Properties of ammonium ion-water clusters: analyses of structure evolution, noncovalent interactions, and temperature and humidity effects.

Although ammonium ion-water clusters are abundant in the biosphere, some information regarding these clusters, such as their growth route, the influence of temperature and humidity, and the concentrations of various hydrated clusters, is lacking. In this study, theoretical calculations are performed on ammonium ion-water clusters. These theoretical calculations are focused on determining the following characteristics: (1) the pattern of cluster growth; (2) the percentages of clusters of the same size at different temperatures and humidities; (3) the distributions of different isomers for the same size clusters at different temperatures; (4) the relative strengths of the noncovalent interactions for clusters of different sizes. The results suggest that the dipole moment may be very significant for the ammonium ion-water system, and some new stable isomers were found. The nucleation of ammonium ions and water molecules is favorable at low temperatures; thus, the clusters observed at high altitudes might not be present at low altitudes. High humidity can contribute to the formation of large ammonium ion-water clusters, whereas the formation of small clusters may be favorable under low-humidity conditions. The potential energy surfaces (PES) of these different sized clusters are complicated and differ according to the distribution of isomers at different temperatures. Some similar structures are observed between NH4(+)(H2O)n and M(H2O)n (where M represents an alkali metal ion or water molecule); when n = 8, the clusters begin to form the closed-cage geometry. As the cluster size increases, these interactions become progressively weaker. The successive binding energy at the DF-MP2-F12/VDZ-F12 level is better than that at the PW91PW91/6-311++G(3df, 3pd) level and is consistent with the experimentally determined values.

Theoretical study of the hydration of atmospheric nucleation precursors with acetic acid.

While atmosphere is known to contain a significant fraction of organic substance and the effect of acetic acid to stabilize hydrated sulfuric acids is found to be close that of ammonia, the details about the hydration of (CH3COOH)(H2SO4)2 are poorly understood, especially for the larger clusters with more water molecules. We have investigated structural characteristics and thermodynamics of the hydrates using density functional theory (DFT) at PW91PW91/6-311++G(3df,3pd) level. The phenomena of the structural evolution may exist during the early stage of the clusters formation, and we tentatively proposed a calculation path for the Gibbs free energies of the clusters formation via the structural evolution. The results in this study supply a picture of the first deprotonation of sulfuric acids for a system consisting of two sulfuric acid molecules, an acetic acid molecule, and up to three waters at 0 and 298.15 K, respectively. We also replace one of the sulfuric acids with a bisulfate anion in (CH3COOH)(H2SO4)2 to explore the difference of acid dissociation between two series of clusters and interaction of performance in clusters growth between ion-mediated nucleation and organics-enhanced nucleation.

Theoretical study of temperature dependence and Rayleigh scattering properties of chloride hydration clusters.

Cl(-)(H2O)n (n = 5-6) clusters were investigated using a basin hopping (BH) method coupled with density functional theory (DFT). Structures, energetics, thermodynamics, and vibrational frequencies were obtained using high level ab initio calculations. DF-LMP2 (second-order Møller-Plesset perturbation theory using local and density fitting approximations) with an appropriate basis set were employed for final optimization and frequency calculation, which has been benchmarked in a recent study. The global minimum of Cl(-)(H2O)5 was verified and the new competitive local minimum of Cl(-)(H2O)6 was offered. Considering the increasing complexity of the large system and the high flexibility of the hydrogen bonding environment, Boltzmann averaged Gibbs free energy was provided taking into account the contributions of local minima on the potential energy surface. Finally, the temperature dependence of the conformational population for isomers of Cl(-)(H2O)n (n = 5-6) and Rayleigh scattering properties of Cl(-)(H2O)n (n = 1-6) have been investigated systematically for the first time.

Study of Cl(-)(H2O)n (n = 1-4) using basin-hopping method coupled with density functional theory.

Cl(-)(H2O)n (n = 1-4) clusters were investigated using a basin-hopping (BH) algorithm coupled with density functional theory (DFT). Structures, energetics, thermodynamics, vertical detachment energies, and vibrational frequencies were obtained from high-level ab initio calculations. Through comparisons with previous theoretical and experimental data, it was demonstrated that the combination of the BH method and DFT could accurately predict the global and local minima of Cl(-)(H2O)n (n = 1-4). Additionally, to optimize larger Cl(-)(H2O)n (n > 4) clusters, several popular density functionals as well as DF-LMP2 (Schütz et al., J. Chem. Phys. 2004, 121, 737) (second-order Møller-Plesset perturbation theory using local and density fitting approximations) were tested with appropriate basis sets through comparisons with MP2 optimized results. DF-LMP2 will be used in future studies because its overall performance in describing the relative binding energies and the geometrical parameters of Cl(-)(H2O)n (n = 1-4) was outstanding in this study.

Observation of linear to planar structural transition in sulfur-doped gold clusters: Au(x)S- (x = 2-5).

We report a joint experimental and theoretical study on the structures of a series of gold clusters doped with a sulfur atom, Au(x)S(-) (x = 2-5). Well-resolved photoelectron spectra are obtained and compared with theoretical results calculated using several density functional methods to elucidate the structures and bonding of Au(x)S(-) (x = 2-5). Au2S(-) is found to have an asymmetric linear global minimum structure with C(∞v) symmetry, while the most stable structure of neutral Au2S is bent with C(2v) symmetry, reminiscent of H2S. Au3S(-) is found to have an asymmetric bent structure with an Au-S-Au-Au connectivity. Two isomers are observed experimentally to co-exist for Au4S(-): a symmetric bent 1D structure (C(2v)) and a 2D planar low-lying isomer (C(s)). The global minimum of Au5S(-) is found to be a highly stable planar triangular structure (C(2v)). Thus, a 1D-to-2D structural transition is observed in the Au(x)S(-) clusters as a function of x at x = 4. Molecular orbital analyses are carried out to obtain insight into the nature of the chemical bonding in the S-doped gold clusters. Strong covalent bonding between S and Au is found to be responsible for the 1D structures of Au(x)S(-) (x = 2-4), whereas delocalized Au-Au interactions favor the 2D planar structure for the larger Au5S(-) cluster.