Rotational spectroscopy meets composite Pisa schemes: Structural determination of the atmospheric pollutants 1- and 2-nitronaphthalene.

Polycyclic aromatic hydrocarbons (PAHs) are widespread air pollutants in the Earth's atmosphere. Understanding their structure is relevant for the study of their interactions with other atmospheric molecules and their participation in aerosol formation. Here we present the investigation of the structures of the most abundant nitrated PAHs in the atmosphere, 1-nitronaphthalene (1NN) and 2-nitronaphthalene (2NN), using high-resolution chirped pulse Fourier transform microwave spectroscopy and quantum-chemical calculations. The rotational constants, centrifugal distortion constants and N nuclear quadrupole coupling constants for both 1NN and 2NN were determined. The observation of the heavy atom isotopologues in natural abundance led to the determination of experimental structures using substitution, effective, and mass-weighted methods. Moreover, semi-experimental equilibrium structures for 1NN and 2NN were determined by using a reduced-cost protocol based on the Pisa Composite Schemes, which combines the experimental heavy-atom isotopologue data with hydrogen atom positions and vibrational corrections obtained from affordable quantum chemical calculations. The resulting semi-experimental equilibrium structures achieve spectroscopic accuracy, demonstrating the robustness and transferability of this strategy for nitroaromatic systems. Comparison of the bond lengths and angles of 1NN and 2NN with those of related substituted naphthalenes enabled identification of common structural trends.