Localized dual-level corrections for anharmonic O-H spectroscopy in hydrogen-bonded hydroxy acids.

Reliable assignments of spectroscopic features for hydrogen-bonded hydroxy acids require more than optimized structures and scaled harmonic spectra, because conjugation, multiple O-H groups, and intramolecular hydrogen bonds make different parts of the force field respond differently to the electronic-structure level. This work develops an assignment protocol in which microwave rotational constants validate the conformer structures, while infrared peak lists identify the local O-H coordinates that require higher-level information. The protocol is tested on a chemically ordered series: phenol, benzoic acid, salicylic acid, gallic acid, and L-ascorbic acid. Affordable second-order vibrational perturbation theory provides the anharmonic spectral background, and higher-level harmonic calculations supply the local correction needed for selected O-H stretching features. The rotational constants show that, once the equilibrium geometry is refined, the lower-level vibration-rotation corrections are adequate; the dominant lower-level errors are therefore concentrated in the geometry and harmonic force field rather than in the vibration-rotation correction itself. Phenol and benzoic acid define one-substituent phenolic and carboxylic baselines, salicylic acid adds a single intramolecular hydrogen bond, gallic acid tests a four-O-H aromatic manifold on two microwave-observed rotamers, and L-ascorbic acid extends the same analysis to three nearly isoenergetic conformers with competing hydrogen-bond networks. The resulting criterion is deliberately narrow: retain the full affordable anharmonic spectrum where it is stable, and import higher-level information only where the local O-H force constant controls the assignment. Direct gas-phase validation, condensed-phase infrared anchors, and genuine predictions are kept separate throughout.