Quantum Mechnical Study of Molecular Processes: From Gas Phase to Nano Phase Open Access
Wang, Zhi (2008)
Abstract
This thesis consists of theoretical studies of molecular processes in the scale from gas phase to nano phase.
The first part of the thesis includes theoretical studies of gas phase molecules. In chapter 2, high level ab initio methods combined with direct quasi-classsical molecular dynamics simulation have been used to study the potential energy surfaces in the photodissociation of HN3 with particular interest on the ring closure process in the formation of cyclic-N3 . In chapter 3, low-lying potential energy states of N3 are studied theoretically with focus on their electronic nature, and possible pathways of production of cyclic-N3 through photoexcitation of linear-N3 and/or suitable -N3 precursors are suggested. Finally in chapter 4, we extend this study and construct the potential energy surfaces for 5 low-lying states of N3 .
The second part of the thesis is mainly about the study of carbon nanotubes. The density functional tight binding (DFTB) method is used throughout all the calculations. In chapter 6 and 7, the origin of the linear relationship between CH2/NH/O-SWNT reaction energies and sidewall curvature is investigated for armchair nanotubes and chiral nanotubes, respectively. Then in chapter 8, 9 and 10, high-temperature quantum chemical molecular dynamics simulations have been performed on model systems of thin SiC crystal surfaces to understand the nanotube formation process. Chapter 8 and 10 focus on the initio nanocap formation stage while chapter 9 focus on the later growth process.
Table of Contents
Abstract...iv Acknowledgments...vi Introduction...xxv 1 Introduction of techniques in studying non-adiabatic processes...2
1.1 Locating the Minimum on the Seam of Crossing (MSX)...2
1.1.1 Newton method with Lagrangian multiplier...3 1.1.2 Direct method without Hessian...4
1.2 Approximate Diabatization Scheme by Analyzing CI Vectors...5 1.3 Constrained Least Square Fitting...11 References...22
2 Ab initio study of the photodissociation of HN3...24
2.1 Introduction...24 2.2 Methodology...27 2.3 Results and Discussion...28
2.3.1 Stationary points on the ground and excited states...29 2.3.2 Adiabatic dissociation on S0, S1, and S2 states...34 2.3.3 Internal conversion between S0/S1 states and S1/S2 states...39 2.3.4 Possible dissociation pathways to the cyclic N3 formation...41 2.3.5 Quasi-classical molecule dynamics simulation...44
2.4 Summary and Conclusion...47 References...49 Tables...53 Figures...56
3 The low-lying doublet excited states of N3...67
3.1 Introduction...67 3.2 Technical Details...69 3.3 Results and Discussion...70
3.3.1 2B2 surfaces...72 3.3.2 2A2 surface...73 3.3.3 2A1 surface...73 3.3.4 2B1 surface...74
3.4 Summary...75 References...76 Tables...79 Figures...80
4 Ab initio potential energy surfaces for N3 low-lying doublet states...84
4.1 Introduction...84 4.2 Ab initio Calculations...86 4.3 Global Fit...87 4.4 Local Fit...90
4.4.1 Formulation of special local t...91 4.4.2 Dene the local CI region...93 4.4.3 Connectting global surface and local surface...95
4.5 Diabatization for CI3...96 4.6 Results and Discussion...98
4.6.1 Fitting errors...98 4.6.2 Stationary points on adiabatical surfaces...101 4.6.3 Conincal intersections...105
4.7 Summary...109 References...110 Tables...113 Figures...118
5 Introduction of DFTB method...129
References...138
6 Origin of the Linear Relationship between CH2/NH/O-SWNT Reaction Energies and Sidewall Curvature: Armchair Nanotubes...139
6.1 Introduction...139 6.2 Computational Details...144 6.3 Results and Discussion...145
6.3.1 Comparison between B3LYP/6-31G(d) and DFTB energetics and geometries of exohedral CH2/NH/O adducts of (n; n) SWNTs with n = 3 to 6...145 6.3.2 One or two local minima on the Ca-Ca PES of exo-addition products...146 6.3.3 Linear relationships between the stabilization energy (ΔE) and the inverse radius (1/d) of exo- and endohedral addition products of CH2/NH/O to (n; n) SWNTs with n=4 to 13 and n = ∞...149 6.3.4 Energy decomposition analysis of ΔE and the origin of the 1/d dependency...152
6.4 Summary and Conclusions...157 References...160 Tables...167 Figures...171
7 Analysis of the Relationship between Reaction Energies of Electrophilic SWNT Additions and Sidewall Curvature: Chiral Nanotubes...177
7.1 Introduction...177 7.2 Computational details...181 7.3 Results and Discussion...182
7.3.1 Comparison between B3LYP/6-31G(d) and DFTB energetics of exo(l) X=CH2/NH/O adducts to (2n; n) SWNTs with n=2-5...182 7.3.2 Two local minima can exist on the Ca-Cx PESs of exo-addition products for X=O for the three t/d/p bond types...183 7.3.3 Linear relationship between the stabilization energy ΔE and the inverse radius 1/d of exo- and endohedral addition products of CH2/NH/O to (2n; n) SWNTs with n=3-8 and n=∞...185 7.3.4 Energy decomposition analysis of ΔE...188
7.4 Summary...192 References...195 Tables...199 Figures...202
8 Theory and experiment agree: Single-walled carbon nanotube caps grow catalyst-free with chirality preference on a SiC surface...209
8.1 Introduction...209 8.2 Computational Methodology...210 8.3 Results...211 8.4 Summary...217 References...218 Tables...221 Figures...223
9 Carbon Nanotubes Grow on the C Face of SiC (0001) during Sublimation Decomposition: Quantum Chemical Molecular Dynamics Simulations...226
9.1 Introduction...226 9.2 Computational Methods...229 9.3 Results and Discussion...230
9.3.1 Cap nucleation and CNT growth initial structures...230 9.3.2 CNT growth...233
9.4 Summary and Conclusions...246 References...249 Tables...255 Figures...256
10 Graphene growth during sublimation decomposition of SiC: A Quantum Chemical Molecular Dynamics Investigation...266
10.1 Introduction...266 10.2 Computational Method...268 10.3 Results and Discussions...270
10.3.1 "Press-and-Bake" simulations...270 10.3.2 Shooting C2 molecules on SiC surfaces...272 10.3.3 Random Si removal from C- and Si-face...275 10.3.4 Si Change Color removal...277
10.4 Conclusions...280 References...282 Tables...285 Figures...286
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