Rotational spectrum of 13C{2}-methyl formate (HCOO13CH{3}) and detection of the two 13C-methyl formate in Orion

Context: Laboratory measurements and analysis of the microwave and millimeter-wave spectra of potential interstellar molecules are a prerequisite for their subsequent identification by radioastronomical techniques. The spectral analysis provides spectroscopic parameters that are used in the assignment procedure of the laboratory spectra, and that also predict the frequencies of transitions not measured in the laboratory with a high degree of precision.
Aims: An experimental laboratory study and its theoretical analysis is presented for ¹³C2-methyl formate (HCOO¹³CH3) allowing a search for this isotopologue in the Orion molecular cloud. The ¹³C1-methyl formate (H¹³COOCH3) molecule was also searched for in this interstellar cloud, using previously published spectroscopic data.
Methods: The experimental spectra of ¹³C2-methyl formate were recorded in the microwave and sub-mm energy ranges (4-20 GHz, 8-80 GHz, 150-700 GHz). The spectra were analyzed using the Rho-Axis Method (RAM), which takes the CH3 internal rotation and the coupling between internal rotation and global rotation into account.
Results: Twenty-seven spectroscopic constants of ¹³C2-methyl formate have been obtained from a fit of 936 transitions of the ground torsional state with a standard (unitless) deviation of 1.08. A prediction of line positions and intensities is also produced. This prediction allowed us to identify 230 ¹³C2-methyl formate lines in the Orion interstellar molecular cloud. We refitted all previously published ground state transitions of the ¹³C1-methyl formate molecule in order to provide a prediction of its ground state spectrum. 234 lines of ¹³C1-methyl formate were detected in the Orion interstellar cloud using that prediction.

Tables A.1-A.5 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/500/1109