Millimeter wave spectra of ethyl isocyanate and searches for it in Orion KL and Sgr B2.

Context: Relatively high abundances of methyl isocyanate (CH3 NCO), a methyl derivative of isocyanic acid (HNCO), found in the Orion KL and Sgr B2 molecular clouds suggest that its ethyl derivative, ethyl isocyanate (CH3 CH2 NCO), may also be present.

Aims: The aim of this work is to provide accurate experimental frequencies of ethyl isocyanate in its ground and excited vibrational states in the millimeter wave region to support searches for it in the interstellar medium.

Methods: The rotational spectrum of ethyl isocyanate was recorded at room temperature from 80 to 340 GHz using the millimeter wave spectrometer in Valladolid. Assigned rotational transitions were analyzed using the S -reduced semirigid-rotor Hamiltonian.

Results: More than 1100 distinct frequency lines were analyzed for the ground vibrational state of the cis conformer as well as for three vibrational satellites corresponding to successive excitation of the lowest-energy C-N torsional mode. Newly determined rotational and centrifugal distortion constants were used for searches of spectral features of ethyl isocyanate in Orion KL and Sgr B2 clouds. Upper limits to CH3 CH2 NCO in these high-mass star-forming regions were obtained.