Thermodynamics and kinetics of large hydrocarbon reaction using ab initio calculations

Ph.D.It remains a long-standing challenge to predict accurate thermodynamics and kinetics of large hydrocarbons by first-principle calculations. This is mainly caused by the very high computational cost for high-accuracy electronic structure calculations and overwhelming complexity for anharmoncity treatment in large reaction systems. Developing or validating accurate and efficient quantum chemistry methods is the prerequisite to remove the hurdles towards large hydrocarbon reaction models.This thesis proposes several new methods to enable cost-effective and accurate calculations of the thermodynamics and kinetics of large hydrocarbons such as bond dissociation energies, enthalpies, entropies and rate constants. Firstly, a two-layer ONIOM-CCSD(T)/CBS method, which treats the high layer with CCSD(T) method and the low layer with DFT method, was developed to calculate the C−C bond dissociations energies of C₆-C₂₀ normal alkanes. Secondly, a novel cascaded group-additivity ONIOM method, namely CGA-ONIOM, was developed to approach the globally accurate electronic energies at the CCSD(T)/CBS theory level. A systematic validation against 79 C₄-C₈ hydrocarbons was implemented to justify the accuracy of CGA-ONIOM. Further application of CGA-ONIOM to 12 representative C₁₀, C₁₂ and C₁₆ aliphatic hydrocarbons was also conducted. Thirdly, the multi-structural 2-dimensional torsion method (MS-2DT) based on minimally coupled approximation was proposed to treat the low-frequency anharmonicity presented in large flexible hydrocarbons. This method has been used to calculate the standard entropies and heat capacities of 18 C₆−C₈ alkanes with 5-7 torsions.Finally, the CGA-ONIOM-DFT and ONIOM-DFT schemes were proposed to calculate the rate constants of medium-to-large hydrocarbon reaction systems. This scheme advocates to adopt CGA-ONIOM or ONIOM for barrier height benchmarks to further select the best-performance DFT method. The DFT method is then employed to study the reaction kinetics. The rate constants of medium-size reaction system, i.e., cyclopentane + OH reaction, were treated by the CGA-ONIOM-DFT scheme with the selected density functional M06-2X. The rate constant of cyclopentane with OH was calculated at 200-2000 K by multi-structural canonical variational transition state theory including small-curvature tunneling (MS-CVT/SCT) method. The predictions are in good agreement with the experimental data at combustion and atmospheric temperatures with a deviation of less than 30%. Moreover, the reaction of JP-10 with OH was selected as the prototypical large complex reaction system to demonstrate the feasibility of the ONIOM-DFT scheme. Site-specific rate constants of JP-10 + OH were predicted by MS-CVT/SCT theory. Our predictions agree well with the experimental measurements by a factor of ~1.5.These studies demonstrate the utility of ONIOM, CGA-ONIOM, MS-2DT and CGA-ONION-DFT and ONIOM-DFT methods to predict the thermodynamics and kinetics of large hydrocarbons. The ONIOM method enables a fast prediction of the BDEs of large hydrocarbons at the CCSD(T)/CBS theory level. The CGA-ONIOM method computes the globally accurate energies of large hydrocarbons. Remarkably, the CGA-ONIOM method improves the computational efficiency by up to four orders of magnitude compared with conventional CCSD(T) calculations and maintains the accuracy. The MS-2DT treats the multi-structural torsional anharmonicity at the reduced computational cost compared to the conventional MS-T method. The CGA-ONIOM-DFT and ONIOM-DFT methods provide an affordable solution to the direct dynamics of large reaction systems. By validating these theoretical methods against the experimental measurements, these methods open up new opportunities of studying the large hydrocarbon systems accurately and efficiently.從第一性原理出發，準確預測大型碳氫反應體系的熱力學、動力學參數面臨長期的挑戰。該挑戰主要在於高精度的電子結構計算耗時巨大，以及大分子反應體系的非諧效應的處理極其複雜。因此，開發準確高效的量子化學方法是發展大型碳氫反應模型的前提。本論文提出了若干新方法以實現碳氫大分子熱力學、動力學參數的高效計算。通過此類方法，諸多關鍵信息如斷鍵能、標準生產焓、標準熵以及速率常數均可得以預測。本論文提出了一種雙層的ONIOM-CCSD(T)/CSB方法，該方法對高層部分展開CCSD(T)計算，而對低層展開了DFT計算。該方法用於計算了正己烷至正廿烷的碳-碳斷鍵能。利用基團貢獻法（GA）和ONIOM方法，本文提出了CGA-ONIOM來獲得精度接近於CCSD(T)/CBS水平的精確的電子能量。該方法被用於計算七十九種丁烷至辛烷分子的電子能量以及標準生成焓，論證了該方法的可行性。隨後，CGA-ONIOM方法被進一步用於計算12種典型的碳氫體系的標準生成焓，包括碳十、碳十二以及碳十六分子。基於最小耦合近似，本文提出了多結構-二維扭轉方法用於處理碳氫柔性大分子的低頻非諧振效應。該方法用於計算己烷至辛烷等十八種碳氫分子的標準熵和比熱容，且所選定的碳氫分子具有五到七個扭轉模式。最後，本文提出了CGA-ONIOM-DFT與ONIOM-DFT計算框架用於獲取中大型碳氫反應體系的速率常數。該框架建議採用CGA-ONIOM或ONIOM方法獲取勢壘高度的基準值，並利用該基準值對一系列DFT方法進行篩選從而獲得性能最佳的DFT方法。該DFT方法隨後用於反應動力學計算。為展示該框架的有效性，選取了環戊烷與羥基這一中型反應進行研究。研究表明M06-2X密度泛函方法對於該反應體系的計算性能最佳。在此基礎上，本文利用多結構-變分過渡態理論以及多維小曲率隧穿方法（MS-CVT/SCT）對200-2000 K範圍內的速率常數進行了計算。本文的理論預測結果與寬溫度範圍內的實驗測量值吻合很好，二者偏差僅在30%以內。此外，本文進一步以JP-10與羥基反應體系這一典型的大型複雜碳氫體系為代表，論證了ONIOM-DFT框架的可行性。本文利用MS-CVT/SCT方法獲取了不同位點的反應速率常數。本文計算的反應速率常數與實驗數據吻合較好，二者進存在1.5倍的差距。上述研究展示ONIOM、CGA-ONIOM、MS-2DT以及CGA-ONIOM-DFT與ONIOM-DFT等方法在大型碳氫反應體系的熱力學、動力學參數計算方法的應用。ONIOM方法可在CCSD(T)/CBS 水平下實現對碳氫大分子斷鍵能的快速預測。CGA-ONIOM方法在保證計算精度的前提下可提高傳統的CCSD(T) 計算效率，最高幅度可達四個量級。與MS-T方法相比，MS-2DT方法以較小的計算量即可實現對多結構-扭轉非諧性的處理。CGA-ONIOM-DFT與ONIOM-DFT方法為大型反應體系的直接動力學模擬提供了可行方案，採用上述兩種框架獲得的速率常數均與實驗吻合良好。因此，以上諸方法使碳氫大分子體系的高效高精度研究成為可能。Wu, Junjun."November 2019."Thesis Ph.D. Chinese University of Hong Kong 2020.Includes bibliographical references (leaves 122-134).Abstracts also in Chinese.Title from PDF title page (viewed on 12, April 2021)