# SOLEIL and ALMA views on prototypical organic nitriles: C$_2$H$_5$CN

### Abstract

The high resolution vibrational spectrum of ethyl cyanide (C2H5CN) has been investigated in the far-IR using synchrotron-based Fourier transform spectroscopy. The assignment was performed using the Automated Spectral Assignment Procedure (ASAP) allowing accurate rotational energy levels of the four lowest fundamental vibrations of the species, namely the $v_{13}=1$ @ 205.934099(8) cm$^{-1}$, and $v_{21}=1$ @ 212.141101(8) cm$^{-1}$, $v_{20}=1$ @ 372.635293(15) cm$^{-1}$, $v_{12} =1$ @ 532.699617(16) cm$^{-1}$ states, to be determined. The analysis not only confirms the applicability of the ASAP in the treatment of (dense) high-resolution infrared spectra but also reveals some of its limitations. Complementary to the infrared study, the pure rotational spectrum of C2H5CN was also studied in selected frequency ranges from 75 to 255 GHz. New observations of a prototypical high-mass star-forming region, G327.3$-$0.6, performed with the Atacama Large Millimeter Array show that vibrational satellites of C2H5CN can be very intense, of order several tens of Kelvin in units of brightness temperature.

Publication
The Journal of Molecular Spectroscopy