Lew Snyder thought that the vacuum we assumed to be space was a lot more than that. He knew that many chemical compounds are dipolar. These have one positive side and one negative side, like a magnet. He also knew that charged particles in motion release energy. If these are in space, floating freely they could spin and emit radio waves. More importantly, each type of molecule would broadcast at a set frequency. These could be received distantly to determine the make up of regions of space.
There were setbacks due to initial resistance, but after Nobel prize-winning physicist Charles Townes identified ammonia molecules in space... it was game on. In 1969 the hunt began in earnest. The first molecules that Snyder's team identified was Formaldehyde (4.8 GHz.) The list grew from there to include over 150 different molecules. More here.
The way this works is that every molecule hypothetically. Linear molecules have a simple microwave spectrum, with lines at integer multiples of a fundamental frequency. Some of the data is direct and some of it is deduced. You cant just look for a molecule; sometimes you have to look for the components that would product a reaction creating a particular molecule. In the case of HCN (Hydrogen Cyanide) they looked for two CH3 + N = HCN + H2 and 3CH2 + N = HCN + H. It sounds complicated because it is. You can find the heavy math here. I'll summarize as best as I can, (since I barely understand it.)
My understanding of thsi comes from spectroscopy. The spectral line is a dark or bright line in an otherwise uniform and continuous spectrum. It could be deficiency or excess of photons in a narrow frequency range so it's "bright" or "dark" in comparison. They are the result of molecules (in this case) interacting with photons. When the photon has the right amount of energy it can be absorbed by the molecule and then re-emitted. If the molecule returns to it's original state this emission happens at the same frequency as the original absorption. But they also occur in the millimeter-submillimeter portion of the spectrum and that is directly related to the result from molecular rotational transitions. The emissions will correlate to the isotopes that the molecules contains. They can have one spectral line, or many. You can see HCl here.
If you happen to score some time on a radio telescope you can start by looking for a few of the known molecules:
- Formaldehyde - 4.8 GHz
- Hydrogen Cyanide - 88.6 GHz
- Isocyanic Acid - 87.9 GHz
- Deuterated Ammonia - 389 GHz
- Semiheavy water (HDO) - 464 GHz
- Acetonitrile - 220.7 GHz
- Sulfur Dioxide - 222 GHz
- HC15N - 258.16 GHz
- Cyanopolyyne - 345.34 GHz