Schematic representation of the structural changes of photoexcited in solution. (a) Following photoactivation a C–I bond is dissociated and the resulting radical either escapes the solvent cage or recombines geminately with to form . (b) The relaxation pathway of involves a thermally induced breakage of the I–I bond and may also lead to radicals either escaping the solvent cage or recombining geminately with to recover the parent molecule. Modified from Ref. 5.
Changes in x-ray diffraction intensities characteristic of specific reaction species, expansion and temperature changes. and are recovered from experimental measurements, whereas the other curves derive from molecular dynamics simulations. Intramolecular I-to-I distances were modeled as 3.58 Å for ; 3.03 Å for ; and 2.73 for . These curves were used to fit the experimental data (Fig. 3) from the temporal evolution (Fig. 4) of the transient photochemical populations.
Representative experimental data (circles) and theoretical predictions (solid line) for the x-ray scattering intensity changes following the photoexcitation of in cyclohexane. (a) Difference scattering intensity changes in the low region which are dominated by solvent expansion effects. (b) The same data presented for higher and magnified to highlight the oscillations present over this domain. Time delays between laser photoactivation and the arrival of the x-ray probe are indicated in (a) and are the same for both panels. Theoretical predictions were calculated using the concentrations shown in Fig. 4(a) and the basis difference spectra shown in Fig. 2.
Transient populations of photochemical species following the excitation of in (a) cyclohexane and (b) methanol. The latter concentrations are modified from those previously published (Ref. 5) due to changes in the reaction scheme (Fig. 1) allowing transient to both relax to and to dissociate to form . Iodine radicals escaping the solvent cage decay via in cyclohexane, whereas in methanol they ultimately combine to form . Reaction product concentrations from this model were used to generate the predicted x-ray intensities shown in Fig. 3.
Transient expansion of cyclohexane following photoactivation of . The measured expansions are shown as circles. The solid line is the predicted temporal evolution using an accepted hydrodynamic model (Ref. 37). At equilibrium a temperature jump of 1.7 K is reached.
Real space representation of the time-resolved difference x-ray scattering data. Transient changes in time recorded from photoactivated in (a) cyclohexane and (b) methanol. In cyclohexane a shift in the peak of to lower with time corresponds to a shift from a population dominated by to a population dominated by . In methanol the depletion of the peak at correlates with the maximal transient population of , which has an intramolecular I-to-I separation similar to that of the parent molecule, and for the population is dominated by .
Structural refinement of the intramolecular I-to-I distances of and . (a) The experimental data and (b) its real space representation are shown for photoactivated in cyclohexane for . Black circles/lines represent the experimental data and the optimal theoretical prediction is shown in red. (c) The experimental data and (d) their real space representations are shown for photoactivated in methanol for . An experimental difference signal due to pure solvent heating has been subtracted from all data. The refined intramolecular I-to-I distances are given in Table I.
Intramolecular I-to-I distances of and measured using time-resolved x-ray diffraction. Bond distances were recovered by real space refinement against data recorded at and 1.0 ns (cyclohexane), and and 1.0 ns (methanol).
Density functional calculations for the intramolecular I-to-I distances of and molecules in their electronic ground state. Optimized geometries for the isolated molecules (vacuum) are compared when those recovered when three different H-bond interactions are modeled with methanol.
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