Rotational spectrum of deuterated and $\mathsf{^{15}}$N ethyl cyanides: CH3CHDCN and CH2DCH2CN and of CH3CH2C$\mathsf{^{15}}$N

Abstract

Context. Ethyl cyanide is an abundant molecule in hot molecular clouds. Its rotational spectrum is very dense and several hundreds of rotational transitions within the ground state have been identified in molecular clouds in the 40−900 GHz frequency range. Lines from 13 C isotopically substituted ethyl cyanide were identified in Orion. Aims. To enable the search and the possible detection of other isotopologues of ethyl cyanide in interstellar objects, we have studied the rotational spectrum of deuterated ethyl cyanide: CH2DCH2CN (in-plane and out-of-plane) and CH3CHDCN and the spectrum of 15 N substituted ethyl cyanide CH3CH2C 15 N. Using these experimental data, we have searched for these species in Orion. Methods. The rotational spectrum of each species in the ground state was measured in the microwave and millimeter-submillimeter wavelength range using a waveguide Fourier Transform spectrometer (8−17 GHz) and a source-modulated spectrometer employing backward-wave oscillators (BWOs) (150−260 and 580−660 GHz). More than 300 lines were identified for each species, for J values in the range 71−80 and Ka values in the range 28−31 depending on the isotopologues. The experimental spectra were analyzed using a Watson’s Hamiltonian in the A-reduction. Results. From the fitting procedure, accurate spectroscopic constants were derived for each of the species. These new sets of spectroscopic constants enable us to predict reliably the rotational spectrum (lines frequencies and intensities) in the 4−1000 GHz frequency range and for J and Ka up to 80 and 31, respectively. Combined with IRAM 30 m antenna observations of Orion, this experimental study allowed us to detect 15 N substituted ethyl cyanide CH3CH2C 15 N for the first time in Orion. The derived column density and rotational temperature are 10 13 cm −2 and 150 K for the plateau and 3 × 10 14 cm −2 and 300 K for the hot core. The deuterated species were searched for but were not detected. The upper limit to the column density of each deuterated isotopologues was 10 14 cm −2 .