Analysis of the terahertz rotational spectrum of the three mono-13C ethyl cyanides (13C–CH3CH2CN) C. Richard, L. Margulès, R. A. Motiyenko and J.-C. Guillemin A&A, 543 (2012) A135 Published online: 11 July 2012 DOI: 10.1051/0004-6361/201219713