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.

Automated summary

This study utilizes the Lego brick approach to accurately determine the rotational constants of PAHs and PANHs, which are important species in space. The approach treats the molecular system as smaller fragments with known equilibrium structures and uses a template molecule approach and linear regression model to account for modifications and correct the linkage between fragments. The results demonstrate that the Lego brick approach can provide rotational constants with high relative accuracy, making it a valuable tool for predicting systems without spectroscopic characterization.