Reversed Freeze Quench Method near the Solvent Phase Transition

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 repre...