Direct Observation of Bond Formation in Solution with Femtosecond X-ray Scattering

초록

Every chemical reaction involves the making and breaking of atomic bonds, thus it is essential to track detailed dynamic evolutions during the processes for complete understanding of chemical reactions. The ultrafast dynamics of bond-breaking process have been intensively investigated by using time-resolved techniques, however, bond-making process have not been thoroughly studied since bimolecular nature of the bond making makes it difficult for its initiation to be synchronized with laser excitation. Here, we chose an appropriate molecular system, [Au(CN)2-]3, to scrutinize bond-making process in solution. [Au(CN)2-]3 contains three gold atoms reside in a same solvent cage by a non-covalent interatomic interaction called aurophilicity. Laser excitation upon the gold trimer complex can initiate covalent bond formations between gold atoms in a same solvent cage, which makes the reaction is free from the diffusion limit. We employed femtosecond time-resolved X-ray solution scattering (TRXSS) to visualize the structural evolution during the bond formation of the gold trimer complex. With the advent of X-ray free electron laser (XFEL), time resolution of X-ray solution scattering experiment has been improved down to subpicosecond, making it possible to keep track of ultrafast bond formation of the gold complex. Kinetic and structural analysis on the experimental data reveals that covalent bond formation and bent-to-linear structural relaxation occur within ~500 fs and subsequent bond shortening and tetramer formation take place with time constants of 1.6 ps and 3 ns, respectively.