2013A&A...558A...6C

New radiotelescopes, such as the very sensitive ALMA, will enable the detection of interstellar molecules in much lower concentrations than previously possible. A successful identification of an interstellar molecule requires that laboratory microwave and millimeter-wave spectra are investigated. Several cyanopolyynes and alkynylcarbonitriles have already been detected in the interstellar medium (ISM). Cyanoacetylene (HC=C-C=N) is abundant in the ISM and its methyl derivative, 2-butynenitrile (CH₃C=C-C=N), is also present. The next derivative, ethyl cyanoacetylene, (2-pentynenitrile C₂H₅C=C-C=N) may also be present in interstellar space. We report the rotational spectrum of the ethyl cyanoacetylene (C₂H₅C=C-C=N). This is hoped to facilitate identifying gaseous ethyl cyanoacetylene in the ISM. We studied the rotational spectrum of C₂H₅C=C-C=N between 13 and 116 GHz with the microwave spectrometer of the University of Oslo. The spectroscopic study was augmented by high-level quantum-chemical calculations at B3LYP/cc-pVTZ and CCSD/cc-pVTZ levels of theory. We present for the first time the rotational spectrum of the ethyl cyanoacetylene (C₂H₅C=C-C=N). We assigned 342 transitions of the vibrational ground state, accurate values were obtained for rotational and centrifugal distortion constants, and the dipole moment was determined as well.