Mapping the emergence of molecular vibrations mediating bond formation

Abstract

Fundamental studies of chemical reactions often draw molecular dynamics along a reaction coordinate in a calculated or suggested potential energy surface (PES) 1-5 . But fully mapping such dynamics experimentally, by following all nuclear motions in a timeresolved manner, that is the motions of wavepackets, is challenging and has not even been realized for the simple stereotypical bimolecular reaction 6-8 of A-B + C → A + B-C. Here we report such tracking of vibrational wavepacket trajectories during photo-induced bond formation in the gold trimer complex [Au(CN)2 -]3 in an aqueous solution, using femtosecond x-ray solution scattering (liquidography 9-12 ) at x-ray free electron lasers 13,14 . We find that the complex forms from an assembly of three monomers A, B and C clustered together through non-covalent interactions 15,16 and with the distance between A and B shorter than between B and C. Tracking of the wavepacket in three-dimensional nuclear coordinates (RAB, RBC, and RAC) reveals that within the first 60 fs 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 fs to give a linear and covalently-bonded trimer complex A-B-C. The trimer exhibits harmonic vibrations that we are also able to map, and unambiguously assign to specific normal modes using only the experimental data. More intense x-rays can in principle visualize the motion of not only highly-scattering atoms such as gold but also of lighter atoms such as carbon and nitrogen, which will open the door for the direct tracking of the atomic motions involved in many chemical reactions.The [Au(CN)2 -]3 complex has served as a valuable model system for studying photoinitiated processes in solution. Irradiation with ultraviolet light excites it from the ground state (S0) to the singlet state (S1), which within 20 fs undergoes intersystem crossing to reach a triplet excited state (T1') 18 . A further transition from T1' to another triplet excited state (T1) then occurs with a time constant of 1~2 ps, completing formation of covalent bonds and transformation of the complex from a bent to a linear structure 9,17,18 (see the Supplementary Information (SI) for details of the notations of electronic states).Formation of the bonds could involve any of the three possible candidate trajectories sketched in Fig. 1b. The equilibrium structure in the ground state determines the position of the

Gold Nanoparticle Formation via X-ray Radiolysis Investigated with Time-Resolved X-ray Liquidography

“…We note that the formation of AuNPs was not observed in our TRXL experiment on K[Au(CN) 2 ] in water, which was performed under similar experimental conditions [ 7 , 9 ]. Since the formation of nanoparticles involves the reduction of the metal and the reduction force is driven by the hydrated electrons generated by X-ray radiolysis, we theoretically evaluated the standard reduction potentials (E°) for the generation of the neutral metal atoms from AuCl 4 − , [Au(tpy)Cl] 2+ , and Au(CN) 2 − in water using density functional theory (DFT) calculations and compared them to the hydration potential (HP) of an electron (E°(e − gas /e − aq )), which was determined experimentally to be −2.71 V [ 53 ] and computed to be −2.64 V [ 53 ].…”

“…The original purpose of the TRXL experiment on the [Au(tpy)Cl]Cl 2 aqueous solution was to investigate the structural dynamics of [Au(tpy)Cl] 2+ . For example, in previous TRXL experiments on [Au(CN) 2 − ] 3 in water [ 7 , 9 ], the bond formation dynamics of a gold trimer complex was unveiled. We expected that analogous bond forming events may afford oligomers and intended to study its associated structural dynamics.…”

“…Information on the structural dynamics of a chemical reaction is crucial for determining its reaction mechanism. Time-resolved X-ray liquidograpy (TRXL), known as time-resolved X-ray solution scattering, is an established technique in this regard, by which the structural dynamics of molecules spanning diatomic molecules to proteins have been successfully investigated [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ]. In a typical TRXL experiment [ 16 , 17 , 18 ], with X-ray facilities such as synchrotrons and X-ray free-electron lasers (XFELs), a liquid solution containing the solute molecule of interest is exposed to a femtosecond laser pulse to initiate a photochemical reaction of the solute molecule, and the structural changes associated with the subsequent reactions of the solute molecule are probed by an X-ray scattering of the solution sample as a function of time delays between the laser pulse and the X-ray pulse.…”

“…In a typical TRXL experiment [ 16 , 17 , 18 ], with X-ray facilities such as synchrotrons and X-ray free-electron lasers (XFELs), a liquid solution containing the solute molecule of interest is exposed to a femtosecond laser pulse to initiate a photochemical reaction of the solute molecule, and the structural changes associated with the subsequent reactions of the solute molecule are probed by an X-ray scattering of the solution sample as a function of time delays between the laser pulse and the X-ray pulse. In most TRXL experiments [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ], X-rays serve well as a means of probing the reaction, but in some rare cases, the X-ray itself induces unwanted responses. Especially when the solute molecule contains metal atoms, they have the potential to be reduced by the stimulation of the X-ray and eventually form nanoparticles.…”

“…In our study, we used [Au(tpy)Cl]Cl 2 as a metal source, whereas HAuCl 4 was used in most of the studies reporting the nanoparticle formation by the pulse radiolysis. In contrast, no formation of nanoparticles was observed in TRXL experiments on the aqueous solutions of K[Au(CN) 2 ] [ 7 , 9 ], K 4 [Pt 2 (P 2 O 5 H 2 ) 4 ] [ 4 ] and [Co(tpy) 2 ](PF 6 ) 2 [ 2 ]. The observed differences in the generation of nanoparticles depending on the metal precursor suggest that the reduction potential (RP) of the cationic metal center to a neutral metal atom, which is determined by the oxidation number of the metal and its surrounding ligands, relative to the hydration potential (HP) of electron, decides the formation of nanoparticles.…”