Biomedical subjects

Wei Quan Tian

Publications and source records attributed to Wei Quan Tian.

9 recordsLinked to original sources

PubMed2006

Search for suitable approximation methods for fullerene structure and relative stability studies: case study with C50.

Local density approximation (LDA), several popular general gradient approximation (GGA), hybrid module based density functional theoretical methods: SVWN, BLYP, PBE, HCTH, B3LYP, PBE1PBE, B1LYP, and BHandHLYP, and some nonstandard hybrid methods are applied in geometry prediction for C60 and C70. HCTH with 3-21G basis set is found to be one of the best methods for fullerene structural prediction. In the predictions of relative stability of C50 isomers, PM3 is an efficient method in the first step for sorting out the most stable isomers. HCTH with 3-21G predicts very good geometries for C50, similar to the performance of B3LYP6-31G(d). The gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital from the predictions of all the density functional theory methods has the following descending order: E(gap)(half-and-half hybrid)>E(gap)(B3LYP)>E(gap)(HCTH)(GGA)>E(gap)(SVWN)(LDA).

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PubMed2006

Ozonization at the vacancy defect site of the single-walled carbon nanotube.

The ozonization at the vacancy defect site of the single-walled carbon nanotube has been studied by static quantum mechanics and atom-centered density matrix propagation based ab initio molecular dynamics within a two-layered ONIOM approach. Among five different reaction pathways at the vacancy defect, the reaction involving the unsaturated active carbon atom is the most probable pathway, where ozone undergoes fast dissociation at the active carbon atom at 300 K. Complementary to the experiments, our work provides a microscopic understanding of the ozonization at the vacancy defect site of the single-walled carbon nanotube.

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PubMed2006

Electronic properties and reactivity of Pt-doped carbon nanotubes.

The structures of the (5,5) single-walled carbon nanotube (SWCNT) segments with hemispheric carbon cages capped at the ends (SWCNT rod) and the Pt-doped SWCNT rods have been studied within density functional theory. Our theoretical studies find that the hemispheric cages introduce localized states on the caps. The cap-Pt-doped SWCNT rods can be utilized as sensors because of the sensitivity of the doped Pt atom. The Pt-doped SWCNT rods can also be used as catalysts, where the doped Pt atom serves as the enhanced and localized active center on the SWCNT. The adsorptions of C(2)H(4) and H(2) on the Pt atom in the Pt-doped SWCNT rods reveal different adsorption characteristics. The adsorption of C(2)H(4) on the Pt atom in all of the three Pt-doped SWCNT rods studied (cap-end-doped, cap-doped, and wall-doped) is physisorption with the strongest interaction occurring in the middle of the sidewall of the SWCNT. On the other hand, the adsorption of H(2) on the Pt atom at the sidewall of the SWCNT is chemisorption resulting in the decomposition of H(2), and the adsorption of H(2) at the hemispheric caps is physisorption.

Adsorption↗

PubMed2006

Binary clusters AuPt and Au6Pt: structure and reactivity within density functional theory.

Within density functional theory with the general gradient approximation for the exchange and correlation, the bimetallic clusters AuPt and Au(6)Pt have been studied for their structure and reactivity. The bond strength of AuPt lies between those of Au(2) and Pt(2), and it is closer to that of Au(2). The Pt atom is the reactive center in both AuPt and AuPt(+) according to electronic structure analysis. AuPt(+) is more stable than AuPt. Au(6)Pt prefers electronic states with low multiplicity. The most stable conformation of Au(6)Pt is a singlet and has quasi-planar hexagonal frame with Pt lying at the hexagonal center. The doping of Pt in Au cluster enhances the chemical regioselectivity of the Au cluster. The Pt atom essentially serves as electron donor and the Au atoms bonded to the Pt atom acts as electron acceptor in Au(6)Pt. The lowest triplet of edge-capped rhombus Au(6)Pt clusters is readily accessible with very small singlet-triplet energy gap (0.32 eV). O(2) prefers to adsorb on Au and CO prefers to adsorb on Pt. O(2) and CO have stronger adsorption on AuPt than they do on Au(6)Pt. CO has a much stronger adsorption on AuPt bimetallic cluster than O(2) does. The adsorption of CO on Pt modifies the geometry of AuPt bimetallic clusters.

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PubMed2006

Special stability of cationic MPb12+ clusters and superalkali character of neutral MPb12 clusters (M = B, Al, Ga, In, and Tl).

The electronic structures and stabilities of cationic MPb12+ clusters (M = B, Al, Ga, In, and Tl) with 50 valence electrons are investigated within density functional theory. It is shown that, at the B3LYP/cc-pVDZ(-PP) and BPW91/cc-pVDZ(-PP) levels of theory, the structures of MPb12+ with icosahedra (I(h)) symmetry are energetically favorable, and their high stabilities may arise from the closed-shell nature of the pi subsystems which are subject to the 2(N(pi + 1)2 rule with N(pi = 1). In addition, the possessing of large nucleus-independent chemical shifts of the five kinds of clusters reflects the common aromatic character of these clusters. From the comparison of our studies on the binding energies and the highest occupied molecular orbital and the lowest unoccupied molecular orbital energy gaps, the cluster AlPb12+ has higher stability than the others and this is consistent with the recent mass-spectrometric discovery of Al-doped Pb(n)+ clusters, in which AlPb12+ is highly abundant. The same methods are used to search for the structures of the neutral MPb12 clusters. The calculations reveal that the most stable geometries of the BPb12 and GaPb12 clusters have I(h) symmetry, the AlPb12 and InPb12 clusters have T(h) symmetry, and the TlPb12 cluster has C5v symmetry. Furthermore, the vertical ionization potentials of the neutral MPb12 clusters are smaller than that of some alkali atoms, indicating that the neutral MPb12 clusters possess superalkali character.

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PubMed2006

Chemical reaction of nitric oxides with the 5-1DB defect of the single-walled carbon nanotube.

In this work, we applied a two-layered ONIOM (B3LYP/6-31G(d):UFF) method to study the reaction of nitric oxides with a 5-1DB defect on the sidewall of the single-walled carbon nanotube (SWCNT). We have chosen a suitable ONIOM model for the calculation of the SWCNT based on the analyses of the frontier molecular orbitals, local density of states, and natural bond orbitals. Our calculations clearly indicate that the 5-1DB defect is the chemically active center of the SWCNT. In the reaction of nitric oxides with the defected SWCNT, the 5-1DB defect site can capture a nitrogen atom from nitric oxides, yielding the N-substitutionally doped SWCNT. We have explored the reaction pathway in detail. Our work verifies the chemical reactivity of the 5-1DB defects of the SWCNTs, indicates that the 5-1DB defect is a possible site for the functionalization of the SWCNTs, and demonstrates a possible way to fabricate position controllable substitutionally doped SWCNTs with a low doping concentration under mild conditions via some simple chemical reactions.

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PubMed2005

Bimetallic clusters Pt6Au: geometric and electronic structures within density functional theory.

Within density functional theory at the general gradient approximation for exchange and correlation (BPW91) and the relativistic 19-electron Los Alamos National Laboratory effective core pseudopotentials and basis sets (3s3p2d), the geometric and electronic structures of Pt(6)Au bimetallic clusters have been studied in detail in comparison with Pt(7). A total of 38 conformations for Pt(6)Au are located. The most stable conformation for Pt(6)Au is a sextet with an edge- and face-capped trigonal bipyramid, in which the Au atom caps an edge of the trigonal bipyramid. Pt(6)Au, in general, prefers a three-dimensional geometry and high spin electronic state with multireference character. The electronic impact of the doping of Au in Pt clusters on the overall chemical activity of the doped bimetallic cluster is not as significant as that of the doping of Pt in Au clusters; however, the doping of Au lowers the chemical activity, thus enhancing the chemoselectivity in the gas phase, of PtAu bimetallic clusters.

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PubMed2005

Theoretical studies on the structure and aromaticity of Ti2P6+.

Cationic cluster Ti2P6+ has been studied within density functional theory. The structure of this cluster is predicted to be a slightly distorted tetragonal prism. The dissociation energy of this cationic cluster is higher than that of the known sandwich compound, [(P5)2Ti]2-, because of the different bonding in these two compounds. In Ti2P6+, the hybridization of P atoms of the ring is sp3. The bonding between the metal atoms and the P ring is mainly sigma-pi. While in [(P5)2Ti]2-, the P atoms take sp2 hybridization, the bonding between the metal atom and the rings is the typical pi-pi interaction. The electronic delocalization is another stabilizing factor for Ti2P6+. The nuclear independent chemical shift indicates that Ti2P6+ is a three-dimensional aromatic molecule. The predicted infrared and NMR help to identify the Ti2P6+ conformations in experiment.

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PubMed2004

Mechanisms of Staudinger reactions within density functional theory.

The Staudinger reactions of substituted phosphanes and azides have been investigated by using density functional theory. Four different initial reaction mechanisms have been found. All systems studied go through a cis-transition state rather than a trans-transition state or a one-step transition state. The one-step pathway of the phosphorus atom attacking the substituted nitrogen atom is always unfavorable energetically. Depending on the substituents on the azide and the phosphane, the reaction mechanism with the lowest initial reaction barrier can be classified into three categories: (1). like the parent reaction, PH(3) + N(3)H, the reaction goes through a cis-transition state, approaches a cis-intermediate, overcomes a PN-bond-shifting transition state, reaches a four-membered ring intermediate, dissociates N(2) by overcoming a small barrier, and results in the final products: N(2) and a phosphazene; (2). once reaching the cis-intermediate, the reaction goes through the N(2)-eliminating transition state and produces the final products; (3). the reaction has a concerted initial cis-transition state, in which the phosphorus atom attacks the first and the third nitrogen atoms of the azide simultaneously and reaches an intermediate, and then the reaction goes through similar steps of the first reaction mechanism. In contrast to the previous predictions on the relative stability of the unsubstituted cis-configured phosphazide intermediate and the unsubstituted trans-configured phosphazide intermediate, the total energy of the substituted trans-configured phosphazide intermediate is close to that of the substituted cis-configured phosphazide intermediate. The preference of the initial cis-transition state reaction pathway is thoroughly discussed. The relative stability of the cis- and the trans-intermediates is explored and analyzed with the aid of molecular orbitals. The effects of substituents and solvent effects on the reaction mechanisms of the Staudinger reactions are discussed in detail.

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