Investigation of Free Singly and Doubly Charged Alkali Metal Sulfate Ion Pairs: M+(SO42-) and [M+(SO42-)]2 (M = Na, K)

Abstract

The authors present a combined experimental and theoretical investigation of alkali metal sulfate ion pairs and their doubly charged dimers in the gas phase: Na{sup +}SO{sub 4}{sup 2{minus}}, K{sup +}SO{sub 4}{sup 2{minus}}, (NaSO{sub 4}){sub 2}{sup 2{minus}}, (KSO{sub 4}){sub 2}{sup 2{minus}}, and NaK(SO{sub 4}){sub 2}{sup 2{minus}}. They produced these anions using an electrospray technique and measured their photoelectron spectra at three photon energies, 355, 266, and 193 nm. The photoelectron spectra of each anion exhibit two detachment features, which approximately correspond to detachment from the HOMO and HOMO-1 of the SO{sub 4}{sup 2{minus}} group perturbed by the cations. The electron binding energies of the dimer dianions are lower than those of the monomers because of the strong electron-electron repulsion in the dianions. The authors also observed the repulsive Coulomb barriers in the dianions and their effects on the photodetachment spectra. They used density functional theory and ab initio molecular orbital theory to obtain the geometric and electronic structure of the ion pairs. The calculated electron binding energies and orbital energy separations are in reasonable agreement with the experimental values. The authors found that the MSO{sub 4}{sup {minus}} ion pairs have C{sub 3v} symmetry with a C{sub 2v} structure slightly higher inmore » energy. In the dimer dianions the two alkali metal cations bridge the two SO{sub 4}{sup 2{minus}} groups.« less