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[en] CH_2Cl–Cl and CH_2Br–Br, photoisomers of CH_2Cl_2 and CH_2Br_2 , were identified in the matrix IR spectra from the precursors exposed to plume radiation of a laser-ablated transition metal. On the other hand, the corresponding photoisomers of CH_2F_2 and CH_2FCl (CH_2F–F, CH_2F–Cl, and CH_2Cl–F) were not observed because of their instabilities. The C[BOND]X bond of the product is unusually strong, leading to a high C[BOND]X stretching frequency. Natural bond orbital (NBO) analysis reveals that it is a true carbon–halogen double bond; in contrast, the X[BOND]X bond is largely ionic (H_2C[DOUBLE BOND]X"δ"+ ∙∙∙X"δ"− ), similar to the previously studied analogs from tri- and tetrahalomethanes. Intrinsic reaction coordinate (IRC) computation reproduces the smooth interconversion between the precursor and the energetically higher photoisomer, consistent with the disappearance of the product during visible photolysis
[en] The hunt for complex organic molecules (COMs) is a major concern for understanding the possible role of interstellar chemistry in the synthesis of the molecules that ultimately may be at the origin of life. A comprehensive screening of the 14 species effectively observed under 32 different isomeric forms in the interstellar medium has been done by means of high-level quantum chemical simulations. Confrontation between calculations and observations shows that when several isomers of the same generic formula are identified, it is always the most stable one that is the most abundant. Moreover, the abundance ratio of the most stable isomer to the other isomers is directly related to their energy difference. What can be seen as a minimum energy principle is verified in molecular clouds, hot cores/corinos, photodissociation regions, and asymptotic giant branch stars. The few exceptions encountered could be rationalized either by the existence of different routes of formation with no intermediate in common and/or specific depletion on the grains of one isomer with respect to the others.
[en] Understanding plant responses to hydrological extremes is critical for projections of the future terrestrial carbon uptake, but much more is known about the impacts of drought than of extreme wet conditions. However, the latter may control ecosystem-scale photosynthesis more strongly than the former in certain regions. Here we take a data-driven, location-based approach to evaluate where wet and dry extremes most affect photosynthesis. By comparing the sensitivity of vegetation greenness during extreme wetness to that during extreme dryness over a 34 year record, we find that regions where the impact of wet extremes dominates are nearly as common as regions where drought impacts dominate. We also demonstrate that the responses of wet-sensitive regions are not uniform and are instead controlled by multiple, often interacting, mechanisms. Given predicted increases in the frequency and intensity of extreme hydrological events with climate change, the consequences of extreme wet conditions on local and global carbon cycling will likely be amplified in future decades. (letter)
[en] This text summarises a talk held by Engelbert Broda at a conference on non-convential energy sources. The talk about photosynthesis and bioconversion is devided in 6 sections: the great physicist and photosynthesis; the influence of photosynthesis on the biosphere (in the past, present and future); the light reactions in photosynthesis; the dark reactions in photosynthesis; bioconversion; respiration and photorespiration. (nowak)
[en] Light induced electron transfer of metal complexes has been studied extensively during the last decade. This interest was stimulated by attempts to develop an artificial photosynthesis for the conversion and chemical storage of solar energy. Even if this goal has not yet been achieved photochemical redox processes of coordination compounds are now much better understood. In this review the various possibilities of photoinduced electron transfer are discussed and illustrated by selected samples. A distinction is made between intra- and intermolecular electron transfer which may occur as a direct optical transition or by an excited state electron transfer mechanism. (author). 52 refs.; 1 fig
[en] Full text:It is known, that the photosynthetic apparatus of plants differs with high sensitivity to action of lines stress factor (for example, high and low temperatures, lack of water or flooding, sanotiazol, pesticides, heavy metals and radionuclides, various pollutants of air) including to action of electromagnetic fields (EMF) practically all sites of a spectrum of radio frequencies. Proceeding from above-stated studying of behaviour of a photo synthesizing cell functioning under action photosynthetic active radiation is interesting at simultaneous action of electromagnetic radiation of high frequency
[en] Pety et al. have reported the detection of eight transitions of a closed-shell, linear molecule (B11244) in observations toward the Horsehead photodissociation region (PDR), which they attribute to the l-C3H+ cation. Recent high-level ab initio calculations have called this assignment into question; the anionic C3H– molecule has been suggested as a more likely candidate. Here, we examine observations of the Horsehead PDR, Sgr B2(N), TMC-1, and IRC+10216 in the context of both l-C3H+ and C3H–. We find no observational evidence of Ka = 1 lines, which should be present were the carrier indeed C3H–. Additionally, we find a strong anticorrelation between the presence of known molecular anions and B11244 in these regions. Finally, we discuss the formation and destruction chemistry of C3H– in the context of the physical conditions in the regions. Based on these results, we conclude there is little evidence to support the claim that the carrier is C3H–.