The 103–360 GHz rotational spectrum of benzonitrile, the first interstellar benzene derivative detected by radioastronomy

Recently, several cm-wave transitions of benzonitrile have been detected in the interstellar medium (TMC-1). In this work, the mm-wave spectrum of benzonitrile (C₆H₅CN, C_2v, μ_a = 4.5 D), a planar asymmetric rotor molecule, is reported in the 103–360 GHz frequency region. Over 3000 rotational transitions have been newly measured for the ground state and were combined with previous data in a global fit, which allows predictions of both low-temperature hyperfine resolved and high-frequency unresolved rotational spectra. Rotational transitions in the lowest fundamentals, ν₂₂ and ν₃₃ (141 and 163 cm⁻¹, respectively), have been observed for the first time, and were complemented by identification of several nominal interstate transitions resulting from strong interstate mixing. Application of a two-state, Coriolis-coupled model accounted, to within experimental precision, for many identified interstate perturbations resulting in ΔE_22,33 = 19.108185(7) cm⁻¹ and |ζ_22,33 ^a| = 0.841(7). This study provides insight into the generic problem affecting the lowest excited vibrational states of this molecular class and describes the techniques allowing its successful analysis. The new data provide the foundation for future remote detection applications of benzonitrile.