Fundamental studies of chemical reactions often consider the molecular dynamics along a reaction coordinate using a calculated or suggested potential energy surface 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 6 – 8 : A–B + C → A + B–C. Here we track the trajectories of these vibrational wavepackets during photoinduced bond formation of the gold trimer complex [Au(CN) 2 − ] 3 in an aqueous monomer solution, using femtosecond X-ray liquidography 9 – 12 with X-ray free-electron lasers 13 , 14 . In the complex, which forms when three monomers A, B and C cluster together through non-covalent interactions 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.

中文翻译：

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绘制介导键形成的分子振动的出现

化学反应的基础研究通常使用计算的或建议的势能面 1 – 5 考虑沿反应坐标的分子动力学。但是，通过以时间分辨的方式跟踪所有核运动（即波包的运动），通过实验完全映射这种动力学是具有挑战性的，即使对于简单的定型双分子反应也尚未实现 6 – 8 : A–B + C → A + B–C。在这里，我们在单体水溶液中使用飞秒 X 射线液相法 9 – 12 和 X 射线自由电子激光器跟踪金三聚体复合物 [Au(CN) 2 − ] 3 在光致键形成过程中这些振动波包的轨迹13、14。在复合体中，当三个单体 A、B 和 C 通过非共价相互作用聚集在一起时形成 15、16、A 和 B 之间的距离比 B 和 C 之间的距离短。 在三维核坐标中跟踪波包显示，在光激发后的前 60 飞秒内，A 和 B 之间形成共价键，得到 A-B + C。 B 和 C 之间的第二个共价键随后在 360 飞秒内形成，以产生线性和共价键合的三聚体复合物 A-B-C。三聚体表现出我们仅使用实验数据映射并明确分配给特定正常模式的谐波振动。原则上，更高强度的 X 射线不仅可以可视化高散射原子（如金）的运动，还可以可视化较轻原子（如碳和氮）的运动，这将为直接跟踪许多化学反应中涉及的原子运动打开大门.