Review of OCS gas-phase reactions in dark cloud chemical models

The association reaction S + CO {\to} OCS + hnu has been identified as being particularly important for the prediction of gas-phase OCS abundances by chemical models of dark clouds. We performed detailed ab-initio calculations for this process in addition to undertaking an extensive review of the neutral-neutral reactions involving this species which might be important in such environments. The rate constant for this association reaction was estimated to be several orders of magnitude smaller than the one present in current astrochemical databases. The new rate for this reaction and the introduction of other processes, notably OH + CS {\to} OCS + H and C + OCS {\to} CO + CS, dramatically changes the OCS gas-phase abundance predicted by chemical models for dark clouds. The disagreement with observations in TMC-1 (CP) and L134N (N), suggests that OCS may be formed on grain surfaces as is the case for methanol. The observation of solid OCS on interstellar ices supports this hypothesis.Comment: Accepted for publication in MNRA

ULTRA-LOW TEMPERATURE KINETICS OF NEUTRAL-NEUTRAL REACTIONS - NEW EXPERIMENTAL AND THEORETICAL RESULTS FOR OH+HBR BETWEEN 295-K AND 23-K

The first determination of the rate of reaction of OH radicals with HBr at temperatures below 249 K is reported. Rate constants measured from 295 to 23 K increase monotonically with decrease in temperature and are faster than has previously been thought at the temperatures present in the mid and low stratosphere. The observed negative temperature dependence is well predicted by a simple formula deduced from quantum scattering calculations employing the rotating bond approximation. © 1994 American Institute of Physics

Theoretical study of the reaction CH(X-2 Pi)+NO(X-2 Pi). 3. Determination of the branching ratios

In this paper, which is the third of a series devoted to the title reaction, we present theoretical calculations of branching ratios for the product channels involved in the reaction. In the first paper of this series (Marchand, N.; Jimeno, P.; Rayez, J. C.; Liotard, D. J. Phys. Chem. 1997, 101, 6077.), we explored the topology of the lowest triplet potential energy surface determined with sophisticated ab initio methods and proposed several reaction paths connecting the reactants to the products. We have used these results to determine the branching ratios using two methods based on multichannel Rice-Ramsperger-Kassel-Marcus (RRKM) calculations: a μVTST/RRKM (μVTST = microcanonical variational transition state theory) method developed by one of us and an ACIOSA/RRKM (ACIOSA = adiabatic capture model using the infinite order sudden approximation) method dealing with a capture rate constant calculation (Marchand, N.; Stoecklin, T.; Rayez, J. C. To be submitted, of this series). Our present results reveal that, at 300 K, HCN + O is the major product channel involved in the reaction (72.0%), the other branching ratios being 13.9% for NCO + H, 8.2% for CO + NH, 3.3% for CNO + H, and 1.4% for CN + OH. All the others channels contribute for less than 1% each. These theoretical results are in agreement with the results of several experimental studies, especially those very recently obtained in our laboratory by Bergeat et al. Moreover, we observe no significant temperature dependence of the branching ratios