Chemical Science Research Frontiers 2020 80 Femtosecond X-ray liquidography captures the birth of molecular vibrations mediating bond formation Trajectories of wavepackets along reaction coordinates, which determine real-time atomic motions in molecules during reactions, are often drawn in calculated or imagined potential energy surfaces (PES). However, it is still challenging to experimentally determine the trajectories of wavepackets on multidimensional nuclear coordinates. In our recent work, we succeeded in achieving this goal using photoinduced bond formation of a gold trimer complex (GTC), [Au(CN) 2 – ] 3 , in an aqueous solution [1]. The equilibrium structure of GTC in the ground state (S 0 ) determines the position of the Franck-Condon (FC) region and the structure in the FC region, where the excited-state wavepacket is initially created, can be considered as the reactants (A+B+C) of the reaction. This wavepacket moves toward the equilibrium structure of T 1 ', which is the product (A-B-C) with two equivalent covalent Au–Au bonds. Regarding the bond formation process of GTC, the three candidate reaction mechanisms can be considered: (i) the concerted bond formation mechanism where two covalent bonds, A - B and B - C, are formed synchronously, and the asynchronous bond formation mechanism where (ii) A - B is formed first or (iii) B - C is formed first. To determine the reaction mechanism among the three candidate mechanisms, it is required to directly observe the initial motions of the wavepacket starting from the FC region on multidimensional PES. To accomplish this task, we performed femtosecond time-resolved X-ray liquidography (TRXL) experiments [2] using X-ray free electron lasers (XFELs) at SACLA BL3 [3] and the X-ray Scattering and Spectroscopy (XSS) beamline of PAL-XFEL [4]. Structural analyses were performed against the TRXL data, and the best fits shown in Fig. 1(a) were obtained when both of the ground and excited states, S 0 and T 1 ', were considered, indicating that wavepacket motions in PESs of both S 0 and T 1 ' contribute to the TRXL signal. Using the time-dependent changes of structural parameters, R AB , R BC , R AC , and Au–Au–Au angle ( q ), shown in Figs. 1(b)–1(c), we reconstructed the trajectories of the excited-state (Figs. 2(a) and 2(c)) and ground-state [1] wavepackets in multidimensional nuclear coordinates, R AB vs R BC vs q . A s s h o w n i n F i g . 2 ( a ) , t h e e x c i t e d - s t a t e wavepacket generated in the FC region ( R AB = 3.13 Å and R BC = 3.38 Å, q = 119°) moves on the PES of T 1 ' toward the equilibrium structure of T 1 ' ( R AB = 2.82 Å (a) (b) (c) 2 2,000 120 2.7 2.8 2.9 5.5 5.6 5.7 5.8 Two symmetric stretching modes (79 cm –1 , 125 cm –1 ) 0 500 1,000 1,500 2,000 500 1,000 1,500 2,000 140 160 180 2.8 2.9 3.0 3.1 3.2 3.3 3.4 5.5 5.6 5.7 5.8 5.9 6.0 1,500 1,000 500 0 –500 –1,000 3 4 5 T 1 ’ R AC T1' R AC T 1 ' R BC T 1 ' R AB T 1 ' R AB & R BC 6 2 3 4 5 6 –1 0 1 Time (fs) (arb. units) Au–Au Distance (Å) Au–Au Distance (Å) Ahgle (°) q (Å –1 ) q (Å –1 ) Time (fs) Time (fs) T 1 ' θ Fig. 1. (a) Experimental TRXL signal (left), and their theoretical fits (right) obtained from the structural analysis. (b) R AB (t), R BC (t), and R AC (t), and Au–Au–Au angle, q , of T 1 ' are represented by black, red, blue, and cyan dots, respectively. (c) The Au–Au distances in the late time range (>360 fs) for T 1' are represented by black open circles with their fits by a sum of two damping cosine functions (red lines). Research Frontiers 2020 81 and R BC = 2.82 Å, q = 180°). Specifically, R AB decreases rapidly down to the covalent Au–Au bond length of the equilibrium T 1 ' (2.82 Å) at 35 fs time delay and becomes even shorter at 60 fs to reach the minimum length along the entire trajectory, whereas R BC is still much longer than the covalent bond length (2.82 Å) at those time delays, as shown in Fig. 2(a). The early- time trajectory reveals that the two covalent bonds are formed in an asynchronous manner, in which the covalent bond is formed earlier in the shorter Au– Au pair of the ground state. With respect to q , the excited-state wavepacket starts from the FC region ( q = 119°) and reaches the equilibrium value of T 1 ' ( q = 180°) in 335 fs, giving the time scale of bent- to-linear transformation. After the initial motions, the wavepacket oscillates around their equilibrium structures in the late time range (> 360 fs), as can be seen in Fig. 1(c). The trajectory of the wavepacket in T 1 ' in the late time range (> 360 fs) is shown in Fig. 2(c). For T 1 ', a sum of two symmetric stretching modes with 79 cm –1 and 125 cm –1 frequencies give satisfactory fits (Fig. 1(c)), and for S 0 , a symmetric stretching mode with 32 cm –1 frequency and an asymmetric stretching mode with 44 cm –1 frequency gives satisfactory fits [1]. In summary, the trajectories of nuclear wavepackets were visualized using femtosecond TRXL and unambiguously provides a direct view of the vibrational motion that drives an asynchronous bond formation. Femtosecond TRXL can be used as a fundamental tool to visualize atomic motions and reveal reaction pathways in many chemical reactions. Jong Goo Kim a,b,c , Shin-ichi Adachi d,e and Hyotcherl Ihee a,b,c, * a Department of Chemistry, KAIST, Republic of Korea b KI for the BioCentury, KAIST, Republic of Korea c Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Republic of Korea d Photon Factory, High Energy Accelerator Research Organization (KEK) e Department of Materials Structure Science, The Graduate University for Advanced Studies *Email: hyotcherl.ihee@kaist.ac.kr References [1] J.G. Kim, S. Nozawa, H. Kim, E.H. Choi, T. Sato, T.W. Kim, K.H. Kim, H. Ki, J. Kim, M. Choi, Y. Lee, J. Heo, K.Y. Oang, K. Ichiyanagi, R. Fukaya, J.H. Lee, J. Park, I. Eom, S.H. Chun, S. Kim, M. Kim, T. Katayama, T. Togashi, S. Owada, M. Yabashi, S.J. Lee, S. Lee, C.W. Ahn, D.-S. Ahn, J. Moon, S. Choi, J. Kim, T. Joo, J. Kim, S. Adachi and H. Ihee: Nature 582 (2020) 520. [2] K.H. Kim et al. : Nature 518 (2015) 385. [3] T. Ishikawa et al. : Nat. Photon. 6 (2012) 540. [4] H.S. Kang et al. : Nat. Photon. 11 (2017) 708. Fig. 2. (a, c) The trajectory of the wavepacket in T 1 ’ in (a) the early time range (< 360 fs) and (c) the late time range (> 360 fs) is represented in the coordinates of R AB versus R BC versus q and R AB versus R BC , respectively. In (a), the positions of the wavepacket at measured time delays are indicated by dots using a color scheme shown at the bottom of each panel. For several time delays, the time delays in femtoseconds units are shown. (b) Transient structures of T 1' at representative time delays. The Au atoms at each time delay are represented by yellow dots while the Au atoms in the FC region are represented by gray dots. Covalent bonds are indicated by black solid lines. The changes of interatomic distance and angle are indicated by red arrows and blue arrows, respectively. The trajectory of the wavepacket in S 0 is now shown due to space limitation. In (c), the wavepacket positions at several time delays are indicated by red dots, and the time delays in femtoseconds units are shown. The normal coordinates of the normal modes are indicated by blue arrows. At the end of each arrow, the representative structure with Au atoms as yellow spheres is shown to indicate displacements of three Au atoms according to the normal coordinate while the equilibrium structures are represented by gray spheres. The red arrows indicate the displacement vectors of Au atoms. (a) (c) (b) 2.8 120 2,000 1,500 1,000 500 0 140 160 180 335 harmonic oscillation (> 360 fs) 160 310 260 60 35 10 10 fs 2,235 fs FC region T 1 ′(eq) 2.8 3.0 3.0 3.2 3.2 3.4 3.4 0 fs 360 fs 60 fs 335 fs Au–Au distance change Au–Au–Au angle change Bond Angle, (°) θ θ Time (fs) R AB (Å) R BC (Å) 2.78 2.80 2.82 2.84 2.86 2.78 2.80 2.82 2.84 2.86 R BC (Å) 1,010 1,135 860 560 360 710 460 Q Q (T (T 1 1 _# #6) 6) Q Q (T (T 1 1 _# #12) 12) T 1 ′(eq)
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