克里奇中間體與水的反應

Abstract

本實驗中，我們以理論計算方法探討克里奇中間體與水的反應。克里奇中間體在大氣中是很強的氧化劑，許多研究指出大氣中的氫氧自由基、過氧化氫、硫酸以及氣膠的生成與克里奇中間體有很大的關聯性。除了氮氣氧氣以外，水氣在大氣中的含量相對於其他氣體是較高的，因此了解其與克里奇中間體反應動力學對我們了解克里奇中間體在大氣扮演的角色是很重要的。我們以RRHO (Rigid Rotor Harmonic Oscillator) 與VPT2 (second order vibrational perturbation theory) 計算分子系統的Vibrational partition function(震動配分函數). 同時，我們也考慮了Hindered rotor 在(H2O)2與CH3 低震動頻率的校正。就量子化學計算而言，我們試了三種方法計算分子系統的能量：B3LYP/6-311+G(2d,2p), QCISD(T)/aug-cc-pVTZ, and QCISD(T)/CBS (complex basis set extrapolation)。最後，我們也做了在不同溫度下反應動力學的研究，並了解水分子與其二聚體對於四種具不同取代基的克里奇中間體的競爭反應。除了理論計算部分，我們也做了anti-CH3CHOO 跟水的動力學實驗，因為anti-CH3CHOO 與水分子的反應很快，故水分子及其二聚體的競爭反應相當明顯。我們做了五個溫度:15, 25, 35, 45, 55 oC，利用MATLAB 曲面擬合可以得到 anti-CH3CHOO 與水分子及其二聚體的反應活化能。

In this work, we use theoretical calculation to understand the kinetics of Criegee intermediates (CIs) reactions with water vapor. CIs are strong oxidizing agents which play an important role of producing OH radical, H2O2, H2SO4, and secondary organic aerosol (SOA) in the atmosphere. Since water vapor is abundant in atmosphere, ([H2O]=1~8 × 1017 cm-3), compared to SO2 ([SO2]=1.2×1012 cm-3 at 50 ppb) and other trace gases), reaction kinetics of CIs with water vapor is important for us to understand the consumption of CIs in atmosphere. To obtain the rate coefficients accurately, we tested rigid rotor harmonic oscillator and second order vibrational perturbation theory method to calculate the vibrational partition function. In addition, hindered rotor is considered for the low vibrational frequency modes for (H2O)2 and CH3 internal rotation. We also use three quantum chemistry approaches: B3LYP/6-311+G(2d,2p), QCISD(T)/aug-cc-pVTZ, and QCISD(T) with complete basis set extrapolation (CBS) to calculate energies. We find that VPT2 partition function correction with QCISD(T)/CBS energies describes our system better than other methods. Lastly, we also study the temperature dependence of the reactions and understand the competition between water monomer and water dimer for the four CIs: CH2OO, anit/syn-CH3CHOO, and (CH3)¬2COO. Except for theoretical calculation, we also did the kinetics experiment on anti-CH3CHOO+(H2O)n. We found that water monomer reaction is very fast that we should consider both water dimer and water monomer in our experimental analysis. Furthermore, by using surface fit on five different temperature data in MATLAB, we can get the activation energies for both monomer and dimer reactions which are very hard to obtain by doing independent fitting for different temperature.