Understanding the Influence of Lewis Acids on CO2 Hydrogenation: The Critical Effect Is on Formate Rotation

Lewis acids (LAs) have been shown to accelerate hydrogenation of C02, but the underlying mechanistic details remain to be elucidated. We have employed computational methods to investigate how LAs affect CO2 hydrogenation with a range of known metal-hydrides (LnIr–H, LnRu–H, LnMn–H, LnCo–H). Our results show that LAs can alter the nature of the hydride–CO2 bond formation step, but do not lower its barrier. Instead, the accelerating effect of LAs is on the subsequent step, the rearrangement of the metal-formate σ-intermediate. These insights are essential for understanding the effect of LA additives on metal-mediated hydrogenations of CO2

THE EFFECT OF FOUR FLAT PLATE REFLECTORS ON LIGHT ENERGY-HARVESTING SYSTEM CHARACTERISTICS

In this paper, the effect of four flat panel reflectors (bottom, top, left and right side reflectors) on the total light radiation on a small-size photovoltaic cell is analyzed. An analytical model for the determination of the optimum inclination angle of the reflectors with respect to the cell’s horizontal surface is presented. The optimal angle was calculated to be 66°. The calculated value was experimentally verified by measuring the short-circuit current of the cell. It was shown that the increase in the short-circuit current of the cell with reflectors in the optimal position was about 60% for the illumination levels between 10 lx and 1000 lx. The cell with reflectors was used to charge the primary capacitor in the energy harvesting circuitry of the wireless sensor node and it was demonstrated that the time needed for the cell to charge the primary capacitor could be reduced 35-40%. UTICAJ ČETIRI RAVNA REFLEKTORA NA KARAKTERISTIKE SISTEMA KOJI SE AUTONOMNO NAPAJA ENERGIJOM SVETLOSTIU radu je analiziran uticaj četiri ravna reflektora (donji, gornji, levi i desni bočni reflektor) na ukupno svetlosno zračenje koje dospeva na solarnu ćeliju malih dimenzija. Dat je analitički model za određivanje optimalnog nagibnog ugla reflektora u odnosu na horizontalnu ravan u kojoj se nalazi solarna ćelija. Izračunati optimalni ugao iznosi 66°. Izračunata vrednost je eksperimentalno proverena merenjem struje kratkog spoja solarne ćelije i pokazano je da povećanje struje kratkog spoja solarne ćelije sa reflektorima u optimalnom položaju iznosi oko 60% za nivo osvetljenosti između 10 lx i 1000 lx. Solarna ćelija je korišćena za punjenje primarnog kondenzatora samonapajajućeg senzora i pokazano je da vreme potrebno za punjenje primarnog kondenzatora može biti smanjeno od 35% do 40%. HIGHLIGHTSThe effect of four flat panel reflectors on the total light radiation on a small-size photovoltaic cell was analyzed;An analytical model for the determination of an optimum inclination angle of the reflectors with respect to the cell’s surface is presented;The optimal inclination angle of the reflectors was determined to be 66°;The reflectors in the optimal position could increase the cell’s short-circuit current up to 60%;The charging time of the primary capacitor was reduced for 35-40%

Uticaj fosfornih đubriva na prinos i sadržaj kadmijuma u krompiru

This study evaluated the effect of different types and rates of phosphorus fertilizers on yield and cadmium content of potato on a slightly alkaline alluvium soil. Results show that the highest yield was obtained under treatment with monoammonium phosphate (manufactured in Russia) and the lowest under Bulgarian TSP treatment. The highest total cadmium was found in potato tubers treated with Bulgarian diammonium phosphate, in contrast to the lowest measured values under Russian DAP treatment.U radu je ispitivan uticaj različitih vrsta i doza fosfornog đubriva na prinos i sadržaj kadmijuma u krompiru, na zemljištu tipa aluvijum, slabo alkalne reakcije. Dobijeni rezultati pokazuju da je primenom monoamonijum-fosfata (poreklom iz Rusije) ostvaren najveći prinos, dok je najmanji prinos postignut sa primenom bugarstkog TSP. Najveći sadržaj ukupnog kadmijuma u krtolama krompira utvrđen je primenom bugarskog diamonijum-fosfata, a najmanji primenom ruskog DAP-a

Uticaj malein hidrazida na proklijavanje crnog luka tokom i nakon skladištenja

Tokom 2018/19. i 2019/20. godine izvedena su ispitivanja uticaja malein hidrazida (MH) i uslova skladištenja na proklijavanje lukovica crnog luka. Ogledi su postavljeni na lokalitetu u Gospođincima. Malein hidrazid je primenjen kao komercijalni preparat Royal MH 30 u količini 12,5 l/ha 21 dan pre vađenja lukovica. Tokom perioda skladištenja svakih mesec dana (I - VIII ocena) je merena masa lukovica i praćen broj isklijalih lukovica, masa i dužina klica, a na kraju ogleda je izračunat gubitak mase lukovica u odnosu na masu u trenutku uskladištenja. Za utvrđivanje proklijavanja lukovica nakon iznošenja iz skladišta 6 nedelja je na svakih 10 dana praćen broj lukovica sa vidljivim klicama. Kod proklijalih lukovica je izmerena dužina klica, dok su lukovice bez vidljivih klica, na kraju ogleda, presečene po dužini na pola kako bi se utvrdio unutrašnji porast klice. Pod uticajem MH, u zavisnosti od godine ispitivanja, masa tretiranih lukovica je bila manja za 9,5-10,6% u odnosu na masu neposredno pre unošenja u skladište. Efikasnost MH u sprečavanju klijanja crnog luka potvrđena je i preko značajno manjeg broja isklijalih lukovica, kao i parametara masa klice i dužina klice. Takođe, nakon iznošenja lukovica iz skladišta (temperatura 18-20 oC) ukupan procenat isklijalih lukovica u kontroli je bio 62-68%, a u tretmanu sa MH 10-16%. Unutrašnji porast klice nije uočen kod 53,3-54,8% tretiranih lukovica, dok je u kontroli odsustvo unutrašnjih klica zabeleženo kod 10,5-18,8% presečenih lukovica

Mechanistic Investigations of the Asymmetric Hydrogenation of Enamides with Neutral Bis(phosphine) Cobalt Precatalysts

The mechanism of the asymmetric hydrogenation of prochiral enamides by well-defined, neutral bis(phosphine) cobalt(0) and cobalt(II) precatalysts has been explored using(R,R)-iPrDuPhos ((R,R)-iPrDuPhos = (+)-1,2-bis[(2R,5R)-2,5-diisopropylphospholano]benzene) as a representative chiral bis(phosphine) ligand. A series of (R,R)-(iPrDuPhos)Co(enamide) (enamide = methyl-2-acetamidoacrylate (MAA), methyl(Z)-α-acetamidocinnamate (MAC), and methyl(Z)-acetamido(4-fluorophenyl)acrylate (4FMAC)) complexes (1-MAA, 1-MAC, and 1-4FMAC), as well as a dinuclear cobalt tetrahydride, [(R,R)-(iPrDuPhos)Co]2(μ2-H)3(H) (2), were independently synthesized, characterized, and evaluated in both stoichiometric and catalytic hydrogenation reactions. Characterization of (R,R)-(iPrDuPhos)Co(enamide) complexes by X-ray diffraction established the formation of the pro-(R) diastereomers in contrast to the (S)-alkane products obtained from the catalytic reaction. In situ monitoring of the cobalt-catalyzed hydrogenation reactions by UV–visible and freeze-quench electron paramagnetic resonance spectroscopies revealed (R,R)-(iPrDuPhos)Co(enamide) complexes as the catalyst resting state for all the three enamides studied. Variable time normalization analysis kinetic studies of the cobalt-catalyzed hydrogenation reactions in methanol established a rate law that is first order in (R,R)-(iPrDuPhos)Co(enamide) and H2 but independent of the enamide concentration. Deuterium-labeling studies, including measurement of an H2/D2 kinetic isotope effect and catalytic hydrogenations with HD, established an irreversible H2 addition step to the bound enamide. Density functional theory calculations support that this step is both rate and selectivity determining. Calculations, as well as HD-labeling studies, provide evidence for two-electron redox cycling involving cobalt(0) and cobalt(II) intermediates during the catalytic cycle. Taken together, these experiments support an unsaturated pathway for the [(R,R)-(iPrDuPhos)Co]-catalyzed hydrogenation of prochiral enamides

ODREĐIVANJE HEMIJSKOG SASTAVA RAZLIČITIH EKSTRAKATA ČUVARKUĆE

Sempervivum tectorum has a similar effect as aloe vera, which is known in the treatment of various skin diseases. This herb is considered one of the safest remedies for a wide range of skin diseases. Due to its anti-inflammatory and antiseptic properties, it also serves as an excellent first aid for burns, stings and bites, because it provides quick relief and calming. Freshly squeezed juice from the leaves of the houseplant is used in the treatment of nervous disorders, epilepsy and restless dreams. The leaves are edible and can be used as an addition to salads or stews. They are not particularly tasty, but as they are rich in water, they can be put in a juicer together with other fruits or vegetables and become a refreshing drink. It is used in folk herbal medicine and as a medicine. The aim of this study was to determine the moisture content, total extracted substances, extract density, vitamin C, organic acids and proteins in house extracts.Publishe

Silicon Differently Affects Apoplastic Binding of Excess Boron in Wheat and Sunflower Leaves

Monocots and dicots differ in their boron (B) requirement, but also in their capacity to accumulate silicon (Si). Although an ameliorative effect of Si on B toxicity has been reported in various crops, differences among monocots and dicots are not clear, in particular in light of their ability to retain B in the leaf apoplast. In hydroponic experiments under controlled conditions, we studied the role of Si in the compartmentation of B within the leaves of wheat (Triticum vulgare L.) as a model of a high-Si monocot and sunflower (Helianthus annuus L.) as a model of a low-Si dicot, with the focus on the leaf apoplast. The stable isotopes 10B and 11B were used to investigate the dynamics of cell wall B binding capacity. In both crops, the application of Si did not affect B concentration in the root, but significantly decreased the B concentration in the leaves. However, the application of Si differently influenced the binding capacity of the leaf apoplast for excess B in wheat and sunflower. In wheat, whose capacity to retain B in the leaf cell walls is lower than in sunflower, the continuous supply of Si is crucial for an enhancement of high B tolerance in the shoot. On the other hand, the supply of Si did not contribute significantly in the extension of the B binding sites in sunflower leaves. © 2023 by the authors

Towards Enantioselective Carboxylation and Hydrogenation Reactions (Quantum Chemical Modelling of Homogeneous Reactions)

With the aid of DFT methods, it is possible to get insights into the mechanistic details of homogeneous reactions, the substrate preferences and activities of catalysts. Computational methods can also help to identify the selectivity-determining factors that govern asymmetric reactions. In this thesis, DFT methods are applied in order to study the enantioselective addition of small molecules, such as CO2 and H2, to alkenes in order to form saturated carboxylic acids and alkanes. Both rhodium-mediated hydrocarboxylation and cobalt-mediated hydrogenation reactions were investigated using the popular DFT functionals PBE and B3LYP, including dispersion corrections. First, the nonselective Rh-cyclooctadiene(COD)-catalyzed hydrocarboxylation of alkenes with CO2 was studied by employing the PBE-D2 functional. Several styrene derivatives and α,β-unsaturated compounds were analyzed. Our computational investigation of Rh-COD-benzyl complexes revealed an unusual TS for the C-CO2 bond formation step, where CO2 does not interact with the metal center and the substrate is coordinated to the metal in a η6-fashion via a phenyl ring. The study was expanded by analyzing the potential of five chiral ligands, (S)-SEGPHOS, (R,R)-BDPP, (R,R)-tBu-BOX, (S)-iPr-PHOX, and (R)-StackPhos, to form Rh-based catalysts for asymmetric hydrocarboxylations. Interestingly, the preferred carboxylation TSs with chiral Rh-complexes display a similar substrate binding mode as with the achiral COD ligand and also show a preference for outer sphere CO2 insertion. However, the results indicate that different CO2 insertion paths, frontside or backside, are possible, dependent on the nature of the ligand. For ligands containing an N-heterocyclic ring, it is shown that CO2 is able to form stacking interactions with the ring, which for several ligands results in a preference for frontside CO2 insertion. Second, a detailed mechanistic investigation of Co-catalyzed asymmetric hydrogenation of enamides was performed at the B3LYP-D3 level of theory. The study of enamides with very different molecular structures shows that two mechanistic pathways appear accessible, both a non-redox Co(II) mechanism proceeding through metallacycle intermediates and a more classical redox Co(0)/Co(II) mechanism. The obtained barriers indicate that these mechanisms may be competing. It is also shown that explicit solvent affects the computed barriers significantly and that its inclusion appears to be crucial for the proper estimation of the enantiomeric excesses of Co-catalyzed hydrogenation of enamides