Fig. 3: Ultrafast structural dynamics of C–C and C–C• bond rotation.

a The reaction processes occurring in the reacting solution of C₂F₄I₂ within 100 ps after photoexcitation, corresponding to the time constants of 130 fs, 1.2 ps, 26 ps, and 292 ps. b The experimental decay-associated difference scattering curves (qDADS(q), black lines) corresponding to the four time constants, superimposed with respective simulated DADSs (qDADS′(q), red lines). We modeled various difference scattering curves corresponding to the candidate transition processes that can occur upon photoexcitation of C₂F₄I₂ (see Supplementary Figs. 8–10 for details) and identified that the four time constants are best attributable to the structural transitions shown in (a). The experimental error and fit residuals are indicated with vertical bars and blue lines, respectively. c The r-space representation of the experimental (black) and simulated (red) DADSs. In the 2^nd DADS, interatomic distances in the anti-C₂F₄I• (light blue lines, r_FI^a = 3.5 Å and 3.8 Å) and gauche-C₂F₄I• (light green lines, r_FI^g = 3.3 Å and 4.3 Å) vanish and emerge, respectively, implying the anti-to-gauche isomerization of the radical. Likewise, in the 3^rd DADS, interatomic distances in the anti-C₂F₄I₂ (blue lines, r_II^A = 5.0 Å, r_FI^A = 2.9 Å and 3.2 Å) and gauche-C₂F₄I₂ (green lines, r_II^G = 4.0 Å, r_FI^G = 2.8 Å and 3.4 Å) emerge and vanish, respectively, supporting the gauche-to-anti isomerization of C₂F₄I₂. We note that only the contributions of solute-related dynamics to the scattering signal are shown in (b) and (c). d Structural parameters of anti-C₂F₄I₂, gauche-C₂F₄I₂, anti-C₂F₄I•, and gauche-C₂F₄I• used for the iterative optimization. The simulated curves in (b) and (c) were generated using these optimized structural parameters. e The time-dependent concentration profiles with respect to the four components: anti-C₂F₄I• (red), gauche-C₂F₄I• (blue), the gauche-to-anti rotation of C₂F₄I₂ (purple), and C₂F₄ (gray). The error bars were estimated using the method described in section 2-3 of the SI, where the SADSs in Supplementary Fig. 12b were used to obtain the concentration profiles as parameterized variables with standard errors. We note that the third component accounts for the time-dependent concentration change of both anti-C₂F₄I₂ and gauche-C₂F₄I₂ simultaneously, as detailed in the methods section. In (e), the results of linear combination fit (LCF) and the least-χ² optimization based on the kinetic modeling are shown with bars and solid lines, respectively.