  Ethyl cyanide is an abundant molecule in hot molecular clouds. Lines from 13C
isotopically substituted ethyl cyanide were 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: CH2DCH2CN (in-plane and out-of-plane) and CH3CHDCN and the
spectrum of15N substituted ethyl cyanide CH3CH2C15N. 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). 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 15N substituted ethyl cyanide 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.
