2012A&A...538A.119T

Astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range to be able to identify them. We obtained very accurate spectroscopic constants in a comprehensive laboratory analysis of rotational spectra. These constants can be used to predict transition frequencies that were not measured in the laboratory very precisely. We present an experimental study and a theoretical analysis of two ¹⁸O-methyl formate isotopologues, which were subsequently detected for the first time in Orion KL. The experimental spectra of both methyl formate isotopologues recorded in the microwave and sub-mm range from 1 to 660GHz. Both spectra were analysed by using the rho-axis method (RAM) which takes into account the CH₃ internal rotation. We obtained spectroscopic constants of both ¹⁸O- methyl formate with high accuracy. Thousands of transitions were assigned and others predicted, which allowed us to detect both species in the IRAM 30m line survey of Orion KL.