We have observed 37 Infrared Dark Clouds (IRDCs), containing a total of 159clumps, in high-density molecular tracers at 3 mm using the 22-meter ATNF MopraTelescope located in Australia. After determining kinematic distances, weeliminated clumps that are not located in IRDCs and clumps with a separationbetween them of less than one Mopra beam. Our final sample consists of 92 IRDCclumps. The most commonly detected molecular lines are (detection rates higherthan 8%): N2H+, HNC, HN13C, HCO+, H13CO+, HCN, C2H, HC3N, HNCO, and SiO. Weinvestigate the behavior of the different molecular tracers and look forchemical variations as a function of an evolutionary sequence based on SpitzerIRAC and MIPS emission. We find that the molecular tracers behave differentlythrough the evolutionary sequence and some of them can be used to yield usefulrelative age information. The presence of HNC and N2H+ lines do not depend onthe star formation activity. On the other hand, HC3N, HNCO, and SiO arepredominantly detected in later stages of evolution. Optical depth calculationsshow that in IRDC clumps the N2H+ line is optically thin, the C2H line ismoderately optically thick, and HNC and HCO+ are optically thick. The HCNhyperfine transitions are blended, and, in addition, show self-absorbed lineprofiles and extended wing emission. These factors combined prevent the use ofHCN hyperfine transitions for the calculation of physical parameters. Totalcolumn densities of the different molecules, except C2H, increase with theevolutionary stage of the clumps. Molecular abundances increase with theevolutionary stage for N2H+ and HCO+. The N2H+/HCO+ and N2H+/HNC abudanceratios act as chemical clocks, increasing with the evolution of the clumps.
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