Infrared spectra of dimethylnaphthalenes in the gas phase

We report the infrared spectra at 0.5 cm−1 resolution of three dimethylnaphthalenes (DMNs), namely 1,5-DMN, 1,6-DMN and 2,6-DMN in the gas phase at an elevated temperature recorded with the help of a variable path-length cell. DFT calculations have been carried out at B3LYP levels of theory with 6-31G* as basis set to determine the harmonic frequencies and intensities of the DMNs to assign the experimentally observed spectra. We have compared the experimentally observed and theoretically calculated spectra of the dimethylnaphthalenes and assignments have been made. The observed and predicted frequencies and relative intensities are generally in good agreement. The intense aromatic C–H out-of-plane bending vibration observed around 800 cm−1 and three methyl C–H symmetric and asymmetric stretching vibrations around 2900 cm−1 have been recognized as unique bands to identify various DMNs in a mixture. The high-resolution IR spectroscopy of these three important polycyclic aromatic hydrocarbons which are present in the atmosphere have been discussed.