Adsorption of organic substances on broken clay surfaces: a quantum chemical study

Hydrogen-bonded interactions between local defect structures on broken clay surfaces modeled as molecular clusters and the organic molecules acetic acid, acetate, and N-methylacetamide (NMA) have been investigated. Density functional theory and polarized basis sets have been used for the computation of optimized interaction complexes and formation energies. The activity of the defect structures has been characterized as physical or chemical in terms of the strength of the hydrogen bonds formed. Chemical defects lead to significantly enhanced interactions with stronger hydrogen bonds and larger elongation of OH bonds in comparison to the physical defects. The type of interaction with the defect structure significantly influences the planarity of the model peptide bond in NMA. Both cases, enhancement of the planarity by increase of the CN double bond character and strong deviations from planarity, are observed.     

Nonadiabatic excited-state dynamics of polar pi-systems and related model compounds of biological relevance

Multireference ab initio dynamics simulations have become available as a tool for the investigation of photochemical processes, mainly for those related to nonadiabatic phenomena taking place in the sub-picosecond time scale. For organic molecules, these phenomena are in many cases deeply dependent on the relaxation of the photoexcited pi-system. We review the latest contributions of our group to this subject and report new results for systems studied previously, grouping them in single pi bonds, chains and aromatic rings. The dynamics of ethylene and substituted ethylenes is discussed mainly in connection to the competition between the two available relaxation paths in the excited states and their relation to the conical intersections in large systems. The trans-cis and the cis-trans dynamics of the pentadieniminium cation is investigated as well. Finally, we discuss the photodynamics of aminopyrimidine starting in the S1 and S2 states and the conclusions, which can be drawn from this for the interpretation of the adenine dynamics.     

MSPD procedure for determining buprofezin,tetradifon, vinclozolin,and bifenthrin residues in propolis by gas chromatography–mass spectrometry

A simple and effective extraction method based on matrix solid-phase dispersion (MSPD) was developed to determine bifenthrin, buprofezin, tetradifon, and vinclozolin in propolis using gas chromatography–mass spectrometry in selected ion monitoring mode (GC–MS, SIM). Different method conditions were evaluated, for example type of solid phase (C₁₈, alumina, silica, and Florisil), the amount of solid phase and eluent (n-hexane, dichloromethane, dichloromethane–n-hexane (8:2 and 1:1, v/v) and dichloromethane–ethyl acetate (9:1, 8:2 and 7:3, v/v)). The best results were obtained using 0.5 g propolis, 1.0 g silica as dispersant sorbent, 1.0 g Florisil as clean-up sorbent, and dichloromethane–ethyl acetate (9:1, v/v) as eluting solvent. The method was validated by analysis of propolis samples fortified at different concentration levels (0.25 to 1.0 mg kg⁻¹). Average recoveries (four replicates) ranged from 67% to 175% with relative standard deviation between 5.6% and 12.1%. Detection and quantification limits ranged from 0.05 to 0.10 mg kg⁻¹ and 0.15 to 0.25 mg kg⁻¹ propolis, respectively.     

Probing the Catalytic Activity of Reduced Graphene Oxide Decorated with Au Nanoparticles Triggered by Visible Light

Hybrid materials in which reduced graphene oxide (rGO) is decorated with Au nanoparticles (rGO–Au NPs) were obtained by the in situ reduction of GO and AuCl₄^?_(aq) by ascorbic acid. On laser excitation, rGO could be oxidized as a result of the surface plasmon resonance (SPR) excitation in the Au NPs, which generates activated O₂ through the transfer of SPR‐excited hot electrons to O₂ molecules adsorbed from air. The SPR‐mediated catalytic oxidation of p‐aminothiophenol (PATP) to p,p′‐dimercaptoazobenzene (DMAB) was then employed as a model reaction to probe the effect of rGO as a support for Au NPs on their SPR‐mediated catalytic activities. The increased conversion of PATP to DMAB relative to individual Au NPs indicated that charge‐transfer processes from rGO to Au took place and contributed to improved SPR‐mediated activity. Since the transfer of electrons from Au to adsorbed O₂ molecules is the crucial step for PATP oxidation, in addition to the SPR‐excited hot electrons of Au NPs, the transfer of electrons from rGO to Au contributed to increasing the electron density of Au above the Fermi level and thus the Au‐to‐O₂ charge‐transfer process.     

On the Gauss mapping of hypersurfaces with constant scalar curvature in ℍ n+1

In this paper, we deal with complete hypersurfaces immersed in the hyperbolic space with constant scalar curvature. By supposing suitable restrictions on the Gauss mapping of such hypersurfaces we obtain some rigidity results. Our approach is based on the use of a generalized maximum principle, which can be seen as a sort of extension to complete (noncompact) Riemannian manifolds of the classical Hopf’s maximum principle.     

Photodynamical simulations of cytosine: characterization of the ultrafast bi-exponential UV deactivation

Deactivation of UV-excited cytosine is investigated by non-adiabatic dynamics simulations, optimization of conical intersections, and determination of reaction paths. Quantum chemical calculations are performed up to the MR-CISD level. Dynamics simulations were performed at multiconfigurational level with the surface hopping method including four electronic states. The results show the activation of four distinct reaction pathways at two different subpicosecond time scales and involving three different conical intersections. Most trajectories relax to a minimum of the S(1) state and deactivate with a time constant of 0.69 ps mainly through a semi-planar conical intersection along the n(O)π* surface. A minor fraction deactivate along ππ* regions of the S(1) surface. Sixteen percent of trajectories do not relax to the minimum and deactivate with a time constant of only 13 fs.