1. Could you explain the significance of your article to the non-specialist? (50-100 words)

Around 135 molecules have been detected in the interstellar medium, most of them in the cores of interstellar clouds. Despite the very low temperature (10-20 K), these species participate in a rich chemistry. Efforts to understand this chemistry proceed by way of large chemical models, which require, as input, the rate constants for chemical reactions between ions and neutral molecules and between pairs of neutral species. Many of the rates cannot be measured in laboratory experiments - at least at the very low temperatures of interest. This paper suggests how low temperature rate constants for reactions between free radicals and unsaturated molecules might be estimated. The reactions of oxygen atoms with alkenes and alkynes could represent an important test case, and ways to determine their rate constants are proposed.

2. What has motivated you to conduct this work?

There were two chief motivations. First, to provide better estimates of rate constants for some potentially important reactions in astrochemical models. Second, to encourage low temperature measurements on oxygen atoms reactions to see how well the simple basis for predicting low temperature reactivity, which is proposed in the paper, stands up in practice.

3. Where do you see this work developing in the future?

Some 'future developments' were already reported at the Discussion meeting. Prompted by this paper, rate constants have been measured for the reactions of oxygen atoms with several alkenes and alkynes in one of the low temperature CRESU apparatuses in Rennes (see remarks by I. R. Sims in the Discussion). It was no real surprise to find that a more sophisticated transition state theory treatment than that described in the paper is required to explain the observed rate constants and their temperature dependence (see remarks by IWMS in the Discussion). We (IRS and IWMS) are currently collaborating with Stephen Klippenstein (a theoretician at Argonne National Laboratory) to provide a detailed explanation of these results.

4. Are there any particular challenges facing future research in this area?

You bet!! The ultimate aim is to devise a model which not only reproduces the abundances of observed molecules but which can also predict the abundances of species that have not yet been observed. Such models might then predict the synthesis of larger molecules than those that have been identified and may also play a role in explaining how stars are formed as dense cloud cores coalesce and heat up under the action of gravity.