Computational chemistry study of O + small alkane molecules

• Main Authors: Nai-Wei Teng, 鄧乃偉
• Other Authors: Ying-Chieh Sun
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/86071000283629471460

碩士 === 國立臺灣師範大學 === 化學研究所 === 91 === In the present thesis, we report the calculated results of O + C3H8 and O + CH3F reactions in crossed molecular beam collision-free environment using ab initio/RRKM calculation. In addition, propane photodissociation and O + C2D6 reactions were examined as well. Calculated results of rate constants, product branching ratios, and mechanism of possible multiple decomposition are presented. In the O + C3H8 reaction, O(3P) is not reactive with C3H8 but O(1D) can insert into a C-H bond of C3H8 to produce the n-propanol or iso-propanol. Considering steric hindrance and only two hydrogen atoms on the geminal carbon, we assume that the ratios of forming n-propanol to i-propanol is either 90/10, or 100/0. With the assumption of 90/10 ratio, product branching ratios of primary decomposition of the activated propanol are 0.5, 0.6, 8.5, 34.3, 36.8, and 19.4% for the H, H2, OH, CH3, C2H5, and H2O formation channels, respectively. The calculated results are in good agreement with available experimental results of 0, 0, 25, 27, and 48% for the H, H2, OH, CH3, and C2H5 products above, respectively. Our calculated results support that H and H2 do not produce in experiment. In the calculation of O + C3H8 for abstraction mechanism, the calculated results showed that this mechanism can contribute formation of OH radical unnegligibly. Furthermore, the theoretical calculation predicts the this O + C3H8 reaction can product H2O product, which was not detected in experiment due to high background noise. To examine further, we carried out calculations for secondary decompositions with 8 kcal/mol collision energy. The calculated results showed that secondary decompositions do not contribute detected product significantly. For the O + CH3F reaction, the calculation gave the percentage of 69.9, 8.0, 2.3, 2.0, 10.1, and 7.7% for the HF, H, H2, OH, F, and H2O, respectively. The calculated results are in good agreement with experiment results of 82, 11, 7, and 0% for HF, H, H2, and OH products. Significant amount of H2O was seen in calculation as well. Also, investigation of the rate constant of secondary reactions for this O + CH3F reaction also showed that the secondary reactions do not contribute to product yield significantly. In addition to the above calculations, the same ab initio/RRKM theoretical method was employed to examine the photodissociation of propane. The calculations gave in detail the potential energy surface and rate constants of primary and secondary decompositions. But we find a difference between the theoretical and experimental branching ratios for some channels. Based on an argument described in text, we think that RRKM theory is not suitable to describe the photodissociation of propane. Finally, we also carried out calculation for the O + C2D6 to examine isotopic effect of O + C2H6 reaction. Except for the OH channel, all of the rate constants decrease due to the deuterium substitution effect. The present theoretical calculated results can be verified in future experiments.