Gas-Phase Far-Infrared and Rotational Spectroscopy of 1- and 2-Cyanonaphthalene: Experiment and Theory

Cyanonaphthalenes (C10H7CN) are bicyclic, nitrogen-substituted polycyclic aromatic hydrocarbons and the first polycyclic species detected in the interstellar medium via radioastronomy. Here, we report a combined experimental and computational study of 1- and 2-cyanonaphthalene, extending and improving the spectroscopic characterization of their rotational and vibrational spectra. Gas-phase far-infrared vibrational spectra were measured in the 50–650 cm–1 region with a Fourier-transform infrared spectrometer, providing the first gas-phase experimental vibrational band centers for these molecules. Pure rotational spectra were recorded between 75 and 220 GHz using chirped-pulse Fourier-transform and source-frequency-modulation spectroscopy, enabling extended observation of transitions in the ground vibrational states as well as the first spectroscopic characterization of low-lying vibrational states of energies lower than 200 cm–1. While density functional theory calculations guided initial spectral assignments, high-level coupled cluster computations were also performed and compared with experiment, yielding highly accurate fundamental vibrational frequencies and rotational constants for both ground and excited vibrational states.