The reactivity of linear alkyl carbonates toward metallic lithium: X-ray photoelectron spectroscopy studies in ultrahigh vacuum

X-ray photoelectron spectroscopy (XPS) was used to examine the reactivity of metallic Li toward gas-phase symmetric and asymmetric alkyl linear carbonates of the form ROCO₂R' (R and R' = -CH₃, -CH₂CH₃) at room temperature in ultrahigh vacuum (UHV). Comparison of the C (1s) and O (1s) XPS features obtained in virtually identical experiments involving methanol and ethanol allowed LiOCH₃ and LiOCH₂CH₃ to be clearly identified as the major products of the reaction between Li and dimethyl (DMC) and diethyl (DEC) carbonates, respectively, and a mixture of the two alkoxides for ethylmethyl (EMC) carbonate. These findings are in agreement with UHV Fourier transform infrared studies of the same systems reported earlier in our laboratory. Also found in the XPS spectra for ROCO₂R'/Li interfaces was an O (1s) peak at 529.6 eV attributed to Li oxide. In the case of DEC and EMC, evidence was obtained for the presence of au additional O (1s) feature at 534.76 eV attributed to Li ethyl carbonate. This behavior was unlike that found for DMC for which no signals due to Li methyl carbonate could be discerned. The conclusions emerging from this study further support the general mechanism for Li reactivity toward ROCO₂R' suggested by Aurbach and co-workers.