Theoretical study of the electron-spin magnetic moments (g-factors) of F2− and Cl2− (X2−), as well as MX2 and M2X2+ compounds with M=Li, Na Original Research Article

The electron-spin magnetic moments, as parameterized by the g-tensors, are calculated for the radicals F2−, LiF2, NaF2, Li2F2+, LiNaF2+, Cl2−, LiCl2, NaCl2 and Li2Cl2+. The approach is based on second-order perturbation theory using a Breit–Pauli Hamiltonian. First-order terms, generally lying around −500 ppm (ppm=10−6), are calculated at the ROHF level. Second-order contributions are obtained as a sum-over-states (SOS) expansion of multireference CI wavefunctions. These expansions are dominated by the magnetic coupling of the ground state with just one or two excited states. For all radicals, the parallel g-shifts (along X–X) are about −400 ppm. For F2− and Cl2−, the perpendicular Δg⊥-value (Δg⊥=g⊥−ge) is 16 900 and 34 000 ppm, respectively. Similar values were obtained for the radicals MX2 and M2X2+. As shown in the case of F2−, Cl2− and LiF2, the Δg⊥-values increase strongly with increasing halogen–halogen bond distance. This geometry dependence is practically linear, and is mainly dictated by the variation of 1/ΔE with R(X2), where ΔE is the 12Πu(X2−) excitation energy. The vertical spectra and oscillator strengths are also briefly discussed. Strong absorption bands of these radicals are confirmed to arise from the transition 12Σ+g←X2Σ+u in F2− and Cl2−, and corresponding transitions in the MX2 and M2X2+ radicals.