Rotational spectrum of chloroform, "grass-roots among the forest of trees": The v2 = 1, v3 = 2, v5 = 1, and v6 = 3 vibrational states of CH35Cl3

Highlights: • First high-resolution rotational analysis of the v₃ = 2, v₂ = 1, and v₅ = 1 levels of CH³⁵Cl₃. • First rovibrational analysis of the ν₅-ν₆ difference band, near 515 cm⁻¹. We have recently analyzed the broadband millimeter-wave rotational spectrum of chloroform CH³⁵Cl₃ in the v₃ = v₆ = 1 (E symmetry) and v₆ = 2 (A₁ + E) excited vibrational states . In the present work, we have used the same spectrum (recorded in the 150 - 330 and 360–660 GHz spectral regions) for a rotational analysis of the v₂ = 1 and v₃ = 2 A₁ states of the same molecule, together with a new analysis of the v₅=1 (E) fundamental vibration. For the v₂ = 1 state (A_1, near 677 cm⁻¹), the body of assigned data comprises 392 rotational transitions. The corresponding frequencies were fitted within an isolated-level model, with a standard deviation of 36.4 kHz, and the obtained parameters are in good agreement with the previous IR work. For the v₃ = 2 vibrational state (A₁, near 734 cm⁻¹), a total of 351 rotational transitions were assigned. Their fit was improved by inclusion of a second-order Coriolis coupling term with the v₅ = 1 fundamental level, providing standard deviation of 40.2 kHz. At the same time, the description of the v₅ = 1 vibration (E symmetry, 776 cm⁻¹), studied previously with high-resolution as an isolated level, was also improved. The quantitative interpretation of the newly assigned 612 MMW data and the significantly extended set of IR assignments necessitated the introduction of the second-order Coriolis interaction with v₃ = 2, but more importantly of an anharmonic interaction with the v₆ = 3 (A₁ + A₂ + E, near 782 cm⁻¹) level. For this latter level more than 200 rotational transitions, together with several series of rovibrational transitions, could finally be assigned and included in the fit.