Anharmonicity in the Vibrational Spectra of Naphthalene and Naphthalene-d₈: Experiment and Theory

In this paper, we report the gas phase infrared (IR) spectra of naphthalene and naphthalene-d₈ recorded in the mid-infrared region (3200-500 cm⁻¹) using a heated multipass long path gas cell. Several combination bands appear as shoulders and satellite peaks in the 3200-2600 cm⁻¹region along with the C-H stretch fundamental bands. Experimental IR spectra of these molecules were systematically analyzed with vibrational self-consistent field (VSCF) theory, vibrational second order perturbation theory (VPT2) and vibrational couple cluster method (VCCM) with two different potential energy surfaces obtained using B3LYP and MP2 methods. A comparative study between these two PESs was made to match the observed spectra. Final assignment of the IR spectra of naphthalene and naphthalene-d₈ was done using the VCCM with MP2 potential, which provided the best match.