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Description
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 40-900GHz frequency range. Lines from ^13^C isotopically substituted ethyl cyanide have been recently identified in Orion. 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: CH_2_DCH_2_CN (in-plane and out-of-plane) and CH_3_CHDCN and the spectrum of ^15^N substituted ethyl cyanide CH_2_DCH_2_C^15^N. The rotational spectrum of these species in the ground state was measured in the microwave and millimeter-submillimeter wavelength range using waveguide Fourier transform spectrometers (4-40GHz) and a source-modulated spectrometer employing backward-wave oscillators (BWOs) (150-260 and 580-660GHz). More than 300 lines were identified for each species for J values up to 71-80 and K_a_ values up to 28-31 depending on the isotopologues. The experimental spectra were analysed using a Watson's Hamiltonian using A-reduction in I^r^ representation. From the fitting procedure, accurate spectroscopic constants were derived for each species. These new sets of spectroscopic constants enable us to compute reliable prediction of the rotational spectrum (lines frequency and intensity) in the 4-1000GHz frequency range and for J and K_a_ up to 80 and 31, respectively. This experimental study allowed us to detect for the first time ^15^N substituted ethyl cyanide CH_2_DCH_2_C^15^N in Orion. The derived column density and rotational temperature are 10^13^cm^-2^ and 150K for the plateau and 3x10^14^cm^-2^ and 300K for the hot core. The deuterated species were search for but are not detected. The upper limit on the column density of each deuterated isotopologues is 10^14^cm^-2^.
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