三氟甲烷的量子化學計算與分子動力學模擬

Abstract

地球溫室效應造成全球暖化是目前最急迫性的問題，研究發現，不僅只有二氧化碳會造成溫室效應，HFCs(Hydrofluorocarbons)也是影響溫室效應的主要氣體，甚至比二氧化碳更嚴重。因此對於HFCs分子產生興趣，我們先選擇HFCs結構為四面體的分子來做研究。在HFCs結構為四面體的分子中，又以三氟甲烷對溫室效應的影響最大，甚至在未來將會比二氧化碳的影響更為嚴重，若二氧化碳的全球暖化能量值(Global Warming Potential)當成1，則三氟甲烷將是二氧化碳的12,000倍。因此我們選擇了三氟甲烷分子來做研究。我們更進一步使用自洽理論(HF)、微擾理論(MP2)、密度泛函理論(DFT)的80種配對組合以6-311++G(3df,3pd)為基底做更進一步的量子化學計算與討論，並得到相當不錯的結果。 我們使用MP2/aug-cc-pVQZ做三氟甲烷的單體結構得到最佳化後，再以HF、MP2及DFT三種量子化學計算方法計算三氟甲烷分子間作用力，我們取出14個具有代表性的構型(A~N)做量子化學計算，並且所有的計算都加入了BSSE進行修正。在完成量子化學計算後，我們使用5 Sites Lennard-Jones potential Model以及庫倫項，來進行與量子化學計算的擬合，將擬合參數帶入分子動力學來進行模擬，並且建構出力場。我們沿著三氟甲烷的相圖之氣化曲線模擬三氟甲烷的速度自相關函數，可以幫助我們了解不同相態下的平衡及動態性質，而積分可得到的擴散係數與實驗相互比較。我們模擬了不同溫度及不同密度下的擴散係數，得到的結果有相當的準確度，這說明了以量子力學計算建構出立場所做出的分子動力學模擬，其在實用性上有一定的精確度。

The global warming due to the greenhouse effect is one of the most important issues in earth. Not only carbon dioxide affects global warming, but also hydrofluorocarbons affect global warming. The research discovered hydrofluorocarbons affect global warning more than carbon dioxide. Therefore we have interest in hydrofluorocarbons. Trifluoromethane is a tetrahedron structure of hydrofluorocarbons, and its Global Warming Potential is 12,000 times more than carbon dioxide. So we start to study trifluoromethane. Quantum chemistry calculations use three methods, HF and MP2 and DFT. HF and MP2 use 11 basis sets, and DFT uses 80 exchange-correlation function to compare to our computation with MP2. All of the computations add BSSE correct. Then we use MP2/aug-cc-pVQZ to do the monomer optimization of trifluoromethane, and choose 14 conformers (A~N). We use 5 site Lennard-Jones potential model with coulomb potential in our fitting to quantum chemistry calculated data. Then construct force field with there parameters. We also take the parameters into our molecular dynamics simulations. We simulate radial distribution function and velocity correlation function. Also, We simulate velocity correlation function along the gasification curve of trifluoromethane phase diagram. And we integrate velocity correlation function to obtain diffusion constant and simulate diffusion constant with different temperature and different density. The simulation results are in good agreement with experiment data. It shows that using quantum chemistry calculations to construct force field, and to do molecular dynamics simulation can yield good result.