Gas-phase kinetics of CH3CHO with OH radicals between 11.7 and 177.5 K

Gas-phase reactions in the interstellar medium (ISM) are a source of molecules in this environment. The knowledge of the rate coefficient for neutral-neutral reactions as a function of temperature, k(T), is essential to improve astrochemical models. In this work, we have experimentally measured k(T) for the reaction between the OH radical and acetaldehyde, both present in many sources of the ISM. Laser techniques coupled to a CRESU system were used to perform the kinetic measurements. The obtained modified Arrhenius equation is k(T = 11.7-177.5 K) = (1.2 ± 0.2) × 10-11 (T/300 K)-(1.8±0.1) exp-{(28.7 ± 2.5)/T} cm3 molecule-1 s-1. The k(T) value of the title reaction has been measured for the first time below 60 K. No pressure dependence of k(T) was observed at ca. 21, 50, 64 and 106 K. Finally, a pure gas-phase model indicates that the title reaction could become the main CH3CO formation pathway in dark molecular clouds, assuming that CH3CO is the main reaction product at 10 K.

Propane and propane-water interactions: a study at cryogenic temperatures

The phase transition of solid propane and a propane-water mixture under ultrahigh vacuum has been investigated using reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption mass spectrometry (TPD-MS). Here, the investigation is divided into two sections: the phase transition of pure propane and the interaction of propane with water. RAIR spectra of pure propane reveal an unknown crystalline phase at 50 K (phase I), which gradually converts to a known crystalline phase (phase II) at higher temperature. This conversion is associated with certain kinetics. Co-deposition of water and propane restricts the amorphous to crystalline phase transition, while sequential deposition (H₂O@C₃H₈; propane over predeposited water) does not hinder it. For an alternative sequential deposition (C₃H₈@H₂O; water over predeposited propane), the phase transition is hindered due to diffusional mixing within the given experimental time, which is attributed to the reason behind the restricted phase transition.

OXIDATION MECHANISM OF THE BUTADIYNYL RADICAL, C4H: ANALOGUE OF C2H OR NOT?

Reactions of the carbon-chain radicals are of great importance in the combustion and astrophysical processes. The kinetics of the butadiynyl radical, C 4 H , has received recent attention. While there has been sufficient knowledge concerning the oxidation of the ethynyl radical, C 2 H , oxidation of the higher even-numbered members C 2n H (n > 1) is hardly known. In this paper, to enrich the C 4 H -chemistry, we report the first study of the oxidation mechanism of C 4 H . At the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(d,p)+ZPVE level, the potential energy surface (PES) survey is presented covering various product channels P1( CO + HC 3 O ) (-152.7 kcal/mol), P2( C 3 H + CO 2) (-117.9), P3( HCO + C 3 O ) (-108.5), P4( HC 4 O +3 O ) (-45.2), and P5( OH + C 4 O ) (-33.2) accompanied by the master equation rate constant calculations. Despite the similarity in the PES, the kinetics of C 4 H +3 O 2 differs dramatically from that of the analogous C 2 H +3 O 2 reaction. For the C 4 H +3 O 2 reaction, the O -abstraction product P4( HC 4 O +3 O ) is almost the exclusive product, whereas the lowest C , O -exchange product P1( CO + HC 3 O ) and other products have little importance. By contrast, the C 2 H +3 O 2 reaction favors the C , O -exchange product HCO + CO . Being overall barrierless and mainly associated with the molecular → atomic oxygen conversion, the C 4 H +3 O 2 reaction should play an important role in the soot formation and interstellar chemistry where C 4 H is involved.