ligands can be transformed selectively into reactive intermediates without destroying the framework. Thus, at 300 °C, Co^II‐azide units in the SBU of Co‐MFU‐4l are converted into Co^II‐isocyanate under continuous CO gas flow, involving the formation of a nitrene intermediate. The reaction of Cu^II‐fluoride units with H₂ at 240 °C leads to Cu^I and proceeds through the heterolytic cleavage of the H₂ molecule.     

Reversible gas-phase redox processes catalyzed by Co-exchanged MFU-4l(arge)

Postsynthetic metal ion exchange in a benzotriazolate-based MFU-4l(arge) framework leads to a Co(II)-containing framework with open metal sites showing reversible gas-phase oxidation properties.     

Pristine Graphene as a Racemization Catalyst for Axially Chiral BINOL

Despite versatile applications of functionalized graphene in catalysis, applications of pure, unfunctionalized graphene in catalysis are in their infancy. This work uses both computational and experimental approaches to show that single-layer graphene can efficiently catalyze the racemization of axially chiral BINOL in solution. Using double-hybrid density functional theory (DHDFT) we calculate the uncatalyzed and catalyzed Gibbs free reaction barrier heights in a number of representative solvents of varying polarity: benzene, diphenyl ether, dimethylformamide (DMF), and water. These calculations show that (i) graphene can achieve significant catalytic efficiencies (▵▵G^≠_cat) varying between 47.2 (in diphenyl ether) and 60.7 (in DMF) kJ mol⁻¹. An energy decomposition analysis reveals that this catalytic activity is driven by electrostatic and dispersion interactions. Based on these computational results, we explore the graphene-catalyzed racemization of axially chiral BINOL experimentally and show that single-layer graphene can efficiently catalyze this process. Whilst the uncatalyzed racemization requires high temperatures of over 200 °C, a pristine single-layer graphene catalyst makes it accessible at 60 °C.     

Formation Mechanism of the First Carbon–Carbon Bond and the First Olefin in the Methanol Conversion into Hydrocarbons

The elementary reactions leading to the formation of the first carbon–carbon bond during early stages of the zeolite‐catalyzed methanol conversion into hydrocarbons were identified by combining kinetics, spectroscopy, and DFT calculations. The first intermediates containing a C?C bond are acetic acid and methyl acetate, which are formed through carbonylation of methanol or dimethyl ether even in presence of water. A series of acid‐catalyzed reactions including acetylation, decarboxylation, aldol condensation, and cracking convert those intermediates into a mixture of surface bounded hydrocarbons, the hydrocarbon pool, as well as into the first olefin leaving the catalyst. This carbonylation based mechanism has an energy barrier of 80 kJ mol^?1 for the formation of the first C?C bond, in line with a broad range of experiments, and significantly lower than the barriers associated with earlier proposed mechanisms.     

NMR spin-lattice relaxation time T1 calculation for molecular reorientation through inequivalent potential barriers in the case of n-fold (n = 2, 3, 4, 6) symmetry axis

It has been shown that if reorientational jumps of molecules or their parts take place through inequivalent potential barriers, it is possible to draw information on the multiplicity of the axis of reorientation from the spin-lattice T ₁ time measurements in the NMR experiment. For the model of n potential wells of which one is deeper than the others, for n = 2, 3, 4, 6 the analytical formulae have been derived for the T ₁ relaxation time as a function of the correlation time and n. The recurrent form of the formulae for the conditional probability of the well population has been obtained, which permits calculation of the relaxation time for any n.     

Supported Intermetallic PdZn Nanoparticles as Bifunctional Catalysts for the Direct Synthesis of Dimethyl Ether from CO‐Rich Synthesis Gas

The single‐step syngas‐to‐dimethyl ether (STD) process entails economic and technical advantages over the current industrial two‐step process. Pd/ZnO‐based catalysts have recently emerged as interesting alternatives to currently used Cu/ZnO/Al₂O₃ catalysts, but the nature of the active site(s), the reaction mechanism, and the role of Pd and ZnO in the solid catalyst are not well established. Now, Zn‐stabilized Pd colloids with a size of 2 nm served as the key building blocks for the methanol active component in bifunctional Pd/ZnO‐γ‐Al₂O₃ catalysts. The catalysts were characterized by combining high‐pressure operando X‐ray absorption spectroscopy and DFT calculations. The enhanced stability, longevity, and high dimethyl ether selectivity observed makes Pd/ZnO‐γ‐Al₂O₃ an effective alternative system for the STD process compared to Cu/ZnO/γ‐Al₂O₃.     

Thermochemistry of phosphorus sulfide cages: an extreme challenge for high-level ab initio methods

Structural Chemistry - The enthalpies of formation and isomerization energies of P4Sn molecular cages are not experimentally (or theoretically) well known. We obtain accurate enthalpies of...     

π–π Catalysis in Carbon Flatland—Flipping [8]Annulene on Graphene

Noncovalent interactions are an integral part of the modern catalysis toolbox. Although stronger noncovalent interactions such as hydrogen bonding are commonly the main driving force of catalysis, π–π interactions typically provide smaller additional stabilizations, for example, to afford selectivity enhancements. Here, it is shown computationally that pristine graphene flakes may efficiently catalyze the skeletal inversions of various benzannulated cyclooctatetraene derivatives, providing an example of a catalytic process driven solely by π–π stacking interactions. Hereby, the catalytic effect results from disproportionate shape complementarity between catalyst and transition structure compared with catalyst and reactant. An energy decomposition analysis reveals electrostatic and, especially with increasing system size, to a larger extent, dispersion interactions as the origin of stabilization.