Electronic structures and electron detachment energies of halogen substituted acetate anions, XCH2COO- (X=F,Cl,Br).

The electronic structures and the halogen inductive effects on the acetate anion were investigated in XCH2COO- (X=F,Cl,Br) by photoelectron spectroscopy (PES) and ab initio calculations. The PES spectra indicated that the electron binding energies increased in the order of F<Cl<Br, contradictory to the known electron affinities of the halogen atoms. The measured adiabatic detachment energies (ADEs) are 3.80, 3.93, and 3.97 eV and the vertical detachment energies (VDEs) are 3.96, 4.10, and 4.13 eV for the F-, Cl-, and Br-substituted species, respectively. Structures of these anions and their neutral species were obtained by full geometry optimizations at the CCSD(T)/aug-cc-pVDZ level of theory, and final energies were calculated at the CCSD(T)/aug-cc-pVTZ level. The calculated ADEs (3.76, 3.88, and 3.91 eV for F, Cl, Br, respectively) and VDEs (4.14, 4.29, and 4.32 eV, respectively) are in good agreement with the corresponding experimental results. Theoretical analysis shows that the increase of ADE/VDE from F to Cl to Br is related to that the matching of the p orbital energy of X with the COO- group is better for Br than that of Cl and F. For comparison, additional calculations were carried out to include halogen substituted ethanol, XCH2CH2OH. Similar trend on electron binding energies was also found. In contrast, the ionization potentials (IPs) of both XCH2COOH and XCH2CH2OH decrease in the order of F>Cl>Br. These systematic changes of detachment energy and IPs were explained by examining the charge redistributions upon detaching electrons.