Hyperfine coupling constants and electron-spin g-factors of B2+, Al2+, Ga2+, BAl+, BGa+, and AlGa+: An ab initio study

Abstract

The hyperfine coupling constants (hfcc) and electron-spin g-factors (magnetic moments) calculated for B2+, Al2+, Ga2+, BAl+, BGa+, and AlGa+ are reported. The hfcc’s are obtained with single-reference configuration interaction, second-order Møller–Plesset, density functional (B3LYP, PW91PW91) methods, and 6-311+G(2df ) basis sets. The 2σg/3σ SOMOs of X 2Σg+(1σg21σu22σg)/X 2Σ+(1σ22σ23σ) mainly have a pσ–pσ composition, leading in most cases to similar values of Adip and Aiso. As a result, |A∥| is up two orders of magnitude larger than |A⊥|. The A⊥’s are slightly negative (ca. −10 MHz) for Al2+, Ga2+, and AlGa+. The g-shifts (Δg=g−ge) are evaluated with multireference CI wave functions, perturbation expansions up to second-order, and 6-311+G(2d) basis sets. Both Δg∥ and Δg⊥ are negative, but Δg∥ lies close to zero. The Δg⊥’s of B2+, Al2+, Ga2+ are about −1 300, −12 800, −97 300 ppm, respectively, while for BGa+, BAl+, AlGa+, they are much smaller (−800, −2 800, −47 400 ppm). The reduced Δg⊥’s for XY+ result from the mutual cancellation between a positive contribution from the 1 2Π(3σ→1π) state but a negative one from 2 2Π(3σ→2π). The positive contribution is at variance with the rule-of-thumb stating that SOMO→virtual MO excitations should contribute negatively. The variation of the hfcc’s with bond distance is analyzed for all systems, and that of the Δg⊥ component for B2+ and BAl+. Experimental or previous theoretical electron-spin resonance data are not available for comparison.