Wang Zhi-gang

Depolymerization dynamics study of poly-alpha-methylstyrene

Polymers are macromolecus composed of repeated monomer subunits, which construct the fabulous world of DNA, protein, cellulose, rubber, plastic, etc. and possess crucial positions in fields like life science, chemistry industry and material preparation with special demands. However, the performance of polymer would decay because of the influence of environment effects and even be depolymerized. While under particular scenarios, such a decomposition process might be valuable. Poly-alpha-methylstyrene (PAMS) is the substrate materials for preparing glow discharge polymer (GDP) shell which coats the fuel target for inertial confinement fusion. The key step for its involvement is to depolymerize into single molecules under high temperature so that it can escape through the space among GDP molecules which has deposited on it, and then leaves only the GDP shell. Besides of this special application, the atomic level understanding of depolymerization processes is also at the key position to modulate other reactions involving polymer materials. Here, through molecular dynamic simulation based on density functional tight-binding methods containing dispersion correction (DFTB-D), we demonstrate the typical dissociation of sequential poly-alpha- methylstyrene (PAMS) tetramer fragments with unsaturation on both ends, on the C- end and on the CH2- end, respectively. DFTB method we used here is an approximation based on the second order expansion of the density functional theory (DFT) total energy with respect to charge density variation relative to a chosen reference density. It promotes the efficiency of DFT method around two to three orders meanwhile remains acceptable precision for electronic structure calculation and large-scale quantum dynamic simulations. The reliability of the method has been admitted in massive researches, especially those on carbon-based molecules and organic systems. Our results show, with the temperature of 500 and 600 K, the dissociation of PAMS fragments is implemented by depolymerization processes, where monomers separate from one of the unsaturated ends one by one, and rising temperature could reasonably accelerate the reaction. Further simulations of a longer hexamer PAMS fragment under 600 K indicates the length effect won’t cause qualitative influence on the depolymerization process. The electronic structures of these three fragments indicate both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are localized at the bond-breaking unsaturated ends, which supports the prediction of dynamic simulations for reaction sits. Furthermore, the DFTB-D electronic structure calculations following the C—C bond breaking steps at different possible sites along the backbone chain also show the preference of the reaction to unsaturated end sites, from the view of potential energy surface. Spin distribution analysis also reflects the energy curves respect to the different depolymerization processes are closely related to the spin-polarized electronic structures of corresponding products. These results give a representative atomic-level prospect about the decomposition process of sequential PAMS under high temperature with detailed interpretation on the reaction sit preference and order, which may be informative for preparation of relative device. Besides, we hope this theoretical study could offer a fundamental reference for understanding the mechanism of decomposition for polymer materials beyond PAMS and spread across the wonderful microscopic world.

All-boron fullerene B 40 : a superatomic structure

By means of the first-principles calculations, we reveal that B40 presents superatomic properties. It not only has superatomic 1S, 1P, 1D and 1F orbitals, but also has superatomic orbitals 2S, 2P, 2D and 2F. The superatomic 2F orbital of B40 is partially occupied, thus adding six electrons to the cluster leads to a superatomic structure of fully occupied shells. The computed nucleus-independent chemical shift values suggest that the B406- is of higher electron delocalization than B40 itself. Our work opens up a new perspective for the superatomic physics in boron fullerene family.摘要文利用第一性原理计算,发现B40表现出超原子特性,不仅有超原子轨道1S, 1P, 1D, 1F,还有超原子轨道2S, 2P, 2D和2F.由于B40的 2F壳层是部分占据, 因此添加6个电子到B4。上将导致一个满占据壳层的超原子结构.核独立化学位移的计算结果显示B406-的电子离域性比 B40高.本工作开创了一个硼富勒烯家族中超原子物理新视角.

Structure and stability of various states of the EuC and EuC2 molecules

B3LYP level density functional theory (DFT) and multiconfiguration self-consistent-field (MCSCF) level ab initio method calculations have been performed on the basis of relativistic effective core potentials to investigate the nature of EuC and EuC2 molecules. The computed results indicate that the ground states of EuC and EuC2 are 12∑+ and 8A2, respectively. Dissociation potential energy curves of the low-lying electronic states of EuC have been calculated using the MCSCF method, and the same level calculation on EuC2 indicates that the dissociation energy of EuC2 of ground state compares well with the available experimental data. The bond characteristic is also discussed using Mulliken populations.

Electronic states of C20 isomers

Using quantum chemistry ab initio method, geometry optimization of the singlet and triplet electronic states are performed for three isomers (ring, bowl and cage structures) of C20. From the optimized geometries, MP2 calculation gives a result which does not agree with Hunds rule, i.e. the singlet state has a lower energy for all the three isomers. Further more, total charge densities and electrostatic potentials of three isomers are given by the UHF wavefunctions, the results indicate negative charges located in the central area of cage and bowl structures and the bond character of ring structure is analysed.

Differential Cross Sections for Elastic Scattering of Low-Energy Electrons by O2

Differential cross sections for the elastic scattering of electrons by oxygen molecule are calculated for selected impact energies 7 eV and 9 eV. The results are compared with other theoretical results and experimental data. The present results are obtained by the momentum–space optical potential method. This method take the polarization of target states into account, which is very important for the scattering problem, particularly at low energies.

Optical potential calculations of elastic collision for electron scattering by H2

Differential cross sections for the elastic scattering of the electrons by H2 at 100 and 150 eV have been calculated and compared with experiments. We use the momentum space method in which the electron–molecule system has a single centre and the effect of higher reaction channels on electron–molecule elastic scattering is approximated by an ab initio equivalent-local potential. It is added to the exact static-exchange potential for e–H2 scattering. \pacs{34.80.Bm