http://swrc.ontoware.org/ontology#Thesis
Quantum Mechanical Approach for Understanding Reaction Mechanisms of Complicated Systems
en
吉川 武宏
博士論文（埼玉大学大学院理工学研究科（博士後期課程））
2014
埼玉大学大学院理工学研究科
Graduate School of Science and Engineering, Saitama University
iv, 95 p.
Although it has been generally accepted that nuclear quantum effects, including tunneling and vibrational quantization, are playing very important roles in light particle transfer reactions, their role has not yet been fully understood in complicated chemical systems. This is simply because it is very difficult to solve quantum mechanical equations of motions expect for simple chemical systems. In this work, in order to understand nuclear quantum effects from the theoretical side, we apply path-integral based computational methods, which can be applied to complicated systems. In particular, we would like to demonstrate that various isotope effects seen in many chemical dynamical systems cannot be understood without consideration of nuclear quantum effects.Double proton transfer for porphycene and isotopic variantsFull-dimensional Path-integral molecular dynamics (PIMD) simulations have been performed for understanding the double proton transfer tautomerization mechanism of the inner two protons in porphycene and its isotopic-substituted molecules. In order to reduce computational costs, the semi-empirical PM6 level combined with specific reaction parameterization have been employed. The obtained results show that double proton transfer of the unsubstituted porphycene at T = 300 K mainly occurs via a so-called concerted mechanism through the D2h second-order saddle point. In addition, we found that both isotopic substitution and temperature significantly affect the double proton transfer mechanism. For example, the contribution of the stepwise mechanism increases with a temperature increase. We have also carried out hypothetical simulations with the porphycene configurations being completely planar. It has been found that out-of-plane vibrational motions significantly decrease the contribution of the stepwise proton transfer mechanism.Diffusion of hydrogen/tritium in Fe (bcc) latticeThe diffusion coefficients of hydrogen (H) and tritium (T) in α-Fe have been computed using two approximate quantum dynamical techniques, i.e. centroid molecular dynamics (CMD) and ring polymer molecular dynamics (RPMD), in the temperature range of T = 100－1000 K using the embedded-atom-method (EAM) potential. It has been found that the RPMD and CMD methods give very similar results. From a further analysis based on quantum transition state theory (centroid density QTST) combined with path integral molecular dynamics (PIMD), it has been clear that there is a crossover between thermal and quantum mechanisms at about T = 500 K and 300 K for H and T diffusions, respectively. The importance of nuclear quantum effects at low temperatures has been illustrated in terms of the effective free energy surface map.Nonadiabatic relaxation dynamics of excited hydrated electron clusterWe have applied a recently-developed hybrid quantum ring-polymer molecular dynamics (RPMD) method to nonadiabatic p → s relaxation dynamics in water anion clusters in order to understand the isotope effects observed in previous experiments. The average relaxation times for (H2O)50-and (D2O)50- were calculated to be 120 and 207 fs, respectively, and are comparable to experimental results. Therefore, we conclude that nuclear quantum effects are playing an essential role in the structural rearrangement dynamics of water anion clusters. The detailed nonadiabatic relaxation mechanisms are also discussed.
1 Introduction 11.1 Importance of nuclear quantum effects in light particletransfer reactions 11.2 Quantum simulation methods 41.3 References 52 Quantum simulation methods for understandingthe hydrogen and proton Dynamics 72.1 Path-integral molecular dynamics (PIMD) 72.2 Harmonic oscillator 92.3 Centroid molecular dynamics (CMD) 102.4 Ring-polymer molecular dynamics (RPMD) 112.5 Efficiency and applicability 122.6 References 163 Double Proton Transfer Mechanism in Porphycene 193.1 Multiple proton transfer 193.2 Intramolecular double proton transfer in porphycene 203.3 Methods 223.3.1 Semiempirical PM6 method 223.3.2 Application of specific reaction parametermethod to the PM6 233.3.3 Simulation conditions 263.4 Potential energy surface of double proton transfer for porphycene 263.5 Proton density distribution for HH-porphycene 283.6 PIMD vs classical MD for two-dimensionalproton density distribution in HH-porphycene 283.7 Proton density distribution for isotopic-substituted porphycenes 313.8 Two-dimensional proton density distributionsfor isotopic substituted porphycenes 333.9 Free energy surface for the double proton transfer 333.10 Correlation of the inner nitrogen motionwith the proton transfer and its isotope effects 373.11 References 394 Diffusion of Hydrogen/Tritium in Fe (bcc) lattice 434.1 Hydrogen/Tritium in Metal 434.2 Diffusion of hydrogen and tritium atom in pure Fe metal 444.3 Methods 454.3.1 Embedded atom model potential function for Fe lattice. 454.3.2 Potential energy surface on Fe (100) plane 474.3.3 Estimation of diffusion constant 494.3.4 Simulation conditions 494.4 Diffusion constants obtained from RPMD, CMDand classical MD simulations 504.5 Arrhenius plots of diffusion constants 534.6 Three dimensional perspective plots 544.7 Free energy surfaces on Fe (100) plane 554.8 Quantum transition state theory 584.9 References 615 Non-Adiabatic Relaxation Dynamics of Hydrated Electron Cluster 665.1 Hydrated electron 665.2 Non-adiabatic relaxation dynamics ofexcited-state hydrated electron and its isotope effects 675.3 Development of RPMD method for describingthe non-adiabatic relaxation dynamics 695.3.1 One electron wave packet propagation method 705.3.2 Hybrid of RPMD with wave packet propagation method 705.4 Methods 725.4.1 TB pseudo-potential function for the interactionbetween an excess electron and water molecules 725.4.2 RWK2-M model potential for the interactionbetween water molecules 745.4.3 Simulation condition 755.5 Snapshots of the relaxation dynamicsalong the representative RPMD trajectory 765.6 Temporal changes of physical quantitiesalong the representative RPMD trajectory 775.7 Statistical aspects of the survival probabilityand VDE for all trajectories of both (H2O)50- and (D2O)50- 795.8 RPMD lifetimes obtained from a different scheme 835.9 Detailed nonadiabatic relaxation mechanisms 835.10 References 866 General Conclusions 906.1 Conclusion for chapter 3 906.2 Conclusion for chapter 4 916.3 Conclusion for chapter 5 91Appendices 93A Velocity Verlet algorithm 93B Discrete Variable Representation (DVR) 94
主指導教員 : 高柳敏幸
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2015-02-04