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Author(s): Jong Goo Kim [sup.1] [sup.2] [sup.3] , Shunsuke Nozawa [sup.4] [sup.5] , Hanui Kim [sup.1] [sup.2] [sup.3] , Eun Hyuk Choi [sup.1] [sup.2] [sup.3] , Tokushi Sato [sup.6] [sup.7] [...], Fundamental studies of chemical reactions often consider the molecular dynamics along a reaction coordinate using a calculated or suggested potential energy surface.sup.1-5. But fully mapping such dynamics experimentally, by following all nuclear motions in a time-resolved manner--that is, the motions of wavepackets--is challenging and has not yet been realized even for the simple stereotypical bimolecular reaction.sup.6-8: A-B + C [right arrow] A + B-C. Here we track the trajectories of these vibrational wavepackets during photoinduced bond formation of the gold trimer complex [Au(CN).sub.2.sup.-].sub.3 in an aqueous monomer solution, using femtosecond X-ray liquidography.sup.9-12 with X-ray free-electron lasers.sup.13,14. In the complex, which forms when three monomers A, B and C cluster together through non-covalent interactions.sup.15,16, the distance between A and B is shorter than that between B and C. Tracking the wavepacket in three-dimensional nuclear coordinates reveals that within the first 60 femtoseconds after photoexcitation, a covalent bond forms between A and B to give A-B + C. The second covalent bond, between B and C, subsequently forms within 360 femtoseconds to give a linear and covalently bonded trimer complex A-B-C. The trimer exhibits harmonic vibrations that we map and unambiguously assign to specific normal modes using only the experimental data. In principle, more intense X-rays could visualize the motion not only of highly scattering atoms such as gold but also of lighter atoms such as carbon and nitrogen, which will open the door to the direct tracking of the atomic motions involved in many chemical reactions. Femtosecond X-ray liquidography is used to track the vibrational wavepacket trajectories of gold atoms in solution, enabling time-resolved observations of the emergence of vibrations and the evolution of the formation of covalent bonds.

Despite extensive studies on the isomer species formed by photodissociation of haloalkanes in solution, the molecular structure of the precursor of the isomer, which is often assumed to be a vibrationally hot isomer formed from the radical pair, and its in-cage isomerization mechanism remain elusive. Here, the structural dynamics of CH 2 I 2 upon 267 nm photoexcitation in methanol were probed with femtosecond X-ray solution scattering at an X-ray free-electron laser. The determined molecular structure of the transiently formed species that converts to the CH 2 I-I isomer has the I-I distance of 4.17 Å, which is longer than that of the isomer (3.15 Å) by more than 1.0 Å and the mean-squared displacement of 0.45 Å 2 , which is about 100 times larger than those of typical regular chemical bonds. These unusual structural characteristics are consistent with either a vibrationally hot form of the CH 2 I-I isomer or the loosely-bound radical pair (CH 2 I˙⋯I˙)., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)