Atmospheric photo-oxidation of myrcene: OH reaction rate constant, gas-phase oxidation products and radical budgets

The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a Large Reaction Chamber). The chemical structure of myrcene consists of one moiety that is a conjugated π system (similar to isoprene) and another moiety that is a triple-substituted olefinic unit (similar to 2-methyl-2-butene). Hydrogen shift reactions of organic peroxy radicals (RO2) formed in the reaction of isoprene with atmospheric OH radicals are known to be of importance for the regeneration of OH. Structure–activity relationships (SARs) suggest that similar hydrogen shift reactions like in isoprene may apply to the isoprenyl part of RO2 radicals formed during the OH oxidation of myrcene. In addition, SAR predicts further isomerization reactions that would be competitive with bimolecular RO2 reactions for chemical conditions that are typical for forested environments with low concentrations of nitric oxide. Assuming that OH peroxy radicals can rapidly interconvert by addition and elimination of O2 like in isoprene, bulk isomerization rate constants of 0.21 and 0.097 s−1 (T=298 K) for the three isomers resulting from the 3′-OH and 1-OH addition, respectively, can be derived from SAR. Measurements of radicals and trace gases in the experiments allowed us to calculate radical production and destruction rates, which are expected to be balanced. The largest discrepancies between production and destruction rates were found for RO2. Additional loss of organic peroxy radicals due to isomerization reactions could explain the observed discrepancies. The uncertainty of the total radical (ROx=OH+HO2+RO2) production rates was high due to the uncertainty in the yield of radicals from myrcene ozonolysis. However, results indicate that radical production can only be balanced if the reaction rate constant of the reaction between hydroperoxy (HO2) and RO2 radicals derived from myrcene is lower (0.9 to 1.6×10-11 cm3 s−1) than predicted by SAR. Another explanation of the discrepancies would be that a significant fraction of products (yield: 0.3 to 0.6) from these reactions include OH and HO2 radicals instead of radical-terminating organic peroxides. Experiments also allowed us to determine the yields of organic oxidation products acetone (yield: 0.45±0.08) and formaldehyde (yield: 0.35±0.08). Acetone and formaldehyde are produced from different oxidation pathways, so that yields of these compounds reflect the branching ratios of the initial OH addition to myrcene. Yields determined in the experiments are consistent with branching ratios expected from SAR. The yield of organic nitrate was determined from the gas-phase budget analysis of reactive oxidized nitrogen in the chamber, giving a value of 0.13±0.03. In addition, the reaction rate constant for myrcene + OH was determined from the measured myrcene concentration, yielding a value of (2.3±0.3)×10-10 cm3 s−1.

Fuel Selections for Electrified Vehicles: A Well-to-Wheel Analysis

Electrified vehicles (xEV), especially the battery electric vehicle (BEV), are burgeoning and growing fast in China, aimed at building a sustainable, carbon-neutral future. This work presents an overview and quantitative comparison of the carbon-neutral vehicles fuel options based on the well-to-wheel (WTW) analysis. A more intuitionistic figure demonstrates the fuel options for greenhouse gas (GHG) emissions and describes the sustainability. Electricity and hydrogen shift the tailpipe emissions to the upstream process, forming larger WTW emissions from a fuel cycle view. The electricity WTW GHG emission reaches as much as twice that of gasoline. However, the high efficiency of the electric drive system improves the WTW emission performance from a vehicle view, making the lowest WTW emission of BEV. The fuel options’ technical and environmental perspectives are presented. Finally, long-term carbon-neutral vehicle development is discussed.

Atmospheric photo-oxidation of myrcene: OH reaction rate constant, gas phase oxidation products and radical budgets

The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR. The chemical structure of myrcene consists of one moiety that is a conjugated π-system (similar to isoprene) and another moiety that is a triple-substituted olefinic unit (similar to 2-methyl-2-butene). Hydrogen shift reactions of organic peroxy radicals RO2 formed in the reaction of isoprene with atmospheric OH radicals are known to be of importance for the regeneration of OH. Structure-activity relationships (SAR) suggest that similar hydrogen shift reactions like in isoprene may apply to the isoprenyl part of RO2 radicals formed during the OH oxidation of myrcene. In addition, SAR predicts further isomerization reactions that would be competitive with bi-molecular RO2 reactions for chemical conditions that are typical for forested environments with low concentrations of nitric oxide. Assuming that OH peroxy radicals can rapidly interconvert by addition and elimination of O2 like in isoprene, bulk isomerization rate constants of 0.21 s−1 and 0.097 s−1 (T = 298 K) for the 3 isomers resulting from the 3'-OH and 1-OH addition, respectively, can be derived from SAR. Measurements of radicals and trace gases in the experiments allowed to calculate radical production and destruction rates, which are expected to be balanced. Largest discrepancies between production and destruction rates were found for RO2. Additional loss of organic peroxy radicals due to isomerization reactions could explain the observed discrepancies. The uncertainty of the total radical (ROx = OH+HO2+RO2) production rates were high due to the uncertainty in the yield of radicals from myrcene ozonolysis. However, results indicate that radical production can only be balanced, if the reaction rate constant of the reaction between hydroperoxy (HO2) and RO2 radicals derived from myrcene is lower (0.9 to 1.6 × 10−11 cm3 s−1) than predicted by SAR. Another explanation of the discrepancies would be that a significant fraction of products (yield: 0.3 to 0.6) from these reactions include OH and HO2 radicals instead of radical terminating organic peroxides. Experiments also allowed to determine the yields of organic oxidation products acetone (yield: 0.45 ± 0.08) and formaldehyde (yield: 0.35 ± 0.08). Acetone and formaldehyde are produced from different oxidation pathways, so that yields of these compounds reflect the branching ratios of the initial OH addition to myrcene. Yields determined in the experiments are consistent with branching ratios expected from SAR. The yield of organic nitrate was determined from the gas-phase budget analysis of reactive oxidized nitrogen in the chamber giving a value of 0.13 ± 0.03. In addition, the reaction rate constant for myrcene + OH was determined from the measured myrcene concentration yielding a value of (2.3 ± 0.3) × 10−10 cm3 s−1.

Synthesis of densely substituted pyridine derivatives from nitriles by a non-classical [4+2] cycloaddition/1,5-hydrogen shift strategy

A novel strategy has been established to assemble an array of densely substituted pyridine derivatives from nitriles and o-substituted aryl alkynes or 1-methyl-1,3-enynes via a non-classical [4 + 2] cycloaddition along with 1,5-hydrogen shift process. The well-balanced affinities of two different alkali metal salts enable the C(sp3)-H bond activation as well as the excellent chemo- and regioselectivities. This protocol offers a new guide to construct pyridine frameworks from nitriles with sp3-carbon pronucleophiles, and shows potential applications in organic synthesis and medicinal chemistry.

Molecular mechanism for rapid autoxidation in α-pinene ozonolysis

Aerosol affects Earth’s climate and the health of its inhabitants. A major contributor to aerosol formation is the oxidation of volatile organic compounds. Monoterpenes are an important class of volatile organic compounds, and recent research demonstrate that they can be converted to low-volatility aerosol precursors on sub-second timescales following a single oxidant attack. The α-pinene + O3 system is particularly efficient in this regard. However, the actual mechanism behind this conversion is not understood. The key challenge is the steric strain created by the cyclobutyl ring in the oxidation products. This strain hinders subsequent unimolecular hydrogen-shift reactions essential for lowering volatility. Using quantum chemical calculations and targeted experiments, we show that the excess energy from the initial ozonolysis reaction can lead to novel oxidation intermediates without steric strain, allowing the rapid formation of products with up to 8 oxygen atoms. This is likely a key route for atmospheric organic aerosol formation.