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[Display omitted] • Ehrenfest Force F (r) applied to the two C-O ring-opening photo-reactions of oxirane. • Ehrenfest Force trajectories and hybrid stress tensor trajectory calculated with the F (r) BCP s. • Torsion of the CH 2 group facilitated the photo-reaction pathway towards the CI. • Torsion of CH 2 group induced symmetry breaking evident from S 0 and S 1 F (r) trajectories. • Agreement found between the Ehrenfest Force and hybrid stress tensor trajectories. Ehrenfest Force F (r) trajectories were constructed for the C-O ring-opening photo-reactions of oxirane. The F (r) trajectories were constructed in an eigenvector-space corresponding to bond-flexing, bond-twist and bond-anharmonicity associated with the least and most preferred directions of charge density accumulation and bond critical point (BCP) sliding respectively. The presence of the torsion of a CH 2 group for one of the photo-reactions led to greater symmetry breaking and greater reaction pathway preference. Consistency was found from hybrid Ehrenfest Force F (r) trajectories that were constructed using the Ehrenfest Force F (r) BCP s and the stress tensor eigenvectors. [ABSTRACT FROM AUTHOR]

Graphical abstract Top (photoreaction pathway leading back to the ring-closed reactant): QTAIM bond-path framework sets for the S 0 state; B 0 = { p 0 , q 0 , r 0 } (left panel), the S 1 state; B 1 = { p 1 , q 1 , r 1 } (middle panel) corresponding a to nudged elastic band (NEB) path length = 13.18 Å. Variation of the local total energy density H (r b) for the S 0 and S 1 states with the NEB of the fissile bond (right panel). Bottom: The corresponding results for the photoreaction pathway leading forward to the ring-opened product. Highlights • QTAIM provided an explanation of the direction of the ring-opening photoreactions. • Bond-path framework sets B compared for MEPs of ring-opening photoreactions in DHCL. • Less efficient MEP explained by "sticky" ring-opening bond resisting ring-opening. • No oscillation in chemical character of ring-opening bond of MEP that led to reactant. • Stress tensor stiffness confirmed explanation of ring-opening pathways. Abstract The factors underlying two possible pathways for the Dihydrocostunolide (DHCL) photochemical ring-opening reaction were investigated; the first pathway returned to the ring-closed conformation of the reactant and the second pathway progressed to the ring-opened product. High-level multi-reference DFT methods were used to optimize the two pathways and the density was analyzed using QTAIM and the stress tensor. Oscillations in the chemical character of the fissile bond were found for the first pathway before and after the conical intersection that steered the reaction back to reactant. Conversely, this behavior was absent for the second pathway that led forward to the product. [ABSTRACT FROM AUTHOR]