Isotopic ethyl cyanide $^{\mathsf{13}}$CH$_\mathsf{3}$CH$_\mathsf{2}$CN, CH$_\mathsf{3}$$^{\mathsf{13}}$CH$_\mathsf{2}$CN, and CH$_\mathsf{3}$CH$_\mathsf{2}$$^{\mathsf{13}}$CN: laboratory rotational spectrum and detection in Orion

Context. Astronomical spectra of hot molecular clouds in the wavelength range from centimeter to submillimeter show a huge number of rotational lines due to the emission of complex organic molecules, and a large fraction of these lines are unidentified. The assignment of these unidentified lines to new molecules, to known molecules in excited states, or to their isotopologues requires a good knowledge of the spectroscopic parameters of these molecules. Aims. We present the experimental study of the spectroscopic properties of 13 C-substituted ethyl cyanide 13 CH 3 CH 2 CN, CH 3 13 CH 2 CN, and CH 3 CH 2 13 CN. Methods. The rotational spectra of the three species in the ground state have been measured in the frequency ranges from 5 to 26 GHz using waveguide Fourier transform spectrometers and from 160 to 360 GHz using a source-modulated spectrometer employing backward-wave oscillators (BWOs). Results. A new accurate set of spectroscopic constants has been determined for each isotopic species. This permits prediction of the position of rotational lines that are best suited for detection with an accuracy of a few hundreds of kHz. The three isotopologues have been detected in an Orion IRc2 IRAM survey via several hundred of lines, illustrating that many "unidentified" bands are definitely due to isotopologues of known molecules.