Kang-Ming Xu

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.

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.

A density functional study of phosphorus-doped gold clusters: AunP− (n = 1–8)

The geometries of phosphorus-doped gold clusters, AunP− (n = 1–8), have been investigated using different density functionals and basis sets. B3LYP and PBE functionals with 4 basis sets (aug-cc-pVDZ, 6-311++G**, CRENBL ECP and LANL2DZ ECP) are chosen for geometry optimisation. Many low-lying structures are obtained for anionic AunP− clusters. For AunP− (n = 1–7) clusters, each level gives the same global minimum structure. It is found that the evolutionary path of phosphorus-doped gold clusters differs from that of pure gold clusters. Phosphorus atoms induce changes in the structure of pure gold clusters in small cluster sizes. Various 2D–3D structures of doped clusters are also investigated. Clusters with an odd number of gold atoms tend to yield planar 2D structures, while those with an even number of gold atoms tend to yield 3D structures.

Observation of linear to planar structural transition in sulfur-doped gold clusters: AuxS− (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.

Probing the 2D-to-3D structural transition in gold clusters with a single sulfur atom: AuxS0,±1 (x = 1–10)

Gold sulfur clusters have received much attention because of the dramatic effect that the gold–sulfide interaction produces in thiol-passivated gold nanoparticles. We present a systematic theoretical study of the electronic properties and geometric structures of AuxS0,±1 (x = 1–10) clusters using the basin-hopping global optimization technique coupled with density functional theory (DFT-BH) methods. Higher-level ab initio calculations are performed to aid in structural assignment. The same species with different electric charges possess different configurations. The 2D-to-3D structural transitions of the global minimum structures of cationic, neutral, and anionic AuxS clusters are found at the sizes of x = 3, 6, and 9, respectively. It is found that the Au5S cluster can be regarded as the building-block unit for the evolution of larger Au–S clusters. The tendency toward planarity of each Au–S cluster species, which is similar to that of bare Au clusters, may be attributed to the strong relativistic effects of Au and the similar electronegativity between Au and S. The trends of the binding energies, electron affinities, and bond parameters with increasing cluster size are studied in detail for each species. The results demonstrate that the binding energies and second-order differences exhibit interesting oscillatory behaviors; it is believed that anionic clusters may be the most suitable for catalysis.

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 studies of the hydration reactions of stabilized Criegee intermediates from the ozonolysis of β-pinene

A theoretical study was performed of the reactions of the stabilized Criegee intermediates (sCIs) of β-pinene with H2O and its dimer. Due to the large size of the biogenic sCIs, the transition states of the hydration reactions were explored with the Monte Carlo Transition State Search Program (MCTSSP), which integrated the Monte Carlo sampling technique with a transition state optimization method. The computations were performed with the M06-2X/6-311+G(2d,p) and B3LYP/6-311+G(2d,p) levels of theory. The relative energies showed that the results of the M06-2X functional are in good agreement with the results of the DF-MP2 and CCSD(T) methods. Both the reactions of the β-pinene-sCI with H2O and the β-pinene-sCI with (H2O)2 were found to be strongly exothermic. Activation barrier calculations indicate that the sink reaction with the water dimer may proceed significantly faster than the reaction with the water monomer despite the low concentration of water dimers in the atmosphere. Therefore, the reaction of sCIs with water vapor that includes large water clusters rather than single water molecules should be studied.

On the properties of Au2P3z (z = −1, 0, +1): analysis of geometry, interaction, and electron density

Au2P3, the only metastable binary phase of gold phosphide, has been discovered to exhibit remarkable semiconductor properties among metal phosphides. A systematic study on the geometry, the transformation of Au2P3 into different valence states and the different interactions among the atoms of the species is performed by using the density functional theory (DFT) method. The global minimum of Au2P3− is a 3D structure with Cs symmetry. This structure could be distorted from a planar configuration of Au2P3 which decreases the steric effect on it and leads to a new stable configuration. An analogous planar configuration, a local minimum rather than a global minimum, is also found in Au2P3+, due to the electron effect acting on the structure. Natural bond orbital (NBO) analysis reveals the re-distribution progression of the charge within the species. The central located Au atom and another no. 5 positioned P atom play significant roles on the structures. P5, as an electron adjuster, balances the electron distribution at different valence states of the structures. Deformation density analysis supplies information about charge transfer and the bonding type between two adjacent atoms as well. Looking deep into the bonding types, as electron localization function (ELF) suggests, the interaction between two adjacent P atoms (P3 and P4) of Au2P3 belongs to a strong covalent bond. The Au–P interactions among the configurations could be classified as weak classical covalent bonds through the atoms in molecules (AIM) dual parameter analysis. And for the first time, the weak interaction between the two adjacent Au atoms (Au1 and Au2) of the charged states of Au2P3 (Au2P3− and Au2P3+), are verified and different from the neutral Au2P3 through the reduced density gradient (RDG) analysis.

Sequential Observation of Alkali‐halide Gas Phase Clusters in High Resolution TOF‐MS and Prediction of Their Structures

Alkali halide clusters are interesting model systems that can provide information about how crystal properties evolve. To study these properties, a high‐resolution atmospheric pressure inlet time‐of‐flight mass spectrometry (APi‐TOF‐MS) study of the sequential sodium halides series, Cl− (NaCl)n and Br− (NaBr)m, has been reported, and the viability of the APi‐TOF‐MS equipped with an electrospray ionization source in determining cluster compositions has been demonstrated. The isotopic patterns were well resolved, as n=4 and 7 were determined to be the magic numbers for Cl− (NaCl)n clusters, which were particularly abundant in the mass spectra. A global minimum search based on density functional theory enabled basin hopping yield the most stable structures for the mentioned series. The structures exhibit several distinct motifs which can be roughly categorized as linear chain, rock salt, and hexagonal ring. This work provides an effective way to discover and elucidate the nonstoichiometry sodium halide clust...