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Infrared Spectroscopy of Radical-Cation Clusters (NH₃)_n=4-6.

Amandeep Singh¹, Arisa Iguchi^2,3, Zane Golpariani¹

¹Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States.

The Journal of Physical Chemistry. A

|October 14, 2025

Summary

Researchers studied ammonia radical-cation clusters ((NH3)n+=4-6) using infrared spectroscopy. Findings reveal an ammonium core (NH4+) solvated by ammonia molecules and an NH2 radical, with evidence of higher-energy isomers in larger clusters.

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Area of Science:

Physical Chemistry
Chemical Physics
Spectroscopy

Background:

Ammonia radical-cation clusters are important in atmospheric and interstellar chemistry.
Understanding their structure and bonding is crucial for chemical reaction mechanisms.

Purpose of the Study:

To investigate the structure and hydrogen bonding of ammonia radical-cation clusters ((NH3)n+=4-6).
To elucidate the role of ammonium core (NH4+) and NH2 radical in cluster formation.

Main Methods:

Production of ammonia radical-cation clusters within helium droplets.
Infrared laser spectroscopy in the NH stretching region.
Quantum chemical calculations for structural analysis and spectral assignment.

Main Results:

Identified global minima structures featuring an NH4+ core solvated by NH3 molecules and an NH2 radical via hydrogen bonds.
Observed strong spectral bands (2300-3200 cm-1) attributed to interactions between hydrogen bond bridge vibrations and bending mode overtones of the NH4+ core.
Detected signatures of higher-energy isomers in (NH3)5+ and (NH3)6+ clusters, characterized by a free NH stretch of the NH4+ core.

Conclusions:

The dominant structures of ammonia radical-cation clusters (n=4-6) consist of an NH4+ core stabilized by NH3 and NH2 radical through hydrogen bonds.
Spectroscopic data provide insights into the vibrational dynamics and hydrogen bonding network within these clusters.
The presence of higher-energy isomers suggests structural flexibility and potential for different reaction pathways.