Using symmetry group correlation tables to explain why erham (and other programs) cannot be used to analyze torsional splittings of some molecules

Groner, Peter

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https://hdl.handle.net/2142/91291 Copy

Description

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

Issue Date

2016-06-21

Keyword(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.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

text

Language

En

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http://hdl.handle.net/2142/91291

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Copyright 2016 by the authors

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