Infrared Spectroscopy of Cyanonaphthalenes under Interstellar Relevant Conditions and Their Potential Connection with Astronomical Aromatic Infrared Bands
Context. Aromatic infrared bands (AIBs) are widely observed in diverse astrophysical environments and are generally attributed to vibrational emission from polycyclic aromatic hydrocarbons (PAHs). The recent interstellar detection of 1-cyanonaphthalene (1-CNN) and 2-cyanonaphthalene (2-CNN) has motivated detailed infrared spectroscopic studies of cyano-substituted PAHs. Aims. We aim to characterize the infrared spectra and vibrational modes of neutral 1-CNN and 2-CNN under cold and gas-phase conditions and to assess their possible spectroscopic relevance to the astronomical AIBs. Methods. The gas-phase infrared spectra of neutral 1-CNN and 2-CNN were measured in a cold molecular beam using ion-dip spectroscopy. The observed bands were assigned with the aid of harmonic and anharmonic calculations at the B3LYP/N07D level. Infrared emission spectra were subsequently simulated from the experimental spectra within a single-photon approximation framework. Results. We report the infrared spectra of neutral 1-CNN and 2-CNN measured under cold and gas-phase conditions relevant to the interstellar medium. Their vibrational features were assigned in detail, including fundamental vibrations as well as overtone and combination bands. The simulated emission spectra exhibit features in several wavelength regions associated with prominent AIBs, including the aromatic CH stretching region near 3.3 micron, the CC stretching region near 6.2 micron, the mixed CH in-plane bending and CC stretching region at 8.6-8.9 microns, and the CH out-of-plane bending region between 10 and 15 microns. Conclusions. The present spectra provide laboratory reference data for small cyano-substituted PAHs and offer useful clues for interpreting selected AIB regions. These results suggest that cyanonaphthalene molecules are promising contributors to the aromatic infrared bands.