Low-Temperature Photochemistry of Previtamin D: A Hula-Twist Isomerization of a Triene

Abstract

What is the standard course of the cis-trans isomerization of nonpolar polyenes? Many results support a pathway in which only a central CH group rotates out of the plane, while the remaining parts of the molecule reorient within the plane ("hula-twist" mechanism). The intermediate structure through which the molecule passes corresponds to the geometry predicted for the conical intersection of the S₁ and S₀ potential energy surfaces (see picture).