Doubly excited (v4=2) vibrational state of 12CH2 35Cl2: Centrifugal distortion and quadrupole splittings in the submillimeter wave spectra up to 1.1 THz

Abstract

The high resolution submillimeter wave spectra of the ${}^{12}$CH${}_2$ ${}^{35}$Cl${}_2$ methylene chloride (six spectra were recorded in different parts of the 0.075–1.1 THz spectral region in the I. Physikalisches Institut, Universität zu Köln (Germany) and Laboratoire PhLAM, Université de Lille (France)) were analyzed in the frame of Watson’s Hamiltonian in $A-$reduction and $I^r-$ representation with the use of the SPFIT/SPCAT package of Pickett. The 2368 unresolved lines (including 878 peripheral lines of triplets) with the maximum values of quantum number $J^{\text{max}}=83$ and $K_a^{\text{max}}=19$ were assigned in the experimental spectra to the $(v_4=2)$ vibrational state. Weighted fit of the rotational and centrifugal distortion parameters, as well as, of the nuclear quadrupole splitting parameters was made. The eleven rotational and centrifugal distortion parameters and three nuclear quadrupole coupling parameters obtained from the fit reproduce values of the experimental multiplet centers with the root mean square deviation of 36 kHz and of the experimental peripheral lines of triplets with $d_{\text{rms}}=47$ kHz. Comparison with the rotational and centrifugal distortion parameters of the $(v_4=2)$ vibrational state known in the literature is made and shown that they reproduce the same set of the experimental line positions with $d_{\text{rms}}$ that is 130 times worse in comparison with the results of the present study. As to the quadrupole coupling $\chi -$parameters, it was found that the three $\chi -$parameters obtained in the fit correlate very well with the corresponding parameters from the recent paper by Z. Kisiel (J Mol Spectrosc 2024;406:111954).