Rotational spectroscopy in the v3 = v6 = 1 and v6 = 2 vibrational states of CH35Cl3

The overtone v6 = 2 (~520 cm−1) and combination v3 = v6 = 1 (~627 cm−1) vibrational levels of chloroform CH35Cl3 have been studied by millimeter wave spectroscopy, in the regions 150-330 and 360-660 GHz, corresponding to J = 22-49 and 55-99. Like the v6 = 1 fundamental vibration, the v3 = v6 = 1 combination level has an accidentally small value of C-B-Cζ ~ −0.0029 cm−1. Hence, a global q22 resonance (Δk = Δl = ±2) connects the (k-1, l6 = -1) and (k+1, l6 = +1) sublevels. As a consequence, several resonant q12 (Δk = ±1, Δl = ∓2) crossings occur, which enabled the precise determination of the q12 interaction parameter for this state. Altogether, 1182 rotational transitions in the v3 = v6 = 1 combination level have been measured and fitted, with a standard deviation of 41.2 kHz. For the v6 = 2 (A1+E) overtone level, the combined effects of the q22 and q12 l-type interactions induce resonant crossings between the (k, l6 = 0)/(k+1, l6 = -2) pairs of levels, for a large variety of k values. Altogether, 1463 rotational transitions in the (l6 = 0) A1 sublevel and 2924 rotational transitions in the (l6 = ±2) E sublevels, together with nine perturbation-allowed transitions induced by the q12 (Δk = ±1, Δl = ∓2) interaction, have been measured and fitted, with a global standard deviation of 41.4 kHz. This resulted in the first accurate determination of the molecular constants of this overtone level.