Terahertz spectroscopy of isotopic acrylonitrile

► Rotational spectra of isotopic acrylonitrile studied up to 1.2 THz. ► Perturbations between g.s. and the v 11 = 1 state fitted for four isotopic species. ► Six new doubly substituted isotopic species of acrylonitrile assigned. ► Semi-experimental equilibrium geometry determined. Rotational spectra of four isotopically enriched, singly substituted species of acrylonitrile have been studied up to 1.2 THz. Extensive analysis of the spectra recorded for H 2 13C CHCN, H 2C CH 13CN, H 2C CHC 15N, and H 2C CDCN, revealed the presence of the same characteristic perturbations between the ground state and the v 11 = 1 excited vibrational state, that have recently been identified in the parent molecule. For this reason transitions in v 11 = 1 have also been assigned in each of the four isotopic species and a coupled state analysis of g.s. and v 11 = 1 was performed on a total of around 3000 lines for each species. The derived precise values of E( v 11 = 1) are found to be consistent with estimates from anharmonic force field calculations. In addition, transitions for six new doubly substituted isotopic species of acrylonitrile, H 2 13C 13CHCN, H 2 13C CH 13CN, H 2C 13CH 13CN, H 2 13C CHC 15N, H 2C 13CHC 15N, and H 2C CH 13C 15N, have been assigned, and their ground state spectroscopic constants have been determined. Rotational constants for all known isotopic species of acrylonitrile have been combined with ab initio calculation of vibration–rotation constants in the first evaluation of the r e SE geometry of this molecule.