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      Bromosubstituted norbornadienes and their reversible photolytic transformation to quadricyclanes

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          Two new model systems for use within the rapidly developing ultrafast time resolved x-ray scattering techniques have been prepared. Their photoisomerisation from norbornadiene to quadricyclane was found to be a suitable reaction to follow. Simulations of scattering patterns (not included in this report) showed that if heavy atoms are included in these molecular structures, then the transformation can be followed by transient X-ray scattering techniques. Two new bromosubstituted norbornadienes were synthesised and characterised. Absorption spectroscopy showed that the norbornadienes are converted quantitatively to quadricyclanes under ultraviolet irradiation. Nuclear magnetic resonance (NMR) studies showed that the process was fully reversible and that the norbornadienes could be completely recovered even without addition of catalysts. Furthermore, it was shown that the formation of quadricyclane from norbornadiene was unaffected by triplet sensitisers. The two new model systems synthesised thus are strong candidates for use in time-resolved X-ray scattering studies in both gas and condensed phases.

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          Time-resolved X-ray scattering of an electronically excited state in solution. Structure of the 3A(2u) state of tetrakis-mu-pyrophosphitodiplatinate(II).

          The structure of the (3)A(2u) excited state of tetrakis-mu-pyrophosphitodiplatinate(II) in aqueous solution is investigated by time-resolved X-ray scattering on a time scale from 100 ps to 1 micros after optical pumping. The primary structural parameter, the Pt-Pt distance, is found to be 2.74 A, which is 0.24 A shorter than the ground-state value. The contraction is in excellent agreement with earlier estimates based on spectroscopic data in solution and diffraction data in the crystalline state. As a second structural parameter, the distance between the P planes in the (3)A(2u) excited state was determined to be 2.93 A, i.e., the same as that in the ground state. This result implies that a slight lengthening of the Pt-P bond occurs following excitation.
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            Analysis of time-resolved X-ray scattering data from solution-state systems.

            As ultrafast time-resolved studies of liquid systems with the laser pump/X-ray scattering probe method have come of age over the past decade, several groups have developed methods for the analysis of such X-ray scattering data. The present article describes a method developed primarily with a focus on determining structural parameters in the excited states of medium-sized molecules (approximately 30 atoms) in solution. The general methodology is set in a maximum-likelihood framework and is introduced through the analysis of the photoactive platinum compound PtPOP, in particular the structure of its lowest triplet excited state ((3)A(2u)). Emphasis is put on structure determination in terms of model comparisons and on the information content of difference scattering signals as well as the related experimental variables. Several suggestions for improving the accuracy of these types of measurements are presented.
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              Spatiotemporal kinetics in solution studied by time-resolved X-ray liquidography (solution scattering).

              Information about temporally varying molecular structure during chemical processes is crucial for understanding the mechanism and function of a chemical reaction. Using ultrashort optical pulses to trigger a reaction in solution and using time-resolved X-ray diffraction (scattering) to interrogate the structural changes in the molecules, time-resolved X-ray liquidography (TRXL) is a direct tool for probing structural dynamics for chemical reactions in solution. TRXL can provide direct structural information that is difficult to extract from ultrafast optical spectroscopy, such as the time dependence of bond lengths and angles of all molecular species including short-lived intermediates over a wide range of times, from picoseconds to milliseconds. TRXL elegantly complements ultrafast optical spectroscopy because the diffraction signals are sensitive to all chemical species simultaneously and the diffraction signal from each chemical species can be quantitatively calculated from its three-dimensional atomic coordinates and compared with experimental TRXL data. Since X-rays scatter from all the atoms in the solution sample, solutes as well as the solvent, the analysis of TRXL data can provide the temporal behavior of the solvent as well as the structural progression of all the solute molecules in all the reaction pathways, thus providing a global picture of the reactions and accurate branching ratios between multiple reaction pathways. The arrangement of the solvent around the solute molecule can also be extracted. This review summarizes recent developments in TRXL, including technical innovations in synchrotron beamlines and theoretical analysis of TRXL data, as well as several examples from simple molecules to an organometallic complex, nanoparticles, and proteins in solution. Future potential applications of TRXL in femtosecond studies and biologically relevant molecules are also briefly mentioned.

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                ScienceOpen Research
                05 September 2014
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                [1 ]Nano-Science Center & Department of Chemistry, University of Copenhagen, København Ø, Denmark
                [2 ]Department of Physics, Technical University of Denmark, Fysikvej 307, 2800 Kgs. Lyngby
                Author notes
                [* ]Corresponding author's e-mail address: klbe@
                © 2014 P. Hammershøj et al.

                This work has been published open access under Creative Commons Attribution License CC BY 4.0 , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Conditions, terms of use and publishing policy can be found at .

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                Figures: 5, Tables: 0, References: 34, Pages: 7
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