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101.
Maranzana A Ghigo G Tonachini G Barker JR 《The journal of physical chemistry. A》2008,112(16):3656-3665
This paper (part 1) and the following one (part 2) aim to assess the viability of some tropospheric oxidation channels for two symmetrical alkynes, ethyne (acetylene) and but-2-yne. Paper 1 defines the features of the DFT(B3LYP)/6-311G(3df,2p) energy hypersurface and qualitatively considers the practicability of different pathways through the estimate of free energy barriers. Paper 2 will assess this in more detail by way of master equation simulations. Oxidized in the presence of HO and O2 (with the possible intervention of NO), ethyne and but-2-yne are known to produce mainly glyoxal or dimethylglyoxal and, to a lesser extent, formic or acetic acid. The initial attack by HO gives an adduct, from which several pathways (1a-c, 2a-e) originate. Pathway 1a passes through the 2-oxoethyl (vinoxyl) radical, or the analogous dimethyl-substituted intermediate, which could in principle undergo O2 addition (and subsequently, but through a demanding step, give the dialdehydes). However, in paper 2 it is assessed that the vinoxyl, as a nonthermalized intermediate, will preferentially follow unimolecular pathways to ketene or acetyl. Pathway 2a is the most important pathway: a very steep free energy cascade, started by O2 addition to the initial HO adduct with a concerted barrierless 1,5 H shift, gives a hydroperoxyalkenyloxyl radical intermediate. Peroxy bond cleavage finally produces the dialdehydes and regenerates HO. Pathways 2b and 2c originate from O2 addition to the initial HO adduct and produce, via different ring closures, either dioxetanyl or alkyl dioxiranyl radicals, respectively. Two subsequent fragmentations occur in both cases and give the carboxylic acids and a carbonyl radical, which can indirectly generate hydroxyl. Two further pathways (1c and 2e) see NO intervention onto the peroxyl radicals formed along pathways 1 and 2. Both could enhance dialdehyde production, while simultaneously depressing the carboxylic acid yield. 相似文献
102.
Ghigo G Causà M Maranzana A Tonachini G 《The journal of physical chemistry. A》2006,110(49):13270-13282
The viability of some nitration pathways is explored for benzene (B), naphthalene (N), and in part pyrene (P). In principle, functionalization can either take place by direct nitration (NO2 or N2O5 attack) or be initiated by more reactive species, as the nitrate and hydroxyl radicals. The direct attack of the NO2 radical on B and N, followed by abstraction of the H geminal to the nitro group (most likely accomplished by 3O2) could yield the final nitro-derivatives. Nevertheless, the initial step (NO2 attack) involves significant free energy barriers. N2O5 proves to be an even worst nitrating agent. These results rule out direct nitration at room temperature. Instead, NO3 and, even more easily, HO can form pi-delocalized nitroxy- or hydroxycyclohexadienyl radicals. A subsequent NO2 attack can produce several regio- and diastereoisomers of nitroxy-nitro or hydroxy-nitro cyclohexadienes. In this respect, the competition between NO2 and O2 is considered: the rate ratios are such to indicate that the NO3 and HO initiated pathways are the major source of nitroarenes. Finally, if the two substituents are 1,2-trans, either a HNO3 or a H2O concerted elimination can give the nitro-derivatives. Whereas HNO3 elimination is feasible, H2O elimination presents, by contrast, a high barrier. Under combustion conditions the NO2 direct nitration pathway is more feasible, but remains a minor channel. 相似文献
103.
Carlo Carlini Marco Martinelli Elisa Passaglia AnnaMaria Raspolli Galletti Glauco Sbrana 《Macromolecular rapid communications》2001,22(9):664-668
Novel catalytic systems based on bis‐(chelate)nickel(II) precursors, such as bis(α‐nitroacetophenonate)nickel(II) [Ni(naph)2] and bis(2,6‐diisopropylbenzenesalicylaldiminate)nickel(II) [Ni(dipbs)2], and methylaluminoxane (MAO) as the cocatalyst were employed for the polymerization of methyl methacrylate (MMA). Reaction parameters were examined. Under proper conditions, the Ni(dipbs)2/MAO system allowed to obtain poly(MMA) with a very high productivity (TOF up to 70 000 h–1) and a remarkable syndiospecificity degree (rr > 80%) at room temperature without addition of an ancillary Lewis base. 相似文献