Identification of Transient Radical Anions (LiClO4)(n)(-) (n = 1-3) in THF Solutions: Experimental and Theoretical Investigation on Electron Localization in Oligomers.

Abstract

Picosecond pulse radiolysis measurements of tetrahydrofuran (THF) solutions containing LiClO4 over a wide range of concentration are performed to investigate the formation of transient species. The (35)Cl NMR measurements of these solutions prior to irradiation show that the salt is in the form of (LiClO4)n oligomers. Kinetics and transient absorption spectra of intermediates in each solution are obtained on the time scale from 10 to 3800 ps. A global spectro-kinetic matrix of the data is analyzed by the multicurve resolution alternated least-squares (MCR-ALS) method. It shows the presence of 3 transient species induced by electron pulse, in addition to the solvated electron. A hybrid Monte Carlo/DFT molecular simulation method is elaborated, using the MPW1K functional for the configuration sampling and B3LYP for the spectra calculations. The maximum of the absorption band of the monomer (LiClO4)(-), dimer (LiClO4)2(-), trimer (LiClO4)3(-), and tetramer (LiClO4)4(-) anions are deduced from the simulations. They enable one to label the MCR-ALS spectra (differences are below 0.1 eV) and to interpret the kinetic data. The simulations show also that Li(I) ion catalyzes the reduction of perchlorate by excess electrons. Only the dimer anion, due to its unique structure with a stable Li2(+) core and two nonbridging perchlorates, presents higher stability toward ClO4(-) reduction into ClO3(-). It corresponds to the long-lived species observed in the experiments.