Unexpected Radiation Chemistry of Borate Buffer. II: Second-Order Decay

In Part I of our study, which deals with the formation of borate radicals, using pulse radiolysis with two-dimensional UV-vis transient absorption, we had confirmed the H-abstraction reaction of •OH and/or •O- with borate monomer anions, and the reaction of the resulting monomer radical anions with boric acid to form dimers. The present manuscript deals with the interesting second-order recombination mechanism involving borate monomer, dimer, and trimer radicals. Linear dependence of second-order decay rate constants vs the inverse of boron concentration obtained at all temperatures signifies the existence of an equilibrium involving dimer radicals. Simultaneously, there is a temperature-dependent but boron-concentration-independent spectrum shift, which signifies an intramolecular rearrangement of the absorbing species. Ultimately, it appears that the transient species that we observe within a few microseconds in 0.2-0.02 M borax solution is mainly a borate trimer dianion radical, with borate dimer radicals present in equilibrium. Lack of any ionic strength effect in the recombination reactions indicates that both dimer anion and trimer dianion radicals are probably strongly ion-paired with sodium. The shift in the visible spectra that was observed experimentally could be explained on the basis of a second equilibrium, which is an intramolecular rearrangement within the trimer radicals. Both dimer and trimer radicals are primarily oxygen-centered, but the oxygen may be attached at either sp2 or sp3 hybridized boron. Time-dependent density functional calculations suggest that sp2-hybridized radical sites are slightly less stable, but absorb much more strongly in the visible than the sp3 hybridized sites. The course of the reaction has been illustrated in the form of a final potential energy diagram showing each elementary step involving the borate monomer, dimer, and trimer radicals.