本論文藉由ab initio理論計算的方法，探討CH3S自由基如何與大氣中的NO2、O3、O2、HO2氣體產生SO2，在MP2/6-311G(d)的計算層級下，設計各種反應形成SO2的反應機制，再比較各種反應路徑的能量以及特性。本文主要分為以下四個部分進行研究：
第一部份：探討CH3S自由基在大氣中與NO2反應的理論計算研究，由計算結果得知，大氣中的CH3S可和NO2進行兩次或三次反應得到SO2，其反應後的能量也相對穩定。
第二部分：我們對CH3S會和臭氧層的O3進行反應。發現大氣中SO2的產生，可經由CH3S和O3進行兩次反應得到，最後形成CH3SO2，再經由熱裂解反應得到。
第三部分：
對CH3S與O2進行反應作一些探討。經由計算結果發現反應形成中間產物後，再經O-O的斷鍵所需能量都相對高，由此可知O2並不太可能和CH3S反應生成SO2。
第四部份：
對CH3S與HO2自由基的反應。經由計算發現，反應形成的中間產物D2和D5會比產物還要穩定許多，由此可知大氣中SO2不太會由CH3S與HO2反應得到。

This thesis deals with the calculation of tropospheric reaction of methylthiyl radical(CH3S) with NO2，O3，O2 and HO2 to check the formation of SO2 by ab initio theory at MP2/6-311G(d) level. There are four major sections to describe the reaction mechanisms.
Section 1: We study the reaction between methylthiyl radical and nitrogen dioxide(NO2) in the gas phase . The reaction proceeds with the participation of NO2 by twice or three times to produced sulfur dioxide. The formation energy is relatively large and the product is stable. Therefore, we think the reaction of methylthiyl radical with nitrogen dioxide to produce sulfur dioxide is very possible.
Section 2: We studied the reaction of CH3S with O3.We found out that with the participation of O3 two times it is possible to form CH3SO2 and than dissociated to SO2.We detect the reaction of methylthiyl radical with ozone twice to produce CH3SO2.After that, we through the thermal decomposition of CH3SO2 and we can get SO2.
Section 3: We studied the reaction of CH3S+O2. We found out that the cleavage energy of O-O bond of intermediate is quite high. As a consequence, it is not possible for the formation of SO2 via this process.
Section 4: We studied the reaction of CH3S+HO2.We found out that the intermediates D2 and D5 are very stable, and it seems not possible to proceed further to produce SO2 in this reaction.

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