环庚烷正离子重排反应的从头算研究

先采用HF方法,基组采用STO-3G,对环庚烷正离子的重排机理进行了初步粗略的从头算研究,较快地找到了反应过程中的部分过渡态。然后再采用MP2/3-21G方法精确计算了整个重排过程中的各个过渡态的几何构型、零点能,同时对反应路径也进行了计算,以作进一步的过渡态验证。得出的结论是:环庚烷正离子的重排是环的缩小过程,在生成甲基环己烷叔正碳离子的过程中,经历了2个过渡态;首先是C(1)C(7)的键长变长、C(1)C6的键长变短,β位H(20)逐渐远离与之相连的C(1),与C(7)形成化学键;然后是与C+相连的H(16)逐渐远离C+,与β位的C(1)形成化学键,产生稳定的甲基环己烷叔正碳离子椅式结构,甲基环己烷叔正碳离子还有可能进一步重排为一个含伯正碳离子的甲基环己烷结构,计算了每一步重排反应所需的活化能。

Studies on the Rearrangement of Cycloheptane Carboncation with Ab Initio Method

The mechanism of rearrangement of cycloheptane carboncation has been studied firstly with HF method and STO-3G* basis set and some transition states are obtained quickly, then the MP2/3-21G* method is chosen to calculate all states of the process of rearrangement including geometries, zero point energies and reaction paths, which are also used to test whether it is a real transition state. It can be concluded that the rearrangement of cycloheptane carboncation is a process of ring-reducing. During the production of methylcyclohexane tertiary carboncation, rearrangement undergoes two kinds of transition states: first of all, the bond length of C(1)–C(7) becomes longer and C(1)–C(6) becomes shorter, thus β-H(20) parts from C(1) to form chemical bond with C(7) and then H(16) leaves C+ to form a bond with β-C(1), forming chair conformation of methylcyclohexane tertiary carboncation. It is possible that methylcyclohexane tertiary carboncation continues to rearrange to produce a primary carboncation of methylcyclohexane. The activation energies of each step of the arrangement reaction are calculated in the end.