Trihalide cations MF₃ ⁺, MCl₃ ⁺ and MBr ⁺₃ , M = B, Al, Ga: pseudo Jahn–Teller coupling, electronic spectra and ionization potentials of MX₃

Abstract

Optimized geometries and energies, vertical excitation energies and vibrational frequencies are reported for nine cations MX₃ ⁺, with M = B, Al, Ga, and X = F, Cl, Br. Density functional theory using long-range corrected functionals, coupled cluster and multireference configuration interaction methods were applied with triple-zeta polarized basis sets. All cations were shown to be distorted from the high D_3h to the lower C_2v symmetry due to a strong pseudo Jahn–Teller effect. Geometry optimizations lead to two ²B₂ states, one with an X(axial)–M–X angle above 120°, to give a structure with one short and two long bonds (1S2L), the other having such angle below 120°, resulting in a structure with two short and one long bond (2S1L). In most cases, the 1S2L structure was found to be more stable than 2S1L, but the stabilization energies of 1S2L and 2S1L differ by no more than 0.2 eV. There is a saddle point at D_3h symmetry. Adiabatic and vertical ionization energies of MX₃ are also reported. Good to excellent agreement with available experimental data was found.