title: Reversed freeze quench method near the solvent phase transition.
abstract: Freeze quenching is a general method for trapping reaction intermediates on a (sub)millisecond time scale. The method relies on a mixing and subsequent rapid freezing of solutions of reactants. If the reaction is limited by diffusion, it may be advantageous to initially mix the reactants under conditions where the reaction does not proceed, e.g., by mixing them at low temperature as solids. The temperature may then be raised close to the melting point of the solvent. Depending on the viscosity of the solvent, the temperature can be raised either by heating or by applying laser pulses of nanosecond length with concomitant conversion of light into heat. A reduction of the dead time and a good control of the reaction speed in comparison to the standard freeze quench technique has been achieved with this method. The feasibility of the method in combination with EPR spectroscopy is verified by examining the important prototypical reductions of benzoquinone and 2,6-dichlorophenolindophenol by ascorbate as representatives for two-step redox reactions. By using light pulses of a laser, the reaction could be driven with rates lowered by 4 orders of magnitude as compared to room temperature reaction rates. This has allowed the observation of previously unobserved radical intermediates: the reduction of DCPIP by ascorbate is found to be strongly pH dependent. It proceeds via two one-electron steps at low pH, whereas at neutral pH, the reduction of DCPIP by ascorbate proceeds in a 1:2 stoichiometry followed by a disproportionation of the ascorbate radicals.
authors.affiliation: Institut für Physikalische und Theoretische Chemie, Rheinische Friedrich-Wilhelms-Universität Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany.
authors.forename: Aliaksandr
authors.initials: A
authors.lastname: Marchanka
authors.forename: Maurice
authors.initials: M
authors.lastname: van Gastel
mesh_terms: D015086:2,6-Dichloroindophenol / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D005615:Freezing; D006863:Hydrogen-Ion Concentration; D010084:Oxidation-Reduction; D044367:Phase Transition; D012997:Solvents / Q000737:chemistry*
chemical_list: D012997:Solvents; D015086:2,6-Dichloroindophenol