  An on-going Arecibo line search between 1.1 and 10 GHz of the prototypical
starburst/megamaser galaxy, Arp 220, has revealed a spectrum rich in molecular
transitions. These include the ``pre-biotic'' molecules: methanimine
(CH$_{2}$NH) in emission, three $v_{2}=1$ direct l-type absorption lines of
HCN, and an absorption feature likely to be from either $^{18}$OH or formic
acid (HCOOH). In addition, we report the detection of two, possibly three,
transitions of $\lambda$4-cm excited OH not previously detected in Arp~220
which are seen in absorption, and a possible absorption feature from the
6.668-GHz line of methanol. This marks the first distant extragalactic
detection of methanimine, a pre-biotic molecule. Also, if confirmed, the
possible methanol absorption line presented here would represent the first
extragalactic detection of methanol at a distance further than 10 Mpc. In
addition, the strong, previously undetected, cm-wave HCN $v_{2}=1$ direct
l-type lines will aid the study of dense molecular gas and active star-forming
regions in this starburst galaxy.
