On the thermal expansion of molecules

Z. Varga, M. Hargittai, Lawrence S. Bartell

Research output: Contribution to journalArticle

9 Citations (Scopus)

Abstract

Experimental results for a variety of molecules have shown that their bond lengths expand appreciably when the molecules are heated, as expected from the asymmetric Morse-like potentials characterizing the bonds. However, in a series of papers on structures determined by gas-phase electron diffraction, Giricheva et al. claimed that, for very hot MX3 molecules, effects of out-of-plane vibrations cancel the thermal expansion of the M-X bonds. This is incorrect. Although the computations to support their claim were correct as far as they went, the authors neglected the effects of asymmetric vibrational modes and centrifugal stretches of the bonds. In the present report, we show that quantum chemical computations for LaI3 reveal the crucial roles played by the terms neglected by Giricheva et al., which terms are responsible for thermal bond stretches of approximately 0.023 Å at 1142 K. In addition, because the iodine atoms in LaI3 are further apart in the mean structure than the sum of their Pauling van der Waals radii, the geminal nonbonded interactions are attractive. This accounts for the fact that the Morse asymmetry constant for the symmetric stretch mode is smaller than that for the asymmetric stretch. It also helps to explain the very large amplitude of the out-of-plane puckering mode, which tends to decrease the La-I bond length during the puckering trajectory.

Original languageEnglish
Pages (from-to)111-121
Number of pages11
JournalStructural Chemistry
Volume22
Issue number1
DOIs
Publication statusPublished - Feb 2011

Fingerprint

Thermal expansion
thermal expansion
Bond length
Molecules
molecules
Morse potential
Electron diffraction
Iodine
iodine
vibration mode
electron diffraction
Gases
Trajectories
asymmetry
trajectories
vapor phases
Atoms
vibration
radii
atoms

Keywords

  • Anharmonicity
  • Electron diffraction
  • Morse potential
  • Thermal expansion of bond length

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Physical and Theoretical Chemistry

Cite this

On the thermal expansion of molecules. / Varga, Z.; Hargittai, M.; Bartell, Lawrence S.

In: Structural Chemistry, Vol. 22, No. 1, 02.2011, p. 111-121.

Research output: Contribution to journalArticle

Varga, Z. ; Hargittai, M. ; Bartell, Lawrence S. / On the thermal expansion of molecules. In: Structural Chemistry. 2011 ; Vol. 22, No. 1. pp. 111-121.
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