手性和尺寸对受限于单壁碳纳米管内的硝基甲烷热解反应的影响

采用组合的量子化学ONIOM(B3LYP/6-311++G**:UFF)方法, 研究了不同直径的扶手椅型(CNT(5,5)、CNT(6,6)、CNT(8,8))和锯齿型(CNT(9,0)、CNT(10,0)、CNT(11,0))单壁碳纳米管(CNTs)的限制作用对硝基甲烷分子结构和热解反应的影响. 分子结构分析表明, 与单体硝基甲烷相比, 受限于直径较小的CNT(5,5)和CNT(9,0)碳纳米管内的硝基甲烷构型发生扭转, Cs对称性消失, C—N键长略微缩短; 而受限于CNT(6,6)、CNT(8,8)、CNT(10,0)和CNT(11,0)内的硝基甲烷结构变化不明显. 热解势能面计算发现, 与硝基甲烷单体的热解是一个无过渡态的解离过程明显不同: 硝基甲烷在CNT(5,5)和CNT(9,0)碳纳米管内沿C—N键的解离经历过渡态结构, 所需克服的活化能比单体的解离能分别下降了约71和58 kJ·mol-1; 在CNT(6,6)和CNT(10,0)碳纳米管内, 硝基甲烷的热解活化能略有下降; 而在直径较大的CNT(8,8)和CNT(11,0)碳纳米管内, 热解活化能基本不变. 研究结果表明, 直径小的碳纳米管的限制作用对硝基甲烷热解活化能影响显著, 碳纳米管的手性对硝基甲烷热解反应影响不明显.

Effect of Chirality and Size on the Thermal Decomposition of Nitromethane Confined inside Single-Walled Carbon Nanotube

The molecular structure and thermal decomposition of nitromethane confined inside armchair (CNT(5,5), CNT(6,6), CNT(8,8)) and zigzag (CNT(9,0), CNT(10,0), CNT(11,0)) single-walled carbon nanotubes (CNTs) with different diameters were investigated using the ONIOM (B3LYP/6-311++C~( * *) :UFF) method. Results showed that the C_s symmetry of nitromethane confined inside CNT(5,5) and CNT(9,0) was destroyed and that the C-N bond was compressed slightly. Confinement in CNT(6,6), CNT(8,8). CNT(10,0), and CNT(11,0) had no evident influence on the molecular structure of nitromethane. By analyzing the potential energy surface along the C-N bond, we found that a transition state existed during the thermal decomposition of nitromethane confined inside CNT(5,5) and CNT(9,0). This was very different from the behavior of the nitromethane monomer as no transition state existed during C-N bond dissociation. The activation energy barriers of the thermal decomposition for nitromethane confined inside CNT(5,5) and CNT(9,0) were found to be lower by about 71 and 58 kJ·mol~(-1), respectively, compared with the bond dissociation energy of the nitromethane monomer. Confinement in CNT(6,6) and CNT(8,8) resulted in a slight decrease in the activation energy. Confinement in CNT(8,8) and CNT(11,0) did not affect the thermal decomposition of nitromethane. We concluded that the activation energy of nitromethane decomposition was significantly reduced by confinement in a carbon nanotube with a small diameter. Additionally, the activation energy was not influenced by the chirality of the carbon nanotube.