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 Title: The Bending-rotation Approach Applied To The Methylene Radical Ch2 Author(s): Coudert, L.H. Subject(s): Mini-symposium: Large Amplitude Motions Abstract: Quasi-linear molecules display a large amplitude bending mode allowing them to sample their linear configuration. This leads to a strong coupling between the overall rotation and the bending mode and to a singularity in their Hamiltonian. Quasi-linearity has been extensively studied in many species including the closed shell water molecule and the open shell amidogen (NH$_2$) and methylene (CH$_2$) radicals. For these three species, the barrier to linearity ranges from 12\,000~cm$^{-1}$ for NH$_2$ to less than 2000~cm$^{-1}$ for CH$_2$.\footnote{Jungen, Hallin, and Merer, {\em Molec.\ Phys.}~{\bf 40} (1980) 25; Bunker, Jensen, Kraemer, and Beardsworth, {\em J.\ Chem.\ Phys.}~{\bf 85} (1986) 3724; Partridge and Schwenke, {\em J.\ Chem.\ Phys.}~{\bf 106} (1997) 4618} Their rovibrational energy levels can be computed with almost spectroscopic accuracy using variational approaches or, if a higher accuracy is required, with reduced dimensionality models such as the effective Bending-Rotation approach,\footnote{Coudert, Marin-Drumel, and Pirali, {\em J.\ Mol.\ Spectrosc.}~{\bf 303} (2014) 36} already applied to treat the anomalous centrifugal distortion of the water molecule$^b$ and of the amidogen radical.\footnote{TD03, Martin-Drumel, Pirali, and Coudert, 72nd ISMS, Urbana-Champaign, June 19--23, 2017} In this talk, the Bending-Rotation approach$^b$ is extended, adding the spin-rotation and spin-spin fine couplings, so as to be used in the case of the methylene radical. The new approach is applied to the fitting of high-resolution data pertaining to this species. In addition to the ground state data previously analyzed,\footnote{Br\"unken, M\"uller, Lewen, and Giesen, {\em J.\ Chem.\ Phys.}~{\bf 123} (2005) 164315} the data set includes FIR transitions belonging to the $\nu_2$ band.\footnote{Sears, Bunker, and McKellar, {J.\ Chem.\ Phys.}~{\bf 77} (1982) 5363; McKellar, Yamada, and Hirota, {\em J.\ Chem.\ Phys.}~{\bf 79} (1983) 1220; and Marshall and McKellar, {\em J.\ Chem.\ Phys.}~{\bf 85} (1986) 3716} 336 transitions were reproduced with a standard deviation of 1.3 using 42 spectroscopic parameters.\footnote{Coudert, {\em J.\ Chem.\ Phys.}~{\bf 153} (2020) 144115} In the talk, the results of this analysis will be reported and the dependence on the bending angle retrieved for the spin-rotation and spin-spin fine couplings will be discussed. We will also try to see if the analysis results$^f$ can be further improved. Issue Date: 2021-06-24 Publisher: International Symposium on Molecular Spectroscopy Genre: Conference Paper / Presentation Type: Text Language: English URI: http://hdl.handle.net/2142/111342 Date Available in IDEALS: 2021-09-24
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