Infrared Spectroscopy of Radical Cation Clusters (NH₃)₂⁺ and (NH₃)₃

Area of Science:

• Physical Chemistry
• Chemical Physics
• Spectroscopy

Background:

• Ion-molecule reactions in condensed phases are crucial but poorly understood.
• Protonated ionic clusters are well-studied, unlike unprotonated radical cation clusters.
• Radical cation clusters present experimental challenges due to low concentrations.

Purpose of the Study:

• To investigate the structure and properties of ammonia radical cation clusters.
• To characterize the infrared spectra of ammonia dimer and trimer radical cations.
• To understand the isomers and dynamics of solvated radical cations.

Main Methods:

• Solvation of ammonia radical cations in helium nanodroplets.
• Infrared spectroscopy to probe cluster structures.
• Density functional theory calculations for spectral assignment and structural validation.

Main Results:

• Observed infrared spectra indicate proton-transferred structures for (NH3)2+ and (NH3)3+ radical cations.
• These structures correspond to the calculated global minima.
• Internal rotation of NH4+ and NH3 moieties was observed with gas-phase-like rotational constants.

Conclusions:

• The study elucidates the dominant proton-transferred structures in ammonia dimer and trimer radical cations.
• Helium nanodroplet isolation facilitates the study of challenging radical cation clusters.
• Findings contribute to understanding fundamental ion-molecule reactions and solvation dynamics.