The ν4, ν9, ν10 and ν6 + ν11 bands of 12CH313CH3 between 1345 and 1557 cm−1

- An FTIR spectrum of 12CH313CH3 at 130 K was obtained at 0.0022 cm−1 resolution.
- Four bands of 12CH313CH3 near 7 μm were assigned and analyzed for the first time.
- Resonances and interactions permitted ν6 + ν11 and weak ν10 lines to be assigned.
- 1350 Line positions are fitted with an RMS of 0.0032 cm−1.
- Measurements and results are provided in two supplemental files.

The infrared spectrum of 12CH313CH3 is measured between 1345 and 1560 cm−1 using high resolution FT-IR with a 13C-enriched gas sample cooled to 130.3 K. The three fundamentals ν4, ν9 and ν10 (at 1374, 1470 and 1468 cm−1 respectively) and one combination band ν6 + ν11 at 1473 cm−1 are analyzed for the first time. The transitions of ν10 (an infrared inactive “g” mode of the ethane) are observed through its resonant coupling with ν9 (corresponding to a “u” mode of normal ethane). Transitions of ν6 + ν11 are seen due to its strong coupling to ν9. In general, torsional splitting produces two components (as in normal ethane), but in both ν9 and ν10, there is an additional small splitting of these components with degenerate torsional symmetry wherever the interacting ν10 levels become infrared active (from “g ↔ u” mixing); this mechanism is investigated. Several ℓ-type resonances with Δℓ = ±2 and Δk = ∓1 are observed within ν9 and between ν9 and ν10 while the strong interaction between ν9 and ν6 + ν11 plays a role in tuning pairs of levels into resonance. As in normal ethane, a detectable K-doubling occurs in the levels k = ±2, ℓ = ∓1 of ν9, and the parallel band ν4 shows an intensity bias, with the R-transitions markedly stronger than the P-transitions, due to its x,y-Coriolis coupling with ν9. The spectrum is analyzed by adopting an appropriate Hamiltonian model, and vibration-rotation-torsion parameters of the four mentioned vibrational states are determined by the least squares process using 1350 observed transition line positions (RMS deviation 3.24 × 10−3 cm−1). The values of rotational and torsional parameters of the vibrational ground state are also improved or determined anew, from data of the present spectrum. Two supplemental files are provided. One shows the fit of line positions, and the other gives the measured positions and intensities at 130 K (with known quantum assignments and lower state energies) so that lines of this molecule can be identified in planetary spectra.