Millimeter-wave spectrum of 4-cyanopyridine in its ground state and lowest-energy vibrationally excited states, ν20 and ν30

Abstract

The rotational spectrum of 4-cyanopyridine (μa = 1.96 D) was recorded from 130 to 360 GHz, and the analysis of the ground state and two lowest-energy excited vibrational states was completed. Over 3900 new rotational transitions were measured for the ground state, allowing the determination of spectroscopic constants for a partial octic, distorted-rotor Hamiltonian. Over 5600 new transitions were measured for the Coriolis-coupled dyad, ν20 and ν30. A coupled-state least-squares fit of the dyad was obtained, which resulted in the precise determination of several parameters addressing the Coriolis coupling between the two states: Ga, GaJ, Fbc, and FbcK. With inclusion of these terms, numerous resonance transitions associated with selection rules ΔKa = 2 or ΔKa = 4 between vibrational states were assigned and fit to low error (σfit < 50 kHz). The precise energy difference between ν20 and ν30 was determined, ΔE = 18.806554 (11) cm−1, along with the Coriolis coupling coefficient ζ20,30a = 0.8432 (8). Determination of the spectroscopic and perturbation parameters for the vibrational states permits comparison to other arenes bearing –CN or –NC substituents and sharing a similar Coriolis-coupled dyad ~150 cm−1 above the ground state. This new data provides the foundation for an astrochemical search for 4-cyanopyridine in the interstellar medium.