Teng Huang

Structure, stability, and electronic property of carbon-doped gold clusters AunC− (n = 1–10): A density functional theory study

The equilibrium geometric structures, relative stabilities, and electronic properties of Au(n)C(-) and Au(n+1)(-) (n = 1-10) clusters are systematically investigated using density functional theory with hyper-generalized gradient approximation. The optimized geometries show that one Au atom capped on Au(n-1)C(-) clusters is a dominant growth pattern for Au(n)C(-) clusters. In contrast to Au(n+1)(-) clusters, Au(n)C(-) clusters are most stable in a quasi-planar or three-dimensional structure because C doping induces the local non-planarity while the rest of the structure continues to grow in a planar mode, resulting in an overall non-2D configuration. The relative stability calculations show that the impurity C atom can significantly enhance the thermodynamic stability of pure gold clusters. Moreover, the effect of C atom on the Au(n)(-) host decreases with the increase of cluster size. The HOMO-LUMO gap curves show that the interaction of the C atom with Au(n)(-) clusters improves the chemical stability of pure gold clusters, except for Au3(-) and Au4(-) clusters. In addition, a natural population analysis shows that the charges in corresponding Au(n)C(-) clusters transfer from the Au(n)(-) host to the C atom. Meanwhile, a natural electronic configuration analysis also shows that the charges mainly transfer between the 2s and 2p orbitals within the C atom.

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.

Hydration of oxalic acid–ammonia complex: atmospheric implication and Rayleigh-scattering properties

A previous study of the binary system (H2C2O4)(NH3)n (n = 1–6) suggested that an oxalic acid–ammonia complex may participate in atmospheric aerosol formations. However, the mechanism of the hydration of these cores is poorly understood. In this study, the hydration of (H2C2O4)(NH3) and (H2C2O4)(NH3)2 cores with up to three water molecules is investigated with respect to different routes of formation. The results may improve understanding of the nucleation of clusters containing oxalic acid in the atmosphere. Acid dissociation is found to occur during the hydration process, leading to a HC2O4−/NH4+ ion pair. In contrast with the (H2C2O4)(NH3)2 core, water molecules appear to be unfavorable with regard to the formation of hydrates with a (H2C2O4)(NH3) core; additionally, temperature is found to affect the formation of clusters and the distributions of different isomers with the same size, but the impact of relative humidity on the hydrates seems insignificant, implying that the formation of these clusters may be more favorable under cold ambient conditions. The monohydrates and dihydrates of the (H2C2O4)(NH3)2 core may be relatively extensive in (H2C2O4)(NH3)m(H2O)n (m = 1–2, n = 1–3) clusters and may contribute to the atmospheric nucleation. Furthermore, this study presents a first attempt at determining the Rayleigh scattering properties of oxalic acid–ammonia–water pre-nucleation clusters; the results show that adding a water molecule could effectively increase the Rayleigh scattering intensity, but a single ammonia molecule may be able to generate a larger increase in the Rayleigh light scattering intensity than a water molecule. This may also indicate that clusters containing oxalic acid and ammonia show high Rayleigh light scattering intensities, but the more ammonia molecules there are in clusters, the higher the Rayleigh light scattering intensity and the greater the contribution to the extinction properties.

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.

Interaction of oxalic acid with dimethylamine and its atmospheric implications

Oxalic acid, which is one of the most common dicarboxylic acids, is expected to be an important component of atmospheric aerosols. However, the contribution of oxalic acid to the generation of new particles is still poorly understood. In this study, the structural characteristics and thermodynamics of (C2H2O4)(CH3NH2)n (n = 1–4) were investigated at the PW91PW91/6-311++G(3df,3pd) level of theory. We found that clusters formed by oxalic acid and methylamine are relatively stable, and the more the atoms participating in the formation of a ring-like structure, the more stable is the cluster. In addition, via the analysis of atmospheric relevance, it can be revealed that clusters of (C2H2O4)(CH3NH2)n (n = 1–4) have a noteworthy concentration in the atmosphere, which indicates that these clusters could be participating in new particle formation. Moreover, by comparison with (H2C2O4)(NH3)n (n = 1–6) species, it can be seen that oxalic acid is more readily bound to methylamine than to ammonia, which promotes nucleation or new particle formation. Finally, the Rayleigh scattering properties of clusters of (C2H2O4)(CH3NH2)n (n = 1–4) were investigated for the first time to determine their atmospheric implications.

Stability of Hydrated Methylamine: Structural Characteristics and H2N···H-O Hydrogen Bonds.

Methylamine is the simplest aliphatic amine found in human urine, blood, and tissues. It is thought to play a significant part in central nervous system disturbances observed during renal and hepatic disease. In this work we have investigated the methylamine hydration clusters using a basin hopping (BH) algorithm with the density functional theory (DFT). The results presented herein yield a detailed understanding of the structure and stability for a system consisting of one methylamine molecule and up to seven waters: the most stable geometries arise from a fusion of tetramer or pentamer rings; by the geometrical parameters and topological parameters analysis, the strengths of the H2N···H-O hydrogen bonds of the global minima increase as the sizes of clusters increase, except for n = 5 where there is a slight fluctuation. This work may shed light on the form mechanism of methylamine existing in organisms and the hydration structures of larger molecules containing amino functional groups and their interaction with the water molecules nearby.