From Gas-Phase Spectra to Transferable Vitamin Building Blocks: Rotational and Infrared Benchmarks

Vitamin-relevant molecular motifs combine heteroatom networks, hydrogen bonds, conjugated polyenes, saturated and fused rings, benzopyran units, and quinone cores. We present an integrated gas-phase validation strategy in which high-resolution rotational spectroscopy and infrared spectroscopy test complementary observables of the same computed structures. Ground-state rotational constants, after anharmonic vibrational correction, provide a quantitative structural anchor through the global mass distribution; gas-phase infrared spectra and tabulated vapor-phase band envelopes probe the local force fields responsible for diagnostic functional-group and fingerprint signatures. Compact fragments are benchmarked against explicitly correlated references, while larger motifs are treated with bond-corrected double-hybrid geometries and validated directly against experiment. The scope is deliberately reference-oriented. Rather than attempting to reproduce physiological condensed-phase environments, the work establishes isolatedmolecule benchmarks for tractable vitamins and transferable building blocks for larger, more flexible, or environment-dependent vitamin systems. The protocol is applied to complete vitamin molecules, nicotinic acid and ascorbic acid, and to vitamin-relevant fragments for the vitamin A, vitamin D, vitamin E, and vitamin K families: cyclohexanone, cis-1,3,5-hexatriene, 2-decalone, isochroman, benzoquinone, and naphthoquinone. The rotational benchmark confirms sub-percent accuracy for transferable non-hydrogen-bonded motifs, while the infrared comparisons validate carboxyl, ketone, polyene, and quinone fingerprints. For vitamin C, validation is structural and rotational because no isolated gas-phase infrared spectrum is available. Together, the rotational and infrared tests define a spectroscopic transferability map that separates intrinsic molecular structure from environmental perturbations and provides calibration targets for lower-cost quantum-chemical protocols and molecular-dynamics force fields.