Mid-infrared characterization of the NH4+∙(H2O)n clusters in the neighborhood of the n=20 “magic” number

Abstract

Vibrational predissociation spectra are reported for size-selected NH4+ (H2O)n clusters (n=5-22) in the 2500-3900 cm(-1) region. We concentrate on the sharp free OH stretching bands to deduce the local H-bonding configurations of water molecules on the cluster surface. As in the spectra of the protonated water clusters, the free OH bands in NH4+ (H2O)n evolve from a quartet at small sizes (n<7), to a doublet around n=9, and then to a single peak at the n=20 magic number cluster, before the doublet re-emerges at larger sizes. This spectral simplification at the magic number cluster mirrors that found earlier in the H+(H2O)n clusters. We characterize the likely structures at play for the n=19 and 20 clusters with electronic structure calculations. The most stable form of the n=20 cluster is predicted to have a surface-solvated NH4+ ion that lies considerably lower in energy than isomers with the NH4+ in the interior.

Interaction between PH3 and small water clusters: Understanding the electronic and spectroscopic properties

“…No frequency scale factor was used for the vibrational frequencies or the zero-point vibrational energy (ZPVE). It is important to note that the studied phosphine-water structures are based on the analogous ammonium complexes [11][12][13][14][15][16][17][18][19].…”

“…Recently, Lane and Kjaergaard [11] also characterized the phosphine-water complex at a high level of theory using the CCSD(T)-F12 and CCSD(T) methods. For the analogous ammonium-water clusters [NH 3 (H 2 O) n ], a very different picture emerges due to the rich source information on these complexes [12][13][14][15][16][17][18][19][20][21]. The limited information on phosphine-water complexes due to a lack of any experimental data emphasizes the need to evaluate the electronic properties of the interaction between phosphine and more than one water molecule.…”

Pressure and temperature dependence on the hydrogen bonding and dynamics of ammonium ion in liquid water: A molecular dynamics simulations study

“…Also, in many natural and agricultural ecosystems, NH 4 + ion is the predominant nitrogen source and an important micronutrient in the world's water system [1]. Hence, a detailed understanding of the solvation process of ammonium ion plays an important role both in chemistry and biology and the subject of extensive experimental [2][3][4][5][6][7][8][9][10] and theoretical [11][12][13][14][15][16][17][18][19][20] studies. The faster rotational dynamics of NH 4 + in water is a central focus of these studies.…”

Hydration of the Calcium Dication: Direct Evidence for Second Shell Formation from Infrared Spectroscopy

“…One strategy to gain a detailed understanding of how ions and water molecules interact is to build up a solution, one molecule of water at a time. [1] Vibrational spectroscopy is a powerful structural tool for investigating hydrated ions, and has been used to investigate the structures of hydrated monovalent cations, including Ag + , [2] H + , [3][4][5][6] Cs + , [7,8] Cu + , [2] K + , [9] Mg + , [10] NH 4 + , [11,12] and Ni + , [13] and hydrated anions, including Cl À , [14,15] e À , [16] F À , [15,17] HO À , [18] and SO 4 2À . [19] Despite serving critical regulatory and structural roles in biology, experimental studies of hydrated, multiply charged ions have lagged behind those of singly charged ions, owing to the great difficulty in producing them in significant abundances; for example, formation of doubly hydrated, divalent calcium by condensation results in rapid dissociation aided by Coulomb repulsion [Eq.…”