Spatiotemporal reaction kinetics of an ultrafast photoreaction pathway visualized by time-resolved liquid x-ray diffraction

doi:10.1073/pnas.0601958103

Spatiotemporal reaction kinetics of an ultrafast photoreaction pathway visualized by time-resolved liquid x-ray diffraction

^†Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea;
^‡European Synchrotron Radiation Facility, Grenoble Cedex 9, France;
^¶National Institute for the Physics of Matter and Department of Physical and Astronomical Sciences, University of Palermo, via Archirafi 36, 90123 Palermo, Italy; and
^‖Fachbereich Physik der Universität Konstanz, Universitätsstrasse 10, D-78457 Konstanz, Germany

Edited by Peter M. Rentzepis, University of California, Irvine, CA, and approved May 9, 2006 (received for review March 10, 2006)

Abstract

We have studied the reaction dynamics for HgI₂ in methanol by using time-resolved x-ray diffraction (TRXD). Although numerous time-resolved spectroscopic studies have provided ample information about the early dynamics of HgI₂, a comprehensive reaction mechanism in the solution phase spanning from picoseconds up to microseconds has been lacking. Here we show that TRXD can provide this information directly and quantitatively. Picosecond optical pulses triggered the dissociation of HgI₂, and 100-ps-long x-ray pulses from a synchrotron probed the evolving structures over a wide temporal range. To theoretically explain the diffracted intensities, the structural signal from the solute, the local structure around the solute, and the hydrodynamics of bulk solvents were considered in the analysis. The results in this work demonstrate that the determination of transient states in solution is strongly correlated with solvent energetics, and TRXD can be used as an ultrafast calorimeter. It also is shown that a manifold of structural channels can be resolved at the same time if the measurements are accurate enough and that global analysis is applied. The rate coefficients for the reactions were obtained by fitting our model against the experimental data in one global fit including all q-values and time delays. The comparison between all putative reaction channels confirms that two-body dissociation is the dominant dissociation pathway. After this primary bond breakage, two parallel channels proceed. Transient HgI associates nongeminately with an iodine atom to form HgI₂, and I₂ is formed by nongeminate association of two iodine atoms.

Footnotes

^††To whom correspondence may be addressed at:
European Synchrotron Radiation Facility 6, Rue Jules Horowitz BP 220, Grenoble Cedex 38043, France.
E-mail: wulff{at}esrf.fr
^‡‡To whom correspondence may be addressed at:
Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, South Korea.
E-mail: hyotcherl.ihee{at}kaist.ac.kr
↵ ^§Present address: Groupe Matiere Condensee et Materiaux UMR6626, Centre National de la Recherche Scientifique, BAT11A Campus de Beaulieu, Universite de Rennes 1, 35042 Rennes Cedex, France.
Author contributions: H.I. designed research; T.K.K., M.L., J.H.L., M.L.R., J.K., M.C., Q.K., S.N., A.P., M.W., and H.I. performed research; T.K.K., J.H.L., J.K., and H.I. analyzed data; and T.K.K., M.W., and H.I. wrote the paper.
Conflict of interest statement: No conflicts declared.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations:

Abbreviations:

TRXD,

time-resolved x-ray diffraction;

RDF,

radial distribution function;

MD,

molecular dynamics.
Freely available online through the PNAS open access option.