Wei-Qiao Deng

Thermal conductivity of diamond and related materials from molecular dynamics simulations

Based on the Green‐Kubo relation from linear response theory, we calculated the thermal current autocorrelation functions from classical molecular dynamics ~MD! simulations. We examined the role of quantum corrections to the classical thermal conduction and concluded that these effects are small for fairly harmonic systems such as diamond. We then used the classical MD to extract thermal conductivities for bulk crystalline systems. We find that ~at 300 K! 12 C isotopically pure perfect diamond has a thermal conductivity 45% higher than natural ~1.1% 13 C! diamond. This agrees well with experiment, which shows a 40%‐50% increase. We find that vacancies dramatically decrease the thermal conductivity, and that it can be described by a reciprocal relation with a scaling as n va , with a50.6960.11 in agreement with phenomenological theory (a51/2 to 3/4!. Such calculations of thermal conductivity may become important for describing nanoscale devices. As a first step in studying such systems, we examined the mass effects on the thermal conductivity of compound systems, finding that the layered system has a lower conductivity than the uniform system.

First-Principles Investigation of Anistropic Hole Mobilities in Organic Semiconductors

We report a simple first-principles-based simulation model (combining quantum mechanics with Marcus-Hush theory) that provides the quantitative structural relationships between angular resolution anisotropic hole mobility and molecular structures and packing. We validate that this model correctly predicts the anisotropic hole mobilities of ruberene, pentacene, tetracene, 5,11-dichlorotetracene (DCT), and hexathiapentacene (HTP), leading to results in good agreement with experiment.

Ab initio and RRKM calculations of o-benzyne pyrolysis

Recently, a new mechanism has been provided in the phenyl pyrolysis, that is, the phenyl dissociation favours the benzyne channel by losing an H atom [H. Wang, M. Frenklach, J. Phys. Chem., 98 (1994) 11465]. In this Letter, the dissociation of o-benzyne has been investigated by means of ab initio theory. The geometries and structures of o-benzyne with its pyrolysis products C4H2, C2H2 and also the transition state were optimized at the UHF/6-31G* level. The single point energies were refined by B3LYP/6-31G* calculations. The unimolecular rate constants for o-benzyne pyrolysis in different pressures were calculated by the Rice-Ramsperger-Kassel-Marcus (RRKM) method

Durability and Activity Study of Single-Walled, Double-Walled and Multi-Walled Carbon Nanotubes Supported Pt Catalyst for PEMFCs

Pt nanoparticles (2-3 nm) supported on carbon black (C), single-walled carbon nanotubes (SWNT), double-walled carbon nanotubes (DWNT) and multi-walled carbon nanotubes (MWNT) are prepared by an ethylene glycol reduction method. The catalytic activity towards the oxygen reduction reaction (ORR) and the durability of the catalysts are evaluated under condition simulating the environment of proton exchange membrane fuel cell (PEMFCs). The Pt electrochemical surface area (ECSA) of Pt/DWNT and Pt/MWNT degrade 32% after accelerated durability test (ADT), compared with 38% for Pt/SWNT and 48% for Pt/C. The degradation rate of Pt/C is about 1.5 times higher than that of Pt/DWNT. The typical ORR polarization curves of Pt/C, Pt/SWNT, Pt/DWNT and Pt/MWNT obtained before and after ADT are also examined. Before ADT, Pt/DWNT shows the highest catalytic activity towards ORR, followed by Pt/MWNT, Pt/C, and Pt/SWNT in that order. After ADT, the sequence of ORR activity of Pt/C and Pt/SWNT has changed and the overall order is: Pt/DWNT > Pt/MWNT > Pt/SWNT > Pt/C. The Pt/DWNT catalyst shows the highest catalytic activity and the best durability for ORR among the C and CNT supported Pt catalysts.

Potential-energy surface ofH·SO2

The potential-energy surface for H·SO 2 has been computed at the HF/6-311G** and MP2/6-311G** levels. The calculated results confirm that trans-planar HOSO is a transition state but not a stable structure. The decomposition channel for the HOSO adduct has been studied by analysing the transition states and stationary points. The kinetic stimulus for the rearrangement and dissociation of the radical HOSO was calculated by means of RRKM theory.

Rotational alignment of products from NOCl+Sr chemiluminescent reaction

The chemiluminescent reactions Sr+NOCl→SrCl(A,B)+NO are studied under the single collision conditions in a beam-gas arrangement. The product rotational alignment 〈P2(Ĵ′⋅k)〉=−0.40±0.05 for SrCl(B) is obtained from Sr+NOCl chemiluminescent reactions. In addition, the electronic state branching ratio and the product vibrational state distribution have been roughly estimated by simulating the chemiluminescent spectra of SrCl. Quasiclassical trajectory (QCT) calculations for the Sr+NOCl reaction as a three-body system have been carried out. The calculated results agree well with the experimental ones.

Theoretical studies of the decyclization /cyclization of the C6H5 radical

Abstract The decyclization/cyclization reactions of the phenyl radical have been investigated by means of ab initio theory. The geometries and structures of the phenyl radical, the open-chain radical and also the transition state were optimized at the UHF/6-31G ∗ level, and their relative energies refined by the spin-projected (UMP3(PUMP3) method with UHF/6-31G ∗ zero-point energy corrections. Evaluations of the unimolecular rate constant for both directions of this isomerization reaction were carried out in terms of Rice-Ramsperger-Kassel-Marcus theory.

Phonon-electron coupling and tunneling effect on charge transport in organic semi-conductor crystals of Cn-BTBT

Cn-[1]benzothieno[3,2-b][1]-benzothiophene (BTBT) crystals show very high hole mobilities in experiments. These high mobilities are beyond existing theory prediction. Here, we employed different quantum chemistry methods to investigate charge transfer in Cn-BTBT crystals and tried to find out the reasons for the underestimation in the theory. It was found that the hopping rate estimated by the Fermi Golden Rule is higher than that of the Marcus theory due to the high temperature approximation and failure at the classic limit. More importantly, molecular dynamics simulations revealed that the phonon induced fluctuation of electronic transfer integral is much larger than the average of the electronic transfer integral itself. Mobilities become higher if simulations implement the phonon-electron coupling. This conclusion indicates that the phonon-electron coupling promotes charge transfer in organic semi-conductors at room temperature.

Magnetic substate transitions in collisions of Sr(5s5p 1P1) with Ar

Abstract The collision induced magnetic substate transitions of Sr(5s5p 1 P 1 ) from M l = 0 to M l = ± 1 with Ar as collision partner are studied in a beam-gas reaction apparatus. The depolarization rate constant k p = (2.5 ± 0.5) × 10 −9 cm 3 molecule −1 s −1 is obtained, and the magnetic substate transition rate constant k E = (1.25 ± 0.25) × 10 −9 cm 3 molecule −1 s −1 is deduced from depolarization rate constants.

COLLISIONAL ENERGY TRANSFER IN HIGHLY VIBRATIONALLY EXCITED C70

Abstract Quasiclassical trajectory calculations of the energy transfer of highly vibrationally excited C 70 colliding with helium and argon have been performed. From these calculations, the character of collisional energy transfer (CET) between C 70 and rare gas atoms was provided by comparing the results with the related systems such as benzene, toluene and azulene. It was found that deactivation of C 70 was significantly less than in toluene and azulene. These results were explained by the temporal dependent analysis of every trajectory.