Triplet–singlet intersystem crossing as the second step of the cycloaddition of triplet penta-1,4-diene. An ab initio MO study

Abstract

The mechanism of the second step in the photosensitized cycloaddition reaction of penta-1,4-diene has been investigated. This step, which starts from an aliphatic cyclic intermediate on the first excited triplet (T₁) surface, consists of an intersystem crossing from the T₁ state to the ground singlet state (S_o) and ring closure on the S_o surface. For two possible reaction paths, cross addition and parallel closure, we have calculated by an ab initio MO method, the lowest point on the crossing seam between the S_o and T₁ states, which can be regarded as the transition state for this second step. The triplet cyclic intermediate for cross addition, 2, requires only ca. 1 kcal mol^–1 to reach the lowest crossing point accompanied by 90° internal rotation of the terminal methylene group with relatively small changes in the skeletal structure. On the other hand, the triplet cyclic intermediate for parallel closure, 4, requires ca. 5 kcal mol^–1, accompanied by a drastic change in the skeletal molecular structure. The calculation confirms that this second step is not the rate-determining step of the entire cycloaddition reaction.