PNNL

Abstract

The hallmark of multiply charged anions is the repulsive Coulomb barrier (RCB), which prevents low energy electrons from being emitted in photodetachment experiments. However, using photoelectron imaging, we have observed persistent near zero-eV electrons during photodetachment of doubly charged dicarboxylate anions, -O2C(CH2)nCO2- (Dn2-, n = 2-8). Here we show that these low energy electron signals are well structured and are independent of the detachment photon fluxes or energies. The relative intensities of these signals are dependent on n, with maxima at n = 2, 4, 6. These near zero-eV electrons cannot come from direct photodetachment of the dianions and are proposed to come from decarboxylation of the product radical anions upon photodetachment of the parent dianions [•O2C(CH2)nCO2- ? CO2 + •(CH2)nCO2-], followed by dissociative autodetachment [•(CH2)nCO2- ? (CH2)n + CO2 + e] or hydrogen-transfer-induced electron detachment [•(CH2)nCO2- ? CH2=CH(CH2)n-2CO2H + e]. Energetic considerations suggest that these processes are exothermic. It is further observed that solvation by one water molecule quenches the low energy electron signals in the spectra of Dn2-(H2O), consistent with the proposed mechanisms. These indirect dissociative autodetachment processes are expected to involve cyclic transition states for n > 2, in agreement with the dependence on the chain length due to the anticipated strains in the intermediate steps. The quenching of the low energy electron signals by one water molecule demonstrates the importance of solvation on chemical reactions. ________________________