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A balanced treatment of the covalent and ionic contributions to the ground and excited states originating from torsion about double bonds is known to be strongly dependent on the presence of dynamic electron correlation. We undertake an analysis of the minimum energy pathways corresponding to deactivation of the first excited singlet state of PSB3. In doing so we consider torsion about the three double bonds including other intramolecular degrees of freedom, such as the bond length alternation. The 3‐D bond‐path analysis provides a new 'bond‐localized orbital‐like' directional interpretation of bonding. Therefore, we present a more sophisticated method of determination of the degree of covalent and ionic contributions known to be responsible for altering the relative stability of the S1/S0 conical intersections. The results presented suggest that the commonly used simplified multi‐reference methodologies that often result in incorrect predictions for the excited state deactivation reaction mechanism. The bond‐path framework set B visualization of the spectrum of the chemical character (covalent–biradical‐ionic) displayed of each of the back‐bone bonds as well as the influence on neighboring bonds. The most preferred directions of accumulation of charge density, q‐ and q′‐paths along the backbone bond‐path (r) corresponding to the torsional C10‐N12 BCP for the S0 (top) and S1 (below) states at a value of the torsion q = 90.0°. [ABSTRACT FROM AUTHOR]

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 S1 and S0 electronic states, there exists an attractive interaction between the ends of the fissile σ‐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. Experimentally observed branching ratio (70:30) of the (1,3‐cyclohexadiene) CHD → HT (1,3,5‐hexatriene) photochemical ring‐opening reaction is investigated by first‐principles calculations and next‐generation quantum theory of atoms in molecules. It is suggested that opening of the ring and formation of the reaction product can only be achieved when there is a sufficient persistent nuclear momentum in the direction of stretching of the fissile bond. [ABSTRACT FROM AUTHOR]