Controlling vibrational energy flow in liquid alkylbenzenes

Area of Science:

• Physical Chemistry
• Molecular Spectroscopy
• Chemical Dynamics

Background:

• Understanding vibrational energy transfer is crucial for controlling chemical reactions.
• Alkylbenzenes offer a model system to study intramolecular energy dynamics.
• Previous studies classified vibrational modes as phenyl, substituent, or global.

Purpose of the Study:

• To investigate vibrational energy transfer pathways in alkylbenzenes (toluene, isopropylbenzene, t-butylbenzene).
• To determine the influence of substituent size on intramolecular energy transfer rates.
• To explore methods for controlling forward-to-backward vibrational energy transfer ratios.

Main Methods:

• Ultrafast infrared (IR) Raman spectroscopy was employed.
• Normal modes were classified as phenyl (ϕ), substituent (S), or global (G).
• Anti-Stokes Raman spectroscopy measured transient energy content in Raman-active modes.

Main Results:

• Phenyl to substituent (Φ → S) energy transfer efficiencies were small and similar for all substituents.
• Substituent to phenyl (S → Φ) energy transfer efficiencies increased with increasing substituent size.
• This trend contradicts predictions based on the density of states.

Conclusions:

• Substituent size plays a key role in controlling intramolecular vibrational energy transfer.
• Increased substituent size enhances S → Φ transfer due to increased local anharmonic couplings.
• Heavier vibrating substituents transfer energy more effectively to the phenyl group.