  The factors underlying the experimentally observed branching ratio (70:30) of
the (1,3-cyclohexadiene) CHD$\rightarrow$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 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 to be
relatively rare during the dynamics, this explains the observed low quantum
yield of the ring-opening reaction.
