The Arsenic Fluorides AsFn (n = 1−6) and Their Anions: Structures, Thermochemistry, and Electron Affinities

The molecular structures, electron affinities, and dissociation energies of the AsF n /AsF n - ( n = 1−6) species have been examined using four hybrid and pure density functional theory (DFT) methods. The basis set used in this work is of double-ζ plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. Three types of energy separations reported in this work are the adiabatic electron affinity (EA ad ), the vertical electron affinity (EA vert ), and the vertical detachment energy (VDE). The first As−F dissociation energies D e (F n - 1 As−F) for AsF n, and both D e (F n - 1 As - −F) and D e (F n - 1 As−F - ) for AsF n - species have also been reported. The best method for predicting molecular structures was found to be BHLYP, while other methods generally overestimated bond lengths. For the closed-shell anions the As−F bond distances are ∼0.1 Å longer than those for the analogous neutrals. In contrast, when the neutral AsF n is a closed shell, the anion As−F distances is ∼0.2 Å longer. The most reliable adiabatic electron affinities, obtained at the DZP++ BHLYP level of theory, are 0.74 eV (As), 0.94 eV (AsF), 1.17 eV(AsF 2 ), 0.80 eV (AsF 3 ), 4.42 eV (AsF 4 ), and 2.79 eV (AsF 5 ), respectively. Those for As and AsF 2 are in good agreement with experiment, but that for AsF is smaller than the available experimental value (1.3 ± 0.1 eV). The predicted vertical detachment energy for AsF 6 - is remarkable, as large as 10.54 eV (BHLYP), indicating AsF 6 - is stable. The general trend for predicting the first dissociation energies is BP86 ∼ BLYP > B3LYP ≫ BHLYP. The first dissociation energies for the neutral arsenic fluorides predicted by the DFT methods except BHLYP are 4.22−4.50 eV (AsF), 4.45−4.74 eV (AsF 2 ), 4.76−5.03 eV (AsF 3 ), 1.46−1.84 eV (AsF 4 ), and 3.87−4.11 eV (AsF 5 ). Compared to the experimental dissociation energies, the theoretical predictions are very reasonable. The anion bond dissociation energies are largely unknown experimentally. The dissociation energy for AsF - → As + F - is predicted to be 1.73 eV (BHLYP), 1.82 eV (B3LYP), 1.93 eV (BP86), and 1.87 eV (BLYP), which values are in good agreement with experiment (1.9 ± 0.2 eV). The predicted bond dissociation energies for D e (F 3 As−F - ) are in the range of 2.45−2.57 eV, which is close to the previous theoretical results using the HF and MP2/ECP methods. For the vibrational frequencies of the AsF n series, the BHLYP method also produces good predictions with the average error only about 10 cm -1 from available experimental values. The other three methods underestimate the vibrational frequencies, with the worst predictions given by the BLYP method.