  We present 3 mm observations obtained with the IRAM 30-m telescope for ten
nearby gas-rich active galaxies spanning three orders of magnitude in infrared
luminosity. Emission lines of HCO+(1-0), HCN(1-0), and C2H(1-0) were
simultaneously detected in most galaxies of our sample. We also tentatively
detected the rare isotopic species, H13CN, HC15N and H13CO+ in up to four
galaxies (IC 694, NGC 3690, NGC 4258 and NGC 6240). Our estimation shows that
the optical depth of HCN is low to moderate in these galaxies (tau ~ 1-5).
After comparing the intensity ratios of different molecular emission lines
(HCN/HCO+, C2H/HCN, and C2H/HCO+) with the infrared luminosity (LIR), we find
that in the infrared luminous sample of five composite (AGN+Starburst)
galaxies, these ratios vary with LIR: the HCN/HCO+ ratio increases with LIR,
which is consistent with previous studies, while there is a slight trend in the
composite galaxies that both C2H/HCN and C2H/HCO+ ratios decrease with
increasing LIR. Although our sample is limited, this trend may possibly imply a
relation between the overall gas density and the infrared luminosity of
galaxies.
