Inelastic neutron scattering spectra of the longitudinal acoustic modes of the normal alkanes from pentane to pentacosane

D. A. Braden, S. F. Parker, J. Tomkinson, B. S. Hudson

Research output: Contribution to journalArticle

42 Scopus citations

Abstract

The inelastic neutron scattering spectra of the polycrystalline n-alkanes, 5<n<25, have been obtained at ∼25 K. At this temperature the n-alkane molecules are extended in their all-trans configuration. The spectral region from 200 to 600 cm-1 is reported with emphasis on the higher frequency longitudinal acoustic modes (LAM). These modes are identified by their nominal translational symmetry eigenvector, κ. A complete assignment of experimental frequencies out to κ≈0.7 is presented for the first time. This assignment is consistent with the known optical data and ab initia Hartree-Fock and DFT calculations. On the basis of these theoretical results, the lower frequency, higher κ LAM modes are also assigned. The LAM sequences exhibit bandhead behavior, i.e., a maximum frequency value, υmax, at some value of κ. The maximum frequency is consistently found at κmax=0.36±0.01 [cf. polyethylene (PE), κmax=0.34] for all chain lengths but the value of the maximum frequency, υmax, increases roughly linearly with n for n>7. At about n = 21 it reaches the value found for PE, 525 cm-1. This variation in frequency with chain length for the same nominal value of κ and also the full set of calculated DFT LAM results is compared with the expectations of a simple zig-zag bead bending model.

Original languageEnglish (US)
Pages (from-to)429-437
Number of pages9
JournalJournal of Chemical Physics
Volume111
Issue number1
DOIs
StatePublished - Jul 1 1999

ASJC Scopus subject areas

  • Physics and Astronomy(all)
  • Physical and Theoretical Chemistry

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