Integrated Computational Workflow for Molecular Spectroscopy: Rotational and Vibrational Signatures of Phenalene and Isomeric Dihydronaphthalenes

Spectroscopic techniques play a central role in the structural characterization and identification of molecular systems. In this context, the combined use of complementary spectroscopic methods is becoming increasingly important, as different observables probe distinct aspects of the molecular structure and dynamics. Achieving accurate and internally consistent theoretical predictions across multiple spectroscopic domains remains computationally demanding for medium-sized molecular systems, including polycyclic aromatic hydrocarbons (PAHs) and their derivatives. Here, we present a standardized computational protocol designed to deliver accurate rotational and vibrational spectra at a computational cost that remains accessible. The approach integrates double-hybrid functionals, one-parameter bond-length corrections, dual-level treatments of harmonic and anharmonic effects, and automated spectral simulations within a single, internally consistent framework. Starting from a SMILES string or a preliminary molecular scaffold, the workflow enables the determination of a comprehensive set of parameters, including equilibrium geometries, dipole moments, rotational constants (with vibrational corrections), centrifugal distortion constants, anharmonic vibrational frequencies, and infrared intensities, with accuracy close to the spectroscopic level. Applications to phenalene and the 1,2- and 1,4-dihydronaphthalene isomers confirm the expected level of accuracy and allow direct comparison of the simulated spectra to their experimental counterparts. Overall, this approach provides a practical tool for the interpretation of laboratory infrared and microwave spectra and the structural characterization of molecular systems of an intermediate size.