Investigation of free singly and doubly charged alkali metal sulfate ion pairs: M+(SO42-) and [M+(SO42-)](2) (M = Na, K)

We present a combined experimental and theoretical investigation of alkali metal sulfate ion pairs and their doubly charged dimers in the gas phase: N a+SO42-, K+SO42-, (NaSO4)(2)(2-), (KSO4)(2)(2-), and NaK(SO4)(2)(2-). We pr oduced these anions using an electrospray technique and measured their phot oelectron spectra at three photon energies, 355, 266, and 193 nm. The photo electron spectra of each anion exhibit two detachment features, which appro ximately correspond to detachment from the HOMO and HOMO-1 of the SO42- gro up perturbed by the cations. The electron binding energies of the dimer dia nions are lower than those of the monomers because of the strong electron-e lectron repulsion in the dianions. We also observed the repulsive Coulomb b arriers in the dianions and their effects on the photodetachment spectra. W e used density functional theory and ab initio molecular orbital theory to obtain the geometric and electronic structure of the ion pairs. The calcula ted electron binding energies and orbital energy separations are in reasona ble agreement with the experimental values. We found that the MSO4- ion pai rs have C-3v symmetry with a C-2v structure slightly higher in energy. In t he dimer dianions the two alkali metal cations bridge the two SO42- groups.