Exploiting the "Lego brick" approach to predict accurate molecular structures of PAHs and PANHs

Polycyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic nitrogen heterocycles (PANHs) are important and ubiquitous species in space. However, their accurate structural and spectroscopic characterization is often missing. To fill this gap, we exploit the so-called "Lego brick" approach [Melli et al., J. Phys. Chem. A, 2021, 125, 9904] to evaluate accurate rotational constants of some astrochemically relevant PAHs and PANHs. This model is based on the assumption that a molecular system can be seen as formed by smaller fragments for which a very accurate equilibrium structure is available. Within this model, the "template molecule" (TM) approach is employed to account for the modifications occurring when going from the isolated fragment to the molecular system under investigation, with the "linear regression" model being exploited to correct the linkage between different fragments. In the present work, semi-experimental equilibrium structures are used within the TM model. The performance of the "Lego brick" approach has been first tested for a set of small PA(N)Hs for which experimental data are available, thus leading to the conclusion that it is able to provide rotational constants with a relative accuracy well within 0.1%. Subsequently, it has been extended to the accurate prediction of the rotational constants for systems lacking any spectroscopic characterization.