The Dimerization of Cyclobutadiene. An ab Initio CASSCF Theoretical Study

Ab initio molecular orbital calculations have been performed on
the dimerization of cyclobutadiene using
the 3-21G and 6-31G* basis sets. Complete active space SCF
calculations with an 8-electron 8-orbital active space
were carried out. The anti dimerization occurs by
either synchronous concerted or asynchronous concerted
pathways
with nearly identical activation energies. The syn
dimerization is highly favored, and this reaction is predicted
to
have a negative activation energy. There is no synchronous
concerted pathway. The stationary points on the
syn
dimerization surface consist of a degenerate stationary point which can
collapse directly to the product in four different
ways or collapse to the transition state for the Cope
rearrangement.

本研究采用3-21G与6-31G*基组，针对环丁二烯（cyclobutadiene）的二聚反应开展了从头算分子轨道（ab initio molecular orbital）计算，并完成了包含8电子8轨道活性空间的完全活性空间自洽场（Complete active space SCF）计算。结果表明，反式二聚反应可通过协同同步或协同异步两条路径进行，二者的活化能几乎相等。顺式二聚反应具有极强的反应倾向性，且计算预测该反应的活化能为负值，且不存在协同同步的反应路径。顺式二聚反应势能面上的驻点（stationary points）为简并驻点（degenerate stationary point），该驻点可通过四种不同路径直接转化为产物，也可经由过渡态发生科普重排（Cope rearrangement）。