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 $3.97eV$ and the vertical detachment energies (VDEs) are 3.96, 4.10, and $4.13eV$ 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 $3.91eV$ for F, Cl, Br, respectively) and VDEs (4.14, 4.29, and $4.32eV$⁠, 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.