An analysis of the rotational ground state and lowest-energy vibrationally excited dyad of 3-cyanopyridine: Low symmetry reveals rich complexity of perturbations, couplings, and interstate transitions

The rotational spectrum of 3-cyanopyridine from 130 to 360 GHz was recorded, and an analysis of the ground state and two lowest-energy excited vibrational states was completed. Almost 6700 new transitions were measured for the ground state and fit to a partial octic, distorted-rotor Hamiltonian with low error (sigma(fit) < 0.05 MHz). The first two excited vibrational states, nu(30) and nu(21), are an isolated dyad that exhibits both a- and b-type Coriolis perturbations and requires a two-state, least-squares fit to fully predict the rotational spectrum and determine accurate spectroscopic constants. Quartic and sextic distortion constants were determined for the dyad, along with seven symmetry-allowed perturbation terms: G(a),G(a)',F-bc(K),G(b), G(b)', and G(b)(K). Numerous resonances, including those following a-type selection rules, Delta K-a = 2 or Delta K-a = 4, and b-type selection rules, Delta K-a = 3 or Delta K-a = 5, were observed and fit. For nu(30) and nu(21), the energy difference (Delta E-30,E-21 = 15.7524693 (37) cm(-1)), both Coriolis coupling constants (zeta(a)(30,21) = 0.8327 (9) and zeta(b)(30,21) = -0.0181 (3)), and vibration-rotation interaction constants were deter-mined experimentally and compared to theoretical values determined computationally. Combined with the work on the vibrationally excited dyads of 4-cyanopyridine, phenyl isocyanide, benzonitrile, and phenylacetylene, the coupling in nu(30) and nu(21) provides an opportunity to compare the Coriolis interactions of these analogous mono-substituted aromatic molecules in unusual detail. Additionally, this work improves the ground-state rotational constants and centrifugal distortion constants of 3-cyanopyridine and provides the fundamental constants needed to support an astronomical search for 3-cyanopyridine in the interstellar medium. (C) 2020 Elsevier Inc. All rights reserved.