正十六烷CHH和CHC五配位正碳离子之间转化反应的分子模拟

Molecular simulation studies on isomerization of CHH and CHC carbonium ions of protonated n-hexadecane

为了研究催化裂化过程中,烷烃分子中的C-H键和C-C键质子化产生具有三中心两电子键结构的CHH和CHC五配位正碳离子之间的转化反应。利用密度泛函理论的量子化学从头算法,计算了正十六烷分子链中C_4-H键和与之相邻的C_3-C_4键和C_4-C_5键质子化形成的C_4HH、C_3HC_4和C_4HC_5五配位正碳离子的结构和能量。结果表明,正十六烷质子化产生的CHH五配位正碳离子能量高于C_3HC_4和C_4HC_5五配位正碳离子。表明CHH五配位正碳离子能够转化为CHC五配位正碳离子。以C_4HH、C_3HC_4和C_4HC_5五配位正碳离子为起点进行过渡态搜索,得到了C_4HH五配位正碳离子转化为C_3HC_4和C_4HC_5五配位正碳离子的过渡态,并对过渡态进行了确认。计算得到的C_4HH五配位正碳离子转化为C_3HC_4和C_4HC_5五配位正碳离子的能垒分别为16.6kJ/mol和13.9 kJ/mol,表明CHH五配位正碳离子很容易转化为CHC五配位正碳离子。

Carbonium ions with three-canter-two-electron bonds would be generated in the protonation of C-H bonds and C-C bonds in alkanes in catalytic cracking process, in order to determine the isomerization of CHH and CHC carbonium ions. The structure and energy of the carbonium ions, protonated n-hexadecane, were studied by DFT methods. Three stable structures were found for the protonated form of C4-H bond, C3-C4 bond and C4-C5 bond in n-hexadecane, the energy of C4HH carbonium ion is higher than the energy of C3HC4 and C4HC5 carbonium ions, which meant that CHH carbonium ion could transform into CHC carbonium ions. Based on the structures of C4HH, C3HC4 and C4HC5 carbonium ions, transition states had been found. Energy barrier of C4HH carbonium ion transformed into C3HC4 carbonium ion was 16.6 kJ/mol, and energy barrier of C4HH carbonium ion transformed into C4HC5 carbonium ion was 13.9 KJ/mol, which showed that the reaction could take place easily.