Infrared Spectroscopic Study on Trimethyl Amine Radical Cation: Correlation between Proton-Donating Ability and...

Area of Science:

• Physical Chemistry
• Molecular Spectroscopy
• Quantum Chemistry

Background:

• Recent studies revealed barrierless intermolecular proton transfer in trimethylamine (TMA) dimer ionization.
• This highlights an enhanced proton-donating capability of the CH bond in the cationic state.

Purpose of the Study:

• To investigate the mechanism behind the enhanced proton-donating ability of the CH bond in cationic TMA.
• To analyze the role of molecular vibrations and electronic structure in this phenomenon.

Main Methods:

• Infrared spectroscopy of neutral and cationic TMA in the CH stretch region.
• Theoretical calculations to model electronic and vibrational properties.

Main Results:

• The CH stretch band in the cation spectrum exhibits a long tail extending to 2600 cm⁻¹.
• This spectral feature is attributed to CH bond hyperconjugation with the nitrogen atom's nonbonding orbital.
• Geometry deformation, specifically CN stretching vibration excitation, effectively promotes this hyperconjugation.

Conclusions:

• Photoionization induces molecular vibrations that enhance CH bond hyperconjugation in TMA cations.
• Hyperconjugation delocalizes σ electrons, significantly increasing the proton-donating ability of the CH bond.
• CN stretching vibrations play a crucial role in facilitating this electronic effect.