Stability and Structure of Protonated Clusters of Ammonia and Water, H⁺(NH₃)_m (H₂O)_n

Abstract

Mass spectrometric experiments show that protonated mixed ammonia/water clusters predominant exist in three forms namely H(+)(NH(3))(4)(H(2)O)(n), H(+)(NH(3))(5)(H(2)O)(n), and H(+)(NH(3))(6)(H(2)O)(n) (n = 1-25). For the first two series the collisional activation mass spectra are dominated by loss of water, whereas ions of the latter series preferably lose ammonia. The quantitative characteristics of these observations are reproduced by quantum chemical calculations that also provide insight into the geometrical structures of the clusters. Although the experiments and the calculations agree that clusters with five ammonia are thermodynamically preferred, this does not indicate a rigid tetrahedral structure with one central ammonium covered with an inner solvation shell of four ammonia molecules, with water outside. Instead, water and ammonia have comparable affinities to the binding sites of the first shell, with a preference for ammonia for the first two sites, and water for the last two. The "leftover" ammonia molecules bind equally strong as water molecules to sites in the second shell due to synergistic hydrogen binding. Finally, it is discussed whether the observation of enhanced stability of the H(+)(NH(3))(5)(H(2)O)(20) in terms of magic numbers and associated geometries may be related to a tetrahedral ammonium core encapsulated in a dodecahedral (H(2)O)(20) structure, typically found in clathrates.

Stabilities of protonated water-ammonia clusters

“…Given that the fragmentation follows from different preparations of the clusters (spontaneous decay here vs. collisional induced dissociation), the agreement is rather good and corroborates the statistical decay ansatz underlying the analysis here and implicitly in Ref. 30. The close but not absolute agreement between branching ratios from spontaneous decays and CID-induced decay is similar to previous observations on silicon clusters.…”

“…27 The subject was revisited in Ref. 30 where the same conclusion was reached. For smaller mixed clusters, the earliest proposed structure was 0021-9606/2018/148(18)/184306/9/$30.00…”

“…four ammonia molecules surrounding an ammonium ion with water and remaining ammonia molecules attached to an outer shell. 29 However, a later computational study 30 suggested that this was a too simple description and that there is a competition between ammonia and water for the last two molecular positions in the inner shell and for all the spaces in the second shell.…”

Computational study of the Rayleigh light scattering properties of atmospheric pre-nucleation clusters

“…Suggestions for the nature of the stabilizer molecule X include ammonia, [5][6][7][8][9] amines, [10][11][12][13][14][15][16] oxidized organics 4, or ions. [38][39][40][41][42][43][44][45][46] The light scattering properties of molecules, clusters and particles in the atmosphere are determined by their size and the wavelength of the incoming light. The scattering of light by particles with the same size as the wavelength of the incoming light is described by Mie theory.…”

Ionization of Ammonia Nanoices with Adsorbed Methanol Molecules

“…23 Ionized ammonia clusters have been investigated quite extensively and it has been conclusively shown that the ammonium cation solvated with 4 ammonia units in the first solvation shell (NH3)4NH + 4 is especially stable. [24][25][26][27][28][29][30][31][32] Pure ionized methanol clusters were studied as well and the PT reactions following the ionization were identified here. [33][34][35][36][37] The proton transfer was also studied for mixed water-ammonia, 38,39 water-methanol 35 or methanolammonia systems.…”