This month we look at the history of efforts to find the trans rotamer of ethyl methyl ether (trans-C₂H₅OCH₃, tEME) in the interstellar medium. As far as astromolecules go, this is a moderately large organic molecule (in reality, organic molecules can be composed of hundreds of atoms). A claimed detection from a 2005 paper was disputed in 2015, but another group offered a more definitive but still tentative detection the same year. In spite of noting matches to 46 unblended lines of tEME in spectra toward Orion KL, Tercero et al. limited their claim to a tentative detection. They made their observations with the IRAM 30m telescope.

The earliest evidence for the existence of interstellar tEME was reported by Charnley et al. in 2001. However, they only found a match for a single line (the 5₃₂-4₂₃ transition) in two sources, which is not sufficient for a molecule with a complex rotational spectrum.

The first detection of tEME based on multiple line matches was reported by Fuchs et al. in 2005. Their experimental spectrum for tEME is shown above. They based their claim on matches to over 20 transitions toward W51e2. However, in 2015 Carroll et al. disputed the claim of detection toward W51e1/e2 and also did not find tEME toward Sgr B2(N-LMH). Carroll et al. found multiple difficulties with the claims of Fuchs et al. regarding line intensities and other issues. Due to the complexity of the spectra of large molecules with little or no symmetry and the high probability of coincidental matches in sources with high line density, extreme care must be taken before claiming a detection.