Excess electrons bound to small ammonia clusters.

Area of Science:

• Physical Chemistry
• Quantum Chemistry
• Computational Chemistry

Background:

• Dipole-bound anions are known for water clusters (H2O)N- (N ≥ 2).
• The smallest experimentally detected ammonia cluster anion is the 13-mer (NH3)13-.
• The nature of electron binding in ammonia clusters is less understood compared to water clusters.

Purpose of the Study:

• Investigate dipole-bound states in small ammonia clusters (NH3)N- for N = 2, 3, 4.
• Determine the smallest ammonia cluster capable of forming a dipole-bound state.
• Examine the electronic structure and binding characteristics of ammonia cluster anions, including potential cavity-bound states.

Main Methods:

• Coupled-cluster ab initio calculations were employed.
• High-level electronic structure methods were used to model the anions.
• Vertical detachment energies were calculated to characterize binding.

Main Results:

• The ammonia trimer (NH3)3- is identified as the smallest cluster forming a dipole-bound state.
• A vertical detachment energy of 27 meV was predicted for the ammonia trimer anion.
• Dipole-bound states with triple-acceptor monomers were found in the ammonia tetramer anion (NH3)4-.
• Ab initio results for the hexamer anion (NH3)6- challenge the concept of a delocalized radical anion in a cavity.

Conclusions:

• The ammonia trimer is the smallest ammonia cluster exhibiting a dipole-bound anion.
• The binding motif in ammonia tetramer anions differs from water cluster anions.
• The study provides new insights into electron localization and binding in ammonia cluster anions, questioning existing models for cavity-bound states.