@phdthesis{oai:sucra.repo.nii.ac.jp:00010314,
 author = {吉川, 武宏},
 month = {},
 note = {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 variants
Full-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) lattice
The 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 cluster
We 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 1
1.1 Importance of nuclear quantum effects in light particle
transfer reactions 1
1.2 Quantum simulation methods 4
1.3 References 5
2 Quantum simulation methods for understanding
the hydrogen and proton Dynamics 7
2.1 Path-integral molecular dynamics (PIMD) 7
2.2 Harmonic oscillator 9
2.3 Centroid molecular dynamics (CMD) 10
2.4 Ring-polymer molecular dynamics (RPMD) 11
2.5 Efficiency and applicability 12
2.6 References 16
3 Double Proton Transfer Mechanism in Porphycene 19
3.1 Multiple proton transfer 19
3.2 Intramolecular double proton transfer in porphycene 20
3.3 Methods 22
3.3.1 Semiempirical PM6 method 22
3.3.2 Application of specific reaction parameter
method to the PM6 23
3.3.3 Simulation conditions 26
3.4 Potential energy surface of double proton transfer for porphycene 26
3.5 Proton density distribution for HH-porphycene 28
3.6 PIMD vs classical MD for two-dimensional
proton density distribution in HH-porphycene 28
3.7 Proton density distribution for isotopic-substituted porphycenes 31
3.8 Two-dimensional proton density distributions
for isotopic substituted porphycenes 33
3.9 Free energy surface for the double proton transfer 33
3.10 Correlation of the inner nitrogen motion
with the proton transfer and its isotope effects 37
3.11 References 39
4 Diffusion of Hydrogen/Tritium in Fe (bcc) lattice 43
4.1 Hydrogen/Tritium in Metal 43
4.2 Diffusion of hydrogen and tritium atom in pure Fe metal 44
4.3 Methods 45
4.3.1 Embedded atom model potential function for Fe lattice. 45
4.3.2 Potential energy surface on Fe (100) plane 47
4.3.3 Estimation of diffusion constant 49
4.3.4 Simulation conditions 49
4.4 Diffusion constants obtained from RPMD, CMD
and classical MD simulations 50
4.5 Arrhenius plots of diffusion constants 53
4.6 Three dimensional perspective plots 54
4.7 Free energy surfaces on Fe (100) plane 55
4.8 Quantum transition state theory 58
4.9 References 61
5 Non-Adiabatic Relaxation Dynamics of Hydrated Electron Cluster 66
5.1 Hydrated electron 66
5.2 Non-adiabatic relaxation dynamics of
excited-state hydrated electron and its isotope effects 67
5.3 Development of RPMD method for describing
the non-adiabatic relaxation dynamics 69
5.3.1 One electron wave packet propagation method 70
5.3.2 Hybrid of RPMD with wave packet propagation method 70
5.4 Methods 72
5.4.1 TB pseudo-potential function for the interaction
between an excess electron and water molecules 72
5.4.2 RWK2-M model potential for the interaction
between water molecules 74
5.4.3 Simulation condition 75
5.5 Snapshots of the relaxation dynamics
along the representative RPMD trajectory 76
5.6 Temporal changes of physical quantities
along the representative RPMD trajectory 77
5.7 Statistical aspects of the survival probability
and VDE for all trajectories of both (H2O)50- and (D2O)50- 79
5.8 RPMD lifetimes obtained from a different scheme 83
5.9 Detailed nonadiabatic relaxation mechanisms 83
5.10 References 86
6 General Conclusions 90
6.1 Conclusion for chapter 3 90
6.2 Conclusion for chapter 4 91
6.3 Conclusion for chapter 5 91
Appendices 93
A Velocity Verlet algorithm 93
B Discrete Variable Representation (DVR) 94, 主指導教員 : 高柳敏幸, text, application/pdf},
 school = {埼玉大学},
 title = {Quantum Mechanical Approach for Understanding Reaction Mechanisms of Complicated Systems},
 year = {2014},
 yomi = {ヨシカワ, タケヒロ}
}