The infrared spectrum of the hydrogen bifluoride anion: unprecedented variation with level of theory

Abstract

High-quality ab initio theoretical methods have been employed to reinvestigate the infrared of the FHF− and FDF− molecular anions in light of the recent experimental v3 (σu+) assignments of Kawaguchi and Hirota. SCF, CISD, and Davidson-corrected CISD (CIDVD) results were obtained for seven basis sets ranging in quality from double-zeta plus polarization (DZP) to triple-zeta plus triple polarization plus diffuse augmented by hydrogen d functions (TZ3P + R + d). Fundamental frequencies were determined using variational techniques and complete quartic force fields. As the level of theory is increased, unprecedented variations in v3(σu+) are observed. At the TZ3P + R + d CIDVD level, v(σg+) = 617, v2(πu) = 1363, and v3(σu+) = 1427 cm−1 are predicted for FHF− and v1(σg+) = 620, v2(πu) = 974, and v3(σu+) = 983 cm−1 for FDF−. These theoretical v3 values are in significant disagreement with the 1848 cm−1 (FHF−) and 1397 cm−1 (FDF−) experimental v3 assignments.