Mechanism of (SCN)₂^·- Formation and Decay in Neutral and Basic KSCN Solution under Irradiation from a Pico- to...

The detailed mechanism of the reaction between SCN^- and the OH^· radical and the formation of the dimer radical (SCN)₂^·- are studied by picosecond pulse radiolysis. First, concentrated SCN^- solutions are used to observe directly the formation and decay of SCNOH^·- in neutral and basic solutions. Then, the spectro-kinetic data, constituting a large matrix of data of the absorbance at different times and different wavelengths, obtained by pulse radiolysis measurements with a streak camera, in neutral and basic SCN^- solutions, are analyzed simultaneously. Data analysis allowed us to deduce the absorption spectra of different radicals with their extinction coefficient and also to determine the rate constants of different reactions involved in the formation and decay of (SCN)₂^·-. Molecular simulations of the absorption spectra of the different species were also performed. The absorption spectrum of the radical SCN^· is determined and is found to be different than that reported previously. It does not present a Gaussian shape centered at 330 nm; the absorption around 310 and 380 nm is not negligible. In addition, in a solution at pH 13, it is found that the (SCN)₂^·- radical is paired with an alkaline cation, inducing a blueshift of the absorption band compared to the free (SCN)₂^·-. Finally, the presence of K⁺ cations catalyzes the disproportionation reaction of (SCN)₂^·- and affects the kinetics.