The 130 – 750 GHz rotational spectrum of 2-cyanopyridine – Analysis of the ground vibrational state and the Coriolis-coupled dyad of its lowest-energy fundamental states

The millimeter-wave rotational spectrum of 2-cyanopyridine was collected from 130 to 750 GHz, and the ground and two lowest-energy excited vibrational states were analyzed. In total, over 20,000 rotational transitions were least-squares fit for the three vibrational states to partial-octic, distorted-rotor Hamiltonians with low error (σ_fit < 50 kHz). For the ground state, the many thousands of newly measured rotational transitions enabled substantial refinement of the rotational constants and determination of the centrifugal distortion constants. The rotational spectrum was collected at room temperature, permitting the observation of the two lowest-energy fundamental modes, ν₃₀ (A″, 154 cm⁻¹) and ν₂₁ (A′, 175 cm⁻¹), and determination of their spectroscopic constants. The two excited vibrational states are Coriolis coupled and require a two-state Hamiltonian. Eight Coriolis-coupling parameters ($G_a,G_a^J,G_a^K,G_a^{\mathrm{JJ}},F_{\mathrm{bc}},F_{\mathrm{bc}}^J,G_b,$and $G_b^J$) have been determined, as well as a precise energy difference of 26.524 312 6 (40) cm⁻¹ between the vibrational states. A comparison of the ground-state spectroscopic constants, as well as the Coriolis coupling-related parameters of analogous dyads is presented for multiple cyanoarenes.