Context. Massive stars play an important role in shaping the structure of galaxies. Infrared dark clouds (IRDCs), with their low temperatures and high densities, have been identified as the potential birthplaces of massive stars. In order to understand the formation processes of massive stars, the physical and chemical conditions in infrared dark clouds have to be characterized. Aims. The goal of this paper is to investigate the chemical composition of a sample of southern infrared dark clouds. One important aspect of the observations is to check, whether the molecular abundances in IRDCs are similar to the low-mass pre-stellar cores, or if they show signatures of more evolved evolutionary stages. Methods. We performed observations toward 15 IRDCs in the frequency range between 86 and 93?GHz using the 22-m Mopra radio telescope. In total, 13 molecular species comprising N2H+, 13CS, CH3CN, HC3N, HNC, HCO+, HCN, HNCO, C2H, SiO, H13CO+, H13CN, and CH3C2H were observed for all targets. Hence, we included in general species appropriate for elevated densities, where some of them trace the more quiescent gas, while others are sensitive to more dynamical processes. Results. We detect HNC, HCO+, and HNC emission in all clouds and N2H+ in all IRDCs except one. In some clouds we detect SiO?emission. Complicated shapes of the HCO+ emission line profile are found in all IRDCs. Both signatures indicate infall and outflow motions and the beginning of star-formation activity, at least in some parts of the IRDCs. Where possible, we calculate molecular abundances and make a comparison with previously obtained values for low-mass pre-stellar cores and high-mass protostellar objects (HMPOs). We show a tendency for IRDCs to have molecular abundances similar to low-mass pre-stellar cores rather than to HMPOs abundances on the scale of our single-dish observations.
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