From Localized to Delocalized OH···O Hydrogen Bonds: Benchmark of Hierarchical Quantum Chemical Methods against...

Intramolecular OH···O hydrogen bonds remain difficult to describe at spectroscopic accuracy. Electronic correlation, vibrational averaging, and conformational flexibility all influence the same rotational constants and the corresponding structural parameters, often on a comparable scale. This is especially critical for oxygen-rich molecules of astrochemical and biochemical interest, where OH···O(R), OH···O═C, and OH···OH contacts coexist and can approach quasi-symmetric proton sharing. Here we benchmark a hierarchy of quantum chemical methods against rotational spectroscopy for hydroxyaldehydes, hydroxyethers, hydroxyacids, polyols, and the limiting case of malonaldehyde. The analysis is organized according to the actual computational layers of the protocol: equilibrium geometries, empirical geometrical corrections, local-correlation approximations, vibrational corrections, and final comparison with experimental ground-state rotational constants. Direct comparison with parent and deuterated rotational constants is combined with semiexperimental equilibrium structures whenever isotopic information is sufficient, thereby separating equilibrium-geometry errors from vibrational contributions. The resulting picture is clear. Double-hybrid and bond-corrected models remain useful low-cost approximations, but their accuracy deteriorates as covalent bond-based transferability breaks down in hydrogen-bonded systems. By contrast, inclusion of local-correlation in the explicitly correlated coupled-cluster ansatz emerges as the most robust reduced-cost approximations in the present set and, especially at the equilibrium-geometry layer, often approaches the quality of the reference conventional model (PCS2). The comparison between these two variants therefore primarily tests the error introduced by the local-correlation approximation, whereas the comparison between bare DFT and its bond-corrected variant addresses the transferability of empirical structural corrections. The structural analysis shows that the dominant residual error is concentrated in the O···H contact, whose variation is typically 1 order of magnitude larger than that of individual covalent parameters. This shift is redistributed over the full hydrogen-bonded pseudocycle, explaining why direct spectroscopic benchmarks are more discriminating than inspection of isolated local coordinates. Because PCS2 already reproduces the rotational constants accurately, it can serve as an internal structural reference within the present accuracy target, and the semiexperimental analysis then identifies the OH···O contact as the natural target for extending local-regression ideas from covalent bonds to hydrogen-bond interactions. At the same time, the present OH···O-specific mapping is intentionally preliminary and should be regarded as a proof of concept based on a limited set of localized systems, not yet as a generally validated correction framework. Malonaldehyde is treated separately in this respect, because its experimental rotational constants are tunneling-averaged dynamical observables rather than direct observables of a single localized equilibrium structure. These results define a practical hierarchy for extending spectroscopic-accuracy structural predictions to larger carbohydrates and related hydrogen-bonded systems.