### 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, υ_{max}, at some value of κ. The maximum frequency is consistently found at κ_{max}=0.36±0.01 [cf. polyethylene (PE), κ_{max}=0.34] for all chain lengths but the value of the maximum frequency, υ_{max}, 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 language | English (US) |
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Pages (from-to) | 429-437 |

Number of pages | 9 |

Journal | Journal of Chemical Physics |

Volume | 111 |

Issue number | 1 |

DOIs | |

State | Published - Jul 1 1999 |

### ASJC Scopus subject areas

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

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## Cite this

*Journal of Chemical Physics*,

*111*(1), 429-437. https://doi.org/10.1063/1.479293