Next-generation quantum theory of atoms in molecules for the ground and excited state of the ring-opening of cyclohexadiene

The factors underlying the experimentally observed branching ratio (70:30) of the (1,3-cyclohexadiene) CHD -> HT (1,3,5-hexatriene) photochemical ring-opening reaction are investigated. The ring-opening reaction path is optimized by a high-level multi-reference DFT method and the density along the path is analyzed by the quantum theory of atoms in molecules (QTAIM) and stress tensor methods. The performed density analysis suggests that, in both S-1 and S-0 electronic states, there exists an attractive interaction between the ends of the fissile sigma-bond of CHD that steers the ring-opening reaction predominantly in the direction of restoration of the ring. It is suggested that opening of the ring and formation of the reaction product (HT) can only be achieved when there is a sufficient persistent nuclear momentum in the direction of stretching of the fissile bond. As this orientation of the nuclear momentum vector can be expected relatively rare during the dynamics, this explains the observed low quantum yield of the ring-opening reaction.