  We have observed 37 Infrared Dark Clouds (IRDCs), containing a total of 159
clumps, in high-density molecular tracers at 3 mm using the 22-meter ATNF Mopra
Telescope located in Australia. After determining kinematic distances, we
eliminated clumps that are not located in IRDCs and clumps with a separation
between them of less than one Mopra beam. Our final sample consists of 92 IRDC
clumps. The most commonly detected molecular lines are (detection rates higher
than 8%): N2H+, HNC, HN13C, HCO+, H13CO+, HCN, C2H, HC3N, HNCO, and SiO. We
investigate the behavior of the different molecular tracers and look for
chemical variations as a function of an evolutionary sequence based on Spitzer
IRAC and MIPS emission. We find that the molecular tracers behave differently
through the evolutionary sequence and some of them can be used to yield useful
relative age information. The presence of HNC and N2H+ lines do not depend on
the star formation activity. On the other hand, HC3N, HNCO, and SiO are
predominantly detected in later stages of evolution. Optical depth calculations
show that in IRDC clumps the N2H+ line is optically thin, the C2H line is
moderately optically thick, and HNC and HCO+ are optically thick. The HCN
hyperfine transitions are blended, and, in addition, show self-absorbed line
profiles and extended wing emission. These factors combined prevent the use of
HCN hyperfine transitions for the calculation of physical parameters. Total
column densities of the different molecules, except C2H, increase with the
evolutionary stage of the clumps. Molecular abundances increase with the
evolutionary stage for N2H+ and HCO+. The N2H+/HCO+ and N2H+/HNC abudance
ratios act as chemical clocks, increasing with the evolution of the clumps.
