Accurate Structures and Spectroscopic Parameters of CN-Substituted Polycyclic Hydrocarbons at DFT Cost

The structures, isomerization energies, and rotational and vibrational spectra of prototypical CN-substituted polycyclic hydrocarbons in the gas phase have been analyzed using a general computational strategy based on Pisa composite schemes (PCS) and second-order vibrational perturbation theory (VPT2). The final results obtained in this way show, in most cases, relative average deviations with respect to experimental rotational constants close to 0.1%, corresponding to errors of around 1 mÅ and 0.1° for bond lengths and valence angles, respectively. At the same time, fundamental IR absorption bands are reproduced with average deviations below 10 cm^-1 without any scaling factor. In addition to the intrinsic interest of the studied molecules, this work confirms that spectroscopic studies of large systems can be supported by unsupervised computational tools that couple accuracy with reasonable cost.