Theoretical Analysis of the <i>n</i> ₁ ν ₁ + ν ₃ Combination Bands in Hydrogen Bihalide Anions, XHX ^– , X = {F, Cl, Br, and I}

The anharmonic vibrational structures of XHX^- anions, X = {F, Cl, Br, and I}, were examined with a focus on the <i>n</i>₁ν₁ + ν₃ combination bands, where <i>n</i>₁ is the excitation quanta of the symmetric X-H stretch (ν₁) and ν₃ is the asymmetric X-H stretch. Full-dimensional potential energy surfaces (PESs) were built by using a multilevel scheme based on CCSD(T) and MP2 levels of <i>ab initio</i> theories. These dual-level PESs were then used to simulate the anharmonic spectra and to assess the extent of vibrational coupling. The vibrational signatures of XHX^- are very similar to those of the previously studied proton-bound noble gas dimers. The combination bands become more visible as the halide becomes heavier. The quantum nature of these combination bands was examined using an adiabatic model. Furthermore, the halide binding energies for XHX^-, X = {F, Cl, Br, and I}, were studied using a modern fragmentation-based method, energy decomposition analysis based on absolutely localized molecular orbitals (ALMO-EDA). The halide binding energy decreases from that of FHF^- to that of IHI^-. Both the geometric distortion and dispersion-free frozen components destabilize these complexes, weakening their binding energies. However, the dispersion, polarization effects, and charge transfer contributions stabilize these complexes.