Filming ultrafast roaming-mediated isomerization of bismuth triiodide in solution

Eun Hyuk Choi,Jong Goo Kim,Jungmin Kim,Joonghan Kim,Jeongho Kim,Hyotcherl Ihee,Seonggon Lee,Yunbeom Lee,Kihwan Yoon,Hosung Ki

doi:10.1038/s41467-021-25070-z

Abstract

Roaming reaction, defined as a reaction yielding products via reorientational motion in the long-range region (3 – 8 Å) of the potential, is a relatively recently proposed reaction pathway and is now regarded as a universal mechanism that can explain the unimolecular dissociation and isomerization of various molecules. The structural movements of the partially dissociated fragments originating from the frustrated bond fission at the onset of roaming, however, have been explored mostly via theoretical simulations and rarely observed experimentally. Here, we report an investigation of the structural dynamics during a roaming-mediated isomerization reaction of bismuth triiodide (BiI3) in acetonitrile solution using femtosecond time-resolved x-ray liquidography. Structural analysis of the data visualizes the atomic movements during the roaming-mediated isomerization process including the opening of the Bi-Ib-Ic angle and the closing of Ia-Bi-Ib-Ic dihedral angle, each by ~40°, as well as the shortening of the Ib···Ic distance, following the frustrated bond fission.

Highlights

Roaming reaction, defined as a reaction yielding products via reorientational motion in the long-range region (3 – 8 Å) of the potential, is a relatively recently proposed reaction pathway and is regarded as a universal mechanism that can explain the unimolecular dissociation and isomerization of various molecules
Since its discovery from the unimolecular dissociation of formaldehyde in the gas phase, the roaming reaction has attracted much attention for its unusual reaction products and their energy distributions, which are difficult to understand with the traditional transition state theory, presuming that the products are generated following the minimum energy pathway[1,2,3,4,5,6,7,8,9,10]
Subsequent experimental and theoretical studies have found that roaming reactions are not restricted to specific molecules and have been generally accepted as a universal mechanism for many unimolecular decompositions and isomerization reactions in the gas phase[11,12,13,14,15]

Summary

Results and discussion

1 ps, with a linear combination of the known scattering curves of solute and solvent, as shown in Supplementary Fig. 8 This result indicates that at time delays earlier than 100 ps must involve an additional intermediate, termed as “X”, whose structure differs from that of the known isomer, iso-BiI2-I. The optimized structure of the early isomer determined from the GFA is shown in Fig. 4a and b showing the side view and top view of the structure, respectively, and compared with the structures of ground-state BiI3 and the late isomer, iso-BiI2-I. From a structural point of view, the early isomer can switch to either ground-state BiI3 or the late isomer primarily by closing or opening of its Bi-Ib-Ic angle, respectively, by ~40° This structural nature of the early isomer partly rationalizes its kinetics obtained from GFA as well.

3.44 Å c b ground state c b

5.64 Å 600 fs

Methods

Code availability