Electronic isomers in [(CO2)nROH]− cluster anions. I. Photoelectron spectroscopy

Photoelectron spectra of [(CO2)n−1ROH]− (R=H and CH3) with 2⩽n⩽7 have been measured at a photon energy of 4.66 eV. Analysis of the photoelectron band envelopes has revealed that the spectra of [(CO2)n−1H2O]− with 3⩽n⩽5 consist of two band components. The maximum of each component corresponds to the vertical detachment energy (VDE) of the relevant anionic species. In each spectrum the VDE values for the two components differ by ≈1 eV. For example, the [(CO2)4H2O]− spectrum is characterized by two VDE values of 2.63±0.04 and 3.71±0.06 eV. From the VDE difference, we conclude that the observed two components arise from isomers having different electronic structures, and that these “electronic isomers” can be designated as C2O4−⋅H2O(CO2)n−3 and CO2−⋅H2O(CO2)n−2. Coexistence of electronic isomers occurs also in [(CO2)n−1CH3OH]−, but only at n=3. The [(CO2)n−1CH3OH]− anions with n≠3 display photoelectron spectra composed of a single broad band, which corresponds to photodetachment from CO2−⋅CH3OH(CO2)n−2 structure. The CO2−⋅ROH(CO2) isomers (n=3) are found to be distinguishable from C2O4−⋅ROH by selective photodepletion at 532 nm during the time of flight in the mass spectrometer, implying that the two electronic isomers of [(CO2)2ROH]− coexist without interconversion. In contrast, it is revealed that [(CO2)4H2O]− (n=5) are fluctuating between C2O4−⋅H2O(CO2)2 and CO2−⋅H2O(CO2)3 structures. Based on these experimental findings, combined with results obtained by recent ab initio calculations, a possible isomerization mechanism operative in [(CO2)n−1ROH]− is proposed.