J. Chem. Phys. 148, 184306 (2018)

Stabilities of protonated water-ammonia clusters

A. E. K. Sundén¹, K. Støchkel², P. Hvelplund², S. Brøndsted Nielsen², B. Dynefors³, and  K. Hansen^1,4

1 Department of Physics, University of Gothenburg, 41296 Gothenburg, Sweden

2 Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark

3 Chalmers University of Technology, 41296 Gothenburg, Sweden

4 Center for Joint Quantum Studies and Department of Physics, Tianjin University, Tianjin 300072, China

Branching ratios of water and ammonia evaporation have been measured for spontaneous evaporation from protonated mixed clusters H⁺(H₂O)_n(NH₃)_m in the size range 0 ≤ n ≤ 11 and 0 ≤ m ≤ 7. Mixed clusters evaporate water except for clusters containing six or more ammonia molecules, indicating the formation of a stable core of one ammonium ion surrounded by four ammonia molecules and a second shell consisting predominantly of water. We relate evaporative branching ratios to free energy differences between the products of competing channels and determine the free energy differences for clusters with up to seven ammonia molecules. Clusters containing up to five ammonia molecules show a very strong scaling of these free energy differences.