title: Simulated annealing and density functional theory calculations of structural and energetic properties of the ammonium chloride clusters (NH4Cl)n, (NH4+)(NH4Cl)n, and (Cl-)(NH4Cl)n, n = 1-13.
abstract: Simulated annealing Monte Carlo conformer searches using the "mag-walking" algorithm are employed to locate the global minima of molecular clusters of ammonium chloride of the types (NH(4)Cl)(n), (NH(4)(+))(NH(4)Cl)(n), and (Cl(-))(NH(4)Cl)(n) with n = 1-13. The M06-2X density functional theory method is used to refine and predict the structures, energies, and thermodynamic properties of the neutral, cation, and anion clusters. For selected small clusters, the resulting structures are compared to those obtained from a variety of models and basis sets, including RI-MP2 and B3LYP calculations. M06-2X calculations predict enhanced stability of the (NH(4)(+))(NH(4)Cl)(n) clusters when n = 3, 6, 8, and 13. This prediction corresponds favorably to anomalies previously observed in thermospray mass spectroscopy experiments. The (NH(4)Cl)(n) clusters show alternations in stability between even and odd values of n. Clusters of the type (Cl(-))(NH(4)Cl)(n) display a magic number distribution different from that of the cation clusters, with enhanced stability predicted for n = 2, 6, and 11. None of the observed cluster structures resemble the room-temperature CsCl structure of NH(4)Cl(s), which is consistent with previous work. Numerous clusters have structures reminiscent of the higher-temperature, rock-salt phase of the solid ammonium halides.
authors.affiliation: Department of Chemistry, The Cooper Union for the Advancement of Science and Art, 41 Cooper Square, New York, New York 10003, USA. topper@cooper.edu
authors.forename: Robert Q
authors.initials: RQ
authors.lastname: Topper
authors.forename: William V
authors.initials: WV
authors.lastname: Feldmann
authors.forename: Isaac M
authors.initials: IM
authors.lastname: Markus
authors.forename: Denise
authors.initials: D
authors.lastname: Bergin
authors.forename: Paul R
authors.initials: PR
authors.lastname: Sweeney
mesh_terms: D000465:Algorithms; D000643:Ammonium Chloride / Q000737:chemistry*; D015394:Molecular Structure; D009010:Monte Carlo Method; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D000643:Ammonium Chloride