Observing the Structural Evolution in the Photodissociation of Diiodomethane with Femtosecond Solution X-Ray Scattering

Resolving the structural dynamics of the initial steps of chemical reactions is challenging. We report the femtosecond time-resolved wide-angle x-ray scattering of the photodissociation of diiodomethane in cyclohexane. The data reveal with structural detail how the molecule dissociates into radicals, how the radicals collide with the solvent, and how they form the photoisomer. We extract how translational and rotational kinetic energy is dispersed into the solvent. We also find that 85% of the primary radical pairs are confined to their original solvent cage and discuss how this influences the downstream recombination reactions.

Figure 1

(a), (b) The difference scattering profiles of refined $\mathrm{I}\cdots \mathrm{I}$ distances in reciprocal ($q$) and real-space ($r$). (c), (d) The difference scattering [$\mathrm{\Delta }S(q)$, black] and difference scattering obtained from the structural refinement (red) are shown for selected time delays in reciprocal and real space. The conversion between reciprocal and real space was achieved by sine-Fourier transformation. The colored bands follow the refined $\mathrm{I}\cdots \mathrm{I}$ distances and are explained in the text.

Figure 2

(a) the $\mathrm{I}\cdots \mathrm{I}$ separation of the geminate pair species as a function of time $t$. The colored areas separated by dashed lines denote different dynamic regimes. The solid black line is a guide to the eye. The red-dotted line represents the instrument response function (see Supplemental Material Fig. S9 [24]) [8]. (b) time dependence of the amplitudes obtained by the structural refinement (Fig. 1, dots) and the least-squares fit of the kinetic model (lines).

Figure 3

Schematic representation of the revised mechanism of the photoisomer formation within 1 ns. The excited state (I) leads to dissociation. In the impulsive regime (II), the rotational and translational speed of the fragments are given, and the time constants in the kinetic regime (III) and (IV) are lifetimes. The delineating orange lines represent the solvation shells of the structures. The black dotted line indicates the positional distribution of the I atom within the solvent shell.