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Methyl rotor dependent vibrational interactions in toluene.

Jason R Gascooke1, Warren D Lawrance

  • 1School of Chemical and Physical Sciences, Flinders University, GPO Box 2100, Adelaide SA 5001, Australia.

The Journal of Chemical Physics
|April 12, 2013
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Summary

This study reveals how methyl rotor states influence Fermi resonance in S1 toluene. The interaction strength changes significantly with methyl rotor states, impacting energy levels and coupling.

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Published on: August 6, 2018

Area of Science:

  • Molecular Spectroscopy
  • Quantum Chemistry
  • Chemical Physics

Background:

  • Fermi resonance is a significant phenomenon in molecular spectroscopy, affecting energy level structures.
  • Previous studies indicated methyl rotor state dependence in S1 toluene's Fermi resonance, but spectral evidence was lacking.

Purpose of the Study:

  • To investigate the methyl rotor dependence of a three-state Fermi resonance in S1 toluene at approximately 460 cm(-1).
  • To provide direct spectral evidence for the methyl rotor state (m) dependence of the Fermi resonance.
  • To explore the absence of resonance at higher methyl rotor states (m=3).

Main Methods:

  • Utilized two-dimensional laser-induced fluorescence spectroscopy.
  • Separated overlapped spectral features corresponding to different methyl rotor states (m=0 and m=1).
  • Probed the Fermi resonance at the m=3 state.

Main Results:

  • Direct spectral evidence obtained for the m dependence of the Fermi resonance.
  • The Fermi resonance was found to be absent at the m=3 state, indicating a significant change in interaction.
  • Deperturbation revealed that the m dependence originates from variations in the energy separations of 'zero-order' coupled states.

Conclusions:

  • The observed m dependence is attributed to long-range torsion-vibration coupling perturbing 'zero-order' states.
  • The coupling selection rules (Δm = ±3n) and rotor energy scaling (m(2)) explain the variation in interaction energy (ΔE) with m.
  • The findings suggest that this methyl rotor dependence of Fermi resonance is likely widespread in molecules with methyl rotors.