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

Abstract 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.