Vibrational perturbations in the spectra of N-substituted polycyclic aromatic heterocycles revealed by rotational...

N-bearing derivatives of polycyclic aromatic hydrocarbons (PANHs) are considered to be relatively abundant in the interstellar medium. Thanks to their significant permanent dipole moment, several pure rotation spectroscopy studies of PANHs were performed in the microwave and millimeter-wave ranges enabling radio astronomical searches, but were unsuccessful. In contrast, very few studies reported rotationally resolved spectra of the vibrational bands of PANHs in the far-infrared region. In the continuity of a recent high resolution rovibrational study of two-ring centrosymmetric PAHs [S. Chawananon, O. Pirali, M. Goubet and P. Asselin, J. Chem. Phys., 2022, 157, 064301], the present work targets four PAHs with one or two N atoms substituted on one ring, namely quinoline and isoquinoline (C₉H₇N) and quinazoline and quinoxaline (C₈H₆N₂), mainly using jet-cooled infrared laser experiments in both in-plane ring C-H bending and C-C stretching regions. Very distinct spectral behaviors are observed: on one hand, quinoline and quinoxaline exhibit unperturbed a-type band contours leading to derivation of accurate spectroscopic parameters for five vibrationally excited states of quinoline and one excited state of quinoxaline. In the case of isoquinoline and, to a lesser extent, quinazoline, the presence of vibrational perturbations revealed by distorted Q-branches could be systematically correlated with the presence of a closely lying vibrational mode and related to vibrational interactions and coupling effects. The rovibrational analyses of these perturbed states underlined substantial variations with respect to calculated rotational constants leading to experimental inertial defect values far from theoretical predictions. The larger number of vibrational modes perturbed in isoquinoline compared to quinoline is finally discussed on the grounds of density of states in both spectral regions investigated.