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 migration and 1,3]-bond formation, are compared. In this study a new phenomenon emerged: when three bonds of the triene are twisted by 90°, a conical intersection between ground and excited state occurs, through which efficient radiationless decay and product formation can take place. The reaction mechanism in which 1,3]-bond formation is the primary step is the more efficient of the two mechanisms studied, but decay via the conical intersection is somewhat more facile.