Mechanical and electrical anharmonicity in the infrared spectra of graphene fragments: From planar polycyclic aromatic hydrocarbons to buckybowls

Abstract

Polycyclic aromatic hydrocarbons (PAHs) and their curved buckybowl derivatives are key motifs in combustion, atmospheric, and astrochemical environments, as well as fundamental building blocks of graphene-based materials. Here, we show that the Pisa composite schemes provide an accurate and affordable framework for predicting equilibrium structures, rotational constants, and vibrational spectra across representative planar (indene and azulene) and curved (corannulene and sumanene) PAHs. Accurate equilibrium geometries are combined with a fully anharmonic treatment based on generalized second-order vibrational perturbation theory, including both mechanical and electrical contributions to infrared intensities. This enables reliable prediction not only of fundamental frequencies but also of overtone and combination-band intensities, which are strictly absent in the harmonic approximation. Indene and azulene validate the structural and spectroscopic accuracy of the protocol for planar π-systems, whereas corannulene and sumanene quantify the effects of bowl curvature on mode mixing, symmetry breaking, and activation of otherwise forbidden transitions. Across all systems, the proposed computational approach yields fundamental frequencies with typical deviations of 5–10 cm−1, reproduces curvature-dependent shifts in C–H stretching and ring-deformation modes, and captures characteristic splitting patterns and intensity redistribution induced by both mechanical and electrical anharmonicity.