Tetroses as a Unique Benchmark for Gas-Phase Carbohydrates: From Cyclic Aldoses to Open-Chain Ketoses

Tetroses are the smallest carbohydrates for which covalent topology, ring closure, and cooperative intramolecular OH···O hydrogen bonding must be treated as a single structural problem. High-resolution gas-phase rotational data anchor both cyclic aldose and open-chain ketose motifs within this family, making tetroses an exacting test of whether the accuracy ladder developed for smaller hydrogen-bonded prototypes transfers to genuine carbohydrates. We combine rotational constants and diagnostic infrared markers through a composite strategy in which each observable is treated at the level that controls its dominant error. Pair-natural-orbital coupled-cluster structures, combined with affordable anharmonic rotational corrections, reproduce the microwave-observed d-erythrose furanoses and the open-chain erythrulose conformer with spectroscopic accuracy. A dual-level vibrational protocol supplies diagnostic band assignments for the observed species and predictive infrared markers for conformers that remain experimentally uncharacterized. The same structural diagnosis motivates a topology-corrected reconstruction based on double-hybrid functionals, with covalent and calibrated H···O contact corrections redistributed through a weighted Cartesian framework. The resulting ladder is validated on d-erythrose and erythrulose and then used to provide search data for d-threose. The central outcome is that cyclic and open-chain tetroses can be described on the same footing without a uniformly expensive treatment of every contribution: bonds, interaction-driven hydrogen-bond motifs, and vibrational averaging are each assigned to the rung at which their dominant error is controlled.