Watching atoms move to form a bond

The direct experimental tracking of nuclear dynamics during chemical reactions is challenging, even for the simple reaction A–B + C → A + B–C. Hyotcherl Ihee and colleagues now show that femtosecond X-ray liquidography (X-ray solution scattering) with X-ray free-electron lasers can map how gold atoms move during photo-induced formation of a trimeric gold complex in an aqueous solution. The measurements show that in the first 60 femtoseconds, an initial cluster of three monomers with unequal distances between them develops a covalent bond between the nearest monomers, and that subsequently a second covalent bond forms, within 360 femtoseconds, to give a linear, covalently bonded trimer complex. The strategy should be applicable to a range of interesting reactions, particularly once next-generation X-ray sources makes it possible to track the movement of not only heavy atoms such as gold but also lighter atoms such as carbon and nitrogen.