Ultrafast X-ray Diffraction of Transient Molecular Structures in Solution

Abstract

We report direct structural evidence of the bridged radical (CH2ICH2.) in a polar solution, obtained using time-resolved liquid-phase x-ray diffraction. This transient intermediate has long been hypothesized to explain stereo-chemical control in many association and/or dissociation reactions involving haloalkanes. Ultrashort optical pulses were used to dissociate an iodine atom from the haloethane molecule (C2H4I2) dissolved in methanol, and the diffraction of picosecond x-ray pulses from a synchrotron supports the following structural dynamics, with approximately 0.01 angstrom spatial resolution and approximately 100 picosecond time resolution: The loss of one iodine atom from C2H4I2 leads to the C-I-C triangular geometry of CH2ICH2.. This transient C2H4I then binds to an iodine atom to form a new species, the C2H4I-I isomer, which eventually decays into C2H4 + I2. Solvent dynamics were also extracted from the data, revealing a change in the solvent cage geometry, heating, and thermal expansion.

Synchrotron ultrafast techniques for photoactive transition metal complexes

“…The main limit of this spectroscopic approach is the possibility of missing intermediate species containing only linear carbonyls (which are buried under the GS signals) and the poor structural sensitivity. TR-XSS is therefore a perfect complement for ultrafast IR characterization since it contains the scattering signal contributions from all the reaction pathways (in proportion to their population and scattering function) and is highly sensitive to intermediate geometries [43,49,58,59,[61][62][63]65,70,[198][199][200].…”

“…Furthermore, TR-XSS was also applied in the investigation of the iodine elimination reaction in haloalkanes, disclosing also the solvent polarity role on the intermediates' structure and their kinetics [58,63,165,166].…”

“…Once all the contributions to the time-dependent differential scattered intensity are properly modelled, the state-of-the-art approach for data analysis consists of the 'global-fitting analysis' [31,58]. This method relies on the simultaneous fitting of TR-XSS data in q-space for all the delays investigated, instead of separately simulating the I(q, τ ) curves for each τ .…”

“…A typical experimental set-up for ultrafast XSS is described in figure 4 in the model of the ID09b beamline [52,[55][56][57][58][59][60][61][62][63] of the European Synchrotron Radiation Facility (ESRF) (ESRF website: http://www.esrf.eu/). Other beamlines dedicated to TR-XSS (for instance, beamline NW14A [64] (NW14 beamline @KEK website: http://pfwww.kek.jp/adachis/NW14/NW14.htm) of PF-AR at KEK (PF-AR@ KEK website: http://www-pfring.kek.jp/PF/index_html) and beamline 14-ID-B (14-ID-B beamline at APS website: http://cars9.uchicago.edu/biocars/pages/idb.shtml) at the Advanced Photon Source (APS) (APS website: http://www.aps.anl.gov/)) employ analogue setups.…”

Diffraction using laser-driven broadband electron wave packets

“…Conventional methods based on diffraction and absorption [1][2][3][4] , using electrons or synchrotron-generated X-rays pulses, routinely achieve sub-angstrom resolutions, but their picosecond duration precludes characteristic, femtosecond-scale dynamical molecular imaging [4][5][6][7][8] . The recent development of femtosecond sources 9,10 of X-rays, for example, free-electron lasers, and electron beams has enhanced the temporal resolving power of these standard probes for ultrafast molecular dynamics [11][12][13] .…”