The Weak Visible n → π* Transition of CF₃NO: Vertical Accuracy versus Spectral Agreement

Trifluoronitrosomethane (CF₃NO), one of the few relatively stable blue gases, owes its color to a weak, structured n → π* absorption near 690 nm. Its small size suggests a straightforward benchmark, yet vertical excited-state methods place this transition very differently. We investigate this discrepancy by treating the vertical excitation, the 0-0 transition, and the finite-temperature band envelope as distinct observables. The benchmark combines full linear-response TD-DFT and TDA for five functionals with EOM-CCSD and EOM-CC3 vertical references, together with CCSD-(T), CASSCF/NEVPT2, spin-orbit, structural, and vibronic analyses. At the reference geometry, S₀ is predominantly closed-shell, whereas S₁ is an isolated, spin-pure state dominated by n-(N) → π*-(NO); these diagnostics argue against pronounced multireference character, ambiguous state identity, or appreciable spin-orbit mixing as dominant explanations of the method dependence at the reference structure. EOM-CC3/aug-cc-pVTZ gives 1.974 eV, and EOM-CCSD lies only about 0.03 eV higher. Full LR-TDDFT is 0.142-0.231 eV lower (red-shifted), whereas TDA raises the same state by 0.102-0.179 eV and brings every tested functional closer to the coupled-cluster reference. The jet-cooled 0-0 origin (1.727 eV) and strongest room-temperature feature (1.789 eV) lie well below the vertical region. The local harmonic vibronic models also underestimate the 0-0 energy and recover the gross experimental envelope only after empirical alignment and broadening. CF₃NO therefore provides a compact benchmark in which vertical accuracy, state character, and spectral agreement can be separated quantitatively rather than collapsed into a single comparison.