Effect of ion-molecule interaction on fermi-resonance in acetonitrile studied by ultrafast vibrational spectroscopy

- We investigate the effect of ion-molecule interaction on the Fermi-resonance.
- Fermi-resonance in acetonitrile is significantly enhanced by the bound cation.
- Fermi-resonance is observed as a strong oscillatory cross-peak in 2DIR spectra.
- Two vibrational modes coupled by the Fermi-resonance are oscillating in phase.

The nitrile stretch (ν2) and the combination (ν3+4) modes of acetonitrile (CH3CN) were found to be strongly coupled by Fermi-resonance. The frequencies of vibrational modes of acetonitrile in CH3CN⋯Li+ complex were significantly varied due to a strong ion-molecule interaction. The intensity of the combination mode of CH3CN in the FTIR spectrum was substantially enhanced in CH3CN⋯Li+ complex because the combination band shifted closer to the nitrile stretch band. Here, Fermi-resonances in free CH3CN and CH3CN⋯Li+ complex were studied by ultrafast infrared pump-probe (IR PP) and two-dimensional infrared (2DIR) spectroscopy. Spectral features of Fermi-resonance observed in IR PP and 2DIR signals were presented and the effect of Li+ ion on Fermi-resonance were discussed.