Yongjun Liu

A theoretical study of silicon-doped boron nitride nanotubes serving as a potential chemical sensor for hydrogen cyanide

In order to search for a novel sensor to detect and control exposure to hydrogen cyanide (HCN) pollutant molecule in environments, the reactivities of pristine and silicon-doped (Si-doped) (8, 0) single-walled boron nitride nanotubes (BNNTs) towards the HCN molecule are investigated by performing density functional theory (DFT) calculations. The HCN molecule presents strong chemisorption on both the silicon-substituted boron defect site and the silicon-substituted nitrogen defect site of the BNNT, which is in sharp contrast to its weak physisorption on pristine BNNT. A remarkable charge transfer occurs between the HCN molecule and the Si-doped BNNT as proved by the electronic charge densities. The calculated data for the electronic density of states (DOSs) further indicate that the doping of the Si atom improves the electronic transport property of the BNNT, and increases its adsorption sensitivity towards the HCN molecule. Based on calculated results, the Si-doped BNNT is expected to be a potential resource for detecting the presence of toxic HCN.

Theoretical investigation on second-order nonlinear optical properties of (dicyanomethylene)-pyran derivatives

Abstract On the basis of the ZINDO program, we have designed a program to calculate the second-order nonlinear polarizabilities β ijk , β 0 and β μ according to the sum-over-states (SOS) expression. A series of new 4-(dicyanomethylene)-2-methyl-6-( p -dithylamino-styryl)-4 H -pyran (DCM) derivatives were designed and their electron spectra and nonlinear optical properties were studied. It is proposed that these compounds possess two important excited states close to each other in energy, both contributing to hyperpolarizability in an additive fashion; 4-(dicyanomethylene)-2,6-bis-( p -donor-styryl)-4 H -pyran derivatives are more nonlinear than 4-(dicyanomethylene)-2,6-bis-( p -donor-phenyl)-azo-4 H -pyran derivatives. The high nonlinearities, good thermal stability and high transparency make them attractive candidates for second-order nonlinear applications such as electro-optic modulators and frequency doublers.

Theoretical Investigation of Second-Order Nonlinear Optical Properties of Substituted Benzothiazole Derivatives

On the basis of the ZINDO program, we have designed a program to calculate the nonlinear second-order polarizabilities βijk, β0, and βμ according to the SOS expression. The second-order nonlinear optical properties of a series of benzothiazole derivatives including 2-(p-donor-β-styryl)-6-nitrobenzothiazole, 2-(p-donor-phenyl)-azo-6-nitrobenzothiazole, and 2-(p-nitro-β-styryl)-6-donor-benzothiazole derivatives were studied. The calculated results show that when the benzothiazole rings are connected to the nitro group (acceptor), the β values are relatively larger. This does not agree with the theory of auxiliary donor–acceptor effects. The 2-(p-donor-β-styryl)-6-nitrobenzothiazole derivatives exhibit more nonlinearity than 2-(p-donor-phenyl)-azo-6-nitrobenzothiazole derivatives. The 2-(p-donor-β-styryl)-6-nitrobenzothiazole derivatives are good candidates as chromophores due to their high nonlinearities and good thermal stability.

Catalytic mechanisms of Au11 and Au11-nPtn (n=1–2) clusters: a DFT investigation on the oxidation of CO by O2

The oxidation of CO catalyzed by clusters of Au11, Au10Pt and Au9Pt2 was investigated using the M06 functional suite of the density functional theory. Au and Pt atoms were described with the double-ζ valence basis set Los Alamos National Laboratory 2-double-z (LanL2DZ), whereas the standard 6-311++G(d,p) basis set was employed for the C and O atoms. Our theoretical model showed that (1) after coordination to Au and Au-Pt cluster, O2 and CO are apparently activated, and Mulliken charges show that the gold atoms in the active sites of Au11 are negatively charged; (2) Au-Pt clusters with 11 atoms can effectively catalyze the oxidation of CO by O2; (3) Au11 exhibits good catalytic performance for the oxidation of CO; (4) oxidation of CO occurs preferably on the Au–Pt active sites in Pt-doped clusters, and the single-center mechanisms are more favorable energetically than the two-center mechanisms; (5) after adsorption, an O2 molecule oxidates two CO molecules via stepwise mechanisms; and (6) the catalytic processes are highly exothermic.

Role of F− in the hydrolysis–condensation mechanisms of silicon alkoxide Si(OCH3)4: a DFT investigation

The hydrolysis and condensation mechanisms of tetramethoxysilane aided by F− were investigated with Gaussian03 program package. The coordination of F− to tetramethoxysilane and the first-order hydrolysis of the fluorotetramethoxysilane anion were studied extensively with both CPCM full optimization and CPCM single-point energy (SPE) calculations, and the single-point energies agree well with those obtained from the full optimization with CPCM solvation model. The coordination of F− decreases the Mulliken charge on the Si atom and the energy gap between the HOMO and LUMO, and alters the shape of the frontier orbitals. Our calculations show that entropic effects elevate potential energy surface (PES) profiles distinctly, but have a minor influence on the free energy barriers. With the aid of F−, the hydrolysis barriers of fluorotetramethoxysilane anion and the dimerization free energy barriers before entropic corrections decrease from 104, 106, 109, 109 and 136, 104, 99, 94 to kJ mol−1 to 84.2, 88.5, 77.2, 81.9 and 82.2, 80.0, 87.7, 88.7 kJ mol−1, respectively, compared with the neutral hydrolysis and condensation barriers. These barriers are much lower than the corresponding neutral SN2 ones, but only slightly higher than the SN1 ones. The role of nucleophile F− is similar to HO−.

Mechanistic investigations of Al(OH)3 oligomerization mechanisms

Aluminum aerogels have extremely low thermal conductivities, and are ideal candidates for use in thermal superinsulators, adsorbents, sensors, catalyst carriers, and inorganic fillers. In the present work, the oligomerization mechanisms of Al(OH)3 were investigated systematically with the Gaussian 03 package at the B3LYP/6-311++G(d,p) level in combination with CPCM single-point energy calculations. The results of our theoretical model showed that: (1) the Al atoms are tetracoordinate and pentacoordinate; (2) in alkaline solution, Al(OH)3 tends to condense into more soluble polyhydroxy compounds; (3) the neutral dimerization of Al(OH)3 and the transfer of the hydrogen on the bridging hydroxyl are energetically favorable, but the most stable geometry is a four-membered Al–O ring structure linked by two bridging hydroxyls; (4) Al(OH)3 is inclined to form tetracoordinate oligomers, which develop into three-dimensional structures connected by four-membered Al–O rings.

Theoretical investigations on the synthesis mechanism of cyanuric acid from NH3 and CO2

AbstractIn the synthesis of cyanuric acid from NH3 and CO2, urea and isocyanic acid OCNH are two pivotal intermediates. Based on density functional theory (DFT) calculations, the synthesis mechanism of cyanuric acid from NH3 + CO2 was investigated systematically. Urea can be synthesized from NH3 and CO2, and cyanuric acid can be obtained from urea or NH3 + CO2. In the stepwise mechanism of cyanuric acid from urea or NH3 + CO2, the energy barriers are relatively high, and the condition of high pressure and temperature does not decrease the energy barriers. Our theoretical model shows that cyanuric acid is actually acquired from OCNH via a one-step cycloaddition reaction.n FigureThe synthesis mechanism of cyanuric acid from NH3 and CO2 was revealed systematically with density functional theory methods relative to 3NH3 + 3CO2

The electron-transfer reaction for Co(H2O)62+/3+

Abstract A theoretical scheme is presented to study the rate of the electron-exchange reaction between transition metal complexes in aqueous solution. In the scheme the activation energy is obtained via an activation model and ab initio technique, and the coupling matrix element at the transition state is determined by using perturbation theory and a numerical integral method. The electron-transfer (ET) reactivity of Co(H 2 O) 6 2+/3+ , a prototype system of the ET reactions between transition metal complexes in solution, is determined by employing this scheme. The theoretical results are compared with the corresponding experimental values and good agreement is found. The further extension of the scheme is also discussed.