Accurate yet Affordable Molecular Structures: Rotational and Vibrational Spectroscopic Parameters of Substituted Aromatic Nitriles

An affordable and scalable computational strategy rooted in the Pisa Composite Schemes (PCS) framework is applied to the challenging case of substituted aromatic nitriles, delivering accurate molecular structures together with rotational and vibrational spectroscopic parameters at a computational cost comparable to that of standard density functional theory. Two semiexperimental (SE) equilibrium structures are derived from high-resolution rotational spectroscopy combined with computed vibrational corrections. These structures are found to be significantly more accurate than literature substitution (rs) geometries, thus providing stringent reference data for method validation. For one system, the fully parameter-free PPCS2 approach also offers an independent and consistent structural cross-check. Building on these benchmarks, a cost-effective dual-level strategy combining double-hybrid equilibrium geometries and harmonic force fields with hybrid-level anharmonic contributions is identified as an efficient operative level of theory (DPCS3//HPCS2). Further refinement of selected bond lengths through one-parameter effective corrections (BDPCS3 model) yields ground-state rotational constants in excellent agreement with the experiment, with typical deviations well below 0.1%. The validated protocol is applied to ethynylbenzonitrile (EBN) and hydroxybenzonitrile (HBN) isomers, yielding anharmonic infrared spectra in outstanding agreement with the experiment for the ortho and para HBN species and providing predictive, high-confidence reference data for the remaining isomers. Overall, this work demonstrates that near-spectroscopic accuracy for both rotational and vibrational observables can be achieved at affordable computational cost by treating electronic correlation and vibrational effects on an equal footing, thereby enabling reliable multispectroscopic characterization of substituted aromatic nitriles of astrochemical relevance.