PHOTOCHEMISTRY OF PLANAR E-HEXATRIENES - AN AM1 STUDY

The photochemical formation of an allylcyclopropene derivative from E- hexatriene is studied with the AM1 semiempirical method. Two possible reaction mechanisms, with a differing sequence of [1,2]-hydrogen migra tion and [1,3]-bond formation, are compared. In this study a new pheno menon emerged: when three bonds of the triene are twisted by 90-degree s, a conical intersection between ground and excited state occurs, thr ough which efficient radiationless decay and product formation can tak e place. The reaction mechanism in which [1,3]-bond formation is the p rimary step is the more efficient of the two mechanisms studied, but d ecay via the conical intersection is somewhat more facile.