Universität zu Köln

Schmiedt, Hanno ORCID: 0000-0001-6450-3713, Schlemmer, Stephan ORCID: 0000-0002-1421-7281 and Jensen, Per ORCID: 0000-0001-5133-7621 (2015). Symmetry of extremely floppy molecules: Molecular states beyond rotation-vibration separation. J. Chem. Phys., 143 (15). MELVILLE: AMER INST PHYSICS. ISSN 1089-7690

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Abstract

Traditionally, molecules are theoretically described as near-static structures rotating in space. Vibrational motion causing small structural deformations induces a perturbative treatment of the rotation-vibration interaction, which fails in highly fluxional molecules, where all vibrational motions have amplitudes comparable in size to the linear dimensions of the molecule. An example is protonated methane (CH5+) [P. Kumar and D. Marx, Phys. Chem. Chem. Phys. 8, 573 (2006); Z. Jin et al., J. Phys. Chem. A 110, 1569 (2006); and A. S. Petit et al., J. Phys. Chem. A 118, 7206 (2014)]. For these molecules, customary theory fails to simulate reliably even the low-energy spectrum [T. Oka, Science 347, 1313-1314 (2015) and O. Asvany et al., Science 347, 1346-1349 (2015)]. Within the traditional view of rotation and vibration being near-separable, rotational and vibrational wavefunctions can be symmetry classified separately in the molecular symmetry (MS) group [P. Bunker and P. Jensen, Molecular Symmetry and Spectroscopy, NRC Monograph Publishing Program (NRC Research Press, 2006)]. In this article, we discuss a fundamental group theoretical approach to the problem of determining the symmetries of molecular rotation-vibration states. We will show that all MS groups discussed so far are isomorphic to subgroups of the special orthogonal group in three dimensions SO(3). This leads to a group theoretical foundation of the technique of equivalent rotations [H. Longuet-Higgins, Mol. Phys. 6, 445 (1963)]. The group G(240) (the MS group of protonated methane) represents, to the best of our knowledge, the first example of a MS group which is not isomorphic to a subgroup of SO(3) (nor of O(3) or of SU(2)). Because of this, a separate symmetry classification of vibrational and rotational wavefunctions becomes impossible in this MS group, consistent with the fact that a decoupling of vibrational and rotational motion is impossible. We discuss here the consequences of this. In conclusion, we show that the prototypical, extremely floppy molecule CH5+ represents a new class of molecules, where customary group theoretical methods for determining selection rules and spectral assignments fail so that new methods have to be developed. (C) 2015 AIP Publishing LLC.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Schmiedt, Hanno UNSPECIFIED orcid.org/0000-0001-6450-3713 UNSPECIFIED Schlemmer, Stephan UNSPECIFIED orcid.org/0000-0002-1421-7281 UNSPECIFIED Jensen, Per UNSPECIFIED orcid.org/0000-0001-5133-7621 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-389476
DOI:	10.1063/1.4933001
Journal or Publication Title:	J. Chem. Phys.
Volume:	143
Number:	15
Date:	2015
Publisher:	AMER INST PHYSICS
Place of Publication:	MELVILLE
ISSN:	1089-7690
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language CH5+; SPECTRUM; QUANTUM Multiple languages Chemistry, Physical; Physics, Atomic, Molecular & Chemical Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/38947