Issue 13, 2005

Rotational tunneling of methyl groups in low temperature phases of mesitylene: potentials and structural implications

Abstract

Mesitylene can be stabilized at He temperature in three solid phases of so far unknown crystal structures. Rotational tunneling of methyl groups is based on rotational potentials and used to characterize structural aspects. In phase III found after the first fast cooling of the sample three nonequivalent methyl rotors with splittings of 2.7, 4.1 and 16.3 μeV are observed. Three other unresolved bands are identified by their librational modes. In the second phase II the metastability is emphasized by tunneling energies still changing at temperatures T ≤ 12 K. Above this temperature tunneling bands at 6.6, 12.5, 15.0 and 18.3 μeV evolve in the manner characteristic of coupling to phonons. In the equilibrium phase I a single tunnel splitting of 10.2 μeV represents all methyl groups. A unit cell containing a single molecule at a site of threefold symmetry explains quantitatively this spectrum. Phases II and III most likely contain two nonequivalent molecules in the unit cell with no local symmetry in phase II and a mirror plane in phase III. The good moderator properties for neutrons are most likely not connected to the low energy tunneling bands but to a dense vibrational phonon density of states.

Graphical abstract: Rotational tunneling of methyl groups in low temperature phases of mesitylene: potentials and structural implications

Article information

Article type
Paper
Submitted
07 Mar 2005
Accepted
16 May 2005
First published
09 Jun 2005

Phys. Chem. Chem. Phys., 2005,7, 2587-2593

Rotational tunneling of methyl groups in low temperature phases of mesitylene: potentials and structural implications

M. Prager, H. Grimm and I. Natkaniec, Phys. Chem. Chem. Phys., 2005, 7, 2587 DOI: 10.1039/B503342J

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