Revised assignments of the v4=1 vibrational level of CH35Cl3: The ν4 and ν4-ν3 rovibrational bands with remarkable clustering effects

The v4 = 1 fundamental vibration of chloroform (C-H bending vibration, E symmetry) has been reinvestigated at high-resolution. For this purpose, FTIR spectra recorded in the regions of the ν4 (1220 cm−1) and ν4 − ν3 (853 cm−1) bands were employed. Spectra were recorded at the Synchrotron Soleil, using a monoisotopic CH35Cl3 sample. More than 6900 transitions were assigned, among which more than 4200 in the difference band. Assignments span the 0 ≤ J ≤ 100 and − 75 ≤ K′′ΔK ≤ 75 values, despite the systematic overlaps of transitions, observed over a wide range of the spectrum. These overlaps, giving rise to remarkable clustering effects, are characteristic of both the fundamental ν4 and the difference ν4-ν3 bands. They are also, sometimes, source of misassignments, especially when the systematic use of the lower state combination differences as a checking is not possible. In this regard, we have to notice here that the indirect assignment checking through fundamental and difference transitions sharing a common upper level allowed us to systematically correct and extend the K-assignments of the rRK(J) transitions in the ν4 band. In the least-squares fit, the ground state parameters were fixed to the most recent experimental values. The parameters of the v3 = 1 level could be refined and improved with respect to previous determination, thanks to about 800 IR data collected from a FTIR spectrum of the ν3 band, together with more than 1300 MMW data in the v3 = 1 level (22 ≤ J ≤ 98 and 0 ≤ K ≤ 79). The theoretical model used for the v4 = 1 fundamental vibration is that of an isolated vibrational level, and ten molecular parameters were refined; the global standard deviation of the fit was of 0.160 × 10−3 cm−1, which is of the order of the accuracy of the experimental data. © 2022 Elsevier Ltd