Method for evaluating vibrational mode assignments in surface-bound cyclic hydrocarbons using sum-frequency generation

Avram M. Buchbinder, Julianne M. Gibbs-Davis, Grace Y. Stokes, Mark D. Peterson, Eric Weitz, Franz M. Geiger*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

16 Scopus citations


A method is proposed and tested in which vibrational sum frequency generation (SFG) spectra are assigned to vibrational modes in cyclic hydrocarbon species at transparent oxide surfaces. This method is especially effective for identifying asymmetric C-H stretches which are particularly difficult to assign in cyclic hydrocarbons because they have frequencies similar to those of overtones and Fermi resonances, and because multiple nondegenerate asymmetric C-H stretches occur. The ratios between SFG signal intensities in two different experimental polarization combinations are compared to the predicted values assuming local C2v symmetry for an asymmetric CH2 stretching vibration. By applying this method to silica surfaces containing covalently linked cyclopropyl, cyclobutyl, cyclopentyl, and cyclopentenyl groups with fixed orientations, and benchmarking to published gas phase spectra, the observed SFG peaks are assigned to either (1) isolated asymmetric stretches of CH2 groups possessing local C2v symmetry, (2) motions of CH bonds coupled throughout the molecule not possessing local C2v symmetry, or (3) symmetric stretching or other C-H stretching motions. The SFG spectra of the less-strained five-membered rings contain peaks that - based on polarization ratios - correspond to isolated asymmetric stretches. For these species, the results contradict a full-molecule normal mode description of molecular vibrations.

Original languageEnglish (US)
Pages (from-to)18284-18294
Number of pages11
JournalJournal of Physical Chemistry C
Issue number37
StatePublished - Sep 22 2011

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Energy(all)
  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films


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