We propose to search for two molecules --CH2 and NCCN-- that
 are not yet detected in the interstellar medium and for
 which ISO due to its high spectral resolution, high
 sensitivity and access outside the atmospheric windows,
 provides the only opportunity to detect them.

 The CH2 radical is a simple hydride molecule, which is an
 important building block for other larger molecules.  Such
 unsaturated hydrides are thought to be formed mostly by
 ion-molecule chemistry.  In contrast, grain-surface
 chemistry produces more saturated species.  From comparison
 of the measured CH2 abundance with those of CH3 and CH4 to
 be obtained in our guaranteed time proposal, we will be able
 to establish the relative importance of the two chemistries.

 Many nitrogen bearing molecules have been found varying from
 the very simple CN to the long HC11N carbon chain.  Given
 the molecular complexity of interstellar and circumstellar
 clouds, there should be many more nitrogen bearing
 molecules, e.g.  NCCN.  NCCN has no dipole moment and
 therefore no strong rotational transitions, leaving only the
 ro-vibrational bands at 42 micron as a way to observe this
 molecule.  This exotic molecule is detected in the
 atmosphere of Titan and can be the first of a completely new
 class of nitrogen-bearing carbon chains to be discovered in
 the ISM.