Observation and Simulation of Transient Anion Oligomers (LiClO₄)_n^- (n = 1-4) in Diethyl Carbonate LiClO₄ Solutions

NMR measurements show that diethyl carbonate (DEC, a solvent with a low dielectric constant) solutions of LiClO₄ contain (LiClO₄)_n oligomers. The reduction of these species by solvated and presolvated electrons is followed by picosecond pulse radiolysis measurements. The data analysis shows that several anions absorbing in the near-infrared (NIR) and visible range are formed after the 7 ps electron pulse. In contrast with tetrahydrofuran (THF) solutions of LiClO₄, the anionic monomer (LiClO₄)^- is not observed in DEC solutions. This is due to the fact that DEC is a nonpolar solvent favoring the clustering of monomers in the nonirradiated solution, as shown by NMR results, and also due to the instability of the anionic monomer. The absorption spectra of the anionic dimer (LiClO₄)₂^-, trimer (LiClO₄)₃^-, and tetramer (LiClO₄)₄^- are clearly observed in NIR and visible ranges. Compared to the results obtained for the same system in THF and in agreement with simulated absorption spectra, the experimental results show that the absorption bands are shifted to the blue end of the spectrum when n increases. The kinetics recorded for the molar LiClO₄ solution indicates that the solute is only in the form of oligomers (LiClO₄)_n with a large n value and that the reduced species absorb weakly in the visible region. Lastly, and contrary to what is known for well-separated ions in polar solvents, it is shown that the (LiClO₄)_n^- anions are not stable with respect to self-reduction, leading to the decomposition of perchlorate anions. In this reaction, the perchlorate anion ClO₄^- is reduced by the Li atom into a chlorate anion ClO₃^-. This is proved by the presence of ClO₃^- and chlorinated species detected by mass spectrometry measurements in irradiated DEC solutions containing LiClO₄.