Probing Ultrafast Structural Dynamics in Proton Transfer Reactions in Solution with Time-Resolved Infrared and X-Ray Spectroscopies
Final Report Abstract
The aim of the project has been to develop methods to probe the hydrogen bond structure of photoacid molecules, before, during and after proton transfer. Because photoacids are optically triggered molecular systems, ultrafast spectroscopy is the obvious methodological tool to study proton transfer dynamics in real-time. The strategic approach has been to develop time-resolved spectroscopic methods with local probes for the hydrogen bonds of the proton-donating group of photoacids (for aromatic alcohols R- OH, or for protonated aromatic amines R-NH3+). One obvious method is ultrafast infrared spectroscopy, where a UV-pump pulse excites the photoacids to the first electronic excited state, and the local hydrogen stretching oscillator (O-H or N-H stretching mode of R-OH or R-NH3+, respectively) can be measured. This builds on the well-known empirical relationships where hydrogen bond distances and hydrogen stretching frequency shifts are well-correlated: the stronger the hydrogen bond, the shorter the heavy atom distance in hydrogen bonds), and the more red-shifted the hydrogen stretching frequency. The problem with the empirical relationship is that both intrinsic hydrogen bond interactions, and coupling with the surroundings (solvent shells) are heavily intertwined. We have been able to disentangle these different contributions of the hydrogen bonded complex of 2-naphthol-CH3CN, and provide a direct correlation between O-H stretching frequency, and (photo)acidity (as characterized with its pKa-value), as well as between O-H stretching frequency and the calculated O∙∙∙N distance. Another finding comes from calculated local charges on the oxygen and hydrogen atoms of the proton donating group, where photoexcitation does not lead to significant changes, supporting the notion that the photoacidity effect is largely driven by electronic charge distribution changes on the conjugate photobase side of the Förster cycle. A second local probe is soft-x-ray spectroscopy of inner shell excitations to unoccupied molecular orbitals, that have been shown to provide insight into the electronic structure of hydrogen bonds. Liquid flatjet technology for soft-x-ray spectroscopy has been developed together with liquid jet experts [4] enabling a free-flowing continuously refreshment of sample guaranteeing x-ray radiation damage free probing of molecular systems in liquid solution. This flatjet has length and width dimensions on a millimeter scale, with thicknesses of micrometers. Several beamtimes in the years 2014-2017 have resulted in publications on the electronic structure, solvation structure and hydrogen bond structure of amines/protonated amines in solution using N K-edge spectroscopy. Additional activities demonstrating the potential of flatjet technology focused on the L-edge of 3dmetal ligand complexes, where the aim was to obtain experimental information of charge and spin densities. With the flatjet technology an extension towards time-resolved soft-x-ray spectroscopy appears straightforward. This is now going to be explored and further developed with the recently obtained European Research Advanced Grant.
Publications
- Ultrafast excited state proton transfer reaction of 1-naphthol-3,6-disulfonate and several 5- substituted 1-naphthol derivatives. J. Phys. Chem. B 117 (16), 4594-4603 (2013)
M. Prémont-Schwarz, T. Barak, D. Pines, E. T. J. Nibbering and E. Pines
(See online at https://doi.org/10.1021/jp308746x) - Ultrafast solvent-assisted electronic level crossing in 1-naphthol. Angew. Chem. 125 (27), 7009–7013 (2013) and Angew. Chem. Int. Ed. 52 (27), 6871–6875 (2013)
F. Messina, M. Prémont-Schwarz, O. Braem, D. Xiao, V. S. Batista, E. T. J. Nibbering and M. Chergui
(See online at https://doi.org/10.1002/anie.201301931) - A liquid flatjet system for solution phase soft-X-ray spectroscopy. Struct. Dyn. 2 (5), 054301 (2015)
M. Ekimova, W. Quevedo, M. Faubel, Ph. Wernet and E. T. J. Nibbering
(See online at https://doi.org/10.1063/1.4928715) - Correlating photoacidity to hydrogen-bond structure by using the local O−H stretching probe in hydrogen-bonded complexes of aromatic alcohols. J. Phys. Chem. A 119 (20), 4800-4812 (2015)
B. T. Psciuk, M. Prémont-Schwarz, B. Koeppe, S. Keinan, D. Xiao, E. T. J. Nibbering and V. S. Batista
(See online at https://doi.org/10.1021/acs.jpca.5b01530) - Aqueous solvation of ammonia and ammonium: Probing hydrogen bond motifs with FT-IR and soft-x-ray spectroscopy. J. Am. Chem. Soc. 139 (36), 12773-12783 (2017)
M. Ekimova, W. Quevedo, L. Szyc, M. Iannuzzi, Ph. Wernet, M. Odelius and E. T. J. Nibbering
(See online at https://doi.org/10.1021/jacs.7b07207) - Soft-x-ray spectroscopy of the amine group: Hydrogen bond motifs in alkylamine/alkylammonium acid-base pairs. J. Phys. Chem. B 122 (31), 7737–7746 (2018)
M. Ekimova, M. Kubin, M. Ochmann, J. Ludwig, N. Huse, Ph. Wernet, M. Odelius and E. T. J. Nibbering
(See online at https://doi.org/10.1021/acs.jpcb.8b05424)
