Search results for "PHOTOCHEMISTRY"
showing 10 items of 2034 documents
Insights into the Mechanism of Cumene Peroxidation Using Supported Gold and Silver Nanoparticles
2013
Due to the considerable industrial implications, an in-depth study of cumene peroxidation using supported gold and silver nanoparticles was carried out to gain more insight into the mechanism of this reaction. Supported gold nanoparticles were found to efficiently catalyze the decomposition of cumene hydroperoxide with a selectivity of 25% at 80 °C when using gold supported on hydrotalcite (AuNP@HT), and 2-phenyl-2-propanol (i.e., cumyl alcohol) was the main product. Further, silver nanoparticles supported on hydrotalcite (AgNP@HT) converted cumene to cumene hydroperoxide at 80 °C with 80% selectivity. Both benchtop and oxygen-uptake experiments were used to probe the reaction mechanism and…
Xylene isomerization and aromatic alkylation in zeolites NU-87, SSZ-33, β, and ZSM-5: molecular dynamics and catalytic studies
2004
The unique pore topology of zeolite NU-87, with 10-membered ring (MR) channels intersected by perpendicular 12-MR cavities, can offer new opportunities for alkylaromatics isomerization, as well as for aromatics alkylation. The catalytic performance has been studied for meta-xylene isomerization and disproportionation, as well as toluene alkylation with methanol, ethanol, propanol, and isopropanol. Product distribution was interpreted on the basis of pore topology and compared to zeolites with 12-MR (β), 10- and 12-MR (SSZ-33), and 10-MR (ZSM-5) zeolites. The presence of cavities allows more space for the location of bulky intermediates and/or products, and also traps the molecules, allowing…
On/Off Photoswitching in a Cyanide-Bridged {Fe2Co2} Magnetic Molecular Square
2013
International audience; A repeatable bidirectional paramagnetic ↔ diamagnetic photomagnetic effect has been observed in the cyanide-bridged Fe-Co square complex {[Fe{B(pz)(4)}(CN)(3)](2)[Co(bik)(2)](2)}(ClO(4))(2)*3H(2)O [B(pz)(4) = tetrapyrazolylborate, bik = bis(1-methylimidazol-2-yl)ketone]. Magnetic measurements and low-temperature single-crystal X-ray diffraction experiments have shown that a complete electron transfer from the diamagnetic Fe(II)-Co(III) state to the paramagnetic Fe(III)-Co(II) metastable state is induced by 808 nm laser light irradiation, whereas the diamagnetic state is recovered in an almost quantitative yield under irradiation at 532 nm.
Light-controlled reversible modulation of frontier molecular orbital energy levels in trifluoromethylated diarylethenes
2017
Among bistable photochromic molecules, diarylethenes (DAEs) possess the distinct feature that upon photoisomerization they undergo a large modulation of their π-electronic system, accompanied by a marked shift of the HOMO/LUMO energies and hence oxidation/reduction potentials. The electronic modulation can be utilized to remote-control charge- as well as energy-transfer processes and it can be transduced to functional entities adjacent to the DAE core, thereby regulating their properties. In order to exploit such photoswitchable systems it is important to precisely adjust the absolute position of their HOMO and LUMO levels and to maximize the extent of the photoinduced shifts of these energ…
Catalytic Oxidation of Alkanes and Alkenes by H 2 O 2 with a μ‐Oxido Diiron(III) Complex as Catalyst/Catalyst Precursor
2015
A new mu-oxo diiron(III) complex of the lithium salt of the pyridine-based unsymmetrical ligand 3-[(3-{[bis(pyridin-2-ylmethyl)amino]methyl}-2-hydroxy-5-methylbenzyl)(pyridin2-ylmethyl)amino] propanoate (LiDPCPMPP), [Fe-2(mu-O)(LiDPCPMPP)(2)](ClO4)(2), has been synthesized and characterized. The ability of the complex to catalyze oxidation of several alkanes and alkenes has been investigated by using CH3COOH/H2O2 (1:1) as an oxidative system. Moderate activity in cyclohexane oxidation (TOF = 33 h(-1)) and good activity in cyclohexene oxidation (TOF = 72 h(-1)) were detected. Partial retention of configuration (RC = 53%) in cis- and trans-1,2-dimethylcyclohexane oxidation, moderate 3 degrees…
A PM3 study of the molecular mechanism for the cycloaddition between cyclopentadiene and protonated pyridine-imine derivatives
2001
Abstract The molecular mechanism of the cycloaddition reaction between cyclopentadiene (CP) and three pyridine-imine derivatives has been studied by means of PM3 semiempirical method. The role of the protonation, endo/exo and π-facial stereoselectivity are analyzed and discussed. In neutral conditions the cycloaddition between CP and the pyridine-imine takes place along a concerted mechanism. Protonation on both nitrogen atoms brings out that the reaction pathway takes with a very low barrier along a stepwise mechanism. PM3 results are capable to find the key factors governing this chemical reaction and to explain the experimental outcome.
The Effect of Vanadium on the Content of Chlorophyll, P-700 and Cytochrome f at Different Light Intensities in Chlorella fusca
1980
Summary Cells of Chlorella fusca, were grown in liquid medium supplemented ,vith O.2% glucose and varied concentrations of vanadium both at 5 different light intensities and in the dark. \Vith increasing concentrations of vanadium the content of chlorophyll, P-700 and cytochrome f increased as well, each, however, in a different way. At the saturating concentration of 20 µg vanadium per litre the pigment content per dry weight was double as high as that of the vanadium-free control; the ratio of chlorophyll a/chlorophyll b increased from 2.1 to 2.9. The content of P-700 at saturating vanadium concentration was four times higher than that of the control when based on dry weight and double as…
Evidence for the operation of a cyanide-sensitive oxidase in chlororespiration in the thylakoids of the chlorophyll c-containing alga Pleurochloris m…
1995
For characterisation of chlororespiration in the chlorophyll c-containing alga Pleurochloris meiringensis, we measured the flash-induced electrochromic absorbance changes between 470 and 545 nm and the redox changes of cytochrome f and cytochrome c553. Cytochrome c553 was shown to be present in high amounts (1 mol cytochrome c553 per 300 mol chlorophyll) in this alga and to function as the obligate electron donor for photosystem I instead of plastocyanin. Whereas salicylhydroxamic acid had no effect on the flash-induced absorbance transients, cyanide enhanced the slow-rising (t1/2≈10 ms) kinetic component of the electrochromic absorbance change. Cyanide also accelerated the re-reduction of …
Measurements of cytochrome f and P-700 in intact leaves of Sinapis alba grown under high-light and low-light conditions
1978
The oxidation and reduction of cytochrome f and P-700 is measured spectrophotometrically in leaves of low-light and high-light plants. After illumination with red light, an induction phenomenon for cytochrome f oxidation is observed which indicates a regulation of photosystem I activity through energy distribution between the pigment systems by the energy state of the membrane. After far-red excitation the reduction of cytochrome f in the dark is much slower in low-light leaves. This shows that cyclic electron transport is not improved in low-light plants under these conditions. P-700 is oxidized on excitation with far-red light. However, with high intensities of far-red light, P-700 is par…
Distinct Photophysics of the Isomers of B18H22 Explained
2012
The photophysics of the two isomers of octadecaborane(22), anti- and syn-B 18H 22, have been studied by UV-vis spectroscopic techniques and theoretical computational methods. In air-saturated hexane, anti-B 18H 22 shows fluorescence with a high quantum yield, Φ F = 0.97, and singlet oxygen O 2( 1Δ g) production (Φ Δ ∼ 0.008). Conversely, isomer syn-B 18H 22 shows no measurable fluorescence, instead displaying much faster, picosecond nonradiative decay of excited singlet states. Computed potential energy hypersurfaces (PEHs) for both isomers rationalize these data, pointing to a deep S 1 minimum for anti-B 18H 22 and a conical intersection (CI) between its S 0 and S 1 states that lies 0.51 e…