Canonical and Rare Tautomers of Nucleobases and Their Taggants in the Gas Phase: A Benchmark Computational Study

We present a comprehensive benchmark of canonical and rare nucleobase tautomers in the gas phase with equilibrium geometries and relative stabilities determined at spectroscopic accuracy. Explicitly correlated composite schemes, including core-valence contributions and complete basis set extrapolation, reproduce experimental data for stable forms and reliably predict structural and energetic features of rare species. A reduced-cost strategy, based on one-parameter bond corrections to geometries optimized with a double-hybrid functional, is validated as an accurate route for larger systems. This benchmark catalogue provides a reference for nucleobase tautomerism and a scalable foundation for systematic studies of nucleic acid fragments.