Experimental Equilibrium Structures: Application of Molecular Dynamics Simulations to Vibrational Corrections for Gas Electron Diffraction

Derek A. Wann, Alexander V. Zakharov, Anthony M. Reilly, Philip D. McCaffrey, David W. H. Rankin

Research output: Contribution to journalArticlepeer-review

Abstract

A general method is described that allows experimental equilibrium structures to be determined from gas electron diffraction (GED) data. Distance corrections, starting values for amplitudes of vibration and anharmonic "Morse" constants (all required for a GED refinement) have been extracted from molecular dynamics (MD) Simulations. For this purpose MD methods have significant advantages over traditional force-field methods, its they can more easily be performed for large molecules, and, as they do not rely on extrapolation from equilibrium geometries, they are highly suitable for molecules with large-amplitude and anharmonic modes of vibration. For the test case Si8O12Me8, where the methyl groups rotate and large deformations of the Si8O12 cage are observed, the MD simulations produced results markedly superior to those obtained using force-field methods. The experimental equilibrium structure of Si8O12H8 has also been determined, demonstrating the use of empirical potentials rather than DFT methods when such potentials exist. We highlight the one major deficiency associated with classical MD-the absence of quantum effects-which causes some light-atom bonded-pair amplitudes of vibration to be significantly underestimated. However, using C2N3Cl3 and C3N3H3 as examples, we show that path-integral MD simulations can overcome these problems. The distance corrections and amplitudes of vibration obtained for C3N3Cl3 are almost identical to those obtained from force-field methods, as we would expect for such a rigid molecule. In the case of C3N3H3, for which an accurate experimental structure exists, the use of path-integral methods more than doubles the C-H amplitude of vibration.

Original languageEnglish
Pages (from-to)9511-9520
Number of pages10
JournalThe Journal of Physical Chemistry A
Volume113
DOIs
Publication statusPublished - 27 Aug 2009

Keywords

  • VALENCE BASIS-SETS
  • ORBITAL METHODS
  • CYANURIC CHLORIDE
  • CRYSTAL-STRUCTURES
  • 2ND-ROW ELEMENTS
  • FORCE-FIELDS
  • AB-INITIO
  • APPROXIMATION

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