H-vinyl conical intersections for dienes: a mechanism for the photochemical hula twists.

The potential role of "H-vinyl" conical intersections in a photochemical process known as the hula twist has been evaluated. The H-vinyl conical intersections of butadiene are explored with complete active space self-consistent field calculations, and proposals about the geometries involved in the hula-twist mechanism are discussed and contrasted with the conventional one-bond flip mechanism of cis-trans isomerization.