Stabilities of protonated water-ammonia clusters

Abstract

Branching ratios of water and ammonia evaporation have been measured for spontaneous evaporation from protonated mixed clusters H(HO)(NH) 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.

Non-scrambling of hydrogen in NH4+(H2O)3 clusters

“…Ammonia loss was also observed in ref. 15 for the pure protium species studied there, likewise at the level of 1%, but ammonia loss fractions were seen for larger clusters. The channel was also seen in ref.…”

“…It has been used previously in ref. [15][16][17][18] and is well tested. The positively charged clusters were produced in a corona discharge source (position (A) in Fig.…”

“…Studies of evaporative branching ratios show that protonated water clusters doped with even a single ammonia molecule have an observable branching ratio to loss of ammonia, and consequently for the transfer of a proton to the water molecules. 15 Given the low equivalent temperature of these clusters (signicantly below room temperature), this fact implies a strong reduction in the effective difference in proton affinity in the cluster. The values of the evaporative free energy differences extracted from the experiments are around 0.05 eV for a range of small mixed water ammonia clusters.…”

“…The values of the evaporative free energy differences extracted from the experiments are around 0.05 eV for a range of small mixed water ammonia clusters. 15 The question of hydrogen scrambling between the ammonia and water molecules is therefore not as trivial as the values of the vacuum proton affinities suggest. The small free energy differences alone suggest a facile exchange of protons, but obviously does not address the question of the value of the energy barrier for the proton exchange in the cluster which, for the scrambling, is then the only relevant parameter.…”

A Tug of War between the Self‐ and Cross‐associating Hydrogen Bonds in Neutral Ammonia‐Water Clusters: Energetic Insights by Molecular Tailoring Approach

“…They further suggested that the difference in the hydrogen-bonded structural arrangement is responsible for the variable dipole moments of anionic and neutral clusters. There are many similar studies on anionic and protonated ammonia-water clusters [56][57][58][59][60][61][62] however, the discussion is avoided for brevity. A large number of studies have been carried out on the structural properties of water clusters using various theoretical as well as experimental methods.…”