The reaction of O((3)P) with C(2)H(4), of importance in combustion and atmospheric chemistry, stands out as paradigm reaction involving not only the indicated triplet state potential energy surface (PES) but also an interleaved singlet PES that is coupled to the triplet surface. This reaction poses great challenges for theory and experiment, owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Crossed molecular beam (CMB) scattering experiments with soft electron ionization detection are used to disentangle the dynamics of this polyatomic multichannel reaction at a collision energy E(c) of 8.4 kcal∕mol. Five different primary products have been identified and characterized, which correspond to the five exothermic competing channels leading to H + CH(2)CHO, H + CH(3)CO, CH(3) + HCO, CH(2) + H(2)CO, and H(2) + CH(2)CO. These experiments extend our previous CMB work at higher collision energy (E(c) ∼ 13 kcal∕mol) and when the results are combined with the literature branching ratios from kinetics experiments at room temperature (E(c) ∼ 1 kcal∕mol), permit to explore the variation of the branching ratios over a wide range of collision energies. In a synergistic fashion, full-dimensional, QCT surface hopping calculations of the O((3)P) + C(2)H(4) reaction using ab initio PESs for the singlet and triplet states and their coupling, are reported at collision energies corresponding to the CMB and the kinetics ones. Both theory and experiment find almost an equal contribution from the triplet and singlet surfaces to the reaction, as seen from the collision energy dependence of branching ratios of product channels and extent of intersystem crossing (ISC). Further detailed comparisons at the level of angular distributions and translational energy distributions are made between theory and experiment for the three primary radical channel products, H + CH(2)CHO, CH(3) + HCO, and CH(2) + H(2)CO. The very good agreement between theory and experiment indicates that QCT surface-hopping calculations, using reliable coupled multidimensional PESs, can yield accurate dynamical information for polyatomic multichannel reactions in which ISC plays an important role.

Sulfur appears to be depleted by an order of magnitude or more from its elemental abundance in star-forming regions. In the last few years, numerous observations and experiments have been performed in order to to understand the reasons behind this depletion without providing a satisfactory explanation of the sulfur chemistry towards high-mass star-forming cores. Several sulfur-bearing molecules have been observed in these regions, and yet none are abundant enough to make up the gas-phase deficit. Where, then, does this hidden sulfur reside? This paper represents a step forward in our understanding of the interactions among the various S-bearing species. We have incorporated recent experimental and theoretical data into a chemical model of a hot molecular core in order to see whether they give any indication of the identity of the sulfur sink in these dense regions. Despite our model producing reasonable agreement with both solid-phase and gas-phase abundances of many sulfur-bearing species, we find that the sulfur residue detected in recent experiments takes up only ∼6 per cent of the available sulfur in our simulations, rather than dominating the sulfur budget.

We report on the determination of primary products and their branching ratios for the combustion relevant O(3P)+allene reaction by the crossed molecular beams method with soft electron-ionization mass-spectrometric detection at a collision energy of 39.3 kJ/mol. We have explored the reaction dynamics of the open channels leading to C2H4+CO, C2H2+H2CO, C2H3+HCO, CH2CCHO+H, and CH2CO+CH2. Because some of the observed products can only be formed via intersystem crossing (ISC) from triplet to singlet potential energy surfaces, from the product branching ratios we have inferred the extent of ISC. The conclusion is that the O(3P)+allene reaction proceeds mostly (>90%) via ISC. This observation poses the question of how important it is to consider nonadiabatic effects for this and other similar systems involved in combustion chemistry. Another important conclusion is that the interaction of atomic oxygen with allene breaks apart the three-carbon atom chain, mostly producing CO and ethylene.

Recent laboratory experiments have demonstrated that, even though contribution from other reaction channels cannot be neglected, unsaturated hydrocarbons easily break their multiple C-C bonds to form CO after their interactions with atomic oxygen. Here we present an upgraded chemical modelling including a revision of the reactions between oxygen atoms and small unsaturated hydrocarbons for different astrochemical environments. A first conclusion is that towards hot cores/corinos atomic oxygen easily degrades unsaturated hydrocarbons directly to CO or to its precursor species (such as HCCO or HCO) and destroys the double or triple bond of alkenes and alkynes. Therefore, environments rich in atomic oxygen at a relatively high temperature are not expected to be rich in large unsaturated hydrocarbons or polycyclic aromatic hydrocarbons. On the contrary, in O-poor and C-rich objects, hydrocarbon growth can occur to a large extent. In addition, new radical species, namely ketyl and vinoxy radicals, generated from other reaction channels can influence the abundances of hydrocarbons towards hot cores. We, therefore, suggest they should be included in the available databases. Hydrocarbon column densities are calculated in the 10 13 -10 15 cm −2 range, in good agreement with their observed values, despite the small number of data currently published in the literature.