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Title:Using symmetry group correlation tables to explain why erham (and other programs) cannot be used to analyze torsional splittings of some molecules
Author(s):Groner, Peter
Subject(s):Spectroscopy of Large Amplitude Motions
Abstract:ERHAM has been used to analyze rotational spectra of many molecules with torsional splitting caused by one or two internal rotors.\footnote{P. Groner, J. Mol. Spectrosc. 278 (2012) 52–67.} The gauche form of dimethyl ether-d$_{1}$ whose equilibrium structure has $C_{1}$ symmetry is an example of a molecule for which ERHAM could not model additional small splittings resolvable for many transitions, whereas the spectrum of the symmetric (anti, trans) form with a $C{_s}$ equilibrium structure could be analyzed successfully with ERHAM.\footnote{C. Richard et al. A\&A 552 (2013), A117.} A more recent example where ERHAM failed is pinacolone \chem{CH_3-CO-C(CH_3)_3}.\footnote{Y. Zhao et al., J. Mol. Spectrosc. 318 (2015) 91–100, with references to all other programs mentioned in the abstract.} In this case, the barriers to internal rotation of the methyl groups within the \chem{-C(CH_3)_3} unit are too high to produce observable internal rotation splittings, but the splittings due to the \chem{CH_3-CO} methyl group could not be modeled correctly with ERHAM nor with any other available program (XIAM, BELGI-$C_{s}$, BELGI-$C_{1}$, RAM36). In the paper, it was speculated that BELGI-$C_{s}$-2tops might be able to the job, but arguments against this possibility have also been put forward. The correlation between irreducible representations of groups and their subgroups according to Watson\footnote{J. K. G. Watson, Can. J. Physics 43 (1965) 1996-2007.} can be used not only to determine the total number of substates (components) to be expected but also to help decide which particular program has a chance for a successful analysis. As it turns out, the number of components of split lines depends on the molecular symmetry at equilibrium in relation to the highest possible symmetry for a given molecular symmetry group. Therefore, for pinacolone, the vibrational ground state is split into 10 torsional substates.
Issue Date:2016-06-21
Publisher:International Symposium on Molecular Spectroscopy
Genre:Conference Paper/Presentation
Rights Information:Copyright 2016 by the authors
Date Available in IDEALS:2017-01-26

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