The 130-370 GHz rotational spectrum of phenyl isocyanide (C₆H₅NC)

The analysis of phenyl isocyanide (C₆H₅NC, μ_a = 4.0 D) in its ground vibrational state and two lowest-energy excited vibrational states, ν₂₂ (141 cm^-1) and ν₃₃ (155 cm^-1), in the 130-370 GHz frequency region has been completed. Over 4500 new rotational transitions have been measured in the ground vibrational state for the most abundant isotopologue, resulting in the determination of the spectroscopic constants for a partial octic Hamiltonian with low error. The Coriolis-coupled ν₂₂-ν₃₃ dyad reported herein, containing over 3500 new transitions for each vibrational state, has been analyzed for the first time. The coupled-state least-squares fit utilizes seven coupling terms (G_a, G_a ^J, G_a ^K, G_a ^JJ, G_a ^JK, F_bc, and F_bc ^K) to address perturbation between the two vibrational states, including resonances and several nominal interstate transitions. This work results in precise determination of the energy separation between the two states, ΔE_22,33 = 9.682 248(3) cm^-1, and the Coriolis coupling coefficient, |ζ_22,33 ^a| = 0.858(9). The precise rotational and distortion constants determined in this work provide the foundation for an astronomical search for phenyl isocyanide across the radio band.