Rotational spectrum of ethyl cyanoacetylene (C

Context. 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. Aims. 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. Methods. 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. Results. 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