超臨界水中的溶解和化學反應受到一系列因素的影響，如溶解能，熵以及溶液密度等，是決定化學平衡的基本熱力學量，同時這些又受到溫度和壓強的控制。爲了解釋這些因素的影響，有必要把量子化學的靜態優化與分子動力學模擬相結合和比較。通過量子化學可以得到0K下的優化結構，而分子動力學模擬可以提高實際時間的勢能面。本論文的研究，主要圍繞在氣態下甲醛分子HCHO和甲酸分子HCOOH跟不同數目水分子H2O結合的水合簇結構，以及在常溫水溶液和超臨界水溶液中，甲醛HCHO和甲酸HCOOH的溶解結構和溫度所帶來的熱效應，最後研究甲酸HCOOH在水催化下的離解反應機理。 / 使用化學計算軟件Gaussian03和密度泛函理論方法，用6-311++G(d,p)基組來計算和研究氣態下甲醛分子和甲酸分子的水簇合物。通過不同數目的水分子所得到的最穩定簇合物的結構和能量，來研究甲醛分子HCHO，甲酸根離子HCOO⁻以及甲酸分子HCOOH與水分子相互結合時的氫鍵作用力強弱和簇合物的穩定性。同時，也考慮了甲酸酸解后的水簇合物結構，通過與沒有酸解的水簇合物的比較，為進一步瞭解水溶解中甲酸的酸解離情況提供寶貴的信息。 / 使用基於贗勢和平面波基組，以及密度泛函理論的從頭計算分子動力學軟件VASP，來模擬和研究甲醛分子HCHO和甲酸分子在水溶液中的溶解情況。根據對半徑關聯函數PRDF的統計結果，可以觀察出溶質的溶解結構，以及溶劑分子之間，或者溶質與溶劑分子之間的氫鍵作用。通過水合數目可以看出氫鍵作用力隨著溫度的提升而減弱。水溶液的溫度在臨界點之上時，其結果證實了甲酸的酸解反應受到嚴重的抑制，與常溫水的結果相反。 / 使用從頭計算分子動力學軟件CPMD中基於Car-Parrinello分子動力學方法的Metadynamics方法對甲酸的反應機理進行系統的研究，包含脫水反應和脫氫反應。解離反應分別包含不同數目的分子，通過對比來研究反應中水分子所起的潛在的催化作用。除此之外，通過300K和700K這兩種不同溫度下的結果對比，來解釋超臨界水溶解中甲酸快速解離的原因。自由能曲面和自由能壘揭示了在不同環境下甲酸的主要解離途徑。 / Solvation and chemical reactions in supercritical water are affected by a number of factors. Solvation energy, entropy, and densities are the basic thermodynamic quantities that determine the chemical equilibriums, which can be controlled by temperature and pressure. To account for these factors, static optimization leading to zero-temperature structures should be combined and compared with molecular dynamics simulation in real time. In my thesis, the solvation structures are studied in gas phase and aqueous phase, to understand the properties of solvent water and the thermal effect on the reactions. / The hydrated clusters of formaldehyde and formic acid in gas phase are explored computationally by density functional theory (DFT) with a basis set 6-311++G(d,p). Investigation on the structures and energies of hydrated HCHO, HCOO⁻ and HCOOH solvated by a number of water molecules is important for understanding the hydrogen bond interactions as the number of water molecules increases. Comparisons between non-dissociated and dissociated clusters of hydrated formic acid provide valuable information on the acidic dissociation of formic acid in aqueous solution. / The solvations of formaldehyde and formic acid in aqueous solution are simulated by density functional theory based ab initio molecular dynamics (AIMD) method with pseudopotentials and a plane wave basis set using Vienna Ab-initio Simulation Package (VASP). The pair radial distribution function is obtained to elucidate the solvation structure and the hydrogen bond interaction among solvent molecules, and between solute and solvent. The hydration number indicates the weakening of the hydrogen bond with increasing temperature. The results at the temperatures above the critical point of water show that the acid dissociation of formic acid is greatly depressed which is different from the results in ambient water. / The mechanisms for the dissociations of formic acid in the gas phase and in aqueous solution are studied by Car-Parrinello (CP)-based metadynamics (MTD) method, implemented in the Car-Parrinello Molecular Dynamics (CPMD) program. The two main dissociations channels of dehydration and dehydrogenation, including zero, one, two and bulk water molecules, respectively, are simulated with a biased external potential to examine the potential catalytic role of water. In addition, the thermal effects at two different temperatures are included to account for the rapid dissociation of formic acid in supercritical water. The free energy surfaces are reconstructed and the barriers are calculated to show the main dissociation pathway of formic acid in different environments. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Chen, Qiubo. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references (leaves 185-194). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese. / TITLE PAGE --- p.i / ABSTRCACT (ENGLISH) --- p.ii / (CHINESE) --- p.v / AKNOWLEDGEMENTS --- p.vii / TABLE OF CONTENTS --- p.viii / LIST OF FIGURES --- p.xiv / LIST OF TABLES --- p.xxiv / Chapter Chapter ONE --- Background / Chapter 1.1 --- Introduction of green chemistry --- p.1 / Chapter 1.1.1 --- Green chemistry --- p.1 / Chapter 1.1.2 --- Supercritical fluid (SCF) and supercritical water (SCW) --- p.2 / Chapter 1.2 --- Formaldehyde and formic acid in supercritical water --- p.8 / Chapter 1.2.1 --- Formaldehyde --- p.8 / Chapter 1.2.2 --- Formic acid --- p.12 / Chapter 1.3 --- Scope of this thesis --- p.17 / Chapter Chapter TWO --- Theories and Calculation Methods / Chapter 2.1 --- General background --- p.19 / Chapter 2.1.1 --- Schrödinger equation --- p.19 / Chapter 2.1.2 --- Born-Oppenheimer approximation --- p.20 / Chapter 2.2 --- Hartree-Fock (HF) approximation and post-Hartree-Fock (post-HF) approximation --- p.22 / Chapter 2.3 --- Density functional theory (DFT) --- p.28 / Chapter 2.3.1 --- Kohn-Sham (KS) scheme --- p.29 / Chapter 2.3.2 --- Local density approximation (LDA) --- p.31 / Chapter 2.3.3 --- Generalized gradient approximation (GGA) --- p.33 / Chapter 2.3.4 --- Hybrid functionals --- p.34 / Chapter 2.4 --- Ab initio molecular dynamics (AIMD) --- p.35 / Chapter 2.4.1 --- Molecular dynamics --- p.35 / Chapter 2.4.2 --- Ab initio molecular dynamics --- p.36 / Chapter 2.4.3 --- Plane waves --- p.42 / Chapter 2.4.4 --- Pseudopotentials (PP) --- p.44 / Chapter 2.4.5 --- Periodic boundary condition (PBC) --- p.48 / Chapter 2.5 --- Metadynamics (MTD) method --- p.48 / Chapter 2.5.1 --- The Algorithm --- p.49 / Chapter 2.5.2 --- Lagrangian metadynamics and the choice of V(t,s) --- p.52 / Chapter 2.5.3 --- The choice of CVs --- p.53 / Chapter Chapter THREE --- Structures of the Hydrated Clusters of Formaldehyde and Formic Acid / Chapter 3.1 --- Introduction --- p.55 / Chapter 3.2 --- Computational details --- p.56 / Chapter 3.3 --- Results and discussions --- p.58 / Chapter 3.3.1 --- Studies of HCHO(H₂O)[subscript n] clusters, n = 0~4 --- p.58 / Chapter 3.3.1.1 --- The structures of HCHO(H₂O)[subscript n] clusters, n = 0~4 --- p.58 / Chapter 3.3.1.2 --- The energies of HCHO(H₂O)[subscript n] clusters, n = 0~4 --- p.63 / Chapter 3.3.2 --- Studies of HCHO⁻(H₂O)[subscript n] clusters, n = 0~6 --- p.65 / Chapter 3.3.2.1 --- The structures of HCHO⁻(H₂O)[subscript n] clusters, n = 0~4 --- p.66 / Chapter 3.3.2.2 --- The energies of HCHO⁻(H₂O)[subscript n] clusters, n = 0~4 --- p.71 / Chapter 3.3.2.3 --- Studies of HCOO⁻(H2O)[subscript n] clusters, n = 5 and 6 --- p.73 / Chapter 3.3.3 --- Studies of HCOOH⁻(H2O)[subscript n] and HCOO⁻(H₃O)⁺(H₂O)[subscript n-1] clusters --- p.76 / Chapter 3.3.3.1 --- Results of cis-HCOOH(H₂O)[subscript n] clusters and trans-HCOOH(H₂O)[subscript n] clusters, n = 0 ~ 4 --- p.76 / Chapter 3.3.3.2 --- Results of trans-HCOOH(H₂O)[subscript n] clusters, n > 4 --- p.82 / Chapter 3.3.3.3 --- Comparisons of non-dissociated trans-HCOOH(H₂O)[subscript n] clusters with dissociated ion pair HCOO⁻(H₃O)⁺(H₂O)[subscript n-1] clusters --- p.84 / Chapter 3.4 --- Summary --- p.87 / Chapter CHAPTER FOUR --- Ab initio Molecular Dynamics Studies on the Solvations of Formaldehyde HCHO and Formic Acid HCOOH in Water Solution at Different Temperatures / Chapter 4.1 --- Introduction --- p.90 / Chapter 4.2 --- Computational details --- p.93 / Chapter 4.3 --- Results and discussions --- p.94 / Chapter 4.3.1 --- The solvation of water solution --- p.94 / Chapter 4.3.1.1 --- The solvation of pure water solution at T = 300 K, 600 K, 700 K and 2000 K --- p.95 / Chapter 4.3.1.2 --- The solvation of proton H⁺ in water solution at T = 300 K and 700 K --- p.101 / Chapter 4.3.2 --- The solvation of formaldehyde HCHO in water solution at T = 300 K, 500 K, 700 K and 900 K --- p.104 / Chapter 4.3.3 --- The solvation of formate ion HCOO⁻ in water solution at T = 300 K, 500 K, 700 K and 900 K --- p.109 / Chapter 4.3.4 --- The solvation of formic acid HCOOH in water solution at T = 300 K, 500 K, 700 K and 900 K --- p.117 / Chapter 4.4 --- Summary --- p.125 / Chapter CHAPTER FIVE --- The Reactions of Formic Acid HCOOH: Insights from Car-Parrinello Based Metadynamics (MTD) Method / Chapter 5.1 --- Introduction --- p.128 / Chapter 5.2 --- Computational details --- p.130 / Chapter 5.3 --- Results and Discussions --- p.132 / Chapter 5.3.1 --- The intrinsic rotation of a single formic acid molecule HCOOH in gas phase at T = 300 K and T = 700 K --- p.132 / Chapter 5.3.2 --- The dehydration of formic acid in gas phase at T = 300 K and T = 700 K --- p.140 / Chapter 5.3.2.1 --- The dehydration of a single formic acid molecule trans-HCOOH --- p.140 / Chapter 5.3.2.2 --- The dehydration of formic acid molecule trans-HCOOH with one water molecule --- p.147 / Chapter 5.3.2.3 --- The dehydration of formic acid molecule trans-HCOOH with two water molecules --- p.152 / Chapter 5.3.3 --- The dehydrogenation of formic acid in gas phase at T = 300 K and T = 700 K --- p.158 / Chapter 5.3.3.1 --- The dehydrogenation of a single formic acid molecule cis-HCOOH --- p.158 / Chapter 5.3.3.2 --- The dehydrogenation of formic acid molecule cis-HCOOH with one water molecule --- p.163 / Chapter 5.3.3.3 --- The dehydrogenation of formic acid molecule cis-HCOOH with two water molecule --- p.167 / Chapter 5.3.4 --- The dissociations of formic acid in water solution at T = 300 K and T = 700 K --- p.171 / Chapter 5.3.4.1 --- The acid dissociation of formic acid in water solution --- p.173 / Chapter 5.3.4.2 --- The dehydration of formic acid in water solution --- p.175 / Chapter 5.3.4.3 --- The dehydrogenation of formic acid in water solution --- p.178 / Chapter 5.4 --- Summary --- p.181 / References --- p.185