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.