Reaction energy profiles for the disrotatory and conrotatory ring openings of cyclohexadiene. Potential energies in the absence of an applied force relative to 1,3-cyclohexadiene are provided in brackets in units of kcal/mol. Labels on the hydrogen atoms are used to identify PPs in the text.
Barriers for the ring opening of 1,3-cyclohexadiene along conrotatory and disrotatory pathways as a function of applied force, F ext. Values obtained through quantum chemical calculations (QC) are shown as symbols and barriers predicted with Eq. (7) are shown as solid curves. The PPs used correspond to (a) H5 and H6, (b) H5 and H8 (termed PP1) and H6 and H7 (termed PP2), (c) H1 and H4, and (d) H2 and H3.
Structures and energies related to the force-induced ring-flip of 1,3-cyclohexadiene. (a) Reactant, transition state, and product of the ring-flip of 1,3-cyclohexadiene. The asterisks indicate the H5 and H8 atoms used as PPs. (b) Predicted structures of 1,3-cyclohexadiene at F ext = 1000, 2000, and 3000 pN using the PPs indicated by asterisks. The data show a trend toward the transition state structure from (a) with increasing F ext. (c) Barriers to the ring-flip of 1,3-cyclohexadiene as a function of F ext obtained with quantum chemical methods and Eq. (7) using H5 and H8 as PPs. (d) Potential energy as a function of PP separation, R, for the reactant and products of the ring flip process at F ext = 0 and 1000 pN. The minimum of the reactant parabola has been set to 0.0 kcal/mol at both values of F ext.
Revised versions of Figures 2(a) and 2(b) incorporating the effects of force-induced instabilities. (a) Barriers with F ext applied between H5 and H6. (b) Barriers with F ext applied between H5 and H8 (PP1) and H6 and H7 (PP2). Symbols denote quantum chemical values, solid curves represent barriers predicted with Eq. (7), and the dashed line corresponds to the predicted barriers along the conrotatory pathway with PP1 including a third-order correction to Eq. (7).
Changes in the energies of 1,3-cyclohexadiene with PP2 and the conrotatory transition state with PP1 on the FMPES as a function of F ext using quantum chemical methods and Eq. (6).
(a) Changes in atomic positions in 1,3-cyclohexadiene due to the application of F ext to H5 and H8. The lengths of the arrows indicate the relative displacements of the atoms. (b) Percent contributions of changes in the bond lengths, bond angles, and torsion angles within the H5-C-C-H8 sub-unit of 1,3-cyclohexadiene to the change in PPs separation, δR, as a function of F ext. (c) Changes in atomic positions in 1,3-cyclohexadiene due to the application of F ext to H6 and H7. (d) Percent contributions of changes in the bond lengths, bond angles, and torsion angles within the H6-C-C-H7 sub-unit of 1,3-cyclohexadiene to the change in PPs separation, δq, as a function of F ext.
Changes PP separations, ΔR 0, in Å, and compliances, ΔC RR, in Å/nN for the application of F ext between different sets of PPs for the ring opening of 1,3-cyclohexadiene along conrotatory and disrotatory pathways.
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